Sources of alkylbenzenes in Lower Cretaceous lacustrine source rocks, West African rift basins

Pedentchouk, Nikolai, Freeman, Katherine H., Harris, Nicholas B., Clifford, David J. and Grice, Kliti (2004) Sources of alkylbenzenes in Lower Cretaceous lacustrine source rocks, West African rift basins. Organic Geochemistry, 35 (1). pp. 33-45.

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Abstract

We conducted molecular biomarker and compound-specific carbon isotope analyses of off-line pyrolyzates from lacustrine samples recovered by ONEZ-1 well (the Kissenda Formation) in the Gabon Basin and Well A (the Maculungo Formation) in the Kwanza Basin in West Africa. Pyrolyzates from ONEZ-1 well contain isotopically depleted 1,2,3,4-tetramethylbenzene (1,2,3,4-TetraMB; -26. 1 to -29.4‰) and 1,2,4-trimethylbenzene (1,2,4-TriMB; -26.4 to -29.7‰), which are isotopically similar to aryl isoprenoids (-29.9 and -31.2‰) and ß-carotane (-28.5 to -31.3‰) identified in the bitumen fractions. Pyrolyzates from 3 samples in the lower sections of Well A contain isotopically enriched 1,2,3,4-TetraMB (ca. -16‰) and 1,2,4-TriMB (ca. -14‰), consistent with an origin from diagenetic products of isorenieratene/chlorobactene biosynthesized by green sulfur bacteria (Chlorobiaceae). Pyrolyzates from five samples in the upper sections of Well A are characterized by the presence of intermediate carbon-isotope values of 1,2,3,4-TetraMB (-20.7 to -22.8‰) and 1,2,4-TriMB (-19.3 to -21.2‰) and isotopically enriched isorenieratane (ca. -14.5‰). Off-line pyrolyzates from these lacustrine source rocks represent two distinct end members: 13C-depleted diagenetic products of ß-carotene derived from a lacustrine algal source and 13C-enriched diagenetic products of isorenieratene/chlorobactene derived from green sulfur bacteria. Organic matter (OM) from ONEZ-1 well shows a significant proportion of algal biomass with a 13C-depleted composition. The presence of 13C-enriched biomarkers derived from carotenoids of Chlorobiaceae from samples in the lower part of Well A indicates the development of photic zone euxinia and a significant biogeochemical role of sulfur. In the upper section of Well A, the 1,2,3,4-TetraMB and 1,2,4-TriMB have d 13C values indicative of contributions from both Chlorobiaceae and algae to the OM.

Item Type: Article
Faculty \ School: Faculty of Science > School of Environmental Sciences
Depositing User: Rosie Cullington
Date Deposited: 01 Jul 2011 13:27
Last Modified: 21 Apr 2020 20:22
URI: https://ueaeprints.uea.ac.uk/id/eprint/33222
DOI: 10.1016/j.orggeochem.2003.04.001

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