A comprehensive review of the characteristics, formation, evolution, resource potential, and ultradeep exploration fields of oil-cracking gas in the Ediacaran–Cambrian formations of the Sichuan super gas basin

Since the past decade, a super-large gas field with reserves of ∼2 trillion cubic meters has been discovered in the Ediacaran–Cambrian Formations (referred to as Ediacaran–Cambrian hereafter) of the Sichuan super gas basin. In carbonate rocks of the Ediacaran–Cambrian, the geological conditions and reservoir formation issues of natural gas have drawn extensive attention, leading to numerous research advances. However, key geological aspects—such as the genesis of the Deyang–Anyue aulacogen, origin and source of natural gas, process of oil and gas formation, mechanism of gas-reservoir pressure formation and evolution, main controlling factors of natural gas enrichment, and calculation method of oil-cracking gas resources—remain unclear. This study systematically reviews the advancements in research on the Ediacaran–Cambrian of the Sichuan Basin, including the geochemical characteristics and genesis of natural gas and solid bitumen, geological conditions responsible for natural gas accumulation, cracking and gas generation processes in paleo-oil reservoirs, accumulation models, and controlling factors of natural gas accumulation. Results revealed that the understanding of the genesis of the Deyang–Anyue aulacogen has been unified, and the genesis and sources of natural gas have been clarified based on extensive geochemical data of natural gas. In addition, based on a detailed explanation of the geological conditions responsible for the formation of large gas reservoirs in the Ediacaran–Cambrian, the review elucidates the formation of oil and gas along with the formation and evolution mechanisms of gas-reservoir pressure. Further, it establishes an accumulation model and identifies the main controlling factors of the accumulation. Moreover, a calculation formula for the natural gas dissipation coefficient is established to address the deficiencies in existing methods used for calculating the resource quantity of oil-cracking gas. Using a genetic method and PetroMod three-dimensional basin simulation software, the resource quantity of cracking gas is estimated following the technical route of oil formation from kerogen, formation of paleo-oil reservoirs, and subsequent cracking to generate gas. In addition, the oil generation history of the basin and five tectonic units is studied, yielding the resource quantity of paleo-oil reservoirs. Finally, the directions for ultradeep natural gas exploration was further pointed out further indicate. Hence, this study is critical for the oil and gas geological theory and exploration activities of ultradeep, ancient strata. Moreover, this study offers a reference, which is expected to promote the exploration of Ediacaran–Cambrian natural gas in domestic basins (such as Sichuan, Ordos, and Tarim), and holds substantial scientific and practical importance for pioneering the exploration of natural gas in ancient strata from the Middle–Upper Proterozoic to the Lower Paleozoic eras worldwide.