Recovery of original quality characteristics and estimation of hydrocarbon expulsion of upper paleozoic source rocks in the Sishen 1 well area, Songliao Basin

Natural gas has been discovered in the basement of the Paleozoic strata in the Sishen 1 well area of the Songliao Basin. However, the organic matter in the source rocks was in the over mature stage. Based on residual TOC and rock pyrolysis, the evaluation accuracy of source rock was reduced significantly, and it could result in erroneous assessments of the oil and gas exploration potential. A bulk of experimental data encompassing rock pyrolysis parameters, isotopic ratios of organic carbon, vitrinite reflectance, organic maceral components and multiple recovery methodologies were employed to restore the original organic matter content and hydrocarbon generation potential of the Paleozoic source rocks. The degradation rate method was selected as the most reliable recovery method based on the recovery results. Subsequently, the expulsion characteristics of the source rocks throughout the thermal maturation process were analyzed, leveraging the hydrocarbon potential method. The findings indicated that the majority kerogen type of Paleozoic source rocks in the Sishen 1 well area were classified as Type I and Type II₁. The average original total organic carbon (TOC) content was 2.56%, while the average original S₂ value was calculated to be 16.11 mg/g. Consequently, the quality of the original source rocks was good to excellent. According to the hydrocarbon expulsion curves of I and II₁ kerogen during thermal evolution, the Paleozoic source rocks in the Sishen 1 well area were found to have entered the hydrocarbon expulsion threshold during the Early Triassic (approximately 230 Ma), characterized by a vitrinite reflectance (R_o) value of 0.7%, and the peak of hydrocarbon expulsion was attained in the Late Triassic (approximately 210 Ma), with an R_o value of 1.1%. Oil accumulation was occurred from the Early Triassic to the Early Jurassic (approximately 230 Ma to 200 Ma), with an estimated expulsion volume of 416.423×10⁸ t. Then, gas generation was happened during the Middle to Late Jurassic (approximately 200 Ma to 165 Ma), with an expulsion volume recorded at 55.093×10⁸ t. The study has shown that that the Upper Paleozoic source rocks in the Sishen 1 well area have provided abundant source of hydrocarbons for the formation of oil and gas reservoirs.