Petrographic, palynologic and geochemical characteristics of source rocks of the Permian Lucaogou formation in Jimsar Sag, Junggar Basin, NW China: Origin of organic matter input and depositional environments

Abstract

The Lucaogou Formation contains significant amount of shale oil and tight oil resources in China, and the target strata studied here is located in the Jimsar Sag, Junggar Basin. A combination of mudstone, silty mudstone, dolomite mudstone and limy mudstone from this formation was investigated for the organic petrology and organic geochemistry and thus organic matter origin, thermal maturity and depositional condition were evaluated. Source rocks indicate good potential in hydrocarbon generation, which were featured by high TOC, S2 and HI. Tmax and biomarker ratios reveal that source rocks are mature and have enter oil generation window. Microscopical analyses reveal that liptinitic organic matter are dominant components, especially the amorphous components, which are commonly accompanied with advanced plant fragments. Organic matter assemblages indicate organic materials originated from aquatic organisms and terrestrial plants, which is evidenced from the discrimination diagrams of Pr/nC17-Ph/nC18 and C27–C29 regular steranes. Vertical variations in the abundance of organic matter from the bottom up reflect the changes of the depositional conditions, which are consistent with the indications of organic geochemical parameters. In the lower Lucaogou (LLF), the depositional conditions varied from the relatively shallow water, moderate-energy, proximal suboxic/dysoxic-anoxic condition to the deeper, low-energy, distal suboxic-anoxic condition, as endorsed by the enhanced OM degradation and the preservation of palynomorphs. In the upper Lucaogou (ULF), the conditions changed gradually from a brackish, stratified, suboxic-anoxic condition to fresh, aerobic condition, followed by a stratified oxygen-depleted setting with a relatively higher saline upwards, as supported by the variation in the organic components. Overall, the conditions of LLF are more reducing than that of ULF, which is demonstrated by Pr/Ph ratios, gammacerane index and ETR. The understanding of the redox conditions and their evolution of the Lucaogou Formation is crucial to expound the formation of source rocks and evaluate the hydrocarbon-generating potential.