Pressure evolution of gas-bearing systems in the Upper Paleozoic tight reservoirs at the eastern margin of the Ordos Basin

Multiple tight gas reservoirs are well developed in the Upper Paleozoic sequences at the eastern margin of the Ordos Basin. An accurate understanding of the pressure evolution process of gas reservoirs will be of guiding value to gaining more insights into tight gas accumulation and achieving high and stable gas production in this region. In this study, drilling, logging, and core fluid inclusion test data, as well as simulations of burial and thermal evolution histories are integrated to reveal the pressure evolution of the Upper Paleozoic gas-bearing systems at the eastern margin of the Ordos Basin. The results show that reservoirs in the study area exhibit underpressure, slightly underpressure, and normal pressure systems from bottom to top. The homogenization temperature and salinity of fluid inclusions exhibit continuous distributions overall, suggesting a continuous hydrocarbon charging process. The Taiyuan, Shanxi, and Lower Shihezi formations demonstrate a positive correlation between the homogenization temperature and salinity of fluid inclusions, suggesting a rapid hydrocarbon charging process following near-source hydrocarbon generation. In contrast, the Upper Shihezi and Shiqianfeng formations exhibit a negative correlation between the homogenization temperature and salinity due to the long-distance fluid migration and charging, as well as the rebalancing of fluid inclusions in gas reservoirs under the influence of the Zijinshan tectono-thermal event. During the Mid-Cretaceous, the study area experienced the generation of large quantities of hydrocarbons, leading to the anomalously high reservoir pressure ranging from 34.89~38.26 MPa, followed by a decrease at later stages under the uplifting of strata. For the pressure drop, 50.31%~57.85% was caused by the decline in formation temperature, 28.25%~41.95% by natural gas swelling-induced gas migration (predominantly in upper strata), and 0.37%~0.79% by pore rebound. The findings of this study systematically reveal the pressure system evolution of the Upper Paleozoic tight gas reservoirs in the Ordos Basin and the origin of the current reservoir pressure formation. These will be of referential value to understanding the enrichment and accumulation patterns of tight gas in the Ordos Basin and the like.