Oil and its biomarkers associated with the Palaeoproterozoic Oklo natural fission reactors, Gabon

Abstract

The 2.1 Ga FA Formation sandstone of the Franceville Basin in Gabon that hosts the Oklo natural fission reactors has been discovered to contain abundant Palaeoproterozoic oil-bearing fluid inclusions inside which syngenetic biomarkers are preserved. The oil occurs within H 2O and CO 2-dominated inclusions trapped in syntaxial quartz overgrowths and intragranular and transgranular microfractures in detrital quartz. Homogeneous fluid inclusion assemblages with oil inside a brine and carbonic fluid indicate that there was intermixing of brines and CO 2 with liquid hydrocarbons prior to trapping. Textural relationships and microthermometry suggest that entrapment of oil occurred first during diagenesis of the sandstone at ca. 115 to 200 °C soon after deposition of the sequence at ca. 2.1 Ga and secondly during a hydrothermal event at 200 to > 285 °C linked to the operation of the reactors ca. 1.98 Ga. A series of sequential outside-rinse blanks of framework grains analysed by gas chromatography–mass spectrometry (GC–MS) prior to fluid inclusion crushing shows a first solvent rinse dominated by hydrocarbons derived from co-occurring solid bitumens, which have a different geochemical distribution compared to the fluid inclusion oil. The crush–leach extract containing oil from fluid inclusions is markedly richer than the final outside-rinse blank, displays significant geochemical differences in compound distribution and is interpreted to include only a very small contribution from solid bitumen. The fluid inclusion oil has a high maturity, is non-biodegraded and lacks clear effects of radiolysis. It is therefore likely that the oil that migrated at temperatures higher than those of diagenesis was generated by hydrothermal alteration caused by reactor-heated fluids rather than by the criticality event itself. Biomarkers include hopanes, 2α-methylhopanes, terpanes and steranes, indicative of bacteria, including cyanobacteria and of eukaryotes. The relatively high concentration of regular steranes suggests that the source rock had a substantial eukaryotic contribution. Trace amounts of C 30 n-propylcholestanes indicate a contribution from marine algae. The oil was likely derived from conformably overlying marine black shales that were deposited during the Great Oxidation Event during which the Earth's atmosphere became oxygenated. The biomarker content of the oil is remarkably similar to that of other oils formed in sedimentary basins between 2.7 and 1.4 Ga. It is thus suggested that hydrocarbons were derived from a uniform range of organisms in this time period, despite apparent fluctuations of atmospheric O 2.