Oil generated overpressure of shale and its effect on tight sandstone oil enrichment: A case study of Yanchang Formation in the Ordos Basin, China

Oil generated overpressure (OGOP) is a key factor in tight sandstone oil enrichment. However, the relationship between the OGOP and oil enrichment remains unclear. On the basis of a physical simulation experiment and an oil injection experiment, an improved OGOP model based on the organic matter enrichment zone (OMEZ) was established, the attenuation of OGOP in shale was discussed, and the oil charging distance under different reservoir conditions was evaluated. The results showed that the OGOP starts from the OMEZ in the shale, is controlled by the OMEZ size, and decays during the expansion of the hydrocarbon expulsion microfractures. The attenuated OGOP acts as the initial migration force to push crude oil into tight sandstone reservoirs. The OGOP magnitude, OMEZ size and reservoir properties together determine the enrichment scale of tight sandstone oil. Driven by OGOP, the oil charging distance for the tight reservoir in the Yanchang Formation is only 66.7 m but 383.5 m for the conventional reservoir. These findings open a window to study the impact of shale internal structure on OGOP. The OGOP mathematical model and calculation method of various parameters in the process of oil accumulation proposed in this study are helpful for further understanding the mechanism of hydrocarbon generation, discharge, and enrichment.