Light hydrocarbon characteristics of petroleum in a tight sandstone gas reservoir and its geological significance: a case study of the Upper Triassic Xujiahe Formation gas reservoir in the northwestern Sichuan Basin

Light hydrocarbons (C₇ and C₅-C₇) of the natural gas and condensate from the Upper Triassic Xujiahe Formation in the northwestern Sichuan Basin indicated that the petroleum was sourced primarily from coaly mudstones and coal rocks that were rich in humic kerogen. K₁ values from the study area average 1.04 and are relatively well correlated, while the K₂ values vary greatly and are relatively poorly correlated. The parent-daughter ratio of the Xujiahe Formation is of the discrete type in the Zitong Sag. The heptane value, isoheptane value and MANGO steady-state catalytic kinetic method indicates that the natural gas is currently in the mature to highly mature stage, with a maturity clearly lower than the measured maturity of the source rocks in the study area and the δ¹³C₁-calculated maturity. The star-plot diagram of light hydrocarbons of natural gas and condensate from the Xujiahe Formation in the Zitong Sag revealed the poor correlation with oil trapped in the same structure but different layers or in the same layer but different structures. This suggested that sand bodies in the reservoirs within the study area were poorly connected and the reservoirs were formed mainly as the result of near-source and short-distance migration and accumulation. Since the early reservoirs in this area have entered the tight evolution stage, natural gas is at different thermal evolution stages, and condensate oil is influenced by the effect of "evaporation and fractionation" of natural gas at different stages of the homologous period, resulting in the discrepancy of the interpretation of some light hydrocarbon parameters.