Formation mechanism of tight coal-derived-gas reservoirs with medium-low abundance in Xujiahe Formation, central Sichuan Basin, China

Although Xujiahe Formation source rocks in the central Sichuan Basin, China were determined to have low gas-generation intensity, several large-scale gas fields have been found associated with this formation. Thus origins and formation mechanisms of natural gas in the Xujiahe Formation have attracted attentions of many researchers. In this study, geochemistry techniques were deployed to analyze natural gas and formation water in the Xujiahe Formation tight gas reservoir, central Sichuan Basin. By considering geologic background of the gas reservoir, its formation mechanism was investigated. Research results show that the Xujiahe Formation reservoir contains an independent petroleum system, with its natural gas exclusively originated from coal-measure source rocks in the Xujiahe Formation. The formation water was determined to be CaCl2 type with high salinity. H and O isotope values are largely deviated from those of meteoric water line, indicating favorable preservation conditions for the formation water. The Xujiahe Formation is composed of multiple coal-measure source rocks and superimposed by tight sandstones. The well-developed formation water provides favorable conditions for generating water-dissolved gas. Structural movements during the Himalayan period induced significant uplifting of the central Sichuan Basin with overlying formations denudated. Consequently, natural gas dissolved in formation water experienced depressurization and exsolution. The released natural gas formed free gas phase in structural highs or provided additional natural gas supplies to existing gas reservoirs.