Geochemistry and depositional environment of fresh lacustrine source rock: A case study from the Triassic Baijiantan Formation shales in Junggar Basin, northwest China

The shales in the Baijiantan Formation are considered to be an important potential source rock in the Junggar Basin. To evaluate this basin's source-rock characteristics, 61 core samples from 14 boreholes were analyzed using total organic carbon and total sulfur analyses and Rock-Eval pyrolysis. Thirty-two core samples were analyzed using solvent extraction, column fractionation, biomarker analysis and the organic petrology of kerogen. The results suggest that the shales in the Baijiantan Formation have fair to excellent hydrocarbon generation potential, and the organic matter types include both Type II and Type III. The shales in the Baijiantan Formation are mainly oil-generating rocks, and better source rocks are mainly distributed in the middle-lower section of the Baijiantan Formation. The total organic carbon and total sulfur analyses, Rock-Eval pyrolysis, and biomarkers indicate that the shales in the Baijiantan Formation formed in a low-salinity to freshwater lacustrine sedimentary environment under reducing conditions. Biomarkers indicate that the organic matter input of the shales exhibits varying contributions from plankton, algae and terrestrial organisms. The biomarker molecular composition of the source rock is related to the organic matter source, salinity, redox conditions and maturation.