Origin of oil and gas

Origin of oil and gas

The studies of origin of oil always concentrated on the determination of organic matter and establishing processes of its transformation. Discussions in the preceding chapters clearly point to the organic origin of all discovered oil and gas accumulations. Chemofossils are present not only in oil but also in coal, oil shale, and bitumen, i.e., biomarkers of organic compounds that preserve the structure of transitional bioorganic molecules.

More than 300 such hydrocarbons are described in crude oils. About the same number of biomarkers were discovered in the sulfur- and oxygen-containing compounds. (Liang 2003 ) Quite often, more than half of the crude oil is represented by biomarkers that are, therefore, not an admixture but an integral constituent of oils. (McQueen 1986)

The main concepts of the organic theory of oil generation [e.g., Mikhailovskiy, 1906; (in: Eremenko and Chilingar, 1996); Arkhangelskiy, 1954; Gubkin, 1915, 1932] are simple. Organic matter is accumulated (mostly in a dispersed state) in predominantly clayey marine deposits. There are two major types of organic matter: humic and sapropelic. It was believed that the latter played a major role in oil generation, whereas the decomposition of humic organic matter resulted in the formation of coal and water-soluble (hence, easily dispersible) substances and gas. (Bill 2006)

The decomposition of sapropelic matter gives rise to the liquid and gaseous compounds including hydrocarbons. The decomposition occurs as a result of heat flow and the energy of the sun accumulated by the organic matter. The hydrocarbons and some other substances formed from the decomposed organic matter are squeezed together with water out of the shales into the reservoir rocks. The hydrocarbons derived from the organic matter float in the water medium (gravitational theory) and move until trapped in the reservoir. (Ballentine 1992 )

Marine origin of oil source rocks appeared to be obvious, although it is unclear why the first oil-bearing sequences developed in different countries were continental or near-shore marine Paleogene and Neogene rocks. The studies of the present-day sedimentation indicated that all marine and almost all continental deposits contain organic matter. It was eventually recognized that only the presence of subaquatic sediments, either of marine or continental origin, was required. (Liang 2003)

Potonie (1920) indicated that there were three ways for organic matter to "burn" in nature: combustion, smoldering, and rotting. The latter process was believed (Mikhailovskiy, 1906; Potonie, 1920; Gubkin, 1932; Strakhov, 1960) to be responsible for the organic matter formation in nature. This is an important issue for at least two reasons: rotting occurs without the supply of oxygen; from the outset, the process is believed to be isothermal.

The most popular theory is known as the Organic Theory. This theory states that oil and gas have biological origins. Small sea creatures from the days when the earth was mostly covered in water died and settled to the bottom of the ocean floor. Layer upon layer of silt, sand and clay built up on top of them over time. Through the process of decay, as well as ever ...