Relationship Between Petroleum Composition and Depositional Environment of Petroleum Source Rocks

Crude oils of nonmarine source can be distinguished from those of marine shale source and from oils originating in marine carbonate sequences by using a battery of geochemical parameters, as demonstrated with a sample suite of nearly 40 oils. A novel parameter based on the presence of C30 steranes in the oil was found to be a definitive indication of a contribution to the source from marine-derived organic matter. A second novel parameter based on monoaromatized steroid distributions was effective in helping to distinguish nonmarine from marine crudes and can be used to gauge relative amounts of higher plant input to oils within a given basin. Sterane distributions were similarly useful for detecting higher plant input but were less effective than monoaromatize steroid distributions for making marine versus nonmarine distinctions. Concentrations of high molecular-weight paraffin can also be effective nonmarine indicators but are influenced by maturation and biodegradation processes. Certain algal-derived nonmarine oils may show little high molecular-weight paraffin response. Oils from carbonate sources (with a few exceptions) can be distinguished by having low pristane-phytane ratios, low carbon preference indexes, and high sulfur contents. Gammacerane indexes and carbon isotope ratios of the whole crude are not effective in distinguishing these types of environmental differences on a global basis.