The Puguang （普光） gas field is the largest gas field found in marine carbonates in China. The Feixianguan （飞仙关） and Changxing （长兴） reservoirs are two such reservoirs that had been buried to a depth of about 7 000 m and experienced maximum temperature of up to 220 ℃ before uplift to the present-day depth of 5 000-5 500 m, with present-day thermal maturity between 2.0% and 3.0% equivalent vitrinite reflectance （Ro）. Bitumen staining is ubiquitous throughout the Feixianguan and Changxing formations, with the greatest concentrations in zones with the highest porosity and perme-ability, suggesting that the solid bitumen is the result of in-situ cracking of oil. According to the distri-bution of bitumen in the core, the paleo-oil boundary can be approximately determined. The paleo-oil resource is calculated to be about （0.61-0.92） × 109 t （average 0.76 × 109 t）, and the cracked gas volume is about （380.80-595.80） × 109 m3 （average 488.30 × 109 m3）; at least 58.74% of cracked gas is preserved in Puguang gas field. The study area experienced not only the cracking of oil but also thermochemical sulfate reduction, resulting in large quantities of nonhydrocarbon gas, with about 15.2% H2S and 8.3% CO2, together with the structural reconfiguration. During the whole process, the great change of vol- ume and pressure compels the PVTsim modeling software to simulate various factors, such as the cracking of oil, the thermochemical sulfate reduction （TSR） and the tectonic uplift in both isolated and open geological conditions, respectively. The results show that although any one of these factors may induce greater pressure changes in an isolated system than in a closed system, the oil cracking and C3+ involving TSR lead to overpressure during the early stage of gas reservoir. Therefore, the tectonic uplift and the methane-dominated TSR, as well as the semi-open system contribute to the reducing pressure resulting in the current normal formation pressure.
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