Organic Origin of Petroleum.

Organic Origin of Petroleum 

From an oil industry and exploration geochemistry point of view it is significant to remember that at one time a diversity of opinion existed about the origin of petroleum. It was the laboratory identification of chemical constituents and physical associations found to persist along the organic matter-to-petroleum pathway:

• organic matter biotic matter deposited in sediments 
• kerogen biotic matter preserved in sediments (insoluble in organic solvents, but soluble in some acids) 
• bitumen soluble petroleum precursors dispersed in source rocks 
• petroleum oil and gas accumulations in reservoirs 

that permitted scientists to verify the organic origin theory. Today, it is this same pathway along which we backtrack to make many exploration geochemistry interpretations. Let us look at some diagnostic chemical constituents and physical associations that document the organic matter-petroleum pathway on which geochemistry is based. 

Occurances of Hydrocarbon Compounds 

Many hydrocarbon and related compounds that are present in petroleum are also present in plants and animals. For example, hydrocarbon compounds make up small fractions of fats, waxes, natural oils, and pigments in living organisms. Even more important both volumetrically and as confirmation of the organic origin of petroleum are the hydrogen/carbon-rich compounds in organic matter which, through minimal biochemical and thermochemical breakdown, have been found to alter to hydrocarbons in the subsurface. Traced in their natural occurrences from organic matter through kerogen and bitumen to petroleum, many of these organic molecules, commonly called biomarkers or geochemical fossils, provide conclusive evidence of the direct line of evolution in the organic matter-to-petroleum sequence (e.g., Moldowan, Seifert, and Gallegos 1985; Risk and Rhodes 1985; Wanli, Yongkang, and Ruiqui 1985). Conversely, if traced in reverse, as in geochemical studies of rock extracts and crude oils, these biomarkers permit us to identify source beds, correlate oil families, and make interpretations about depositional environments and postdepositional histories.

Carbon Isotope Properties 

Carbon isotope concentrations in organic matter and petroleum are similar (e.g., Silverman 1967; Sackett 1968; Fuex 1977). In addition, these concentrations are different from those of many other substances. Within organic matter the two stable isotopes of carbon, carbon 12 (12C) and carbon 13 (13C), do not have isotopic ratios that are similar to that fixed between carbon isotopes at the time of the earth's formation ( Figure 1 , Stable carbon isotope ratios: Carbon isotope values of organic and inorganic materials relative to primordial carbon and atmospheric carbon dioxide).

This is because organic processes and reactions that yield carbon from carbon dioxide, as during photosynthesis, favor the lighter 12C isotope at the expense of the heavier 13C isotope ( Figure 2 , Fractionation of carbon isotopes through photosynthesis). As a consequence, natural fractionation creates carbonaceous compounds that are depleted of 13C relative to earth's primoidal, fixed 13C-12C ratio.

To carry the discussion one step further, terrestrial plants are even more effective at 13C-12C fractionation than are marine plants. Terrestrial organic processes generally involve an additional fractionation step of putting carbon dioxide into solution, thereby generating organic matter further depleted of 13C.

The depletion of 13C in petroleum in the same ranges as both terrestrial and marine organic matter is interpreted to reflect the transformation of organic matter to kerogen and bitumen in buried sediments, followed by the migration of bitumen to form petroleum in reservoirs. This relationship is one of those upon which exploration geochemistry relies when using carbon isotopes to correlate crude oils, pair crude oils and source rock, or interpret terrestrial versus marine progenitors.

General Relationships 

In addition to tracing organic compounds and isotope ratios directly along the organic matter-to-petroleum evolutionary pathway, a number of other general relationships are present that support the concept of organic origin of petroleum. These are applied in a variety of ways in geochemical interpretations.

Organic-rich sediments A most obvious relationship is the association of petroleum occurrences with sedimentary rocks. This broad association, in fact, gave initial impetus to the theory of the organic origin of petroleum. Studies by scientists from nearly all countries that are active in oil and gas exploration have consistently shown that sedimentary rocks with a minimum or threshold concentration of organic matter must be present in the sedimentary package before petroleum accumulations are to be expected. Although this minimal level can vary with rock type, or with organic matter type, or with the manner in which organic matter is distributed throughout the rock, a fundamental requisite for petroleum occurrence is the presence of adequate carbonaceous organic source material. It is a logical step, then, when exploring for oil and gas to use exploration geochemistry to backtrack on this organic matter-to-petroleum sequence and confirm the presence of source rocks.

Chemical balance Another association that documents organic origin of petroleum is the persistence with which a general chemical balance is maintained within organic materials and products as reactions progress irreversibly along the organic matter-kerogen-bitumen-petroleum pathway.

In the kerogen-to-bitumen transition, for example, approximately twice as much hydrogen as carbon is used up to form bitumen. The kerogen that remains unconverted to bitumen shows this preferential loss of hydrogen (Tissot et al. 1974) ( Figure 3 , General evolutionary pathway of bitumen-generative kerogen during maturation). More mature kerogen consists of more hydrogen-depleted (e.g., cyclical, condensed, aromatized) molecules.

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