The exploration firm UK Oil and Gas Investments (UKOG) announced that there could be up to 100 billion barrels of onshore oil beneath the south of England. Based on data recovered from a well drilled at Horse Hill near Gatwick in Sussex it suggested that the area could hold 158million barrels of oil per square mile.
The find is being heralded as a very significant discovery and the largest onshore in the UK for the past 30 years. UKOG says that the majority of the oil lies within the Upper Jurassic Kimmeridge Formation at a depth between 2,500ft (762m) and 3,000ft (914m).
So what does the geological sequence look like beneath Gatwick?
In 1964 a borehole was drilled at Collendean Lane, Norwood Hill close to Horse Hill. The borehole was commissioned by Esso Petroleum Company Ltd and the information remained confidential until it was declassified in 1977. The log shows a geology very similar to what has been reported in the press and is likely to be representative of the recent discovery.
You can download the borehole log here from the BGS website.
The site of the Esso borehole lies on approximately 150m thickness of Weald Clay that contains bands of siltstone, limestone and sands. The Weald Clay is not a designated aquifer but does host localised groundwater bores and wells that take water from these more permeable lenses interbedded with the clay.
Underlying the Weald Clay is the Hastings Formation signified by sandstones, siltstones and shales. The Hastings Formation is considered a secondary aquifer in the Weald with private abstractions tapping the Tunbridge Wells Sand Formation. The Hasting Formation is approximately 300m thick and overlies the Purbeck Formation. The Purbeck Formation is made up of interbedded mudstones, limestones and evaporites.
The table below shows the geological sequence identified in the 1964 borehole.
A core in the Esso borehole recovered from the Portland Formation described a ‘Sandstone non porous to tight grey to dark grey very fine grained, bleeding oil with H2S odour…’ at a depth of approximately 630m. This is at the top of the Kimmeridge which was where UKOG reported their find.
‘Sandstone non porous to tight grey to dark grey very fine grained, bleeding oil with H2S odour…’
The geology profile is shown in the graphic below and includes the comparison of where oil was seen in the Esso Borehole versus the UKOG report. The graphic also shows the decrease in water quality with depth, i.e. increased salinity as measured in 1964 using Total Dissolved Solids. The water quality in the Hastings Formation is markedly better than that found at depth.
Oil has been produced onshore in the South of England for decades. There are currently around a dozen oil production sites across the Weald and these sites have Environmental Permits and operate under strict monitoring conditions.
Inland onshore oil extraction can potentially harm groundwater resources if not managed carefully. Spills and leaks of product may occur at depth or at surface including process fluids. In some cases aquifer units of varying water quality can become interconnected through drilling of wells causing the mixing of groundwater bodies resulting in an overall decline in water quality.
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