Sedimentary Paleoenvironment and Hydrocarbon Generation Potential Evaluation of Middle Jurassic Coal-Measure Source Rocks in the Southern Beishan Area.

The Middle Jurassic coal seams in the Beishan region represent an important target for hydrocarbon exploration; however, research on the associated coal-measure source rocks remains limited. The coupling between depositional paleoenvironment and hydrocarbon generation potential among various coal-measure lithologies (including coal, associated dark mudstone, and carbonaceous mudstone) within the same or similar sedimentary facies belts is still poorly understood. In this study, the paleoenvironment was analyzed using major and trace element geochemistry to assess its influence on source rock development. Subsequently, organic geochemical methods were employed to evaluate the hydrocarbon generation potential of the source rocks. The results indicate the following: (1) Major and trace element data show that the coal-measure source rocks are enriched in CaO, Sr, and U, with Sr contents decreasing in the order of dark mudstone > coal > carbonaceous mudstone. (2) The Middle Jurassic lacustrine basin experienced cyclic water-depth fluctuations (shallow-deep-shallow), with paleosalinity dominated by saline to brackish conditions, a warm and humid paleoclimate, and predominantly reducing paleo-redox conditions. (3) Organic geochemical analyses reveal that dark and carbonaceous mudstones have higher organic matter abundances than coal. The organic matter is primarily Type II kerogen, with minor contributions from Type I and Type III. Vitrinite reflectance (Ro) values range from 0.54% to 0.84%, and the average value is 437 °C, indicating a low-maturity to early mature thermal evolution stage. These findings demonstrate that, within the same coal-measure source rock system, lacustrine-swamp facies dark and carbonaceous mudstones interbedded with coal represent high-quality hydrocarbon source rocks. Their enhanced hydrocarbon generation potential is attributed to elevated paleosalinity, which promotes the preservation of hydrogen-rich macerals such as desmocollinite. Currently at a low to early mature stage of thermal evolution (Ro: 0.54-0.84%), these source rocks possess significant potential for generating low-maturity oil and gas. Notably, their hydrocarbon generation potential surpasses that of the coal layers, offering critical theoretical support for Middle Jurassic coal-measure source rock exploration in the Beishan region.