Hydrocarbon Generation within Source Rock
Not much kerogen is needed to make up source rock, usually, concentrations as low as 1-3% are sufficient to make source rock suitable for commercial exploitation of crude oil and gas. The most abundant kind of source rock is black shale. Typically, oil source rocks can contain up to 40% organic matter and the level can come close to 100% for some types of coal. As source rock, which is rich in organic matter, undergoes burial, where additional sediments are laid above it, the rock becomes hotter. This phenomenon reflects the geothermal gradient of the earth. Ground temperature is relatively stable (11°C) from the surface to a depth of about 60 meters, from 60 meters to about 122 meters the temperature varies depending on atmospheric influences and groundwater which is constantly circulating. Overall, temperature plays a crucial role in the generation of oil and gas from kerogen.
As the 400-meter mark is surpassed, the temperature rises steadily with depth, which varies depending on location. Usually, it is 1.5-2°C per 30.5 meters. Oil window, a term used to describe the range of temperature or depth where most of the oils complex constituents are produced as shown below. This window is typically 80-220°C or 2,200-5,500 meters.
Kerogen in source rock eventually undergoes conversion to petroleum. This process is called maturation and it depends on certain factors such as pressure, which is imposed by overlaying rock and sediment, presence of heat-tolerant bacteria that can act on oil, and the presence of water and hydrogen. Peak conversion from kerogen to oil happen at 100°C. When the temperature reaches 130°C, crude oil breaks into even smaller molecules, creating gas. Initially, this will be wet gas and condensate, with high levels of hydrocarbons. As temperature increases further, the gas becomes drier and contains more of the lighter hydrocarbons.
A term worth mentioning in this article is abiogenic hydrocarbons. Around 60 years ago, Russian and Ukrainian scientists proposed an alternative mechanism for the production of hydrocarbons. The abiogenic, or inorganic, the process does not involve biological material but transforms hydrogen and carbon into oil and gas deep within the outer mantle of the earth. This process has to take place at temperatures greater than 1,500°C and pressured 50,000 times that at the earth’s surface. These conditions are usually found at a depth of 120 thousand meters. Thomas Gold, a U.S astronomer and geoscientist, was really fond of this theory. Nevertheless, despite promising results from laboratories and field experiments, the theory itself remains highly speculative.