Geological conditions , reservoir forming evolution and favorable exploration directions of marine ultra-deep oil and gas in China

China’s marine ultra-deep oil and gas resource is mainly distributed in the Sichuan, Tarim and Ordos cratonic basins, and its exploration prospects and strategic status are attracting increasing attention. The breakup-convergence cycles of the supercontinent controls the evolution of intracratonic rifts, cratonic marginal rifts, and intracratonic depressions, providing favorable tectonic backgrounds for the development of multiple sets of source rocks of Nanhua, Sinian, Cambrian, Ordovician, Silurian, etc. The early high-energy sedimentary environment and the later dissolution and faulting control the development of high-quality carbonate reservoirs. The regional caprocks are dominated by gypsum salt rocks, shale, and tight carbonate rocks. By examining the formation and evolution of oil and gas reservoirs of Sinian-Cambrian in the central Sichuan Basin, Cambrian to Ordovician in the northern Tarim Basin, and Changcheng System in the Southern Margin of Ordos Basin, it is found that the accumulation of oil and gas in ultra-deep marine formations often experiences complex evolution involving multiple stages of hydrocarbon generation, charge, and adjustment. The ultra-deep oil and gas fields in China have usually experienced hydrocarbon-accumulation processes of two stages of oil-reservoir forming, thermal (complete or partial) cracking of paleo-oil reservoir, and late accumulation of kerogen-cracked gas (high-over mature oil and gas). The oil and gas accumulation is controlled by static and dynamic geological elements jointly. Major hydrocarbon-generation center, high quality and large scale reservoir-caprock assemblage, and stable trapping and preservation conditions are the key factors for enrichment of ultra-deep oil and gas. Based on petroleum geological conditions, factors affecting hydrocarbon accumulation and evolution, and the latest exploration knowledge, we propose that 3 favorable exploration directions, i.e. intracratonic rifting periphery, intracratonic depression periphery, and craton margin.