Abstract:

There are three key scientific issues in the marine petroleum exploration in China, i.e., the prototypes of China's three major marine basins and their controls on the petroleum source material conditions; the control of Paleozoic multistage tectonic movements and Mesozoic/Cenozoic differential-superposition transformation on the hydrocarbon accumulation/enrichment in the marine carbonate strata; and the dynamic reservoir-forming processes and enrichment rules of marine petroleum in the complex structural areas and ultra-deep layers. Based on our prototype-reconstruction analyses, the basin prototypes and sedimentary models of the three cratons of China (South China, North China, Tarim) obviously differ between different periods under the effects of plate tectonic cycles, and can be divided into three combination types: intra platform depression, passive continental margin and rift (depression). The passive continental margin can be further divided into steep slope and gentle slope, with various source-reservoir allocation relationships formed in the marine strata. Several rounds of global plate tectonic reorganization take place since the Phanerozoic; and since the Late Triassic, especially, under the multiplate subduction-convergence tectonic framework, the three marine cratonic basins—Tarim, Sichuan, Ordos—undergo four superposition transformations and form the corresponding continental basin prototypes, with various types of complex geostructures developed including piedmont belt thrust-fold, differential subsidence-uplift, multi-detachment structures, deep strike-slip faults, as well as structures formed by tectonic magmatism. In the Tarim Basin, the inhibition of hydrocarbon generation due to great burial depth, high pressure and low ground temperature control the petroleum distribution, where the distribution depth of liquid hydrocarbon can exceed 10 000 m. The upper Middle and Late-Caledonian/Hercynian paleo structure controlls the early hydrocarbon migration and accumulation, while the active strike-slip fault zones of the Late Yanshanian to Himalayan control hydrocarbon enrichment in the later stage. In the Sichuan Basin, the ancient uplift/slope controll the initial hydrocarbon accumulation and form the ancient oil reservoir under an unique hydrocarbon source environment; cracked gas becomes the main gas source in the large and medium-sized gas fields. After the Late Cretaceous, large-scale fold, uplift and denudation result in the reorganization and reconstruction of the basin's fluid system. According to the degree of transformation, the models of petroleum accumulation/preservation are divided into (1) weak modification, where conventional/unconventional gas can be preserved in situ for a long time; (2) medium transformation, where conventional gas adjusts and re-accumulates while large-scale shale gas enrichment occurs; and (3) strong transformation, where conventional gas reservoirs are destroyed by intense modification while some unconventional gas reservoirs are preserved. In the Ordos Basin, Ordovician natural gas has two sources from three directions, where natural gas accumulation is closely related to unconformity and stratigraphic sequence boundary as well as lithological and lithofacies changes, while gas reservoirs generally experienced one adjustment process.

Key words: basin prototype, superimposed transformations, petroleum system, orderly accumulation, Tarim Basin, Sichuan Basin, Ordos Basin