Hydrocarbon Expulsion Characteristics of Coaly Measure Source Rocks during Tectonic Uplift and Their Hydrocarbon Significance

Abstract

As the primary parent material for global natural gas, coaly measure source rocks have rendered research on their hydrocarbon expulsion characteristics a longstanding focus in petroleum geochemistry. However, the Yanshanian and Himalayan movements have subjected global petroliferous basins to varying degrees of denudation and modification, leading to a complex hydrocarbon expulsion and evolution process. Therefore, taking the coaly measure source rocks of the fifth member of the Xujiahe Formation (abbreviated as xu-5) in the Xinchang structural belt of the Western Sichuan Depression as a case study, this paper conducted methane adsorption–desorption tests and methane solubility analysis based on the geological and geochemical characteristics of the source rocks. The effects of temperature and pressure on the content of natural gas in different phase states were clarified. The results indicate that the xu-5 coaly measure source rocks have an average total organic carbon content (TOC) of 2.21%, with kerogen belonging to Type III organic matter and a mean vitrinite reflectance (Ro) of 1.59%, suggesting high-over maturity. Affected by the Yanshanian and Himalayan movements, this area has experienced regional tectonic uplift since the end of the Late Cretaceous (65 Ma ago). During this period, the burial depth of xu-5 decreased from 4,250 m to 2,968 m, the formation temperature dropped from 169.91 to 80.31 °C, and the formation pressure decreased from 67.38 to 52.24 MPa. Both methane solubility and adsorption characteristics are jointly influenced by temperature and pressure. At the maximum burial depth stage, the dissolved and adsorbed gas contents were 0.068 and 1.343 m3/(m3 rock), respectively. Currently, these values are 0.034 and 2.799 m3/(m3 rock), respectively. In addition, the adsorbed gas content increased by 0.737 m3/(m3 rock), while 0.016 m3/(m3 rock) of dissolved gas was released, and 10.35 m3/(m3 rock) of free gas was expelled during the tectonic uplift stage. On this basis, it is calculated that there are still 0.619 trillion cubic meters of recoverable adsorbed gas and 0.757 billion cubic meters of recoverable dissolved gas in xu-5 in the study area, which can serve as important sources for shale gas and intrasource tight sandstone gas in this formation. The research results hold significant theoretical implications for the exploration of shale gas and tight sandstone gas in coal-measure source rocks.