Pressure evolution and weak low pressure genesis of Shanxi Formation sandstone reservoir in Xinzhao area, northern Ordos Basin

The Upper Paleozoic sandstone reservoirs in Xinzhao area, northern Ordos Basin are rich in natural gas and generally develop abnormal low pressure. The understanding of paleo-pressure evolution and current subnormal pressure formation mechanism is not very clear, which constraine the understanding of tight sandstone gas accumulation and increase of natural gas reservoir and production. In this paper, the petroleum charging history in the second member of Shanxi Formation was comprehensively analyzed by means of fluid inclusion petrographic observation, micrometry and laser Raman analysis, and the paleo-pressure in the key period of reservoir formation was obtained. The paleo-pressure evolution history was recovered by basin simulation method, and the coupling relationship between paleo-fluid pressure evolution and petroleum charging was established. The relationship between the causes of subnormal pressure and tight gas accumulation is further discussed. The results show that: (1) CO2 was captured in the second member of Shanxi Formation in Xinzhao area from 170 to 180Ma, when the source rock was in the middle to low maturity stage, and the methane inclusion was captured in the peak of hydrocarbon generation from 138 to 121Ma. (2) The overpressure in the second member of Shanxi Formation began to develop in the early Jurassic, and reached the maximum in the end of the Early Cretaceous. The paleo-pressure and paleo-pressure coefficient were about 50MPa and 1.31 respectively. (3) Due to temperature decrease, pore rebound and gas diffusion, the decrease of formation pressure in the second member of Shanxi Formation accounted for 49%, 14.5% and 36.5% of the total formation pressure, respectively. The tight gas reservoir of the second member in Shanxi Formation has undergone the evolution process of normal pressure-medium overpressure-normal pressure and low pressure. Hydrocarbon generation supercharging and pressure conduction are the main reasons for the formation of ancient overpressure. Temperature drop and natural gas diffusion are the main factors for the formation of abnormal low pressure in the second member of Shanxi Formation.