Discussion on high hydrocarbon generation efficiency of saline lacustrine source rocks with low TOC: A case study of the second member of Funing Formation, Qintong Sag, Subei Basin

Significant progress has been made in shale oil exploration within the second member of the Funing Formation in the Qintong Sag, Subei Basin. However, geologists have noted that the measured Total Organic Carbon（TOC） contents are generally below 2%. Despite this, it is believed that source rocks in saline lacustrine basins can generate substantial amounts of petroleum even with low organic matter abundance, a concept known as the “low TOC” and highly efficient hydrocarbon generation mode of saline lacustrine source rocks. As hydrocarbon generation and expulsion occur during thermal maturation, the TOC levels in source rocks decrease. Therefore, accurately restoring the original TOC of these source rocks is crucial for a proper understanding of their hydrocarbon generation capacity, as well as for evaluating petroleum resources and supporting reserve growth and production enhancement. This study focuses on the typical mudstones and shales of the second member of the Funing Formation in the Qintong Sag. Techniques such as hydrocarbon generation simulation experiments, rock pyrolysis data, TOC and productive carbon content evolution during thermal simulation, and an element mass conservation method were employed to restore the original TOC of these mudstones and shales. The findings reveal that the TOC recovery coefficient of these saline source rocks can exceed values between 3 to 4, and this coefficient is significantly influenced by the lithology. The study found that: the TOC recovery coefficient increases with thermal maturity, the coefficient for laminated shale is higher than that for massive mudstone, and the TOC recovery coefficients for mudstones in the Ⅳ sub-member and shales in the Ⅰ to Ⅲ sub-members of the second member of the Funing Formation range from 1.1~1.5 and 1.5~3.0, respectively. These variations are attributed to differences in the type of organic matter and the hydrocarbon generating activation energy, leading to a higher hydrocarbon transformation rate and TOC recovery coefficient in shales compared to mudstones.