World’s geologically oldest shale gas play: Cambrian shale gas accumulation in the Yangtze craton, China

ABSTRACT Ground-breaking discoveries in the Cambrian Qiongzhusi Formation of the Sichuan Basin have unveiled the world’s geologically oldest shale gas play. An integrated analysis of lithofacies, sedimentology, reservoir properties, thermal maturity, and hydrocarbon generation-expulsion history indicates that most productive shale gas reservoirs are predominantly located around the paleouplifts of the Yangtze craton, where equivalent vitrinite reflectance (EqVRo) values are below 3.5%. During the early Cambrian, deep-water depositional environments dominated the Yangtze craton, leading to the widespread accumulation of black shale (20–80 m thick) across the central Sichuan Basin, Huangling, and central Guizhou regions. Unlike other cratonic regions, these areas later evolved into paleouplifts, resulting in relatively shallow burial depths and lower present-day thermal maturity (EqVRo < 3.5%). As a result, the black shales adjacent to these paleouplifts have limited oil generation and expulsion, thereby preserving higher porosity and enhanced shale gas potential. Two distinct reservoir types have been identified: organic-rich shale (total organic carbon content >2%) and organic-poor silty shale (total organic carbon <0.5%). The organic-rich shale includes the deeply buried, overpressured Qiongzhusi shale within the Deyang-Anyue intracratonic rift trough in the central Sichuan Basin, and the normally pressured Niutitang shale outside the Sichuan Basin which is buried in shallower depth. In comparison, the organic-poor silty shale is overpressured and occurs along the Deyang-Anyue rift trough slopes. In conclusion, overpressured Cambrian shale gas reservoirs with EqVRo < 3.5% represent the most prospective exploration targets in the Sichuan Basin, offering valuable insights for global Cambrian shale gas exploration.