OR/18/012 Appendix 6 – Case studies

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Loveless, S, Lewis, M A, Bloomfield, J P, Terrington, R, Stuart, M E, and Ward, R S. 2018. 3D groundwater vulnerability. British Geological Survey Internal Report, OR/18/012.

Case Study 1: Conventional oil and gas, Southeast England

Hydrocarbon source and extraction method

Portland Group, East Sussex (Figure A6.1), conventional oil and gas reservoir approximate location shown by the letter ‘T’ in Figure A6.1 and Figure A6.2.
AOI
Extending to 2 km from vertical borehole.
Geological setting
The AOI lies on the boundary between the Weald Basin and the Wessex Basin, on the north side of the South Downs. In the Wessex Basin a thin (~50 m in thickness) layer of the Triassic-aged Mercia Mudstone Group overlies a basement of Dinantian (Carboniferous) and Devonian-aged rocks, which decrease in depth southwards from ~1600 m below OD beneath the Weald Basin to ~1000 m below OD beneath the South Downs and the English Channel. Sedimentary rocks of Jurassic to Cretaceous age (Lias Group to Wealden Group) overlie the Mercia Mudstone Group. The Wealden Group rocks crop out in the Weald Basin. The thickness of the Cretaceous is about 1600 m. This sequence becomes thinner (600 m in thickness beneath the English Channel) and shallower (base of sequence 1000 m below OD beneath the English Channel) to the south. Here, the Wealden Group and Purbeck Group are truncated by an unconformity and covered by younger Cretaceous rocks (Lower Greensand to Chalk Group). While the Lias Group to Kimmeridge Clay Formation are relatively horizontal over the basement platform, the Lower Greensand Formation, Wealden Group and Chalk Group dip to the south (Figure A6.2). There are a number of large scale faults in the area particularly those associated with large scale east-west monoclines in the Wessex Basin.
Conceptual model
The conceptual geological model for the AOI across (north-northeast – south-southwest) and along (west-northwest – east-southeast) strike, is shown in Figure A6.3. Vulnerability and risk have been assessed for both the overlying and the underlying units in this AOI because the activity would be <1200 m bgl. The AOI lies along the LFV section UK_Reg8_Sec220 (Figure A6.1). A number of boreholes (1 km to the southwest, 4 km to the west and 2 km to the north) terminate in the Lower Greensand, thus providing some evidence regarding the depth and thickness of the Chalk and Gault. These borehole records indicate that there is little variability in the thickness of these units across the AOI, although the topography impacts the Chalk thickness (it is thicker under higher topography). No boreholes in the area penetrate the base of the Lower Greensand, thus the conceptual model from the top of this formation downwards is based on the LFV cross section. Consequently, there is some uncertainty regarding the geometry of units beneath the base of the Gault. Nevertheless, similar to the top two units, there appears to be little variation along or across strike in the AOI. A general geological sequence and unit descriptions are shown in Table A6.1.

A number of large-scale faults (marked on the 1:625 000 geological map) strike west-northwest – east-southeast about 6 km to the northeast of the centre of the AOI. These faults cut the Wealden Group outcrop, between the Weald and Wessex Basins. The AOI also lies approximately along-strike of an east–west trending fault which appears to have offset the Wealden Group. On the 1:10 000 geological map there is a 2 km long north-northeast–south-southwest striking fault which is mapped as having offset the Chalk Group laterally by about 80 m (vertical throw <= 10 m to west — thus, this could be a normal fault or a strike-slip fault) in the southeast of the AOI. This is drawn as having ~50 m vertical throw in the conceptual model in Figure A6.3 because the cross section is beyond the mapped surface extent of the fault and the vertical throw cannot be determined from the map. A number of other, north-northwest – south-southeast striking faults mapped as about 600 m in length, but with a similar horizontal displacement of the Chalk Group, could cross the along-strike conceptual section in the east but there is no map evidence for these crossing into the AOI.

Baseline methane
Bell et al. (2015)[1] sampled for methane concentrations in aquifers in the southeast as part of the methane baseline survey of Great Britain. Six sites were sampled within the area shown in Figure A6.1; two from the Chalk and four from the Wealden Group (Hasting Subgroup; Tunbridge Wells Sand Formation and the Ashdown Formation). One location is about 8 km west of the AOI, along strike. It was found that methane concentrations were above the detection limit in the region and lower in the Chalk than in the Wealden Group. Two samples from the same borehole in the Wealden Group (the sample was repeated) in the northeast of the region shown in Figure A6.1 were over the groundwater equivalent LEL for methane (Baseline methane). The authors describe a known zone of shallow methane in this region and hydrocarbon well logs report significant gas in the shallow Cretaceous sandstones of this area. While additional analysis implied a thermogenic source it was not possible to clarify using isotopic investigations. The authors conclude that given the shallow nature of the gas, the methane could be thermogenic, having migrated up from depth, or biogenic, having formed from thin lignite layers in the Weald Clay. They state that the spatial distribution, source and hydrogeological controls on this methane occurrence are poorly understood.
Potential receptors Classification
Aquifer designations were obtained from the LFV, based on EA aquifer designations (Figure A6.4). Where model units were classified as variable aquifers (Wealden, Portland, Corallian and Lias groups), EA aquifer designation maps were used to identify the designation in this particular region and with a comparable lithology. In this instance the outcrops of these units were predominantly along the coast to the west of the AOI, from the Isle of Wight to Charmouth. These units have the same lithologies, despite being up to 160 km from the AOI (for the Lias). For the Wealden Group, the outcrop is much closer, in the Weald Basin, but there are multiple aquifer designations for this group depending on the formation. This is also the case for the Lias in the Charmouth area. Where there are multiple designations, the most sensitive designation has been applied to the whole unit.
Chalk A – principal aquifer <400 m bgl, a record from a borehole 1 km to the south of the AOI drilled into the Chalk to a depth of 13 m indicates a groundwater TDS of 583 mg/l. Another borehole drilled to 39 m bgl had a measured TDS of 412 mg/l.
Gault D – unproductive strata
Lower Greensand A – principal aquifer <400 m bgl, borehole record in the south of the AOI indicates a TDS of 447 mg/l in this unit.
Wealden Group B – secondary aquifer <400 m bgl
Purbeck Group B – secondary aquifer <400 m bgl
Portland Group A – principal aquifer <400 m bgl
Kimmeridge Clay D – unproductive strata
Corallian Group C – secondary aquifer >400 m bgl
Kellaways and Oxford Clay D – unproductive strata
Great Oolite Group B – principal aquifer >400 m bgl
Inferior Oolite Group B – principal aquifer >400 m bgl
Lias C – secondary aquifer >400 m bgl
Hazard Score
Release mechanism of hydrocarbon No permeability enhancement (passive) for conventional oil and gas.
Head gradient driving flow Incomplete picture of groundwater head distributions in the AOI, or region, at depth. The Hydrogeological Map of the South Downs and adjacent parts of the Weald (IGS, 1978[2]) shows that shallow groundwater heads in the Chalk Group largely follow topography, suggesting groundwater flow in this unit is broadly towards the centre of the upper cross section and to the west in the lower cross section (Figure A6.3). Groundwater head contours for the Lower Greensand appear to be less variable locally, and suggest groundwater flow from the north-east to south-west.

Borehole records 4 km west and 6 km northeast of the AOI indicate that there can be upwards head gradients leading to artesian conditions in the Lower Greensand. It is not known from what depth this upward gradient applies so an upwards gradient from the source (hydrocarbon source unit) to overlying potential receptors cannot be ruled out. There is also a lack of evidence regarding the direction of head gradients from the hydrocarbon source unit reservoir to the underlying units, so a downwards gradient from the hydrocarbon source unit to these units is also not excluded.

Intrinsic vulnerability
Vertical separation distance between source and base of receptor Depth of the Chalk and the Gault potential receptor units are relatively well known due to their limited lateral variability and availability of records from a number of boreholes. There are no borehole records below the Gault, and greater uncertainty associated with the depth of these potential receptors. Nevertheless, the limited area (diameter 4 km) and the little variability across the LFV sections indicate there is probably little variability across the AOI. The vulnerability assessment has been performed for the centre of the AOI.

The confidence is low due to the unknown depth to potential receptors beneath the Lower Greensand and to the hydrocarbon source unit itself.

Lateral separation distance between source and receptor A fault 1 km to the east, across-strike (Figure A6.3), could bring the Wealden Group into contact with the Portland Group hydrocarbon source unit. Small-scale variability could also bring the vertically adjacent units (Purbeck Group and Kimmeridge Clay) into contact with the hydrocarbon source unit and can also be considered laterally connected. The thickness of the Kimmeridge Clay is too great to allow for lateral connectivity of the Corallian Group and the hydrocarbon source unit across the fault.

The confidence is medium because there is little variability in depth and thickness of units across the AOI. However, the throw on the fault is not known.

Mudstones and clays in intervening units between source and receptor The composition of the Chalk, Gault and Lower Greensand were assessed from borehole records. Units underlying this do not have borehole records in the region so their lithology was identified from the geological sheet memoir (Lake et al., 1987[3]).

Units directly above or below the hydrocarbon source unit are not separated by any intervening units. Above the hydrocarbon source unit, the Purbeck Group (limestone and mudstone) and Wealden Groups (mudstone, sandstone and siltstone) are estimated to be 50% mudstone, the Lower Greensand predominantly sandstone (estimated 0% mudstone from borehole records) and the Gault 100% mudstone (although there are occasional sandstone beds). The cumulative mudstone thickness increases up the sequence with distance from the hydrocarbon source unit, with the class ‘A’ potential receptors — the Lower Greensand and the Chalk expected to have about 98 and 178 m of mudstone, respectively, between their bases and the top of the hydrocarbon source unit formation.

There are a number of thick mudstone units in the geological sequence underlying the hydrocarbon source unit, including the Kimmeridge Clay directly beneath the hydrocarbon source unit (203 m mudstone), and the Kellaways and Oxford Clay formations (221 m mudstone).

The confidence level for this factor is medium because there are no borehole logs nearby which indicate the unit lithologies below the Lower Greensand and confidence of the correct assignment of these is only moderate.

Groundwater flow mechanism in intervening units between source and receptor, including the receptor The Portland and Purbeck Groups are carbonates, thus there is potential for these units to have solutionally-enhanced fracture networks which are well connected. Permeability in the Wealden Group is likely to be low and dominated by intergranular flow. The Wealden Group will dominate the cumulative flow type above the hydrocarbon source unit at this point (>50% intergranular flow) because of the large expected thickness (140 m). The Lower Greensand also has high intergranular permeability. The Gault is not a potential receptor class A to C and therefore is not included in the cumulative flow type. The Chalk also has a potential for solutionally enhanced fracture networks and is therefore likely to have well connected fractures but this will not alter the cumulative flow type of the interval above the hydrocarbon source unit due to its limited thickness in comparison to the units dominated by intergranular flow.

Beneath the hydrocarbon source unit, the Kimmeridge Clay and the Kellaways and Oxford Clay Formations are not included in the groundwater flow assessment because they are not potential receptors A to C. The Corallian, Great and Inferior Oolite groups are likely to have well connected fracture networks. The Lias is expected to be fractured but not generally well connected. Beneath the hydrocarbon source unit the cumulative flow type is >50% well connected fractures.

The confidence level for this factor is medium because there is no borehole information for most of the units.

Faults cutting intervening units and receptor The AOI lies approximately along-strike of an east–west trending fault which appears to have offset the Wealden Group. On the 1:10 000 geological map there is a 2 km long north-northeast – south-southwest striking fault which is mapped as having offset the Chalk by about 80 m along-strike (thus this could be a normal fault or a strike slip-fault) in the southeast of the AOI. This is shown as having ~50 m vertical throw in the conceptual model in Figure A6.3 because the cross section is beyond the mapped extent of the fault and the vertical throw cannot be determined from the map. This fault is about 1 km from the hypothetical hydrocarbon activity. A number of other, north-northwest – south-southeast striking, 600 m long, faults with a similar horizontal displacement of the Chalk could cross in the far east of the along-strike cross section but there is no mapped evidence for them crossing into the AOI and they are therefore considered >2 km from the lateral extent of the activity. There is no evidence to suggest whether the fault 1 km from the activity is transmissive.

The confidence level for this factor is medium because the maps point to some evidence for faults; however they are not mapped directly within the AOI, and there is no information regarding their hydraulic properties.

Solution features in intervening units and receptor Many of the geological units have potential for developing solution or karst features (Farrant, 2008[4]) in the AOI due to their predominantly carbonate-based compositions. These include the hydrocarbon source unit — the Portland Group — and the Purbeck, Chalk and the Corallian groups. There are records of karst features in the Chalk Group from a nearby borehole at 56 m bgl where chippings were lost into a fracture system.

Because there is no evidence to support this factor for most of the units, the confidence is medium.

Anthropogenic features-mines close to site of interest No recorded mines in AOI. The confidence for this factor is high.
Anthropogenic features-boreholes close to site of interest Because the hydrocarbon source unit is only 335 m below OD in the AOI, even shallow (<100 m bgl) boreholes within the area of interest are within 600 m vertically of the hydrocarbon source unit, although no boreholes in the AOI extend to within 200 m of the hydrocarbon source unit. Three boreholes are within 0.5 km laterally of the hydrocarbon activity (the drill location). The confidence level in this factor is high.
Potential receptor Intrinsic vulnerability score Specific vulnerability score Risk group Confidence
Chalk 41.5 83 Medium/low Low
Gault 41.5 83 Low
Lower Greensand 43 86 Medium/low
Wealden Group 54 108 Low
Purbeck Group 69.5 139 Low
Portland Group 69.5 139 Medium/low
Kimmeridge Clay 61.5 123 Low
Corallian Group 37.5 75 Low
Kellaways and Oxford Clay 36 72 Low
Great Oolite Group 29.5 59 Low
Inferior Oolite Group 29.5 59 Low
Lias 28 56 Low
Figure A6.1    Hypothetical location of conventional hydrocarbon extraction in East Sussex with geology and LFV sections in the region. T indicates the approximate location for the hydrocarbon source unit.
Figure A6.2    Cross section UK_Reg14_Sec220 from LithoFrame Viewer with the approximate location of the hypothetical hydrocarbon source unit area shown by ‘T’. Cross section location is shown in Figure A6.1 and is across strike of the basin structure. The near horizontal black line indicates 1000 m bgl — the shallowest level allowed for shale gas activities in England and Wales. Rock codes are described in Table A6.1.
Table A6.1    Rock units present in the hypothetical Southeast AOI. Descriptions are from the regional guide, colours correspond with those used in the LFV section (Figure A6.2) and the AOI conceptual model (Figure A6.3). * indicates the hydrocarbon source unit. ** indicates description from BGS Lexicon, otherwise descriptions are from the BGS sheet memoir (Lake et al., 1987[3]).
Model Unit Age Description
Chalk (CK) Cretaceous White Chalk is chalk with flint, the Grey Chalk is marly chalk.
Gault Formation (GUGS, stands for Gault and Upper Greensand formations) Cretaceous In this region only the Gault Formation is present, comprising clay and is silty in parts.
Lower Greensand (LGS) Cretaceous Glauconitic silts and sands.
Wealden Group (W) Cretaceous Comprises Weald Clay; clay with thin limestones and sands. Hastings Beds Subgroup; Tunbridge Wells Sands, Wadhurst Clay and Ashdown Beds.
Purbeck Group (PB) Jurassic/Cretaceous Purbeck Beds, mudstones and limestones with gypsum and anhydrite at base.
Portland Group (PL)* Jurassic Mudstones, siltstones, sandstones and limestones.
Kimmeridge Clay Formation (KC) Jurassic Mudstones and cementstones.
Corallian Group (CR) Jurassic Mudstones, siltstones and argillaceous limestones.
Kellaways and Oxford Clay Formations (KLOX) Jurassic Predominantly Oxford Clay (mudstone), with underlying sandstone with silt and mudstone.
Great Oolite Group (GOG) Jurassic Limestones and oolites overlying argillaceous beds.
Inferior Oolite Group (INO) Jurassic Oolitic limestone.
Lias (Li) Jurassic Predominantly the Lower Lias mudstones and limestones in this area, with the Middle and Upper Lias comprising mudstones and siltstone.
Mercia Mudstone Group (MMG) Triassic Calcareous mudstone, mudstone and silty mudstone with subsidiary anhydrite-stone, gypsum stone, halite, sandstone and siltstone and trace breccia **.
Dinantian Rocks (DINA) Carboniferous Limestone and sandstone with subsidiary dolostone and mudstone.
Devonian Rocks (DEV) Devonian Conglomerate, limestone and mudstone**.
Figure A6.3    Conceptual model of the AOI for the hypothetical conventional oil and gas site in the Southeast. The hydrocarbon source unit is the Portland Group.
Figure A6.4    Potential receptor classifications for units within the conceptual model of the AOI.
Figure A6.5    Conceptual model for the AOI in the southeast of England for potential oil and gas extraction from the Portland Group. Top to bottom; potential receptor classifications, intrinsic vulnerability scores, specific vulnerability scores and risk group for each potential receptor. See Table A6.1 for unit code translations. The confidence for this assessment is low. Boundaries used for intrinsic and specific vulnerability and risk groups are used for preliminary purposes.

Summary of Case Study 1: Conventional oil and gas in the Southeast

  • Important potential receptors are found at a range of depths, interspersed with units with lower potential receptor classifications. The hydrocarbon source unit (Portland Group) remains a potential receptor class ‘A’ as there is no information to the contrary. Nevertheless, if it is a hydrocarbon source unit the water quality within this unit is unlikely to be potable and would therefore be downgraded to a ‘B’ or, more likely, ‘C’. Large distances between the AOI and the outcrop of many of the potential receptors (up to 160 km away for the Lias) suggests that groundwater quality is likely to be relatively poor thus if there were groundwater quality data it is expected that their classification would be down-graded, and consequently also the risk group. More local information regarding the rock properties and water quality of potential receptors needs to be obtained to be confident about their classifications.
  • Intrinsic vulnerability scores for the potential receptors are quite varied, ranging from 28 to 69.5 — about average for all of the case studies. The intrinsic vulnerability of units underlying the hydrocarbon source unit are lower (between 37.5 and 28), primarily due to the absence of anthropogenic disturbance by boreholes at these depths. A fault could provide a potential contamination pathway for all of the units within the AOI.
  • The specific vulnerability scores for the potential receptors are low (56 to 139) as a result of the expected low hazard nature of conventional hydrocarbon extraction activities compared to other technologies. This is despite an assumed head gradient from the source to the potential receptors both above and beneath the hydrocarbon source unit. In the AOI, the risk group, which considers both the potential receptor classification and the specific vulnerability score, is medium/low for the potential receptors classified as ‘A’; the Chalk and Lower Greensand and the Portland Group. This is the lowest risk group possible for a class ‘A’ potential receptor and recognises that there is always an element of risk when interacting with the subsurface. It is important to improve the understanding of water quality of these potential receptors, since the downgrading of the units, particularly in the case of the Portland Group, to B or C would lower the risk group, which is more realistic for this unit based on the assumption that it is likely to already contain hydrocarbons. The risk group for all other potential receptors is low, due to their low specific vulnerability scores.
  • The confidence level in the intrinsic vulnerability scores is low because of the uncertainty associated with the depths and thicknesses of units below the Gault. To increase the overall confidence, it would be advisable to use additional information from new boreholes, or to improve understanding of the variability of thicknesses and depths of units within the geological sequence in the region. The confidence in the head gradient is also low.
  • The National Methane Baseline Survey (Bell et al., 2015[5]) indicated that there are some naturally occurring areas of high methane concentrations in the region, specifically in the Wealden Group. It is currently unclear as to the source of this methane (biogenic or thermogenic), and it could be the Wealden Group, but methane might also have travelled from greater depth. This should be investigated further because, if the latter case, it might indicate that there are migration pathways in the region and in the AOI.

Case Study 2: Coal bed methane, West Midlands

Hydrocarbon source and extraction method

Pennine Coal Measures Group with 2 km lateral wells assumed in any direction. The release mechanism has been specified as CBM. Coal beds are towards the top of the Coal Measures Group.
AOI
Extending to 2 km from lateral borehole, total radius of 4 km
Geological setting
The AOI lies at the northern margin of the Cheshire Basin, north West Midlands, the approximate location is shown by the letter ‘T’ in Figure A6.6 and Figure A6.7.

The Cheshire Basin is a deep basin underlying most of Cheshire, and towards the north under Manchester and south under Shropshire. The basin fill is primarily Permian and Triassic sandstones and mudstones, with some halite beds. The Permo-Triassic infill reaches up to 4 km depth in some places. Coal Measures of variable thickness underlie the Permian-aged rocks across much of the basin and are at outcrop around the margins of the basin. Along the northern margin of the basin the Coal Measures can be more than 1300 m in thickness. At the location of the AOI the Coal Measures are covered by Triassic aged-rocks and have a regional dip to the southeast, towards the centre of the basin. The 1:625 000 geological map shows that rocks in the area are cut by numerous north-northwest – south- southeast trending faults (Figure A6.6).

The Cheshire Basin has a history of oil and gas exploration, with many formations belonging to the Sherwood Sandstone having potential as oil reservoirs (DECC, 2013a[6]). The Mercia Mudstone (present across the centre of the basin) has been identified as having potential as an oil reservoir and source rock. The Halesowen Formation of the Warwickshire Group is a known reservoir. There is an oil seep from Westphalian-aged sandstones near Ironbridge in the Cheshire basin. The Millstone Grit is also a potential reservoir in the Cheshire basin. The Namurian Holywell/Bowland Shales of the Craven Group are source rocks in the Cheshire Basin.

Conceptual model
The geological sequence was determined from cross sections in the 3D LFV project and a number of deep boreholes north of the centre of the AOI (Figure A6.6). There were no deep boreholes to the south of the AOI and there is consequently lower confidence in the depth and thickness of units in the south. There is a high variability in the thickness and depth of geological units in the sequence which introduces greater uncertainty as to their depth and thickness. The general geological sequence and lithological descriptions are shown in Figure A6.8 and Table A6.2.

The Sherwood Sandstone outcrops across the AOI. The Mercia Mudstone crops out for 0.5 km of the southerly part of the AOI. Throughout the AOI the hydrocarbon source unit and overlying units dip to the south-southeast. In the north, the hydrocarbon source unit is at depths <400 m below OD and in the south >1500 m below OD. The hydrocarbon source unit thickness is ~500 m throughout the AOI. The overlying Warwickshire Group is about 50 m in thickness, the Permian Appleby Group (Collyhurst Sandstone) is 150 m thick in the north to 300 m thick in the south, the Cumbrian Coast Group (Manchester Marls) is 100 m thick in the north to 150 m thick in the south, and the Sherwood Sandstone is from 20 m thick in the north to 1000 m thick in the south.

From the west to east cross-section it can be seen that the centre of the AOI lies in a small graben. Units are shallower to the west than the east; the hydrocarbon source unit is at about 500 m below OD in the west and 1000 m below OD in the east. A number of borehole logs document faults at depth, including the fault to the west of the centre of the AOI which has about 300 m throw recorded. The Permian Appleby Group is ~50 m thinner in the west than the east, and it thickens (to ~300 m) into the fault immediately east of the centre of the AOI. The Warwickshire Group decreases in thickness west to east across the central graben.

A number of large faults (marked on the 1:625 000 geological map) cross the AOI in a north-south direction, and are shown to cut all of the units. Two of these are also identified in borehole logs. To the west and east of the centre the faults possibly bring the Collyhurst Sandstone into horizontal contact with the hydrocarbon source unit. Another fault runs approximately east-west about 2 km north of the AOI, the throw is thought to be smaller on this fault.

The vulnerability assessment is made for the hydrocarbon source unit towards the potential receptor units overlying the hydrocarbon source unit. Due to the variability an assessment has been made for the north, centre and south of AOI.

Baseline methane
Bell et al. (2015)[5] sampled two sites within the area shown in Figure A6.6, from the Sherwood Sandstone. One location is about 30 km south of the AOI and the other 25 km northeast, both of these were from the unconfined aquifer. It was found that methane concentrations were above the detection limit in the region, although well below the groundwater equivalent LEL (Section 6.1). The highest methane concentration recorded in the region was to the northeast, although this site is close to central Manchester and beneath a cover of 6 m boulder clay that could cause reducing conditions in the aquifer. However, the maximum recorded methane is well below other areas.
Potential receptors Classification
The Warwickshire Group and Cumbrian Coast Group (Manchester Marls) were classified as variable aquifers but both are designated secondary aquifers in this region.

A borehole record immediately to the east of the north-south trending fault which is east of the graben (centre) records water that ‘overflowed at the surface and is very saline’. A borehole in the central graben (but only 300 m from the estuary) also recorded slightly saline water (SEC at 25°C of >7300 µS/cm, thus TDS approximately 4000 mg/l). The first borehole is 1.2 km away from the estuary. Across the fault to the west of the central graben, a borehole record states a TDS of 316 mg/l from a depth of about 60 m bgl. This suggests that there could be separation of groundwater flow across the western fault, and it may be acting as a barrier to cross-fault flow.

While the evidence for differing flow systems across the north-south trending faults is limited, this conceptual model will be used to demonstrate the process and impacts of down-grading potential receptors based on groundwater chemistry. Here, potential receptors classified as ‘A’ on the east of the western fault have been downgraded to potential receptor class ‘B’. Evidence suggests that this would not be appropriate west of the fault, and the salinity of groundwater north of the east-west trending fault is not known therefore potential receptors have not been downgraded here either (Figure A6.9).

Mercia Mudstone Group B – secondary aquifer, <400 m bgl
Sherwood Sandstone Group A – principal aquifer, <400 m bgl but B east of western fault due to recorded salinity
Cumbrian Coast Group B – secondary aquifer, <400 m bgl
Appleby Group A – principal aquifer, <400 m bgl but B east of western fault due to recorded salinity
Warwickshire Group C – secondary aquifer, >400 m bgl
Pennine Middle Coal Measures Group C – (secondary aquifer, >400 m bgl)
Millstone Grit Group C – (secondary aquifer, >400 m bgl)
Craven Group C – (secondary aquifer, >400 m bgl)
Hazard Score
Release mechanism of hydrocarbon Water table lowering and depressurisation (CBM)
Head gradient driving flow A borehole record from the east of the eastern north-south fault states that groundwater ‘overflowed at surface and is very saline’ (the formation is not stated). Although the source of this water is not known, there could be upwards flow from the hydrocarbon source unit formation towards the overlying potential receptors.

Downing et al. (1987)[7] state that the Permo-Triassic sandstone of the Mersey Valley (which the AOI is located within) is one of the main outlets for groundwater in the northwest of England. However there is limited direct evidence for flow from the Upper Palaeozoic rocks into the Permo-Triassic sandstones. There is also no evidence for a heat flow anomaly in the Cheshire Basin, suggesting that there is not significant rising groundwater. This supports the hypothesis that the regional groundwater flow in the Cheshire Basin is primarily around deeper parts of the basin in essentially horizontal directions towards outlets in the Mersey Estuary (Downing et al., 1987[7]).

However, the AOI is on the north side of the Mersey Estuary, and groundwater is more likely to flow from the north. It is also possible that the fault between the borehole and the site of interest leads to different groundwater flow patterns. It is not clear if groundwater flow is still in an upwards direction from the underlying potential receptors to the source. Therefore, a worst case scenario with, groundwater head direction from the hydrocarbon source unit towards potential receptors, is assumed.

Intrinsic vulnerability
Vertical separation distance between source and base of receptor There are better controls on the depth of the units in the north of the AOI than in the south due to the presence of a number of boreholes in the Coal Measures. However, there is a lot of variability in the depth and thickness of units in the AOI due to a number of faults, and the impact of these is not completely known.

There is a large difference in the depth and thickness of units from the north to the south, and so the vulnerability assessment was conducted in the centre, the north and the south of the AOI.

In the north of the AOI, the hydrocarbon source unit occurs at shallower depths than in the centre (500 m below OD versus 1000 m below OD). Therefore, an assessment has also been made for the underlying Millstone Grit in the north. In this case, vertical separation is calculated between the base of the hydrocarbon source unit and the top of the Millstone Grit. Here, it is 0 m because the Millstone Grit directly underlies the hydrocarbon source unit.

The confidence levels attributed to the assessments in the centre and north of the AOI are medium, since there is some borehole control, but also a high degree of variability. However, the confidence attributed to the assessment in the south of the AOI is low because of a lack of borehole information and assumed high variability.

Lateral separation distance between source and receptor The Cumbrian Coast, Appleby, Warwickshire groups and Upper Coal Measures Formation are brought to the same horizontal level as the hydrocarbon source unit within 0.2 km of the sub-surface activities due to the regional dip to the south (Figure A6.8). The other units are not expected to be at the same horizontal level within the AOI. The confidence level for this factor is medium and is dependent on the conceptual model.
Mudstones and clays in intervening units between source and receptor The composition of the units was assessed from borehole records and the regional guide.

Units directly above or below the hydrocarbon source unit are not separated by any intervening units. Above the hydrocarbon source unit, the Upper Coal Measures and Warwickshire Group have been assessed as being approximately 50% mudstone. The Appleby Group in this area comprises the Collyhurst Sandstone — a coarse-grained sandstone. The Cumbrian Coast Group comprises the Manchester Marl. The Sherwood Sandstone comprises predominantly sandstones. There are no boreholes penetrating the Millstone Grit, and it is assumed from regional information that this unit is 50% mudstone.

In the centre of the AOI, the Sherwood Sandstone has up to 350 m mudstone in the intervening unit between it and the hydrocarbon source unit, resulting from the thick Manchester Marls and the variable composition of the Coal Measures (including the Warwickshire Group). In the north of the AOI, the intervening mudstone thickness is only 230 m.

The confidence level for this factor is medium because there are a number of borehole logs nearby which indicate the unit lithologies but this factor is also dependent on the thickness of the units.

Groundwater flow mechanism in intervening units between source and receptor, including the receptor The Sherwood Sandstone and Appleby Group (Collyhurst Sandstone) are expected to be dominated by intergranular flow. The finer grained and older units (the Cumbrian Coast, Coal Measures and the Millstone Grit groups) are likely to be dominated by poorly connected fracture flow. The dominant flow type changes from >50% fractured, poorly connected or mixed fracture and intergranular flow below the Sherwood Sandstone to >50% inter-granular flow for the Sherwood Sandstone.
Faults cutting intervening units and receptor A number of large faults (marked on the 1:625 000 geological map) cross the AOI in a north-south direction, and are shown to cut all of the units. Two of these are identified in borehole logs. Another fault runs approximately east-west about 2 km north of the AOI; the throw is thought to be smaller on this fault. The closest mapped fault is about 0.5 km to the east of the centre. The confidence level for this factor is high.
Solution features in intervening units and receptor The only unit which is specified as having potential solution features in the AOI is the Mercia Mudstone. This is known to contain halite and gypsum in the Cheshire Basin. However, this is only present at the southern boundary of the AOI and will not impact the units below. A borehole record at the northern boundary of the AOI reports a cavity within the Bunter (Sherwood) Sandstone. The cause of this cavity is not reported and it is not clear from the log whether halite or gypsum are present, although these are a possibility in Triassic units. It will, therefore, be assumed that there could be solution features in this unit but this will have a low confidence level.
Anthropogenic features-mines close to site of interest There are mines recorded in the northern part of the AOI. The confidence in this factor is high.
Anthropogenic features-boreholes close to site of interest There are many boreholes within 0.5 km of the AOI including one within 200 m vertical distance from the hydrocarbon source unit, as determined from the borehole layer in the GIS project. The confidence level in this factor is high.
Potential receptor Intrinsic vulnerability score Specific vulnerability score Risk group Confidence

North

Sherwood Sandstone Group 57.5 230 Medium/low Low
Cumbrian Coast Group 58 232 Low
Appleby Group 76.5 306 Medium/low
Warwickshire Group 76.5 306 Medium/low
Pennine Upper Coal Measures Group 85 340 Low
Millstone Grit Group 85 340 Low

South

Mercia Mudstone Group 44.5 178 Low Low
Sherwood Sandstone Group 51 204 Low
Cumbrian Coast Group 51.5 206 Low
Appleby Group 68.5 274 Medium/low
Warwickshire Group 70 280 Medium/low
Pennine Upper Coal Measures Group 28 340 Low
Figure A6.6    Hypothetical location of CBM extraction in the West Midlands with geology and LFV sections in the region. T indicates the approximate location for the hydrocarbon source unit.
Figure A6.7    LFV sections with the case study site in the West Midlands for CBM along strike of ‘T’. The hydrocarbon source unit is the Pennine Middle Coal Measures Formation (PMCM). See Table A6.1 for units described by codes, WEN is Wenlock rocks and ORD is Ordivician rocks. The near horizontal black line indicates 1000 m bgl, the shallowest level allowed for shale gas exploitation in England and Wales.
Table A6.2    Rock units present in the hypothetical West Midlands AOI. Descriptions are from the sheet memoir, colours correspond with those used in the LFV sections (Figure A6.7) and the AOI conceptual model (Figure A6.8). * indicates the hydrocarbon source unit.
Model Unit Age Description
Mercia Mudstone Group (MMG) Triassic Mudstone and siltstone, gypsiferous
Sherwood Sandstone Group (SSG) Triassic Comprises the Helsby Sandstone Formation (sandstone, slightly or well cemented), Wilmslow Sandstone Formation (sandstone, slightly cemented), Chester Pebble Beds Formation (sandstone with pebbles, moderately cemented), Kinnerton Sandstone Formation (sandstone, slightly cemented)
Cumbrian Coast Group (CCO) Permian Comprises the Manchester Marls Formation (mudstone, gypsiferous)
Appleby Group (APY) Permian Comprises Collyhurst Sandstone Formation (coarse-grained sandstone)
Warwickshire Group (WWAK) Carboniferous Mottled mudstone with common beds of sandstone, and Etruria Marl Formation (fine- grained mudstone)
Pennine Coal Measures Group* (PUCM/ PMCM/ PLCM) Carboniferous Mudstone, sandstone seatearth and coal.
Millstone Grit Group (MG) Carboniferous Sandstone with mudstone common throughout.
Figure A6.8    Conceptual model of the AOI for the hypothetical CBM site in the West Midlands. The hydrocarbon source unit is the Pennine Coal Measures Group.
Figure A6.9    Receptor classifications for units within the conceptual model of the AOI in the West Midlands.
Figure A6.10    Conceptual model for the AOI in the West Midlands of England for CBM from the Pennine Coal Measures Group. Top to bottom; potential receptor classifications, intrinsic vulnerability scores, specific vulnerability scores and risk group for each potential receptor. Intrinsic vulnerability scores are provided for the north, centre and south of the AOI. The risk group is only shown for the centre. See Table A6.2 for unit code translations. The confidence for this assessment is low. Boundaries used for intrinsic and specific vulnerability and risk groups are used for preliminary purposes.

Summary for Case Study 2: Coal bed methane, West Midlands

  • In the AOI the Sherwood Sandstone and the Appleby Group (Collyhurst Sandstone) have been downgraded from potential receptor class ‘A’ to ‘B’ in the south and east, based on borehole evidence of water chemistry. Evidence suggests that this would not be appropriate west of the fault. The salinity of groundwater north of the east-west trending fault is not known; therefore potential receptors have not been downgraded here either. More information is needed to be certain of these classifications. Intrinsic vulnerability scores are relatively high for all of the potential receptors, ranging from 85 to 44.5 resulting from the limited vertical separation between the Pennine Middle Coal Measures Formation and the potential receptors. The depth of the Coal Measures varies from 500 m below OD in the north to 1000 m below OD in the south, resulting in an intrinsic vulnerability score difference of up to 5 for the units furthest from the hydrocarbon source unit. However, this does not change the intrinsic vulnerability categories for the potential receptors with these preliminary boundaries.
  • The specific vulnerability scores for the potential receptors are lower (178 to 340) as a result of the assumed relatively low hazard nature of CBM activities, despite an assumed head gradient from the source to the potential receptors.
  • The risk group, for the potential receptors classified as ‘A’ (the Sherwood Sandstone and the Appleby Group to the north and west) is medium/low but where these units are downgraded to potential receptor class ‘B’, they are in the low risk group, indicating the importance of correct potential receptor classification.
  • The difference in hydrocarbon source unit depth of 500 m does not change the risk group of the units but does change the specific vulnerability scores. The assessment would improve from a greater understanding of the head distribution and groundwater flow paths and from further identification of faults and fault behaviour in the region, in addition to understanding the geological variability. It should be noted that The National Methane Baseline Survey (Bell et al., 2015[5]) did not observe areas of high naturally occurring methane concentrations in aquifers in the region.

Case Study 3: Coal bed methane, East Midlands

Hydrocarbon source and extraction method

Pennine Coal Measures Group, for CBM, Nottinghamshire with 2 km lateral wells (Figure A6.11). Here, the Pennine Coal Measures Group comprises the Pennine Lower and Middle Coal Measures Formations. The top of the hydrocarbon source unit is identified as the uppermost point in the Pennine Coal Measures in which coal seams begin to increase in prevalence within the succession.
AOI
Extending to a distance 2 km from lateral boreholes.
Geological setting
East Midlands region of the Carboniferous-aged Pennine Basin, which stretches from the Midlands to the Scottish Border (T in Figure A6.11, middle of the cross sections in Figure A6.12). The region has a history of conventional oil and gas extraction and coal mining and has been extensively explored with boreholes and seismic surveys.

Carboniferous and older rocks become deeper eastwards from the Pennines where they are at the surface, towards the North Sea where they are overlain by post Carboniferous rocks which also dip to the east.

Coal Measures are thicker in the west than the east (>1000 m compared with ~200 m in some places in the east). The maximum depth of the Coal Measures from east to west is quite constant (~700 m); however the unit deepens slightly to the northeast, reaching 900 m below OD. The AOI is located 11 km to the east of the outcrop of the Pennine Coal Measures Group (Figure A6.11).

Below the Coal Measures are the Craven Group and Millstone Grit Group (both Carboniferous-age) which can reach thicknesses of 1400 m and 500 m, respectively, but are closer to 50 m in thickness in the east, above the Eakring Anticline (Figure A6.12).

The Permian and younger rocks overlying the Coal Measures dip gently to the east towards the North Sea and retain relatively uniform thicknesses (Figure A6.12). The base of this sequence outcrops just east of the line of section UK_Reg8_Sec143. The lowest unit in the succession is a thin (<5 m thick) unit of Permian Basal Breccia which overlies the Carboniferous age rocks (although it is not possible to see this in the sections due to its thinness). Over most of the area the Zechstein Group overlies this unit (<150 m thick), although it is thinner and sometimes absent in the south. Overlying this is the Triassic Sherwood Sandstone (~150 m in thickness). This is the second most important aquifer in the UK. To the east of the AOI the Sherwood Sandstone is overlain by the largely low permeability Mercia Mudstone (~140 m thick).

Conceptual model
The conceptual geological model for the AOI across (west-east) and down dip is shown in Figure A6.13. Because the AOI is not close to any of the LFV sections (Figure A6.11), these were only used to gain an understanding of the regional geology. More detailed information was obtained from three deep boreholes within the AOI (Figure A6.11); one 4 km to the north-northwest, one 1.5 km to the southwest and one 1.5 km to the northeast. The general geological sequence and unit descriptions are shown in Table A6.3.

The Pennine Middle Coal Measures Formation and the Pennine Lower Coal Measures Formation are combined in this analysis as the Pennine Coal Measures Group. The upper units of the Coal Measures only have occasional coal beds. The frequency of recorded coal beds increases between ~200 and 400 m bgl indicating the location of the hydrocarbon source unit interval. However, there is no conclusive evidence for a systematic variability in the depth of increased coal occurrence across the AOI. Therefore, while a depth of 300 m to the top of the hydrocarbon source unit interval has been used in the ‘best-guess’ conceptual model, scenarios in which the top is at a depth of 200 m and 400 m were also tested.

The total thickness of the Coal Measures varies between 700 m in the north and west, and 300 m east of the centre of the AOI. The base of the Coal Measures is likely to have a maximum depth of ~900 m and a minimum depth of 500 m, being shallower in the east and south. In the conceptual model, a thin (~2 m in thickness at the hypothesised borehole location) unit of Permian Basal Breccia overlies the Coal Measures (this unit is too thin to see at the scale of the conceptual diagrams and cross sections). It thickens (up to 5 m) to the west and is not present towards the east.

Overlying the Permian Basal Breccia is the Zechstein Group (described as Permian Marls in the borehole logs), which has a fairly uniform thickness of about 50 m across the AOI. This unit dips to the east along with the overlying Sherwood Sandstone. The Sherwood Sandstone thickens from about 200 m in the west to 300 m in the east and it is overlain by up to 200 m of the Mercia Mudstone in the east of the AOI.

Vulnerability has also been assessed for the underlying strata. Since no boreholes penetrate these units in the AOI there is a very high level of uncertainty regarding their thickness and depth. Nevertheless, they are thought to become shallower to the east over the Eakring Anticline. The Millstone Grit underlies the Coal Measures and varies from 300 m in thickness in the north to 50 m in the south, and is located at a depth of 900 m bgl to 500 m bgl. The Craven Group varies from 500 m to 200 m in thickness and 1200 m to 700 m bgl in depth.

The geological map (1:50 000) shows an inferred 600 m-long fault about 500 m to the southeast of the hypothetical drilling site, although it is not clear which of the geological units it cuts. In addition, sections UK_Reg8_Sec287 and 171 show that a large fault could pass ~1 km to the east of the drilling site, cutting from the basement to the base of the Pennine Middle Coal Measures with an offset of up to 400 m. Another fault passes about 200 m to the west of the site and cuts from the basement to the base of the Pennine Lower Coal Measures.

Baseline methane
Bell et al. (2015)[5] sampled for methane concentrations in aquifers in the East Midlands. Samples from 14 sites were collected from the area shown in Figure A6.11, from the Pennine Coal Measures Group, Zechstein Group and Sherwood Sandstone Group. One location is very close to the AOI. Bell et al. (2015)[5] found that methane concentrations were above the detection limit in all aquifers, but none exceeded the groundwater equivalent LEL (Section 6.1) and very few exceeded 10 µg/l. In the area, methane concentrations were generally lowest in the Sherwood Sandstone. Very slightly higher methane concentrations were noted in the confined compared with the unconfined Sherwood Sandstone aquifer, although the highest concentration (465 µg/l) was from the unconfined aquifer. This is thought to be due to the reducing conditions beneath the thick glacial sediment cover which are conducive to elevated methane concentrations. There are no glacial deposits present in the AOI.
Potential receptors Classification
Where model units were classified as variable aquifers in the LFV project (Figure A6.12) (the Mercia Mudstone and Zechstein Group), the EA aquifer designation maps were used to identify the designation based on the closest outcrop to the AOI.
Mercia Mudstone Group B – secondary aquifer, top of the unit <400 m bgl.
Sherwood Sandstone Group A – principal aquifer, top of unit <400 m bgl. Classification supported by electrical conductivity of 640 µS/cm at 60 m bgl within 10 km of the AOI.
Zechstein Group A – principal aquifer, tops of the unit <400 m bgl.
Permian Basal Breccia A – principal aquifer, tops of the unit <400 m bgl
Pennine Middle Coal Measures Group B – secondary aquifer, top of the unit is <400 m bgl.
Millstone Grit Group C – secondary aquifer, top of the unit is >400 m bgl.
Millstone Grit Group C – (secondary aquifer, >400 m bgl)
Craven Group C – secondary aquifer, top of the unit is >400 m bgl.
Hazard Score
Release mechanism of hydrocarbon Water table lowering and depressurisation (CBM)
Head gradient driving flow No direct information in the AOI or region. Bullard and Niblett (1951)[8] found an elevated heat flow in six boreholes, over the Eakring Anticline, 10 km to the northeast of the AOI. This heat flow anomaly was not present under the Kelham Hills to the south. They concluded that this is more likely to be from heat transport from groundwater which has recharged the Carboniferous Limestones in the Pennines to the west and flowed through fissures eastwards to Eakring where it is forced upwards over the anticline. They calculate that the measured heat flow could be achieved with waters flowing over thousands of years through rocks with only a 1% porosity. The relatively low TDS (3262 mg/l) of waters in the Carboniferous Limestones shown at Mansfield Number 1 borehole at 1329 m bgl, might also indicate a relatively short residence time and flow within these rocks.

The Eakring Anticline is 10 km to the northeast of the AOI; however the conceptual models indicate that the Carboniferous-aged rocks become shallower towards the east and therefore there is a chance that the waters begin to rise in this area. A borehole at Popplewick, 4 km to the south of the AOI shows an elevated geothermal gradient in the Coal Measures which might indicate upwards fluid flows. Nevertheless, temperatures measured in the Blidworth Colliery indicate a low geothermal gradient (22.9°C/km), although this may be due to local flow pathway perturbations caused by the collieries.

Because of the uncertainty and chance that there could be upward flow in the AOI an upwards head gradient is assumed as a worst case scenario. The confidence in this is medium.

Intrinsic vulnerability
Vertical separation distance between source and base of receptor Borehole logs indicate that the top of the hydrocarbon source unit interval is highly variable in the AOI so vulnerability assessments were conducted for three separation scenarios; scenario 1 was the ‘best guess’, with the top of the hydrocarbon source unit at 300 m bgl, scenario 2 was a worst case scenario with the top of the hydrocarbon source unit at 200 m bgl, and scenario 3 was a best case scenario with the top of the hydrocarbon source unit at 400 m bgl.

The depth of potential receptor units overlying the hydrocarbon source unit are relatively well known in the area due to their limited lateral variability and the availability of records from a number of boreholes.

For potential receptor units underlying the hydrocarbon source unit (Millstone Grit and Craven Group) it was assumed that the coals could be exploited to the base of the unit. Very few boreholes penetrate units below the Coal Measures and therefore the depths of these units are not well constrained.

The Mercia Mudstone was included in the vulnerability assessment since this is present in the eastern part of the AOI. However the vertical separation was not calculated since is not expected to directly overlie the activity.

The confidence in this factor is low due to the unknown depth to the top of the hydrocarbon source unit interval and to the depths of the units that are underlying the hydrocarbon source unit.

Lateral separation distance between source and receptor In the AOI the lateral separation does not apply to most units. However, there is a fault which brings the Craven Group into contact with the Coal Measures below the area of proposed activity. The Mercia Mudstone occurs within 2 km of the lateral sub-surface extension of the activity and has therefore been given a lateral separation.

The confidence for this factor is medium because there is less variability across the AOI.

Mudstones and clays in intervening units between source and receptor The thickness of mudstones and clays in the intervening layer was calculated according to the average composition of the interval based on the borehole logs.

Above the hydrocarbon source unit the Coal Measures contain a high proportion (80%) of mudstone which provides a large thickness of intervening mudstones between the hydrocarbon source unit and the potential receptors. Borehole records indicate that the Zechstein Group is also comprised predominantly of marl in this area, and so it is considered a mudstone.

The borehole record from the Mansfield number 1 borehole indicates a very high proportion of the Millstone Grit is shale. The shale content has been estimated as 90% providing roughly 270 m of mudstone/clay in between the hydrocarbon source unit and the Craven Group.

The confidence for this factor is medium because there are borehole logs which provide an indication of the unit’s composition nearby.

Groundwater flow mechanism in intervening units between source and receptor, including the receptor Groundwater flow is likely to be intergranular in the Sherwood Sandstone and Basal Permian Breccia. In the Zechstein Group groundwater flow through fractures is more common, but fractures are not likely to be well connected in the marls. Fractures are also known to control groundwater flow in the Coal Measures and older units. These are also likely to be poorly connected. Overall, poorly connected fractures dominate groundwater flow in this sequence upwards until it reaches the Sherwood Sandstone where intergranular flow becomes an important flow mechanism and flow changes to mixed-fracture and intergranular. The confidence for this factor is medium.
Faults cutting intervening units and receptor An inferred 600 m-long fault runs about 500 m to the southeast of the hypothetical drilling site. It is not clear which units it cuts. In addition, sections UK_Reg8_Sec287 and 171 show that a large fault could pass ~1 km to the east of the drilling site, cutting from the basement to the base of the Pennine Middle Coal Measures with an offset of up to 400 m. Another fault passes about 200 m to the west of the site and cuts from the basement to the base of the Pennine Lower Coal Measures. There is no evidence to suggest that any of these faults are transmissive. The confidence for this factor is medium.
Solution features in intervening units and receptor In some places, the Edlington Formation of the Zechstein Group, present in the AOI, can have solution features developed in anhydrite and gypsum beds. However, there is no direct evidence for anhydrites or gypsum beds in the Nottinghamshire area (Bullard et al., 1951[8]) or in the surrounding boreholes. Other units are unlikely to have solution features; therefore all units have been classified as having no potential solution features. The confidence for this factor is medium.
Anthropogenic features-mines close to site of interest A large proportion of the region and the AOI is part of the coal mining reporting area (Figure A6.11) and therefore there are likely to be coal mines in the area.

The confidence for this factor is high.

Anthropogenic features-boreholes close to site of interest There are two boreholes within the AOI (1.5 km to the northeast and 1.5 km to the southwest) which penetrate the Coal Measures and therefore are within 200 m vertically of the hydrocarbon source unit. The confidence for this factor is high.
Potential receptor Intrinsic vulnerability score Specific vulnerability score Risk group Confidence

300 m scenario

Mercia Mudstone Group 41.5 166 Low Low
Sherwood Sandstone Group 61 244 Medium/low
Zechstein Group 64.5 258 Medium/high
Permian Basal Breccia 64.5 258 Medium/high
Pennine Coal Measures Group 85 340 Medium/low
Millstone Grit Group 85 170 Low
Craven Group 71 142 Low

200 m scenerio

Mercia Mudstone Group 45 180 Low Low
Sherwood Sandstone Group 66 264 Medium/low
Zechstein Group 73 292 Medium/high
Permian Basal Breccia 73 292 Medium/high
Pennine Middle Coal Measures Group 85 340 Medium/low
Millstone Grit Group 85 170 Low
Craven Group 71 142 Low

400 m scenario

Mercia Mudstone Group 41.5 166 Low Low
Sherwood Sandstone Group 59.5 238 Medium/low
Zechstein Group 59.5 238 Medium/low
Permian Basal Breccia 59.5 238 Medium/low
Pennine Middle Coal Measures Group 85 340 Medium/low
Millstone Grit Group 85 170 Low
Craven Group 71 142 Low
Figure A6.11    Hypothetical location of CBM in the East Midlands, T indicates rough location for the hydrocarbon source unit.
Figure A6.12    Cross sections surrounding hypothetical hydrocarbon source unit area of CBM (hydrocarbon source unit is roughly in the centre of these cross sections). Locations of the cross sections are shown in Figure A6.1. Cross sections UK_Reg8_Sec287 and UK_Reg8_Sec171 are across strike of the basin structure and UK_Reg8_143 and UK_Reg8_Sec157 along strike. Vertical lines are the locations of intersecting cross sections and the near horizontal black line indicates 1000 m bgl, the shallowest level allowed for shale gas exploitation in England and Wales. Rock codes shown on UK_Reg8_Sec287 are described in Table A6.3.
Table A6.3    Rock units present in the hypothetical East Midlands CBM AOI. Colours correspond with those used in the LFV sections (Figure A6.21) and conceptual model (Figure A6.13). * indicates the hydrocarbon source unit which belongs to the Pennine Coal Measures Group. ** indicates description from BGS Lexicon, otherwise descriptions are from the BGS sheet memoir (Howard et al., 2009[9]).
Model Unit Age Description
Mercia Mudstone Group (MMG) Triassic Mudstone and siltstone with beds of gypsum and dolomitic mudstone and siltstone.
Sherwood Sandstone Group (SSG) Triassic Fine to medium grained locally coarse and pebbly to conglomerate sandstone.
Zechstein Group (ZG) Permian Mudstone, siltstone and sandstone, with some dolostone and conglomerate.
Permian Basal Breccia (PBB) Permian Breccia in a dolomitic limestone matrix.
Pennine Middle Coal Measures Formation (PSMCM)* Carboniferous Mudstone, siltstone and sandstone with numerous coal seams and seatearths.
Pennine Lower Coal Measures Formation (PSLCM) Carboniferous Mudstone, siltstone and sandstone with numerous coal seams and seatearths.
Millstone Grit Group (MG) Carboniferous Mudstone and siltstone with thick sandstone beds.
Craven Group (CG) Carboniferous Mudstone and siltstone.
Dinantian (DINA) Carboniferous Limestone and dolostone
Ordovician (ORD) ** Ordovician Mudstone, siltstone and sandstone
Lower Palaeozoic Rocks (LPRU) ** Lower Palaeozoic Undefined
Figure A6.13    Conceptual model of the AOI for the hypothetical CBM site in the East Midlands. The hydrocarbon source unit is the Pennine Coal Measures Group (combined unit).
Figure A6.14    Potential receptor classifications for units within the conceptual model of the AOI for CBM in the East Midlands.
Figure A6.15    Conceptual model for the AOI for potential CBM in the Pennine Coal Measures Group with upper limit of coal exploitation at 300 m bgl with units identified as potential receptors, intrinsic and specific vulnerability scores, and risk group for each potential receptor. See Table A6.3 for code translations. The confidence for this assessment is low. Boundaries used for intrinsic and specific vulnerability scores and risk groups are used for preliminary purposes.
Figure A6.16    Conceptual model for the AOI for potential CBM in the Pennine Coal Measures Group with upper limit of coal exploitation at 200 m bgl with intrinsic and specific vulnerability scores, and risk group for each potential receptor. See Table A6.3 for code translations. The confidence for this assessment is low. Boundaries are used for intrinsic and specific vulnerability scores and risk group are used for preliminary purposes.
Figure A6.17    Conceptual model for the AOI for potential CBM in the Pennine Coal Measures Group with upper limit of coal exploitation at 400 m bgl with units identified as potential receptors, intrinsic and specific vulnerability scores, and risk groups for each potential receptor. See Table A6.3 for code translations. The confidence for this assessment is low. Boundaries used for intrinsic and specific vulnerability scores and risk group are used for preliminary purposes.

Summary for Case Study 3: Coal bed methane, East Midlands

  • In this AOI there is a general decrease in potential receptor rank with depth, with the exception of the Mercia Mudstone which is classified as ‘B’, because it is a secondary aquifer. The Zechstein Group is classified as potential receptor ‘A’ but it is clear from local borehole logs that it primarily comprises Permian Marls which may not supply significant quantities of groundwater and therefore a review of this classification would be necessary.
  • Intrinsic vulnerability scores for the potential receptors in the ‘best-guess’ (hydrocarbon source unit at 300 m depth) scenario are quite varied (41 to 85) and relatively high compared with the other case studies. Intrinsic vulnerability scores are highest for units closer to the hydrocarbon source rock and with the smallest mudstone thickness in the intervening units. Potential pathways for all of the units include the faults within the AOI, one of which is thought to cut
  • all of the units, the presence of mining in the area and also boreholes which could connect the hydrocarbon source unit and potential receptors.
  • The intrinsic vulnerability scores for potential receptors overlying the coal units were sensitive to the depth of the coal unit demonstrating the sensitivity of the assessment to proximity of potential receptors and the hydrocarbon source unit and thus the importance of reducing uncertainty regarding the geometry. However, this is not a linear relationship; the closer the potential receptor to the hydrocarbon source unit the larger the potential differences in intrinsic vulnerability score with the same difference in separation distance.
  • Specific vulnerability scores are all relatively low (77 to 340) as a result of the assumed relatively low hazard nature of CBM compared to some other technologies. Specific vulnerability scores are higher in the potential receptors overlying the hydrocarbon source unit (than those underlying it) reflecting the probability that the head gradient could be upwards. Apart from this, the potential receptors with the highest intrinsic vulnerability scores also have the highest specific vulnerability scores.
  • The overall confidence in the intrinsic and specific vulnerability scores is low because of the uncertainty associated with the minimum depth at which coal units could be exploited. The confidence in all other factors is medium.
  • The scenario influenced the risk group of some potential receptors; the Zechstein Group and Permian Basal Breccia are in the medium/high risk group in the 300 and 200 m scenario, but the medium/low risk group in the 400 m scenario. The Sherwood Sandstone is in the medium/low risk group for the 300 m and 400 m scenario but medium/high for the 200 m scenario. The Pennine Middle Coal Measures are in the medium/low risk group for all scenarios. The scenario has not impacted the risk group for the other potential receptors.
  • The proximity of the potential receptor to the hydrocarbon source unit is a major uncertainty that will impact the intrinsic vulnerability score, specific vulnerability score and risk group of potential receptors. This should be addressed through further investigations. The assessment would improve from a greater understanding of the head distribution and groundwater flow paths and further identification of faults and fault behaviour in the region. The quality of the groundwater, groundwater flow system, abstractions and outflows from the potential receptors should be assessed in greater detail, in particular in those potential receptors in the medium/low to high risk groups.

Case Study 4: Shale gas, Northwest

Hydrocarbon source and extraction method

Bowland Shale Formation, part of the Craven Group, Lancashire with 2 km lateral wells (Figure A6.18).
AOI
Extending to 2 km from lateral borehole of 2 km
Geological setting
The Carboniferous-aged Bowland shale lies within the Fylde of the West Lancashire Basin. This is a low lying area west of the Pennines. These rocks outcrop to the east, at an elevation of ~130 m OD, but dip below younger Carboniferous and Permo-Triassic aged rocks in the Lancashire Basin. The Mercia Mudstone and the Sherwood Sandstone outcrop across the west and east of the region, respectively. At the coast there is up to 1200 m of Triassic-aged sediments.

The Fylde is structurally complex with a variety of faults of different ages cutting and trending north-northeast–south-southwest, and a large variability in unit thicknesses (see Figure A6.19). Towards the coast and under the AOI, the top of the hydrocarbon source unit can be more than 2000 m below OD. There is a southwest trending synclinal fold in the centre of the Fylde area (Sage and Lloyd, 1978).

Conceptual model
The conceptual geological model for the AOI across (west-east) and along (north-south) strike is shown in Figure A6.20. The conceptual model is based on three boreholes in the centre and west of the AOI. The boreholes were located in the centre, 6 km to the west and 7 km to the northwest of the AOI. LFV sections (Figure A6.19) were used to provide an understanding of the regional geology and the possible variability in the area. The general geological sequence and unit descriptions are as shown in Table A6.4.

There is little information regarding the depth and thickness of units in the AOI and a high degree of variability. In particular, little is known about the south and east of the area, or about the nature of the faults. Nevertheless, the Woodsford Fault in the east of the AOI is expected to be significant since this forms the boundary between the outcropping Mercia Mudstone to the west and the Sherwood Sandstone to the east. LFV sections show that the Appleby Group and the Millstone Grit are not present to the east of this fault. The Craven Group (hydrocarbon source unit) is expected to be much shallower in the footwall of the fault, only 300 m below OD compared with 1800 m below OD west of the fault.

The Mercia Mudstone, Sherwood Sandstone and Cumbrian Coast Group become deeper to the west of the AOI. Boreholes indicate that the Appleby Group is thinner to the west and north of the AOI (ranging from 1000 m in thickness in the hanging wall of the Woodsford Fault to 200 m thickness in the northwest). To the north, this decrease in thickness corresponds with an increase in thickness of the Cumbrian Coast Group (from 200 to 600 m). The Millstone Grit has a fairly uniform thickness and depth across the AOI (350 m at ~1400 m depth). The borehole to the northwest also recorded about 50 m of Coal Measures.

The faults shown on the conceptual model are from the 1:50 000 geological map. They strike north-northeast–south-southwest. The fault shown in the north-south cross section is the most central fault in the west-east section. The easterly fault is expected to be large, and an approximate throw has been obtained from the LFV sections. The throw on the other faults is not known, but they do not offset geological units at the surface. In the assessment for the area east of the Woodsford Fault the Craven Group overlying the target depth (1800 m) has been included as an intervening unit.

Baseline methane
Five sites were sampled by Bell et al. (2015)[5] within the area shown in Figure A6.18, one west of the AOI from superficial deposits to the west and four from the Sherwood Sandstone 10 to 12 km to the east, where the aquifer is unconfined. No samples were from the AOI. Bell et al. (2015)[5] found that methane concentrations were above the detection limit, but none exceeded the groundwater equivalent LEL (Lower Explosive Limit). Methane concentrations were typically lower in the Sherwood Sandstone than in the superficial deposits.
Potential receptors Classification
Receptor classification was initially based on aquifer designations obtained from the LFV sections (Figure A6.19). Where units were classified as variable aquifers in the LFV (Figure A6.19) (Mercia Mudstone and Cumbrian Coast Group), the EA aquifer designation maps were used to identify the designation in this particular region. All these units are secondary aquifers in this region.
Mercia Mudstone Group B – secondary aquifer, top of unit <400 m bgl and 8 km west of the AOI, at the Gas Works in Blackpool, a salinity of ~955 mg/l was recorded at 57 m bgl.
Sherwood Sandstone Group A and D – principal aquifer, top of unit <400 m bgl but groundwater beneath the Mercia Mudstone at Kirkham (to the west of the Woodsford Fault), at a depth of ~370 m bgl, has TDS of >100 000. Therefore, west of the fault the Sherwood Sandstone has been downgraded to potential receptor class ‘D’. Where not confined by the Mercia Mudstone, borehole evidence at Salwick, 3 km east of the fault, suggests a much lower TDS (350 mg/l), consistent with the potential receptor class ‘A’. 5 km southeast of the AOI saline water was encountered in the Clifton Marsh Landfill borehole at a depth of 61 m in the Sherwood Sandstone group but no concentration is recorded and this borehole is close to the Ribble estuary. The difference in salinity over such a short distance points to the barrier-like behaviour of the fault within the Sherwood Sandstone and therefore the assessments will also be conducted both west and east of the fault with the Sherwood Sandstone as a potential receptor class D and A, respectively.
Cumbrian Coast Group C – secondary aquifer, top of unit >400 m bgl
Appleby Group B – principal aquifer, top of unit >400 m bgl
Millstone Grit Group C – secondary aquifer, top of unit >400 m bgl
Craven Group C – secondary aquifer, top of unit >400 m bgl
Hazard Score
Release mechanism of hydrocarbon Shale gas and high volume hydraulic fracturing.
Head gradient driving flow No information on groundwater head gradients from boreholes within the AOI or surrounding area. Sage and Lloyd (1978)[10] suggest from a general piezometric map of the Fylde that some groundwater flow enters the Permo-Triassic sandstones from the Carboniferous sequence at a rate of ~30000 m3/day along a front of about 15 km (Downing et al., 1987[7]). The sandstones are thought to form one of the main outlets for groundwater, although direct evidence for flow from Upper Palaeozoic rocks into the sandstones is limited (Downing et al., 1987[7]). In the south, however, the presence of Permian marls (Manchester Marls/Cumbrian Coast Group) prevents a uniform groundwater flow from the east into the sandstones (Sage and Lloyd, 1978[10]). Downing et al. (1987)[7] also suggest that the available evidence, including salinity >1000 mg/l in Triassic sandstones confined by the Mercia Mudstone (Sage and Lloyd, 1978[10]), appears to preclude significant flow to the west below the Mercia Mudstone. Downing et al. (1987)[7] suggest that the occurrence of oil at Formby implies that, near the coast, there is, or has been, a groundwater discharge zone originating in the Carboniferous, but apart from the possibly anomalous heat-flow values at Kirkham, heat flow in the Fylde is not above average — although the data are sparse. While the above evidence is not conclusive, an upward hydraulic gradient in this area is conceivable and therefore a score of two is appropriate in this case for all units.
Intrinsic vulnerability
Vertical separation distance between source and base of receptor The depth of potential receptor units and hydrocarbon source unit are not particularly well known in the area due to their lateral variability and limited availability of borehole records. Therefore, the confidence in this factor is low.
Lateral separation distance between source and receptor In the current conceptual model of the AOI no units (with the exception of the directly overlying Millstone Grit) would be brought into lateral contact with the exploitation activity within 2 km. The confidence for this factor is, again, low because the geometry of the units around this fault is

uncertain.

Mudstones and clays in intervening units between source and receptor The thickness of mudstones and clays in the intervening layer between the top of the hydrocarbon source unit and the base of the potential receptor was based on the average composition of the interval recorded in a borehole log 1 km to the northwest of the AOI.

Above the hydrocarbon source unit, the Millstone Grit is 51% mudstone (the remainder sandstone), providing 150 m of mudstone thickness to the overlying units. The borehole log shows the Appleby Group as a 50% mudstone, providing a cumulative mudstone thickness in the intervening units of 375 m for the overlying units. The Cumbrian Coast Group and Mercia Mudstone are all mudstone, and the Sherwood Sandstone is sandstone only.

The confidence level for this factor is medium because the information was obtained from nearby borehole logs. However, there remains some uncertainty in the thickness of the units.

Groundwater flow mechanism in intervening units between source and receptor, including the receptor In the first unit above the source, the Millstone Grit, groundwater flow is expected to be predominantly through poorly connected fracture flow.

The overlying Appleby Group is expected to be dominated by units between intergranular flow, thus changing the cumulative groundwater flow to >50% intergranular flow. While groundwater flow in the Cumbrian Coast Group is likely to be through poorly connected fractures, the limited thickness, in comparison to the thickness of the underlying Appleby Group, means that the unit does not change the flow type category. Groundwater flow in the Sherwood Sandstone is also likely to be intergranular. This remains the dominant groundwater flow type in the sequence, despite the Mercia Mudstone likely be dominated by fracture flow. For the assessment to the east of the fault the sequence is assumed to be dominated by fracture flow from the Craven Group.

Faults cutting intervening units and receptor A fault cuts all units to the east of the AOI. There are also a number of north-northeast – south-southwest oriented faults in the western part of the AOI. The closest fault is within the hypothetical footprint of the hydrocarbon activity. The hydrogeological impact of these faults is not known. The confidence in this factor is medium.
Solution features in intervening units and receptor Gypsum and anhydrite have been recorded in boreholes penetrating the Mercia Mudstone and Sherwood Sandstone in the AOI. In addition, one borehole log specifies ‘a few small voids’ which might have resulted from the dissolution and removal of halite in these units. The confidence level for this factor is medium.
Anthropogenic features-mines close to site of interest There are no recorded mines in the AOI. The confidence level for this factor is high.
Anthropogenic features-boreholes close to site of interest There are two boreholes with depths of ~450 m within 0.5 to 2 km laterally from the vertical drill location, therefore a value of 1 has been attributed to all units. The confidence level for this factor is high.
Potential receptor Intrinsic vulnerability score Specific vulnerability score Risk group Confidence

West of fault

Mercia Mudstone Group 30 240 Low Low
Sherwood Sandstone Group 32 256 Low
Cumbrian Coast Group 28 224 Low
Appleby Group 39.5 316 Medium/low
Millstone Grit Group 72 576 Medium/low
Millstone Grit Group 85 170 Low
Craven Group 72 576 Medium/low

East of fault

Sherwood Sandstone Group 33 264 Medium/high Low
Cumbrian Coast Group 29 232 Low
Craven Group 72 576 Medium/low
Craven group (target) 23.5 188 Medium/low
Figure A6.18    Hypothetical location of shale gas, T indicates rough location for the hydrocarbon source unit.
Figure A6.19    Cross sections surrounding hypothetical hydrocarbon source unit area of shale gas (hydrocarbon source unit is roughly in the centre of these cross sections). Locations of the cross sections are shown in Figure A6.18. The hydrocarbon source unit is the Bowland Shale, within the Craven Group (CG), shown by ‘T’. UK_Reg8_Sec141 and UK_Reg8_Sec144 are across strike, to the north and south respectively. UK_Reg8_Sec146 is along strike, to the east. See Table A6.4 for unit codes. Vertical lines are the locations of intersecting cross sections and the horizontal black line indicates 1000 m bgl, the shallowest level allowed for shale gas exploitation in England and Wales.
Table A6.4    Rock units present in the hypothetical Northwest AOI. Descriptions are from the sheet memoir, colours correspond with those used in the LithoFrame Viewer sections (Figure A6.19) and the AOI conceptual model (Figure A6.20). * indicates the hydrocarbon source unit. Units below the hydrocarbon source unit horizon are not described.
Model Unit Age Description
Mercia Mudstone Group (MMG) Triassic Mudstone and siltstone with gypsum and some breccias
Sherwood Sandstone Group (SSG) Triassic Medium-grained sandstone with thin, impersistent mudstone
Cumbrian Coast Group (CCO) Permian Comprises the Manchester Marls Formation (mudstone, locally with thin, interbedded gypsum or anhydrite, and dolostone)
Appleby Group (APY) Permian Comprises Collyhurst Sandstone Formation (coarse-grained sandstone)
Coal Measures (CM) Carboniferous Mudstone (from borehole log)
Millstone Grit Group (MG) Carboniferous Mudstone, siltstone and sandstone (~25%)
Craven Group* (CG) Carboniferous Calcareous mudstone, limestone and mudstone
Figure A6.20    Conceptual model of the AOI for the hypothetical shale gas site in the Northwest. The hydrocarbon source unit is the Craven Group (Bowland Shale).
Figure A6.21    Receptor classifications for units within the conceptual model of the AOI for shale gas in the Northwest.
Figure A6.22    Conceptual model for the AOI for potential shale gas exploitation activities in Northwest England with units identified as potential receptors, intrinsic vulnerability scores, specific vulnerability scores and risk groups for each potential receptor for west and east of the fault. See Table A6.4 for code translations. The confidence for this assessment is low. Boundaries used for intrinsic and specific vulnerability and risk groups are used for preliminary purposes.

Summary of Case Study 4: Shale gas, Northwest

  • Intrinsic vulnerability scores for the potential receptors (28 to 72) are quite varied compared with some case studies. The potential receptors with the highest intrinsic vulnerability scores are the Craven Group and Millstone Grit due to their proximity to the hydrocarbon source unit. The potential receptor with the next highest intrinsic vulnerability score is the Appleby Group (Collyhurst Sandstone) (39.5) reflecting the 300 m vertical separation from the hydrocarbon source unit and 150 m mudstone in the intervening interval. The intrinsic vulnerability scores of the remaining units are all below 33 and are comparatively low due to the relatively large vertical separations, but in particular, the thick mudstone within the intervening units.
  • The intrinsic vulnerability score for the Sherwood Sandstone and Cumbrian Coast Group are slightly higher in the east rather than the west of the fault due to the > 50% fracture flow groundwater mechanism in the Craven Group.
  • Potential contamination pathways exist for all of the units from a number of faults and deep boreholes which could connect the hydrocarbon source unit and potential receptors. In addition, there are known solution features in the upper two units.
  • The specific vulnerability scores for the potential receptors are relatively higher, between 224 and 576, as a result of the assumed relatively higher hazard nature of shale gas (in particular high volume hydraulic fracturing) compared to other technologies.
  • The confidence levels in the intrinsic and specific vulnerability scores are low because of the uncertainty associated with the depth and thickness of the units, particularly in the south and east of the AOI where there are no boreholes, and also the head gradients for the latter score. Geophysical and additional borehole information should be used to constrain the subsurface geometry of the AOI, including the size of the faults. The confidence in all other factors is medium.
  • The risk group is medium/high for the Sherwood Sandstone to the east of the fault due to its potential receptor classification (‘A’) and specific vulnerability score. This is despite a vertical separation of 1600 m, and reflects the groundwater flow mechanism and potential for solution features within the unit particularly. However, west of the fault, it is in the low risk group. The Appleby Group (potential receptor class ‘B’) has a medium/low risk, Millstone Grit and Craven Group are in the medium/low risk group predominantly due to their high specific vulnerability scores despite the fact that they are unlikely to be used as aquifers at these depths (> 1.5 km).
  • The large difference in risk grouping east and west of the fault for the Sherwood Sandstone, and the medium/low risk grouping for the Millstone Grit and Craven Group highlights the importance of correctly classifying these units. Further information about the groundwater quality in this area, particularly on the east side of the fault should be obtained.

Case Study 5: Shale gas and conventional hydrocarbons, Northwest England

Hydrocarbon source and extraction method

1: Shale gas from the Bowland Shale Formation, part of the Craven Group, in the Vale of Pickering, Yorkshire (Figure A6.23). Lateral well extending to 2 km.

2: Natural gas from the Zechstein Group, in the Vale of Pickering, Yorkshire (Figure A6.23). The well is assumed to be vertical.

AOI
1: Extending to 2 km from lateral borehole

2: Extending to 2 km from vertical borehole

Geological setting
The Vale of Pickering is a low-lying (15–35 m above OD) east-west trending basin, approximately lying between Scarborough in the east and Helmsley in the west (Figure A6.23). It is bound by the North York Moors to the north, the Howardian Hills to the southwest and to the south the chalk downlands of the Yorkshire Wolds (Newell et al., 2018), formed by the Market Weighton High (Brenchley and Rawson, 2006[11]). The western margin is marked by the southern North Sea Basin. The bedrock outcrop across most of the Vale of Pickering is the Jurassic-aged Ampthill/Kimmeridge Clay. Underlying this are Triassic sediments (Newell et al., 2018[12]). Numerous cross-cutting faults trend broadly east-west across the Vale of Pickering with average throws in the order of 50 to 100 m, and a maximum of 300 m (A Newell, pers comm.).

A number of rock units in the Vale of Pickering have been exploited for conventional oil and gas. Structural interpretation of the Vale of Pickering was undertaken in 1987 by Kirby et al. using seismic reflection data of varying age and quality and tied to wells. There is thus a reasonable amount of geological information about the area. The structural analysis was used to produce the recent geological model of the Vale of Pickering from the post-Permian upwards by Newell et al. (2018)[12]. At present the model extends to the top of the Permian Zechstein but the authors state that further work should be undertaken to model the lateral thickness and facies variations within the Zechstein in order to understand the sealing capacity of the salt. Modelling of the underlying Carboniferous would also be desirable to understand the relative position of hydrocarbon source rocks.

Conceptual model
The approximate location of the AOI is shown by the letter ‘T’ in Figure A6.23 and Figure A6.24. The AOI lies towards the south of the Vale of Pickering, in the flat-lying land north of the River Derwent. The southern boundary of the AOI is marked by the south-dipping Gilling Fault, which forms the east- west trending graben-like structure of the Gilling-Gap, with the opposing Kilburn Fault (Newell et al., 2018[12]), with approximately 50 m and 150 m offset, respectively. In the area, the shale gas hydrocarbon source unit, the Carboniferous-aged Craven Group rocks containing the Bowland Shale, lies at depths of 1500 m or more. A number of boreholes penetrate to the top of this unit, but not to the base. Overlying the Craven Group are rocks of the Millstone Grit Group which are around 400 m in thickness. The overlying Permian Zechstein Group rocks are about 350 m in thickness and contain a high proportion of anhydrite and dolomite. The Sherwood Sandstone and Mercia Mudstone overlie this, and are about 200 m in thickness. The overlying Lias can be up to 450 m in thickness, the Ravenscar Group 250 m, the Oxford Clay Formation 50 m and the Corallian Group 100 m in thickness, towards the centre of the basin. These units thin to the south as the sequence becomes thinner over the Yorkshire Wolds/Market Weighton High where they are truncated by the Chalk. The Kimmeridge and Ampthill clay, which is at the top of the sequence and outcrops across most of the region, is between 100 and 250 m in thickness and also thicker in the north.

The conceptual geological model for the AOIs for both hydrocarbon source units, across (north-south) and along (west-east) strike, are shown in Figure A6.25. The AOI was developed for hydrocarbon source unit 1 and a smaller volume used for hydrocarbon source unit 2. There are a number of deep (>400 m) boreholes in the AOI; in the centre, to the northeast, southeast and southwest (Figure A6.23). The boreholes terminate in Carboniferous-aged (Namurian) units, thus providing evidence for the depth and thickness of all of the younger units, across much of the AOI. The records of four of these boreholes were used to produce the conceptual model.

In the conceptual model of the AOI the geological sequence becomes shallower to the south due to stepping across north-dipping normal faults. There is little east-west variability in the depth and thickness of units. There is a slight dip from west to east along-strike, with a difference in depth of units of about 100 m. Hydrocarbon source unit 1 (Craven Group) in the AOI lies between 2200 m in the north and 1700 m in the south although the depth is not well constrained as only the borehole in the centre of the AOI differentiates this and the overlying Millstone Grit. None of the boreholes penetrate to the base of hydrocarbon source unit 1 and therefore the thickness is not known. However, this is not important for the risk assessment since calculations use the top of the unit. In the central borehole the Millstone Grit is over 350 m in thickness, and contains approximately 50% mudstone. Since there is no more information this thickness is applied across the AOI. The depth of the Millstone Grit varies from 1800 m below OD in the north to 1400 m in the south. The overlying Zechstein Group comprises between 350 and 500 m of anhydrite (up to 60% in the north of the AOI), dolomite (~35%) and a small amount of claystone. The Sherwood Sandstone overlies this. This unit decreases in depth to the south but increases in thickness, from just over 100 m thickness in the north at a depth of ~1200 m below OD to ~400 m in thickness in the south, at a depth of ~600 m below OD. Borehole records indicate that this unit is predominantly sandstone, with some anhydrite. The Mercia Mudstone overlies this and, similarly, is thicker but shallower in the south; from ~200 m thick at a depth of ~1000 m below OD in the north to ~ 500 m thick at a depth of 300 m below OD in the southwest. Borehole records indicate that this unit is shale with some gypsum and anhydrite, and some sandstone and siltstone. The Lias, Ravenscar Group and Oxford Clay and Kellaways formations and the Corallian Group lie between the Mercia Mudstone and the outcropping Kimmeridge Clay. The Lias is thicker and deeper to the north (400 m thick at 600 m below OD) than the south (150 m thick at 150 m below OD). The thickness of the Ravenscar Group, Oxford Clay and Kellaways Formations and the Corallian are relatively constant across the AOI, but the units are approximately 300 m shallower in the south. The Kimmeridge Clay is approximately 200 m thicker in the north of the AOI. The Cromer Knoll Group (Speeton Clay Formation) is present to the east of the AOI, but not within it.

There is slightly less variability in the unit depths and thicknesses in the AOI for hydrocarbon source unit 2 as there are no lateral boreholes and hence the AOI is smaller. The hydrocarbon source unit (Zechstein Group) varies from a depth of 1400 m below OD in the north to 1000 m below OD in the south (Figure A6.25). The overlying units are also deeper in the north than in the south, with the greatest difference in depth for the top of the Mercia Mudstone which is 900 m below OD in the north and 400 m below OD in the south. The Lias is thicker in the north than in the south (~400 m and ~150 m respectively). There is little difference in thickness and depth along-strike.

A number of large-scale faults (marked on the 1:625 000 geological map and included in Newell et al., 2018[12]) strike roughly west-east in the AOI. Newell et al. (2018)[12] indicate that these faults offset the post-Permian bedrock units but it is not known whether or not these faults are hard-linked to the Carboniferous-aged units or are listric with a base in the more-ductile anhydrite units of the Zechstein Group. In this conceptual model the faults have been assumed to continue to depth and cut the pre-Permian units, as a worst-case scenario.

The Gilling Fault forms the southern boundary of the AOI. Other large faults lie ~1 km south of the centre of the AOI and a northwest-southeast striking fault lies ~2 km north of the centre of the AOI. Two additional faults are marked on the 1:50 000 geological map, immediately north of the centre. These faults are currently modelled as relatively simple planes but may, in reality, be more complex (Newell et al., 2018[12]). Newell et al., pers comm states that most of the faults are believed to have throws of 50 to 100 m.

Baseline methane
Bell et al. (2015)[5] sampled for methane concentrations in aquifers in the northeast as part of the Methane Baseline Survey of Great Britain. Five sites were sampled within the area shown in Figure A6.23; three from the Corallian Group and two from the West Walton Formation (below the Corallian Group), all where the aquifers outcrop at rockhead. Sample locations for the Corallian Group aquifer are about 7 km to the south, 8 km to the west and 30 km to the east of the hydrocarbon source unit 1 AOI. Sample locations in the West Walton Formation are about 2 km to the north and 32 km to the east of the hydrocarbon source unit 1 AOI. It was found that methane concentrations were above the detection limit in the region but no samples exceeded the groundwater equivalent LEL (Section 6.1).

Smedley et al. (2017)[13] investigated the baseline chemistry of groundwater from a shallow Quaternary/Kimmeridge Clay aquifer and the Corallian aquifer. High concentrations of dissolved methane were observed in the superficial aquifer groundwaters (up to 37 mg/l). These waters were also confined and highly reducing. While the methane appears to be of mixed biogenic-thermogenic origin, further work is needed to determine whether the source includes a deeper hydrocarbon reservoir contributing via fractures, or a shallower source in the Quaternary or Kimmeridge sediments (Smedley et al. 2017[13]).

Potential receptors Classification
Receptor classification was initially based on aquifer designations obtained from the LFV sections, according to EA aquifer designations (Figure A6.26). Where model units were classified as variable aquifers (Corallian Group, Lias, Mercia Mudstone and Zechstein Group) the EA aquifer designation maps were used to identify the designation based on outcrops with similar lithologies. For the Corallian Group this was 2 km to the west of the hydrocarbon source unit 1 AOI, the Lias 9 km to the north, over the north York Moors and also 8 km to the southwest over the Market Weighton High, and the Mercia Mudstone 12 km to the southwest. The furthest located outcrop was for the Zechstein Group, 40 km to the west, at the foot of the Pennines.
Kimmeridge and Ampthill Clay Formations B – designated as unproductive. A 47 m deep borehole to the eastern side of the AOI, had a TDS of 1140 mg/l although a smell of H2S was recorded and it was highly corrosive. Another borehole also within the AOI suggests that the water quality in this unit is ‘fairly good’. A borehole 5.6 km to the west of centre, 91 m deep had an EC of 513 µS/cm (TDS ~266 mg/l). There are a number of boreholes which abstract water from this unit, with one yielding ~5 l/s and potable water quality.
Corallian Group A – principal aquifer, <400 m bgl; a 61 m deep borehole 1 km to the north of the AOI 1 indicates a TDS of 310 mg/l
Kellaways and Oxford Clay Formations D – unproductive strata
Ravenscar Group B – secondary aquifer, <400 m bgl.
Lias C – secondary aquifer, >400 m bgl.
Mercia Mudstone Group C – secondary aquifer, >400 m bgl.
Sherwood Sandstone Group B – principal aquifer, >400 m bgl.
Zechstein Group D – principal aquifer, >400 m bgl, but ~16 km to the northeast of the hydrocarbon source unit 1 AOI at depths of 1647 to 1702 m bgl, TDS ranges from 67 100 to 306 200 mg/l. Records from a borehole in the centre of the AOI indicate ‘saline’ water in this unit, and another in the southwest of the AOI records that the water is ‘black and sulphurous’.
Millstone Grit Group C – secondary aquifer, >400 m bgl
Craven Group C – secondary aquifer, top of unit >400 m bgl
Hazard Score
Release mechanism of hydrocarbon 1: Shale gas and high volume hydraulic fracturing.
2: Conventional hydrocarbons
Head gradient driving flow There is little information on groundwater head distributions at depth in the AOI, or region. Groundwater was found to be artesian in a borehole drilled into the Corallian Group aquifer 1 km to the north of the AOI for hydrocarbon source unit 1, with head 2 m above ground level. Artesian conditions were also found in the Corallian Group 4 km to the southwest of the AOI for hydrocarbon source unit 1. There are no records of hydraulic head in other formations in the AOI. The East Yorkshire Hydrogeological map (IGS, 1980[14]) does not have groundwater head information on these units.

Downing et al. (1987)[7] suggest that, north of the Vale of York, the Triassic sandstones are the main outlet for groundwater. Groundwater is thought to flow south along the Vale of York, along the line of the Yorkshire Ouse (west and south of the AOI). They suggest that there may be some deep regional flow to the east within the Triassic sandstones, ‘particularly along the line of the Vale of Pickering’, draining the groundwater from deep Upper Palaeozoic rocks and therefore an upwards gradient.

While there is some evidence of upwards head gradients at shallow depths in the AOI, there is no information regarding groundwater head at greater depths, for example, from the hydrocarbon source unit depths. Nevertheless, it is not possible to rule this out and therefore the worst case scenario remains that the head gradient might be from the source to the potential receptor for all units. This is given a medium confidence level for the upper two units and a low confidence level for the underlying units.

Intrinsic vulnerability
Vertical separation distance between source and base of receptor There are a number of deep boreholes in and around the AOI so there is reasonable information about the depth and thickness of the units.

For hydrocarbon source unit 1, there is a difference of about 550 m depth for the top surface for the Craven Group between the north and south of the conceptual model. Consequently, three vulnerability scenarios were tested; centre of the AOI, minimum separation (south) and maximum separation (north). It was found that the vertical separation to the base of the potential receptors does not change systematically for all of the units due to the variation in thickness of the intervening units. The vertical separation to the base of the potential receptors is greatest in the south, then the north and then the centre of the AOI. The biggest difference in vertical separation is for the Lias, in which the vertical separation with the hydrocarbon source unit is 430 m greater in the south and 80 m greater in the north, than in the centre of the AOI. For the Ravenscar Group the vertical separation is 370 m greater in the south and 295 m greater in the north. The vertical separation difference is greater for units overlying the Ravenscar Group than underlying it.

Only one scenario was tested for hydrocarbon source unit 2 due to the small differences in results for hydrocarbon source unit 1. The confidence in the vertical separation distance is medium due to the presence of deep boreholes in the AOI.

Lateral separation distance between source and receptor In the AOI the lateral separation distance factor does not apply since no units are brought into horizontal contact with the hydrocarbon source unit. The exceptions are the Millstone Grit and the Sherwood Sandstone which directly overlie the hydrocarbon source units. Other units are not brought to the same horizontal level in the AOIs due to the relative thickness of the overlying Millstone Grit and Zechstein Group and the comparatively limited throw of the faults.

The confidence in the horizontal separation distance is medium because there is little variability in depth and thickness of units across the AOI. However, the actual throws on the faults are not known.

Mudstones and clays in intervening units between source and receptor The composition of all units was assessed from borehole records. The Millstone Grit is estimated to be 50% mudstone. The Zechstein Group is predominantly anhydrite and dolomite with little mudstone,. The Lias is predominantly mudstone. The Ravenscar Group is predominantly sandstone (with some mudstone). The Oxford Clay is predominantly mudstone. The Corallian Group limestone. The Ampthill and Kimmeridge clays are mudstone.

The cumulative mudstone thickness increases up the sequence with distance from the hydrocarbon source units. For hydrocarbon source unit 1, the large thickness of the Millstone Grit, and the fact that 50% of this is mudstone, results in all of the overlying potential receptors having a mudstone thickness of 184 m in the intervening interval between them and the hydrocarbon source unit. Receptors overlying the Mercia Mudstone have more than 350 m of mudstone thickness in the intervening interval. The only class ‘A’ potential receptor (Corallian Group) is separated from the hydrocarbon source unit formation by a thickness of more than 700 m mudstone in the intervening units.

For hydrocarbon source unit 2, there are no mudstones in the intervening interval for the Sherwood Sandstone or the Mercia Mudstone, but there are thicknesses of over 200 m and 400 m for the Lias and Ravenscar Groups, respectively. This separation increases further up the geological sequence.

The confidence level for this factor is medium since there are borehole records for all of the units in the AOI.

Groundwater flow mechanism in intervening units between source and receptor, including the receptor Permeability in the Millstone Grit is likely to be predominantly through fracture flow due to its age, but fractures are likely to be poorly connected. There is likely to be limited permeability in the Zechstein Group anhydrite which has a tendency to re-seal fractures; however the dolomite is brittle and could be fractured. The Sherwood Sandstone is probably dominated by intergranular flow. The Mercia Mudstone and Lias are also likely to be fractured but not well connected and the Ravenscar Group is dominated by intergranular flow. While the Corallian is likely to be fractured, well-connected (e.g. Reeves et al., 1978), this unit is only 15 to 100 m thick and therefore will not have a particularly large influence on the cumulative flow type. The cumulative flow type is therefore likely to be >50% potential receptor class ‘A’ to ‘C’ fractured, poorly connected or mixed fracture and intergranular flow, for both hydrocarbon source units.

The confidence level for this factor is medium because borehole records do not provide this information for most of the units.

Faults cutting intervening units and receptor A number of large-scale faults (marked on the 1:625 000 geological map and included in Newell et al., 2018[12]) strike roughly west-east in the AOI. Newell et al. (2018)[12] indicate that these faults offset the post-Permian bedrock units. They state that it is not known whether or not these faults are hard-linked to the Carboniferous-aged units or are listric with a base in the more-ductile anhydrite units of the Zechstein Group. In this conceptual model the faults have been assumed to cut the pre-Permian units as well as those above as a worst-case scenario.

The Gilling Fault forms the southern boundary of the AOI. Other large faults lie ~1 km south of the centre of the AOI and a northwest-southeast striking fault lies ~2 km north of the AOI. Two additional faults are marked on the 1:50 000 map, immediately north of the AOI. These faults are currently modelled as relatively simple planes but may, in reality, be more complex (Newell et al., 2018[12]). Newell et al., pers comm states that most of the faults are believed to have throws of 50 to 100 m.

Reeves et al. (1978) state that the bulk of groundwater discharge from the Corallian aquifer in the Vale of Pickering takes place from a series of large springs whose positions are governed by faulting, where the aquifer passes beneath the impermeable clay cover of the centre of the Vale. Sometimes these break through the line of the fault. However, the documented springs are to the north and west of the AOI. Reeves et al. (1978)[15] also state that faulting has completely or partially reduced hydraulic continuity between the confined aquifer and the outcrop. It thus appears that faults can behave as conduit-barriers in the region. Since there is no other evidence to suggest whether the faults are transmissive the category has not been changed based on this evidence.

The confidence level for this factor is medium because the maps point to some evidence for faults, but there is no information regarding their hydraulic properties. In addition, the depth to which they penetrate is not known. The confidence is slightly higher for hydrocarbon source unit 2 since it is known that the faults do penetrate to this level.

Solution features in intervening units and receptor A number of the potential receptors have potential for developing solution or karst features (Farrant, 2008[4]) in the AOI. These include the Zechstein Group — due to the presence of anhydrite and dolomite, as well as Sherwood Sandstone and Mercia Mudstone where anhydrite has been documented in boreholes in the AOI, and gypsum in the latter to the southwest. The Corallian also has potential for solution features with swallow holes common at outcrop where it acts as a karstic aquifer. Mud losses were documented in this formation during drilling of the borehole in the northeast of the AOI. Because there is little evidence to support this factor for most of the units the confidence is medium.


Anthropogenic features-mines close to site of interest There are no recorded mines in the AOI. The confidence level for this factor is high.
Anthropogenic features-boreholes close to site of interest There are about ten boreholes drilled to the hydrocarbon source unit in the AOI with a number within 200 m vertically of both hydrocarbon source units. The confidence level in this factor is high.
Potential receptor Intrinsic vulnerability score Specific vulnerability score Risk group Confidence

Shale gas

Kimmeridge and Ampthill Clay Formation 36.5 292 Medium/low Low
Corallian Group 36.5 292 Medium/High
Kellaways and Oxford Clay Formations 34.5 276 Low
Ravenscar Group 34.5 276 Medium/Low
Lias 36 288 Low
Mercia Mudstone Group 39.5 316 Low
Sherwood Sandstone Group 41 328 Medium/Low
Zechstein Group 44 352 Low
Millstone Grit Group 71 568 Medium/Low
Craven Group 71 568 Medium/Low

Conventional oil and gas

Kimmeridge and Ampthill Clay Formation 38 76 Low Low
Corallian Group 38 76 Medium/low
Kellaways and Oxford Clay Formations 37.5 75 Low
Ravenscar Group 37.5 75 Low
Lias 44 88 Low
Mercia Mudstone Group 57.5 115 Low
Sherwood Sandstone Group 71 142 Low
Zechstein Group 71 142 Low
Figure A6.23    Hypothetical location of shale gas and natural gas extraction in the Vale of Pickering, Northeast England with outcrop bedrock geology, LFV sections, deep (>400 m) boreholes and mines in the region. T indicates the approximate location for the hydrocarbon source units.
Figure A6.24    Cross sections from LFV with the approximate location of the hypothetical hydrocarbon source units, T1 is hydrocarbon source unit 1 – Craven Group and T2 is hydrocarbon source unit 2 – Zechstein Group. Cross section locations are shown in Figure A6.1. UK_Reg9_154 is approximately across the strike of the basin and UK_Reg9_153 is approximately along strike. The near horizontal black line indicates 1000 m bgl – the shallowest level allowed for shale gas exploitation in England and Wales. Rock codes are described in Table A6.5.
Table A6.5    Rock units present in the hypothetical southeast AOI. Descriptions are from the sheet memoir (Powell et al., 1992[16]) and the BGS Lexicon. Colours correspond with those used in the LithoFrame Viewer section (Figure A6.24) and the AOI conceptual model (Figure A6.25). *1 indicates the hydrocarbon source unit 1 and *2 indicates hydrocarbon source unit unit 2. ** indicates unit not in AOI.
Model Unit Age Description
Cromer Knoll Group (Speeton Formation)** Cretaceous Clay and mudstone with subsidiary argillaceous, muddy limestone/cementstone/calcilutite and calcareous mudstone.
Kimmeridge and Ampthill Clay Formation (AMKC) Jurassic Comprises the Kimmeridge Clay Formation in this region; mudstone with carbonate concretions in lower part.
Corallian Group (CR) Jurassic Limestone (sometimes oolitic), sand and sandstone, and siltstone.
Kellaways and Oxford Clay Formations (KLOX) Jurassic Siltstone and silty mudstone
Ravenscar Group (RAG) Jurassic Sandstone, mudstone and siltstone.
Lias (Li) Jurassic Mudstone and silty mudstone.
Mercia Mudstone Group (MMG) Triassic Mudstone, silty mudstone with siltstone and thin sandstone with beds of gypsum.
Sherwood Sandstone Group (SSG) Triassic Sandstone with beds of siltstone.
Zechstein Group (ZG)*2 Permian Dolomite, limestone, evaporites, mudstone and siltstone.
Millstone Grit Group (MG) Carboniferous Mudstone, siltstone and sandstone.
Craven Group (CG)*1 Carboniferous Calcareous mudstone, limestone and mudstone.
Figure A6.25    Conceptual model of the AOI for the hypothetical shale gas site in the Vale of Pickering, Northeast England. The AOI for hydrocarbon source unit 1 (Bowland Shale Formation, part of the Craven Group) is the whole conceptual model, the AOI for hydrocarbon source unit 2 (Zechstein Group) is within the box with the dotted lines.
Figure A6.26    Receptor classifications for units within the conceptual model of the AOI. Whole model is AOI for hydrocarbon source unit 1 (Bowland Shale, Craven Group) and blue dotted box indicates AOI for hydrocarbon source unit 2 (Zechstein Group).
Figure A6.27    Conceptual model for the AOI in the northeast of England for potential shale gas extraction from hydrocarbon source unit 1, the Craven Group (Bowland Shale Formation). Top to bottom; potential receptor classifications, intrinsic and specific vulnerability scores risk group, for each potential receptor. See Table A6.5 for code translations. The confidence for this assessment is low. Boundaries used for intrinsic and specific vulnerability and risk groups are used for preliminary purposes.
Figure A6.28    Conceptual model for the AOI in the northeast of England for conventional hydrocarbon extraction from hydrocarbon source unit 2, the Zechstein Group. Top to bottom; potential receptor classifications, intrinsic and specific vulnerability and risk groups, for each potential receptor. See Table A6.5 for code translations. The confidence for this assessment is low. Boundaries used for intrinsic and specific vulnerability and risk groups are used for preliminary purposes.

Summary of Case Study 5: Shale gas and conventional hydrocarbons, Northeast England

  • For hydrocarbon source unit 1, intrinsic vulnerability scores for the potential receptors are quite varied, ranging from 34.5 to 71 (Figure A6.27).
  • There was very little difference in the intrinsic vulnerability score (maximum score difference of 2) for the assessments completed for the north and south of the AOI, and this does not impact on the risk group.
  • The minimum intrinsic vulnerability scores for hydrocarbon source unit 2 were slightly higher, ranging from 37.5 to 71, due to the closer proximity of the hydrocarbon source unit (Figure A6.28).
  • The intrinsic vulnerability scores generally decrease with vertical separation from the hydrocarbon source unit. The slightly higher intrinsic vulnerability in the Corallian Group and Kimmeridge Clay results from the known solution features in the Corallian Group.
  • It has been assumed that multiple faults could provide potential pathways for all of the units within both AOIs. However it is possible that these might not penetrate the Zechstein Group anhydrites (Newell et al., 2018[12]). There is some evidence for transmissive faults in the region (Reeves et al., 1978[15]).
  • The specific vulnerability scores for the potential receptors for hydrocarbon source unit 1 (Bowland Shale) are higher than other case studies (276 to 568) as a result of the assumed higher hazard nature of shale gas extraction activities compared to other technologies.
  • For hydrocarbon source unit 2, the specific vulnerability scores are relatively low (75 to 142) as a result of the assumed lower hazard nature of conventional hydrocarbon extraction.
  • The confidence level of the intrinsic vulnerability scores is medium because there are a number of deep boreholes in the AOI which record the depths and thicknesses of all the units. The confidence for the specific vulnerability score remains low due to the uncertainties associated with the direction of the head gradients.
  • The risk group, is medium/high for the Corallian Group for hydrocarbon source unit 1 and medium/low for hydrocarbon source unit 2.
  • The Kimmeridge Clay, Ravenscar Group, Sherwood Sandstone, Millstone Grit and Craven Groups are also in the medium/low risk group in relation to hydrocarbon source unit 1.
  • All of the potential receptors, with the exception of the Corallian Group, are in the low risk group in relation to hydrocarbon source unit 2.
  • Downgrading of the potential receptor class of units at depth (such as the Sherwood Sandstone, Millstone Grit and the Craven Group) would lower the risk group for the hydrocarbon source unit 1 to low, which is potentially more realistic at these depths. The quality of groundwater and hence potential receptor type is a major uncertainty in assessing the risk group for the aquifers.
  • The assessment would benefit from a greater understanding of the head distribution and groundwater flow paths. If it could be shown that faults do not cut the Zechstein Group rocks then the risk group could be reduced for overlying units. An improved understanding of the fault behaviour in the region would also be useful.
  • Smedley et al. (2017)[13] found high concentrations of dissolved methane in the Quaternary/Kimmeridge Clay aquifer of the Vale of Pickering. It is currently unclear as to the source of this methane (biogenic or thermogenic) and requires further work to investigate the origins since the latter might indicate that permeable pathways could pre-exist in the region, or AOI.

References

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