Significant contribution of haloalkaliphilic cyanobacteria to organic matter in an ancient alkaline lacustrine source rock: A case study from the Permian Fengcheng Formation, Junggar Basin, China

Abstract

Alkaline lakes are productive aquatic environments, and related source rocks appear to possess considerable hydrocarbon generation potential. The Fengcheng Formation (Fm.) source rock is a good case to investigate organic matter enrichment and hydrocarbon generation characteristics of such alkaline lacustrine source rocks. However, the biological contributor to organic matter of the Fengcheng Fm. is generally controversial. Based on geochemical, biological, and theoretical evidence, a new understanding of the biological contributors to organic matter of alkaline lacustrine source rocks was reassessed. The depositional paloenvironment of the Fengcheng Fm. was reconstructed and compared to the Tumbiana Fm. (a typical alkaline lake case) by using alkaline minerals, trace elements, nitrogen isotopes, and biomarkers. The results of abundant alkaline minerals, Pr/Ph < 1, Sr/Ba > 1, B/Ga > 7, gammacerane index >0.2, and δ15N > 10‰ (published data) show that the Fengcheng Fm. was deposited in a reducing, hypersaline, alkaline lake, similar to that of the Tumbiana Fm. The biological contribution to organic matters of the Fengcheng Fm. was comprehensively investigated by biomarkers, fossils, and microbial community structure analysis. The low terrigenous/aquatic ratio, abundant 2-methylhopanes, and high αααC29cholestane-20R proportion of the Fengcheng Fm. provide geochemical evidence for a larger cyanobacteria input than land plants and algae. Identified stromatolites and Chroococcus fossils provide biological evidence for the active participation of cyanobacteria during deposition. The investigation of microbial community structures of modern alkaline lakes provides a theoretical evidence for the fact that cyanobacteria are the dominant phytoplankton in alkaline lakes. The dominance of cyanobacteria in alkaline lakes relates to the following aspects: the extreme resistance of cyanobacteria, their inorganic carbon-concentrating mechanism, their adequate sodium and carbon supply from alkaline lake water, and the inhibited growth of some species of algae within alkaline waters. In summary, haloalkaliphilic cyanobacteria are the primary contributors to organic matter of the Permian Fengcheng Fm. that was deposited in an ancient alkaline lake.