Shale gas is natural gas formed and held within shale formations. It is a fossil fuel originating from plant, algal or other remains, and composed primarily of methane (CH4). Generally, shale formations are composed of fine-grained (less than 0.0039 mm in diameter) laminated sedimentary rocks made up of clay minerals (at least 30%), quartz, and small quantities of fossils, organic matter, carbonates, feldspars and other minerals. Gas shale formations are normally black due to the high organic matter content, although other colours can occur depending on their mineralogical composition.
Shales are formed by the weathering of rocks and the transport and eventual deposit of fine debris into lakes, lagoons, river deltas and the sea floor. Vast quantities of dead plankton or aquatic plant material are incorporated into the deposit, where anaerobic bacteria convert the remains into a waxy substance called kerogen.
Depending on movement in the Earth’s crust and changes in climate, there are variations in the rate and type of debris, and other sedimentary rocks such as sandstones or limestones can be deposited. As sediments are buried deeper underground, they are subjected to increased temperature due to the Earth’s subsurface temperature gradient, and increased pressure due to the weight of accumulated sediments. This causes the sediments to compact and cement into rock. At temperatures above 50°C (122°F), kerogen begins converting into oil (catagenesis).
Origins of oil and natural gas. Source: Oil & Gas UK.
If the shale formation is buried deeper underground, and reaches a depth where the temperature is above 150°C (302°F), oil begins converting into natural gas (metagenesis). Ongoing burial, rock compaction and earth movement together with continued hydrocarbon generation causes the migration of oil and natural gas from shale.
However, some oil or natural gas will always be retained within the shale, adsorbed on to kerogen and clay particles, and filling pore spaces and natural fractures. It is this process that forms a potential shale oil or shale gas reservoir.