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Variations in the chemical composition of crude oils in the Gulf Coast of the United States are related to the depositional environments of the rock successions from which they are produced (a measure of source type), and to the temperature to which the oil has been subjected (a measure of maturation). The chemical composition of 2,105 Gulf Coast crude oils was calculated from physical properties determined by the U.S. Bureau of Mines. The relative proportions of paraffin, naphthene, and aromatic compounds in these oils revealed two clusters of crude oil composition. The first cluster contains an average of 70% paraffin, 20% naphthene, and 10% aromatic compounds and is the most common type of crude oil produced from Mesozoic reservoirs. The second cluster contains an aver ge of 43% paraffin, 45% naphthene, and 12% aromatic compounds and is the most common type of crude oil produced from Cenozoic reservoirs.
The importance of source material in determining crude oil composition is demonstrated by (1) the close association of high-wax crude oils with Mesozoic and Cenozoic rocks formed in deltaic environments; (2) the association of crude oils rich in aromatic compounds with rocks formed in interdeltaic environments; and (3) the occurrence of high-sulfur oils in Mesozoic reservoirs which are not associated with delta systems. Deltaic environments provide greater sources of terrigenous organic material versus interdeltaic areas, which contain more marine organic material.
The effects of thermal maturation are shown by the relation between reservoir temperature and the relative proportion of naphthenes in the crude oil. Oils having greater than 55% naphthenic compounds are produced from Cenozoic reservoirs which have lower temperatures than those which produce paraffin-rich oils.
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