Origin of Marine Sour Natural Gas and Gas-Filling Model in the Puguang Giant Gas Field,Sichuan Basin,China

Taking the geology and tectonic evolution characteristics of the Sichuan Basin into account, the chemical and stable isotopic compositions of natural gas, and biomarker compounds in the reservoir bitumen in the Puguang giant gas field, are investigated to identify the genetic type of marine sour natural gas, take the gas-source correlation, and set up the gas-filling model of the Puguang giant gas field in the Sichuan Basin. The alkane gases in the field are dominated by methane, ranging from 22.06% to 99.64% with an average value of 76.52%, and the low content of heavy hydrocarbon gases are dominantly ethane and little propane. The H₂S contents occur among the marine carbonate gas reservoirs, ranging from 0 to 62.17%, wherein the H₂S contents in the Upper Permian Changxing Formation and Lower Triassic Feixianguan Formation range from 6.9% to 34.72% (average value=15.27%) and from 0% to 62.17% (average value= 13.4%), respectively, indicating that both are H₂S-enriched reservoirs. The chemical and carbon isotopic compositions of marine natural gases show that the alkane gas in the Puguang giant gas field is dominantly oil-cracking gas at high maturity stage, and the biomarker characteristics of reservoir bitumen indicate that the major source rocks are the Upper Permian Longtan Formation sapropelic matters. Moreover, various levels of thermochemical sulfate reduction (TSR) were present in the process of oil-gas transformation, not only increasing the content of non-hydrocarbon gas components (CO₂ and H₂S) and decreasing the content of heavy hydrocarbon gases, but also causing the reversal of carbon isotope compositions of methane and ethane and the heavier carbon isotope of methane. The recovery of structural configurations over geological time investigates that the gas-filling history of Puguang giant gas field can be divided into three stages: formation of paleo-oil accumulation from the middle-late Indosinian period to the early Yanshanian period, thermal cracking of paleo-oil and TSR alteration from the early to the middle Yanshanian period, and adjustment of gas accumulation from the late Yanshanian to the early Himalayan period. The gypsum of the Lower Triassic Jianglingjiang Formation and the Middle Triassic Leikoupo Formation plays the most important role as effective seal to the gas preservation in different periods.