In the Weald Basin, the main interest of the hydrocarbon companies is the Kimmeridge Clay Formation (KCF), a thick deposit of shale rock which extends across most of the region. According to the British Geological Survey and the Oil and Gas Authority, a close analogue is the Bakken Formation of North Dakota. Oil was first discovered within the Bakken in early 1950s, but the area is most known for its recent oil boom enabled by hydraulic fracturing and horizontal drilling technologies. Since the early 2000s, the state has undergone massive industrialisation as thousands of wells have been drilled and fracked.
The “Kimmeridge Limestones” are not Limestones
At Brockham, the KCF is around 400m thick, with its upper and lower boundaries at around 750 and 1150 meters below ground. The KCF at Brockham contains three bands of micrite (9 to 25 meters thick), which could more accurately be described as a coccolith bearing shale rock deposit. The industry likes to call them limestones, but this is scientifically unsound and misleading.
Table showing the Kimmeridge layers, depth and thickness in meters (based on Nutech study of well Brockham #1, Sept 2016).
According to Angus Energy, the new sidetrack at Brockham was to “initially produce from a 200 meter (“m”) naturally fractured section of a 385m thick, Kimmeridge interbedded shale and limestone layer.”
At Horse Hill, the first target is the micrite layers, before targeting the entire Kimmeridge clay/shale, which is apparently estimated to contain 80bn barrels of oil.
Widespread Layer of Low Permeability
The permeability of the Kimmeridge micrites is between 0.02 and 0.032 milliDarcy (and that of the shale layers is even lower) – well below the generally accepted boundary of 0.1 mD between conventional and unconventional hydrocarbon extraction. Below 0.1 mD is generally considered to require unconventional extraction methods such as hydraulic fracturing or acid stimulation.
The Kimmeridge reservoir (and the source rock at the same time) is distributed throughout a widespread layer with no clear-cut boundaries. On the other hand, conventional reservoirs are found in finite and well-defined traps, with the source rock below, and cap rock above.
Schematc geology of gas resources, from US Energy Informaton Administraton.
David Smythe – Emeritus Professor of Geophysics in the University of Glasgow – has studied in-depth developments targeting the Kimmeridge at Horse Hill, Broadford Bridge, Leith Hill and Brockham. In his expert opinion for Brockham, he concludes that Angus Energy’s arguments that stimulation of the KCF – whether by acidisaton or by fracking – will not be necessary are incredible, because they run counter to the known geomechanical properties of the KCF, and because they conflict with the extensive experience of similar unconventional plays in the USA.