Co-evolution of paleo-environment and bio-precursors in a Permian alkaline lake, Mahu mega-oil province, Junggar Basin: Implications for oil sources

The co-evolution of paleo-environment and bio-precursors in alkaline lakes is of significance to understand the extreme environment system and associated enigmatic hydrocarbon potential. Here we used organic petrology and biomarker geochemistry to investigate the bio-precursors in a Permian alkaline paleo-lake in the Mahu mega-oil province within about a hundred miles of the Mahu Sag, Junggar Basin, China, and its effect on oil generation and accumulation. In general, the bio-precursors in the alkaline lacustrine source rocks of the Fengcheng Formation were mainly bacteria and algae, with a low abundance of higher plants. Therefore, these source rocks were mainly prone to oil generation. Two distinctive hydrocarbon-generating bio-precursors— Dunaliella -like algae and cyanobacteria—were identified. In addition to fossil evidence for these bio-precursors, the former results in a high C 28 /C 29 sterane ratio and β-carotane abundance, and the latter results in the formation of medium-chain monomethyl alkanes in terms of biomarkers. The nature of the bio-precursors varied with the sedimentary paleo-environment, and was controlled by the salinity and stratification of the lake. Dunaliella -type source rocks were deposited in the central area of the alkaline lake, and cyanobacteria-type source rocks were formed around the lake margins. The crude oils in different parts of the Mahu mega-oil province within about a hundred miles have different sources. The bio-precursors in the saline lacustrine source rocks were jointly controlled by the age and water salinity of the source rocks. The physiological synthesis of carotenoids and sterols by haloalkaliphilic green algae may have affected the evolution of ancient green algae.