High abundance of carotanes in the brackish-saline lacustrine sediments: a possible cyanobacteria source?

The organic-rich oil shales of low maturity and tight oils from the Middle Permian Lucaogou Formation were formed in a brackish-saline lacustrine environment in the Jimusaer Sag, Junggar Basin. They are characterized by remarkably abundant β-carotane and γ-carotane. The biological sources and favourable sedimentary environments of carotanes formation remain obscure. Organic geochemical analyses of 17 oil shales and 20 tight oils from the Jimusaer Sag reveal that reducing, evaporitic and carbonate-rich, shallow water environment is favourable for β-carotane accumulation. Geochemical studies show that carotane abundance is controlled by both source of organic matter and sedimentary environment rather than thermal maturity in the study area. Even though both higher plants and microalgae are reported to be precursors of carotanes, cyanobacteria may be the main biological sources of carotanes in the Lucaogou Formation based on cyanobacteria-derived biomarkers, such as the C₃₉ 2,6-dimethyl benzene-substituted isoprenoid, 2α-methylhopanes, 7- and 8-monomethylhepadecanes. The airborne volcanic ash containing ample elements (e.g. Mg and Fe) caused by the intermittent volcanic eruptions during the Middle Permian most likely contributed to the flourish of cyanobacteria in the study area.