Hydrocarbon expulsion model and resource potential evaluation of high-maturity marine source rocks in deep basins: Example from the Ediacaran microbial dolomite in the Sichuan Basin, China

Hydrocarbon expulsion features and resource potential evaluation of source rocks are crucial for the petroleum exploration. High-maturity marine source rocks have not exhibited a hydrocarbon expulsion mode owing to the lack of low-maturity source rocks in deep petroliferous basins. We considered the Ediacaran microbial dolomite in the Sichuan Basin, the largest high-maturity marine gas layer in China, to exhibit a method that quantitatively characterizes the hydrocarbon expulsion of high-maturity marine source rocks. The experiment of fluid inclusion, rock pyrolysis, and vitrinite reflectance (Ro) of 119 microbial dolomite core samples obtained from the Dengying Formation were performed. A hydrocarbon expulsion model of high-maturity source rock was established, and its resource potential was evaluated. The results showed that the Ediacaran microbial dolomite in the Sichuan Basin is a good source rock showing vast resource potential. The hydrocarbon expulsion threshold is determined to be vitrinite reflectance at 0.92%. The hydrocarbon expulsion intensities in the geologic history is high with maximum of 1.6 × 107 t/km2. The Ediacaran microbial dolomite expelled approximately 1.008 × 1012 t of hydrocarbons, and the recoverable resource was 1.5 × 1012 m3. The region can be categorized into areas I, II, III, and IV, in decreasing order of hydrocarbon expulsion intensity. Areas with a higher hydrocarbon expulsion intensity have a lower drilling risk and should be prioritized for exploration in the order I > II > III > IV. Two areas, northern and central parts of Ediacaran in the Sichuan Basin, were selected as prospects which had the drilling priority in the future gas exploration. The production data of 55 drilled wells verified the high reliability of this method. This model in this study does not require low-maturity samples and can be used for evaluating high-maturity marine source rocks, which has broad applicability in deep basins worldwide.