Quality and oil-bearing properties of argillaceous hydrocarbon source rocks across different lithofacies of Permian Lucaogou Formation in Jimsar Sag, Junggar Basin: a case study of well J10025

The study on the quality and oil-bearing properties of argillaceous hydrocarbon source rocks across different lithofacies is crucial for understanding shale oil enrichment patterns and predicting sweet spots. 57 mudstone samples from the Permian Lucaogou Formation in the well J10025 of the Jimsar Sag, Junggar Basin were selected in this study. Based on lithofacies classification, the quality and oil-bearing properties of those argillaceous hydrocarbon source rocks across different lithofacies were systematically investigated through X-ray diffraction (XRD), thin-section observation, rock pyrolysis, organic petrology, vitrinite reflectance (R_o) measurement, and multi-temperature pyrolysis. The results indicated that 7 lithofacies were predominantly developed in the Lucaogou Formation of the Jimsar Sag, including massive felsic mudstones, massive calcareous mudstones, massive dolomitic mudstones, laminated felsic mudstones, laminated calcareous-bearing mudstones, laminated calcareous mudstones, and laminated dolomitic mudstones. Distinct quality differences were observed in those source rocks. The massive felsic mudstones and massive calcareous mudstones exhibited poor quality, while massive dolomitic mudstones were moderate-quality source rocks. The laminated felsic mudstones demonstrated good quality, whereas laminated calcareous-bearing mudstones, laminated calcareous mudstones, and laminated dolomitic mudstones were high-quality hydrocarbon source rocks. The massive dolomitic mudstones exhibited relatively favorable oil-bearing capacity and mobility, making them potential sweet spot lithofacies. In the well J10025, mudstones with relatively good oil-bearing properties were deposited at depth intervals of approximately 3 500 to 3 570 m and 3 700 to 3 750 m. However, its overall poor mobility restricted the development of the mudstone-type shale oil sweet spots. The main factors influencing the oil-bearing capacity and mobility of mudstones included hydrocarbon-generating capacity, hydrocarbon expulsion efficiency, and total organic carbon (TOC) content. The content of free hydrocarbons was significantly controlled by the hydrocarbon-generating capacity of mudstones. Meanwhile, the widespread efficient hydrocarbon expulsion in mudstones directly led to a reduction of free oil content. Additionally, high TOC content resulted in a large amount of adsorbed oil, which constrained shale oil mobility.