Forming mechanism of quasi-continuous tight sandstone reservoir of Chang 6 oil-bearing formation, Ordos Basin

The Chang 6 reservoir in the Yanchang Formation in the Shanbei slope of the Ordos Basin is a typical quasi-continuous unconventional tight sandstone reservoir. It has 7 main features as followed. (1) The formation and distribution of the Chang 6 reservoir were not restricted by traps. Oils spread for big area and were connected. (2) Regional relatively stable tectonic background and tight sand bodies connected for big area were the external factors for the formation and distribution of the Chang 6 reservoir. (3) Matured hydrocarbon source rocks in the Chang 7 formation were connected widely. High-strength hydrocarbon generation and expulsion were the basis for the formation and distribution of the Chang 6 reservoir. (4) The Chang 6 reservoir mainly resulted from primary migration and secondary migration of short distance. Vertical cracks and laterally-connected sand body effective combinations provided favorable pathways for petroleum migration. (5) The abnormal overpressure caused by hydrocarbon generation was the main force for petroleum migration. Buoyancy effect was very weak and played an unimportant effect in controlling the formation and distribution of the reservoir. (6) Oil and water distributions were complex in the Chang 6 reservoir. Pure oil was not found. Oil/water, water and dry layers coexisted. There was no unified oil/water boundary or pressure system. The reservoir had strong heterogeneity. Oil saturation was not unified. Reserve abundance was low. Cookies were found locally. (7) The Chang 6 reservoir had been compacted before the mass migration of petroleum, serving foundation for the formation of the Chang 6 quasi-continuous tight sandstone reservoir.