Organic matter enrichment and palaeo-sedimentary environment reconstruction of lacustrine shale in the second member of the Middle Jurassic Qiketai Formation in the Shengbei Sub-sag, Turpan–Hami Basin, NW China

Abstract

Lacustrine organic-rich shales in the second member of the Jurassic Qiketai Formation (J 2 q 2 ) in the Shengbei Sub-sag, Turpan–Hami Basin, NW China exhibit strong heterogeneity due to frequent alternations of sedimentary environments. The distinct shale environments present in both upper and lower units of J 2 q 2 provide an ideal example for studying the enrichment mechanism of organic matter (OM) under a complex sedimentary background. In this study, petrological, mineralogical, major/trace element and isotopic analyses were used to reconstruct the palaeo-environment and reveal the mechanisms of OM enrichment. The results indicate that the J 2 q 2 shale was deposited in a lacustrine mixed carbonate–siliciclastic environment and the palaeo-environment indicators suggesting an oxic–dysoxic, mildly brackish water condition. Based on a comprehensive comparison of both members of J 2 q 2 , an OM enrichment model is established and the main controlling factor of the formation of the organic-rich shale is elucidated. Under the background of a warm-humid climate, the lower unit of J 2 q 2 was deposited in a deeper and more restricted water body with stronger chemical weathering, resulting in limited terrestrial input compared to that of the upper unit. Inorganic geochemical analysis indicates a higher primary productivity in the lower unit of J 2 q 2 with local fluctuations. High primary productivity and favourable preservation conditions domain the OM enrichment in the study area. Supplementary material: Raw data of carbon and oxygen isotopic analysis, TOC, Rock-Eval pyrolysis, and major and trace element analysis are available at https://doi.org/10.6084/m9.figshare.c.6901480