OR/16/020 Introduction

Loveless, S, and Smith, N. 2016. Summary of potential oil and gas formations in England for use in groundwater vulnerability assessments. British Geological Survey Internal Report, OR/16/020.

The joint Environment Agency (EA) and BGS project ‘3D Groundwater Vulnerability’ (3D GWV) will develop a methodology for attributing vulnerability of groundwater to pollution from sub-surface oil and gas exploration and production activities, including unconventional, conventional and hybrid plays. It will also take account of Coal Bed Methane (CBM) and Underground Coal Gasification (UCG) exploration, in addition to both near surface and deeper aquifers and groundwater. Outputs will include descriptions and visual representations of potential sources of sub-surface contamination of groundwater from different sources of hydrocarbons. These outputs are designed for use by the EA, Defra, other government departments, local planning authorities, environmental consultants and the public. The project will involve creating an attributed version of UK3D for England with source rock formations and aquifers identified.

This report describes the hydrocarbon bearing units in England. The units have been identified primarily from three BGS reports commissioned by DECC (the Department for Energy and Climate Change) in 2013 (DECC, 2013a; 2013b; 2013c) and three additional area-specific reports on shale gas prospectivity in the Bowland Shale (Andrews, 2013), the Weald (Andrews, 2014) and the Wessex area (Greenhalgh, 2016). Note that this report is not intended to be an exhaustive summary of the occurrence of hydrocarbon units in England, rather a high-level overview for hydrogeologists interested in the potential for groundwater contamination. If further detailed information is required about the hydrocarbon characteristics of the units the reader should refer to the source documents (and references therein). The reports identify units that have potential as conventional oil and gas reservoirs and source rocks (DECC, 2013a), for Coal Bed Methane (CBM) (DECC, 2013b) and shale gas (DECC, 2013c, Andrews, 2013; 2014; Greenhalgh, 2016).There is no similar report for Underground Coal Gasification (UCG) and therefore coal units have been identified from (DECC, 2013b).

This report gives a summary of the potential hydrocarbon resource types; identifies or attributes specific hydrocarbon source rocks on the Generalised Vertical Section (GVS), which identifies the main geologic units for England from BGS’ National Geological Model (NGM) (UK3D v2015 and Waters et al., 2016); and, summarises features of each of the units in the context of the £D GWV project.

Potential hydrocarbon resource types

Conventional hydrocarbons — Conventional hydrocarbons include known petroleum systems for both oil and gas. Source rocks are rocks that have a high proportion of organic material and have been buried deep enough to reach sufficient temperatures to form hydrocarbons, this is higher for gas than for oil. Reservoirs are rocks that oil or gas has migrated into and generally have high porosity and permeability. Reservoirs have relatively impermeable cap rocks and or faults that bound their upper surfaces preventing the upward migration of the free phase oil and gas. Stimulation of the rocks is generally not needed in order to extract the oil or gas.

Coal Bed Methane (CBM) — Gas can be bound within coal by adsorption — in which gas molecules adhere to the surfaces within the coal. In CBM a well is drilled into the coal seam and water is pumped out to lower the pressure in the seam. This allows methane to desorb from the internal surfaces of the coal and diffuse into the cleat where it is able to flow, either as free gas or dissolved in water, towards the production well. (DECC, 2013b). CBM production can be subdivided into three categories: Coal Mine Methane, CMM (from new or operational mines), Abandoned Mine Methane, AMM (from abandoned mines) and (virgin seam) Coal Bed Methane, (V)CBM (produced via boreholes from virgin coal seams). Permeability (imparted mainly by the cleat) is necessary to achieve CBM production. The natural permeability of coal seams can be low, so some CBM wells are stimulated (hydrofractured) to improve connectivity between the borehole and the cleat system. Wells may have many subsurface horizontal or multilateral side tracks drilled from one surface location to penetrate more coal (DECC 2013b).

Shale gas and oil — Gas can be found in the pores and fractures of shales and also bound to the matrix, also by adsorption. During enhanced fracture stimulation drilling technology, fluid is pumped into the ground to make the reservoir more permeable, then the fractures are propped open by small particles, and can enable the released gas to flow at commercial rates. By drilling multilateral horizontal wells, a greater rock volume can be accessed (DECC, 2013c).

Underground Coal Gasification — Underground coal gasification (UCG) is the process whereby the injection of oxygen and steam/water via a borehole results in the partial in-situ combustion of coal to produce a combustible gas mixture consisting of CO₂, CH₄, H₂ and CO, the proportions depending on temperature, pressure conditions and the reactant gases injected. This product gas is then extracted via a producing well (Jones, et al., 1999).

An up to date guide to the geologic units and areas that are currently licensed or under consideration for hydrocarbon exploration and production can be found on the UK Government’s website www.ogauthority.co.uk/data-centre/data-downloads-and-publications/licence-data.