Generation of Petroleum (Crude oil)

Although exactly how crude oil originated is not established, it is generally agreed that crude oil is derived from marine animal and plant debris subjected to high temperatures and pressures. It is also suspected that the transformation may have been catalysed by rock constituents. Regardless of their origins, crude oil is mainly constituted of hydrocarbons mixed with variable amounts of sulphur, nitrogen, and oxygen compounds. Metals in the forms of inorganic salts or organometallic compounds are present in the crude mixture in trace amounts, the ratio of the different constituents in crude oil, however, varies appreciably from one reservoir to another.
Petroleum generation occurs over long periods of time—millions of years. In order for petroleum generation to occur, organic matter such as dead plants or animals must accumulate in large quantities. The organic matter can be deposited along with sediments and later buried as more sediments accumulate on top. The sediments and organic material that accumulate are called source rock. After burial, chemical activity in the absence of oxygen allows the organic material in the source rock to change into petroleum without the organic matter simply rotting. A good petroleum source rock is a sedimentary rock such as shale or limestone that contains between 1and 5% organic carbon. Rocks occur in many environments, including lakes, deep areas of the seas and oceans, and swamps. The source rocks must be buried deep enough below the surface of the earth to heat up the organic material, but not so deep that the rocks metamorphose or that the organic material changes to graphite or materials other than hydrocarbons. Temperatures of less than 302°F (150°C) are typical for petroleum generation. Geologists often refer to the temperature range in which oil forms as an “oil window” below the minimum temperature oil remains trapped in the form of kerogen, and above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Although this temperature range is found at different depths below the surface throughout the world, a typical depth for the oil window is 4–6 km. Sometimes, oil which is formed at extreme depths may migrate and become trapped at much shallower depths than where it was formed. The Athabasca Oil Sands is an example of this. According to generally accepted theory, petroleum is derived from ancient biomass. The theory was initially based on the isolation of molecules from petroleum that closely resemble known biomolecules . A number of geologists in Russia adhere to the abiogenic petroleum origin hypothesis and maintain that hydrocarbons of purely inorganic origin exist within Earth’s interior. Astronomer Thomas Gold championed the theory in the Western world by supporting the work done by Nikolai Kudryavtsev in the 1950s.