Distribution and formation of Mesozoic low permeability underpressured oil reservoirs in the Ordos Basin, China

The stratum pressure characteristics of Mesozoic reservoirs in the Ordos Basin were first studied. It was found that the Mesozoic reservoirs were mainly ultra-underpressured reservoirs with an average stratum pressure coefficient of 0.63-0.86 and the differences in the abnormal underpressure between the different regions and layers were distinct. The results showed that with increases in the eroded stratum thickness and temperature decrease in the reservoirs, the stratum pressure coefficients showed a decreasing trend. The pore water volume contraction in the Yanchang Formation was from 0.82% to 1.94% after tectonic uplift and stratum temperature reduction. It was proposed that because of the strong uplift of the basin for a long time at the end of the Cretaceous, the function of stratum erosion and paleotemperature reduction resulted in the formation of underpressured reservoirs. It is considered that this underpressured closed system of the Mesozoic in the Ordos Basin was advantageous for reservoir preservation and might have played an important role in adjustment, reenrichment of hydrocarbons by migration, and oil and water distribution in reservoirs. The formation of underpressured anhydrous sand lens reservoirs in the Chang 7 subsection could be related to the distribution of such an underpressured closed system.