Organic geochemical characterization of bitumens, seeps, rock extracts and condensates from Tanzania

• Published in: Organic geochemistry Vol. 21; no. 3; pp. 359 - 371
• Main Authors: Mpanju, F., Philp, R.P.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Oxford Elsevier Ltd 1994; Elsevier Science
• Subjects: Applied sciences; biodegradation; bitumens; Crude oil, natural gas and petroleum products; Earth sciences; Earth, ocean, space; Energy; Exact sciences and technology; Fuels; GCIRMS; Geology and geochemistry. Geological and geochemical prospecting. Petroliferous series; hopanes; Hydrocarbons; Isotope geochemistry; Isotope geochemistry. Geochronology; Prospecting and exploration; Prospecting and production of crude oil, natural gas, oil shales and tar sands; Sedimentary rocks; seeps; steranes; Tanzania; East Africa; Tanzania; GCIRMS; seeps; bitumens; Tanzania; biodegradation; hopanes; steranes; organic residues; stable isotopes; Mesozoic; C-13/C-12; maturity; biomarkers; aromatic hydrocarbons; gas chromatography; sandstone; mass spectroscopy; oil seeps; sedimentary rocks; stereochemistry; condensates; Cenozoic; clastic rocks
• ISSN: 0146-6380; 1873-5290
• DOI: 10.1016/0146-6380(94)90198-8

Several bitumens, oil seeps, and rock extracts from several areas of Tanzania have been characterized by a variety of geochemical techniques. Samples analysed included liquid seeps from Pemba, Ruvuma, bitumen extracts from sandstone outcrops from the Wingayongo area, cores and cuttings from Kimbiji East-1, Mnazi Bay-1, Pemba-5 wells and a condensate from Songo Songo-1 well. Samples from two areas, Wingayongo and Msimbati (Ruvuma) were particularly noteworthy and are discussed in detail in this paper. The extracts of the Wingayongo sandstone outcrop are virtually devoid of n-alkanes and steranes; hopanes are dominated by isomers with the ßß-stereochemistry suggesting an unusual source material or maturity history for this sample. The Msimbati seep samples are also characterized by high concentrations of immature hopanoids present as both hydrocarbons and fatty acids. The carbon isotopic compositions for the saturate and aromatic fractions are exceedingly light with δ 13C values of −47 and −59%, respectively. From the results of this study it is proposed that these seeps do not represent the biodegraded residues of mature crude oil but rather are a type of paraffin dirt where bacteria have utilized thermal methane as a substrate and biosynthesized the various compounds found in these samples. Evaluation of the biomarkers in a condensate from Songo Songo suggests the source rocks contained Type II/III kerogen. The only successful correlation that could be made was between a seep on Pemba Island and the Campanian/Maastrichtian formation of the Kimbiji East-1 well. This same seep did not correlate with extracts from the nearby Pemba-5 well and similarly the Ruvuma seeps did not correlate with extracts from the nearby Manzi Bay-1 well.