30-NOR-17 [alpha] (H) - hopanes and their applications in petroleum geochemistry

Abstract

A suite of samples consisting of twenty-two crude oils and eight sedimentary rocks has been analysed for biological marker compounds by GC-MS. The sedimentary rocks are rich in carbonate minerals and the crude oils were reported to have been derived from carbonate source rocks. These samples are from a variety of geographical origins, geological ages and depositional environments. They consistently contain a homologous series of 30-nor-17 [alpha] (H)-hopanes. Seven homologues (C28 - C34) of the 30-nor-17 [alpha](H)-hopane series have been identified. These compounds appear to be useful biological markers for samples having carbonate associations.A series of 25,30-bisnor-17 [alpha] (H)-hopanes has been observed in a severely biodegraded crude oil of probable carbonate origin. This observation, together with the well-established enrichment of normal hopanes demethylated at position 25 in severely biodegraded crude oils, suggests that the presence of this series of hopanes indicates severe biodegradation of crude oils originating from carbonate-rich source rocks.Another series of hopanes which was previously unreported, the 2-methyl-30-nor-1 [alpha] (H)-hopanes, has also been observed in the carbonate samples. Seven members (C29-C35) of this homologous series have been identified in this study. This series appears to be associated with carbonate rocks deposited under extreme reducing conditions.The biological marker compounds in another sample suite comprising twelve sediments and three crude oils from the North Sumatra Basin, Indonesia, have also been analysed by GC-MS as part of a correlation study. Sediment samples classified as shales, carbonaceous shales and calcareous shales have been shown to contain very different biomarkers. These distinctive biomarkers have enabled the source characteristics of the crude oils to be inferred. Two crude oils have been recognised with similar biomarker characteristics to the shales and one crude oil has the characteristics of the calcareous shale. The distinctive features of the carbonaceous shale were not observed in the crude oils. This study therefore provides an excellent example of how the 30-nor-17 [alpha] (H)-hopane compounds can be useful in oil-source rock correlation studies.