Sources of natural gas in the Middle-Permian carbonate succession in the central-northern Sichuan Basin, Southwest China

Abstract

Natural gas-source correlation is a challenging aspect of natural gas exploration in deep strata. The Middle-Permian carbonate succession in the Sichuan Basin, southwest China, particularly in the Maokou Formation, is a hotspot for natural gas exploration. However, determining the natural gas source in the Maokou Formation is challenging because of the high maturity and complexity of potential source rocks. In this study, we analyzed the genesis and sources of natural gas in the central-northern Sichuan Basin while considering the geological conditions and systematically utilizing the gas components, stable and rare-gas isotopes, and solid bitumen biomarkers. The results indicate that (1) the natural gas in the northern Sichuan Basin comprises a mixture originating from the source rocks of the Maokou and Wujiaping formations; (2) the Qiongzhusi Formation is the major source of natural gas in the north slope (and the Maokou Formation source rock could be used as an important supplement); (3) the natural gas in the Moxi-Longnvsi structure comprises a mixture originating from the source rocks of the Qiongzhusi and Maokou formations. From south to north and west to east, the major source rocks of the gas reservoirs changed from belonging to the Qiongzhusi Formation to the Maokou and Wujiaping formations. This study is the first to combine multiple geochemical methods and consider different geological settings to identify the gas source of a deep carbonate gas reservoir. Our study provides a reliable method for determining the natural gas-source correlation of gas reservoirs that contain highly mature and complex gas sources.