Abstract
Vein, lens and stratiform natural sedimentary bitumen deposits were analysed by combined geochemical and micropetrographic techniques. Reflectances in oil, air and water, fluorescence spectral properties, solubility in dichloromethane, elemental analysis and bulk chemical composition of soluble fractions by chromatographic separation indicate that these materials are generally gilsonite bitumen, although some, showing evidence of significant alteration and weathering, are closer to albertite in composition. Optical properties show that the bitumens are immature and fall within the field of asphaltites on the bituminization curve. Fluorescence spectra generally show two maxima, again a feature associated with immature gilsonite bitumen. Capillary-column g.c. of saturates fractions indicates the absence of normal and acyclic isoprenoid alkanes, these fractions being dominated by a series of C 27-C 34 pentacyclic triterpanes of the hopane family. The saturates fraction composition is characteristic of biodegraded crude oils, and molecular biomarker stereochemistry indicates a moderately mature petroleum source. All the bitumens are shown to have a common genetic origin and are thought to be the result of in situ alteration and biodegradation of oil infillings in joints, fractures and bedding planes, resulting from fracturing and faulting of reservoir and overlying cap rocks associated with a period of Neogene tectonism.
Published Version
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