Methods and limitations

This page describes how the maps were produced and considerations when using them.

How are the principal aquifers defined?

To help protect groundwater, the Environment Agency (EA) in England and Wales has identified different types of aquifer, which is the name for underground layers of water-bearing, permeable rock from which groundwater can be extracted. These are principal aquifers and secondary aquifers. Other rock types from which groundwater cannot be readily extracted are designated as non-productive strata. The aquifer designations are consistent with the European Water Framework Directive and reflect the importance of aquifers in terms of groundwater as a resource or drinking water supply, and also their role in supporting surface water flows and wetland ecosystems.

Principal aquifers are designated by the EA as strategically important rock units that have high permeability and water storage capacity, and are based on geological mapping provided by BGS.

Because the principal aquifers are defined by rock units that may be found both near the ground surface as well as at significant depth, large parts of these geological units may not be exploited as aquifers and may not have significant interaction with rivers and surface water. As a result, although designated as principal aquifers, these units do not always reflect the location of usable aquifers. However, the aquifer maps show the full extent of the geological units that are designated as principal aquifers by the EA, not just those parts of the rock units that are actively exploited as an aquifer.

How have the shales been selected?

The shale maps show the full extent of six major shale units in England and Wales. These are not the only shale units in England and Wales. They were chosen based on the findings of previous studies by DECC (2012) 7.8 KB pdf and Smith et al. (2010). They were selected because they are regionally extensive shales where some part of each unit may be investigated for future shale gas development.

How have the aquifer and shale gas surfaces and separation maps been calculated?

A low resolution, national scale model of the 3D geology of the UK, the Bedrock Fence Diagram (UK3D) has been used to estimate the position of aquifer and shale surfaces. UK3D consists of a series of vertical geological sections, typically about 30 km apart, across England and Wales. These sections show how the sequence of rocks along their length varies with depth. A geological modelling tool (GSI3D) has been used to estimate the locations of the base of the principal aquifers and the tops of the shales from the information in the geological sections in the National Geological Model (NGM), and then to construct surfaces for each unit. These modelled surfaces are then processed in a geographical information system (GIS) to produce the principal aquifer and shale occurrence maps. The vertical separation maps are also produced in the GIS and have been calculated by subtracting the depth to the base of the aquifer from the depth to the top of the underlying shale unit.

Although the maps of the principal aquifers show the full extent of the rock units that define the aquifers, the aquifer/shale separation maps that show the vertical separation between aquifers and shales use a modified form of the aquifer maps. They have been modified to take into account the recommendation of the UK Technical Advisory Group on the Water Framework Directive (UK TAG, 2011) where the maximum depth of groundwater bodies has been defined as 400 m below ground level. Note that at about 400 m below ground level it may become increasingly difficult to obtain adequate quantities of groundwater from the aquifers and groundwater becomes increasingly mineralised (eg more saline) so that it is typically not suitable for public or private supply. The vertical separation maps that include the Bowland and Craven Groups (shales) show separations based on the prospective area of the Bowland and Craven Groups described in the resource estimate report by Andrews (2013) rather than the full extent of those shale units.

How accurate are the maps?

The maps represent our best understanding based on the NGM. However, it is important to understand that the key boreholes used to produce the vertical geological sections in the NGM may be tens of kilometres apart. So, unlike traditional geological maps, (with mapping at scales of 1:50 000 to 1:625 000 based on surface observations) there is much less information at the national scale for the deep subsurface maps and hence much more uncertainty.

Consequently, to reflect the level of certainty in the modelled geological surfaces, a 3 km by 3 km grid has been used as the basis of the mapping. In addition, the maps showing vertical depth separations between pairs of aquifer/shale surfaces are reported in five depth classes from separations of less than 200 m to separations of greater than 1600 m.

Considerations when using the maps

The maps and associated supporting information are intended to provide consistent national-scale geological and hydrogeological information. They may be used along with other information to help to inform strategic assessments and planning related to aquifers and shales, or as part of a Green Leaves-type environmental risk assessment. The maps and information can be used to identify regions where both aquifers and shales or clays are present, and to assess the broad spatial relationships between these geological units. Because of the scale and resolution of the data use to construct the maps, their use and application has a number of limitations described below.

Specific limitations on use of the maps

The maps and associated information have been designed to provide information at a national scale.
On their own, the maps are not suitable for any form of resource or hazard assessment.
They are not designed to be a stand-alone decision tool.
The maps only consider the principal aquifers, namely: the Crag; Chalk; Lower Greensand; Spilsby Sandstone; Corallian Limestone; Oolites; Triassic sandstone; Magnesian limestone; Permian sandstone; Carboniferous Limestone and the Fell Sandstone and Border Group. Other secondary aquifers are present across England and Wales, but maps are not yet available for these aquifers.
The maps only cover England and Wales: aquifers, shales and clays are present in other regions of the UK but are not currently shown.
The maps and information only considers six major shale units, namely: Kimmeridge and Ampthill Clays; Kellaways, Oxford Clay and Osgodby Formations; the Lias; the Marros Group; Bowland and Craven Groups and the Upper Cambrian shales. Other shale units, such as Devonian and older shales, are present across England and Wales, but are not included here.
The associated supporting information about the intervening strata between pairs of aquifer/shale and aquifer/clay units does not include information about fracturing in the sub-surface or on possible pathways for fluid flow.

References

Andrews, I J 2013. The Carboniferous Bowland Shale gas study: geology and resource estimation. British Geological Survey for Department of Energy and Climate Change, London, UK.

DECC. 2012. The unconventional hydrocarbon resources of Britain's onshore basins - shale gas. 7.8 KB pdf

Mathers, S J, Terrington, R L, Waters, C N, and Leslie, A G. 2014. GB3D — a framework for the bedrock geology of Great Britain. Geoscience Data Journal, Vol 1. pp 30–42.

Smith, N J P, Turner, P and Williams, G. 2010. UK data and analysis for shale gas prospectivity. In: Vining BA and Pickering SC (Eds.). Petroleum Geology: From Mature Basins to New Frontiers. Proceedings of the 7th Petroleum Geology Conference. 1087–1098. DOI:10.1144/0071087.

UK TAG. 2011. Defining and reporting on groundwater bodies. UK Technical Advisory Group on the Water Framework Directive, working paper V6.21/Mar/2011