Page 166 - Geology and Geochemistry of Oil and Gas

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Chapter 8

ORIGIN OF OIL AND NATURAL GAS

8.1. INITIAL ORGANIC MATTER AND ITS TRANSFORMATION

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 accumu-
lations. Especially significant is Chapter 5 on the oil composition. 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 com-
pounds. 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.
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 predom-
inantly 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.
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 hydro-
carbons derived from the organic matter float in the water medium (gravitational
theory) and move until trapped in the reservoir.
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.
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 respon-
sible 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.