Fluid inclusion records of oil-cracked wet gas in Permian carbonate reservoirs from the Eastern Sichuan Basin, China.

Oil-cracked gas is an important gas resource in petroliferous basins. In theory, oil-cracked gas should be rich in C 2+ (i.e., wet gas). However, gas reservoirs of oil cracking origin in the Sichuan Basin (South China) are mainly composed of dry gas. Fluid inclusions record hydrocarbon generation and evolution. Obviously, identifying wet gas inclusions is the key to solving the above controversy. Here, we calibrate the Raman spectroscopic detection limits of ethane and propane (0.007 and 0.016 MPa, respectively) and demonstrate that Raman spectroscopy is a powerful method to identify C 2+ -bearing inclusions. Based on this technique, C 1 –C 2 –C 3 -bearing wet gas inclusions (C 1 /ƩC i = 0.90–0.93) are identified in calcite cements of the Permian Changxing Formation carbonate reservoir in the Eastern Sichuan Basin. These inclusions are direct geological evidence of oil-cracked gas because the calcite cement precipitated after the formation of pyrobitumen. Furthermore, reconstruction of the temperature-pressure-composition evolutions of calcite-hosted (Changxing Formation) and quartz-hosted gas inclusions in the Ordovician Wufeng–Silurian Longmaxi Formation black shale indicates that high pressure can effectively retard the cracking of C 2+ gas hydrocarbons. Differences in gas composition and δ13CCH 4 signature between reservoir gas and inclusion gas lead us to conclude that large-scale oil cracking occurred during the Early–Middle Jurassic in the Permian Changxing carbonate reservoir. Nevertheless, oil-cracked gas escaped during the evolution of the paleo-gas reservoir. Formation of the current dry gas reservoir can be attributed to the continuous mixing of late-stage kerogen pyrolysis gas. • Raman spectroscopy is a powerful method to identify C 2+ -bearing inclusions. • Pristine signature of oil-cracked wet gas was preserved in fluid inclusions. • Fluid inclusion observations supports that high pressure retards C 2+ cracking. • The reservoir dry gas is a mixture of oil-cracked gas and kerogen pyrolysis gas. [ABSTRACT FROM AUTHOR]