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

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 β-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 β-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 δ 13C values indicative of contributions from both Chlorobiaceae and algae to the OM.