Tight oil accumulation mechanisms of Triassic Yanchang Formation Chang 7 Member, Ordos Basin, China

The sedimentary environment, petrology, pore throat structure, migration and accumulation power, and accumulation process are analyzed to study the oil accumulation mechanism and distribution of Triassic Chang 7 Member tight oil in Ordos Basin. Chang 7 tight oil reservoirs are mainly semi-deep lake – deep lake sandy debris flow deposits and delta front underwater distributary channel deposits. The results of field emission scanning electron microscopy, CT scans and other high-resolution microscopy techniques show the physical properties of Chang 7 oil reservoirs are poor, micro- and nano-meter pore throats are developed, inter-granular pores and feldspar dissolved pores are the main pores, and tube- and strip-shaped pores are favorable reservoir space. The hydrocarbon generation and accumulation simulation of tight oil reservoirs shows that the hydrocarbon charging power of Chang 7 high-quality source rocks is strong. The oil saturation of the Chang 7 were more than 50% in the accumulation phase and even reached 70% or more with the continued power increase and oil enrichment. The source and reservoir rocks of Chang 7 tight oil coexist, so the hydrocarbons migrated under high pressure to migrate along relatively high permeable superimposed sands, reticular fractures and other pathways, and formed large-area continuous or quasi-continuous lithologic oil reservoirs.