OR/18/012 Appendix 1 - 3DGWV screening methodology, spreadsheet tool and example for low vulnerability scenario

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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.
Geological unit Lithology Depth top (M OD) Depth base (M OD) Thickness of unit (M) Vertical separation between source and base of receptor (M) Unit mudstone thickness (M) Cumulative mudstone thickness (M) in intervening units EA aquifer designation Potential receptor classification Unit flow type (for receptor classes A to C) Cumulative flow type (for receptor classes A to C) including receptor Notes
Chalk Group Chalk with subsidiary mudstone, flint and limestone 81 -10 91 325 45.5 178 Principal A Fractured, well connected >50% intergranular flow
Gault Formation Clay, mudstone and sandstone with subsidiary calcareous mudstone, chalk, conglomerate, limestone, sand, silt and siltstone -10 -90 80 245 80 98 Unproductive in this region D Not A to C >50% intergranular flow
Lower Greensand Formation Sand and sandstone with subsidiary clay and silt -75 -139 64 196 0 98 Principal A Intergranular flow >50% intergranular flow
Weald Group Mudstone, sandstone and siltstone -139 -279 140 56 70 28 Secondary in this area B Intergranular flow >50% intergranular flow Closest outcrop in Weald where both secondary and unproductive aquifer, take most sensitive —  secondary
Purbeck Group Limestone and mudstone with subsidiary gypsum-stone and non-carbonate salt rock -279 -335 56 0 28 0 Secondary B Fractured, well connected >50% fractured, well connected
Portland Group Limestone, sand and sandstone with subsidiary calcareous sandstone, chert and mudstone -335 -352 17 0 0 0 Principal and secondary A Fractured, well connected >50% fractured, well connected Isle of Purbeck where limestone is Principal
Kimmeridge Clay Mudstone with subsidiary argillaceous, muddy limestone/cementstone/calcilutite, limestone, sand, sandstone, sapropelite, silt and siltstone -352 -555 203 0 203 0 Unproductive D Not A to C >50% fractured, well connected
Corallian Group Mudstones, siltstones and argillaceous limestones -555 -607 52 203 26 203 Principal and secondary C Fractured, well connected >50% fractured, well connected Closest outcrop near Weymouth, where it is Secondary
Kellaways and Oxford Clay Formations Silicate-mudstone, silicate-sandstone and silicate siltstone -607 -828 221 255 221 229 Unproductive D Not A to C >50% fractured, well connected
Great Oolite Group Calcareous mudstone, limestone, mudstone and ooidal limestone -828 -902 74 476 0.5 450 Principal B Fractured, well connected >50% fractured, well connected
Inferior Oolite Group Limestone, ooidal limestone -902 -963 61 550 0 450.5 Principal B Fractured, well connected >50% fractured, well connected
Lias Group Calcareous mudstone, mudstone and silty mudstone -963 -1200 237 611 237 450.5 Principal, secondary and unproductive, closest outcrop is east of Lyme Regis, both secondary and unproductive C Fractured, not well connected >50% fractured, well connected Secondary as most sensitive classification and no principal in this area

Conceptual geological model for the southeast England, conventional oil and gas, in the spreadsheet tool. Unit highlighted in red indicates the target formation.
Blue indicates squares where data is inputted, grey are calculated and yellow indicates notes and justifications.

Factor Release mechanism of hydrocarbon (H1) Head gradient driving flow (H2) Hazard score Confidence
Geological unit Ranking Confidence Rating Confidence
Chalk Group 2 low 2 low
Gault Formation 2 low 2 low
Lower Greensand Formation 2 low 2 low
Weald Group 2 low 2 low
Purbeck Group 2 low 2 low
Portland Group 2 low 2 low
Kimmeridge Clay 2 low 2 low
Corallian Group 2 low 2 low
Kellaways and Oxford Clay Formations 2 low 2 low
Great Oolite Group 2 low 2 low
Inferior Oolite Group 2 low 2 low
Lias Group 1 high 2 low 2 low
Notes

Hazard score for the southeast England, conventional oil and gas, from the spreadsheet tool. Green squares indicates cell has been carried forward, orange is an option with a pull-down menu.

Factor Vertical separation between source and base of receptor Lateral separation between source and receptor Mudstones and clays in intervening units between source and receptor
Weighting (w) 1.5 3 3.5
Confidence low medium medium
Geological unit
Chalk Group 5 7.5 0 0 2 7
Gault Formation 6 9 0 0 3 10.5
Lower Greensand Formation 7 10.5 0 0 3 10.5
Weald Group 8 12 2 6 4 14
Purbeck Group 8 12 4 12 5 17.5
Portland Group 8 12 4 12 5 17.5
Kimmeridge Clay 8 12 4 12 5 17.5
Corallian Group 7 10.5 0 0 2 7
Kellaways and Oxford Clay Formations 6 9 0 0 2 7
Great Oolite Group 4 6 0 0 1 3.5
Inferior Oolite Group 4 6 0 0 1 3.5
Lias Group 3 4.5 0 0 1 3.5
Notes Only upper three units are penetrated by boreholes
Factor Groundwater flow mechanism in intervening units between source and receptor, including the receptor Faults cutting intervening units and receptor Solution features in intervening units and receptor Anthropogenic features — mines close to site of interest Anthropogenic features — boreholes close to site of interest Intrinsic vulnerability score (V)
Weighting (w) 3 4.5 2 8 4
Confidence medium medium medium high high low
Geological unit
Chalk Group 1 3 2 9 2 4 0 0 2 8 38.5
Gault Formation 1 3 2 9 1 2 0 0 2 8 41.5
Lower Greensand Formation 1 3 2 9 1 2 0 0 2 8 43
Weald Group 1 3 2 9 1 2 0 0 2 8 54
Purbeck Group 3 9 2 9 1 2 0 0 2 8 69.5
Portland Group 3 9 2 9 1 2 0 0 2 8 69.5
Kimmeridge Clay 3 9 2 9 1 2 0 0 0 0 61.5
Corallian Group 3 9 2 9 1 2 0 0 0 0 37.5
Kellaways and Oxford Clay Formations 3 9 2 9 1 2 0 0 0 0 36
Great Oolite Group 3 9 2 9 1 2 0 0 0 0 29.5
Inferior Oolite Group 3 9 2 9 1 2 0 0 0 0 29.5
Lias Group 3 9 2 9 1 2 0 0 0 0 28
Notes Fault possibly 1 km from activity Portland group has potential for solution features, and evidence from BH log in chalk Below target, no boreholes.
Boreholes in village.

Intrinsic vulnerability for units between the hydrocarbon source unit and potential receptor,
and the potential receptor itself, for southeast England, conventional oil and gas.

Geological unit Receptor classification Vulnerability score (V) Risk Score (R)
Chalk Group A 41.5 83
Gault Formation D 41.5 83
Lower Greensand Formation A 43 86
Weald Group B 54 108
Purbeck Group B 69.5 139
Portland Group A 69.5 139
Kimmeridge Clay D 61.5 123
Corallian Group C 37.5 75
Kellaways and Oxford Clay Formations D 36 72
Great Oolite Group B 29.5 59
Inferior Oolite Group B 29.5 59
Lias Group C 28 56

Confidence

Low Low

Risk score for potential receptors for southeast England, conventional oil and gas.