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Origin of solid bitumens, with emphasis on biological marker results

1986; Elsevier BV; Volume: 10; Issue: 1-3 Linguagem: Inglês

10.1016/0146-6380(86)90054-9

1873-5290

Joseph A. Curiale,

NMR spectroscopy and applications

Solid bitumens are allochthonous, non-disseminated organic matter found associated with sedimentary rocks. Common worldwide examples include gilsonite, grahamite and ingramite. Their occurrence in petroliferous basins makes knowledge of their origin of interest to organic geochemists and petroleum explorationists. Using current analytical methods, it is usually possible to assign an origin to these materials, based on chemical characteristics of the whole material and its extractable fraction. In this study, 27 solid bitumens and their extracts have been studied, and the following analyses were performed: elemental (H, C, N, Ni, and V) and isotopic (σ13C) analyses of whole bitumen and extracts thereof; pyrolysis-gas chromatography of whole bitumen, and gas chromatography of extractable aliphatic hydrocarbons; carbon-13 NMR aromaticity measurements of the whole bitumen, using CP-MAS techniques; and GCMS studies of biological markers in the distillable (400–900°F) hydrocarbons. Results which have a bearing on the genesis of these materials are presented and discussed. Results indicate that (a) the classical generic names assigned to each bitumen, based on solubility, fusibility and H/C ratios, are neither source-distinctive nor indicative of genesis; (b) the relative amounts of n-hydrocarbons in chromatograms and pyrograms are useful in assessing both origin and the nature of secondary alteration (e.g. biodegradation); (c) the solid bitumens examined can be successfully classified using a genetic scheme, based largely upon molecular and bulk maturation parameters. Using NMR-derived ƒa values, H/C atomic ratios, moretane/hopane ratios, and ratios of sterane isomers, solid bitumens associated with oil source rocks can be genetically classified as either pre-oil or post-oil. Pre-oil solid bitumens are early-generation (immature) products of rich source rocks, probably extruded as very viscous fluids, which have migrated minimal distances to fractures. In contrast, post-oil solid bitumens are products of the alteration of a once-liquid oil, which was generated and migrated from a mature source rock. Following generation and expulsion, both pre-oil and post-oil solid bitumens are subject to the same modification processes, namely biodegradation, devolatilization and thermal alteration.