Abstract

A detailed organic geochemical study of over 150 samples from two cores with a total combined length of 320 m (1,050 ft) through sequences of interbedded source rock-type shales (0.84% R m maturity) and reservoir sandstones allowed recognition and quantitation of a number of migration effects. Thin shales interbedded in sands and the edges of thick shale units are depleted in petroleum-range hydrocarbons to a much higher degree than the centers of thick shale units. For the alkanes, expulsion occurs with pronounced compositional fractionation effects: shorter chain length n-alkanes are expelled preferentially, and isoprenoid alkanes are expelled to a lesser degree than their straight-chain isomers. Based on material balance calculations, expulsion efficiencies were determined and found to be very high in certain instances. For thin interbedded shales, they decrease from about 80% around C 15 to near zero in the C (sub 25+) region. There is no evidence for significant redistribution of steranes and triterpanes in the two sequences. Compared to C 15 to C 25 n-alkanes, they appear relatively immobile. The composition of the hydrocarbons impregnating parts of the reservoir sandstones is in agreement with expulsion occurring with pronounced fractionation based on molecular chain length. Hence, consideration of bulk expulsion efficiencies gives an unrealistic picture. Furthermore, the impregnation of a siltstone cap rock from an underlying hydrocarbon accumulation seems to have occurred by bulk-oil migration and without significant fractionation. The degree of hydrocarbon depletion of some of the shales of both sequences appears to be controlled by compaction, and the primary migration process appears to have occurred with chromatographic separation. The migration phenomena observed in both sequences lead us to propose that the main phase of expulsion can be preceded by an earlier stage, during which the edges of thick shales and thin interbedded shales appear to be slowly and continuously depleted by the chromatographic processes. The composition of the hydrocarbons in the reservoirs at this stage appears to be controlled primarily by physical processes rather than by the type and maturity of the organic matter. By the mechanism, the origin of accumulations of light oils and gas-condensates in low mature sequences bearing predominantly terrestrial-derived organic matter can be explained. Finally, the migration effects documented in this study have some consequences for interpretation of geochemical data (e.g., the pristane/n-C 17 ratio, a commonly accepted maturity parameter, has been shown to be also controlled by the degree of hydrocarbon expulsion).--Modified journal abstract.

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