Geology of Onshore Oil and Gas
Oil and natural gas was formed millions of years ago
Oil and gas can be trapped by rocks or within rocks
Oil and gas can be found anywhere in the UK
All of the oil and gas we use today began as microscopic plants and animals living in the ocean millions of years ago. When they died, they sank to the bottom and over the years, layer after layer built up forming a spongy material called kerogen.
This kerogen was covered by sand, clay and other minerals which over time turned into rock called shales.
After oil and natural gas was formed, it travelled through tiny pores in the surrounding rock until it either reached the surface or was trapped under or against a layer of rock that it couldn’t move through. Very slowly, the oil and gas built up. As it did, reservoirs were formed. These trapped deposits are where we find oil and natural gas today, for example, in the North Sea and many parts of onshore Britain.
In some cases the oil and gas became trapped within the shale rock itself and did not form traditional conventional reservoirs. This type of hydrocarbon is typically found at far greater depths than conventional oil and gas. As the shale is much less permeable (or easily penetrated by liquids or gases), it requires a lot more effort to extract the hydrocarbons from the rock.
The key outward difference is the rate at which the natural gas flows or the ‘flow rate’. Drilling into a conventional accumulation would normally result in at least some flow of oil and gas immediately. An accumulation trapped within the rock has to be stimulated in some way before it will even begin to flow.
Hydraulic fracturing is merely a method used to stimulate the oil or gas either to begin or continue flowing. A conventional well can also be hydraulically fractured to help extraction by improving flow rates. This does not mean it has become an unconventional well as the nature of the hydrocarbon accumulation has not changed.
Coal bed methane is a form of natural gas found in underground coal seams. Methane is adsorbed to the surface of the coal.
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.