Sedimentary environment and organic matter enrichment of black mudstones from the upper Triassic Chang-7 member in the Ordos Basin, Northern China

Abstract

The black mudstone of the Chang-7 Member is the most important source rock in the Ordos Basin. In order to understand the paleo-climate, weathering, redox conditions, salinity, water temperature, water depth, and primary productivity under which black mudstone of the Chang-7 Member was deposited, we have analyzed total organic carbon (TOC), rock pyrolysis parameters, major elements, and trace elements of 60 black mudstone samples from the Chang-7 Member of the Yanchang Formation in the Ordos Basin. Geochemical indicators such as TOC, volatile hydrocarbon and residual hydrocarbon (S1 + S2), hydrogen index (HI), chloroform asphalt “A”, maximum pyrolysis peak temperature (Tmax), Vitrinite reflectance (Ro), kerogen macerals, and other organic indicators, show that the kerogen type of the Chang-7 Member was type I–II1, the thermal evolution of organic matter was in the mature stage, and it has good hydrocarbon generation potential. The major and trace element characteristics, such as C-value, Sr/Cu, Rb/Sr and Mg/Ca ratios, chemical alteration index(CIA), Sr/Ba, B, Ga-B-Rb trace element triangle, B/Ga, 1000 × Rb/K2O, V/(V + Ni), pristane/phytane ratio (Pr/Ph), foraminiferal Mg/Ca, and Co values indicate that the sedimentary environment of the Chang-7 Member was semi-arid and semi-humid, with continental freshwater, a reductive sedimentary environment, and an average paleotemperature of 16 °C. Based on the analysis of Baxs and Ba/Al ratio, the Chang-7 Member has medium–high primary productivity, which together with the sedimentary environment, played an important role in the enrichment and preservation of organic matter.