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