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

摘要

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 -29.4parts per thousand) and 1,2,4-trimethylbenzene (1,2,4-TriMB; -26.4 to -29.7parts per thousand), which are isotopically similar to aryl isoprenoids (-29.9 and -31.2parts per thousand) and beta-carotane (-28.5 to -31.3parts per thousand) 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. -16parts per thousand) and 1,2,4-TriMB (ca. -14parts per thousand), 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 -22.8parts per thousand) and 1,2,4-TriMB (-19.3 to -21.2parts per thousand) and isotopically enriched isorenieratane (ca. -14.5parts per thousand). Off-line pyrolyzates from these lacustrine source rocks represent two distinct end members: C-13-depleted diagenetic products of beta-carotene derived from a lacustrine algal source and C-13-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 C-13-depleted composition. The presence of C-13-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 delta(13)C values indicative of contributions from both Chlorobiaceac and algae to the OM. (C) 2003 Elsevier Ltd. All rights reserved. [References: 37]