Characteristics of Fluid Inclusions and its Significance to the Coaliferous Gas Reservoirs of the Xujiahe Formation in the Central Sichuan Basin,China

Abstract

In the central Sichuan Basin, the maturity of source rocks in the Xujiahe Formation of the Upper Triassic is not high, with R_o values ranging from 0.8% to 1.2%. However, the gas reservoirs nowadays show characteristics of high gas-oil ratios and low condensate oil content (3.8–91.9 g/m³ on average). There are abundant gas hydrocarbon inclusions developed in the reservoir, with few liquid hydrocarbon inclusions, which indicates that the coal measures gave priority to gas rather than oil. In the fluid inclusion group, the CH₄ and C₂H₆ contents are 79.62–96.42% and about 10% respectively, while the content of C₃H₈ ranges from 3% to 5%. The carbon isotope values are δ¹³C₁= −44.59 ∼ −39.31‰, δ¹³C₂= −24.82 ∼ −28.05‰, δ¹³C₃= −21.39‰, δ¹³C₄= −22.05 ∼ −20.2‰, δ¹³Cco₂= −9.14 ∼ −13.86‰. The component and isotope compositions of gas in fluid inclusions coincide with those in the reservoirs, indicating that the gas hydrocarbon in the fluid inclusions can represent the residual gas in the reservoirs. The coal-measure source rocks can generate gas all the time during its burial process. The formation of salt-water inclusions associated with the gas inclusions can be divided into three stages. Firstly, the subsidence stage was at 170∼100 Ma, with the temperature of 70∼120°C. Secondly, the deep burial stage was during 100∼60 Ma, with the temperature of 120∼150°C. Thirdly, the uplift stage was at 60∼0 Ma, with the temperature of 90∼120°C. The time range reflected from gas inclusion is very wide, showing that the coal measures gave priority to gas hydrocarbon and can generate and expel gas continuously in a long period.