Study on gas generation process of high-over mature coal-measure source rocks in southern Ordos Basin

The Upper Paleozoic Carboniferous Taiyuan-Shanxi Formation acts as the main hydrocarbon supply layer for tight sandstone gas in the Ordos Basin. The hydrocarbon generation and expulsion characteristics as well as the hydrocarbon generation potential of coal, carbonaceous mudstone, and dark mudstone are crucial matters in the exploration and development of tight sandstone gas in the southern part of the basin. Considering that the maturity in the southern Ordos Basin is generally above 2.0%, to restore the original hydrocarbon generation potential, in this study, low-mature samples of three types of coal-measure source rocks from the Carboniferous Taiyuan Formation in the Chengning Uplift of the Huanghua Depression in the Bohai Bay Basin were collected from the same strata. Different simulation temperatures ranging from 350 to 700 °C with a gradient of 50 °C were set. Hydrocarbon generation and expulsion simulation experiments in a closed system were conducted, and the residual and expelled hydrocarbons of the source rock simulation samples under different maturity gradients were obtained. The maturity of each simulated temperature point was calibrated by coal samples. Additionally, the source rock samples before and after the simulation were subjected to mass weighing, total organic carbon (TOC) analysis, rock pyrolysis (Rock-Eval), and quantitative analysis of hydrocarbon expulsion. The results indicate that as the simulation temperature increases, the oil generation process mainly occurs before 400 °C (Ro = 1.13%). Meanwhile, gas generation continuously increases from 300 °C (Ro = 0.83%) to 700 °C (Ro = 4.35%) without reaching a peak. The percentage of methane gradually rises and reaches over 95%. Combined with basin modeling, it is determined that the Late Jurassic to Early Cretaceous is the main gas generation period. At an experimental temperature of 700 °C in the closed system, the organic carbon conversion rate of dark mudstone is 77.1%. In contrast, coal has an organic carbon conversion rate of less than 20%, and carbonaceous mudstone has a rate of less than 30%. This shows that coal and carbonaceous mudstone still have significant hydrocarbon generation potential under high-temperature conditions.