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Organic matter occurs ubiquitously in the geological column, but the occurrences of commercial petroleum accumulations bear a relation to types and amounts of hydrocarbons found in equivalent rock strata. The distribution of organic carbon in a sedimentary rock unit, as well as in recent sediments, varies both vertically and lateraly and appears to be dependent on parent material, lithology, environment of deposition, and post-depositional processes. Clays deposited in a nearshore marine environment exhibit a high organic carbon content. The Mowry shale of Lower Cretaceous age in Wyoming has an organic matter content which reaches a maximum in an area where sediments are thought to have been deposited in a relatively shallow marine environment. The frequency of occurrence of oil fields in the Lower Cretaceous in the general area where the content of organic matter is high, and the enhanced concentration of hydrocarbons in sand lenticles in the Gulf of Paria sediments (Kidwell and Hunt, 1958) suggest that a portion of the petroleum generated in a source rock can be and is accumulated in close proximity to the locale of petroleum generation. The chemical constitution of Cretaceous crude oils from the Clearwater group on the eastern flank of the Alberta basin and from the McMurray sands at Athabaska strongly indicates the correlation of these petroleums, suggesting an equivalent source, probably Lower Cretaceous shales. Although there is considerable evidence for lateral migration of petroleum, crude oil correlation studies suggest that vertical migration is restricted, except possibly along fractures in the rock system. This view is further supported by consideration of the variations in benzene concentration around an oil pool. There is abundant evidence for mgiration of benzene several miles laterally, but vertical migration through overlying fine-grained competent rock is negligible. It is clear from the composition, properties, and distribution of organic matter that oil pools do not occur randomly in the subsurface, but, rather, they occur in those parts of the geologic column where petroleum has been generated, where reservoir rocks are available, and where suitable trapping conditions exist.
From considerations of the disposition of organic matter in a rock system, it should be possible to determine quantitatively the amount of petroleum generated in a given rock unit and which parts of the geological section are most likely to be productive. More directly, the types and amounts of hydrocarbons and their distribution in the subsurface can be used to evaluate the existence of an undiscovered oil pool.
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