Gas–Water Distribution and Gas Reservoirs Accumulation Properties of Upper Paleozoic Sequence in the Western Yishan Slope, Ordos Basin, NW China

Gas-water distribution is significant in the determination of hydrocarbon accumulation mechanisms in gas reservoirs, especially for the exploitation of tight sandstone reservoirs. One of such examples are the gas reservoirs in the Yishan Slope in China, where the internal relationship between gas-water distribution is poorly understood. The pattern and controlling factors for gas-water distribution in tight sandstones gas reservoirs in the Yishan Slope have been examined from macro (such as sedimentary and anticlinal structures) and micro (such as pore throat size, heterogeneity) perspectives, using data from rock eval pyrolysis, sedimentary structure, sediment diagenesis, gas migration, mercury injection experiments, and well logs. The results showed that distribution of fluids is relatively complicated in the study area, and the gas wells and water wells are distributed across the plane. These wells are with no obvious boundaries; In the vertical plane, the Shan 1 Formation penetrated the gas layer, and water production gradually increases from the Shan 1 Formation to the He 8 Formation. The gas-water distribution is subject to hydrocarbon-generating intensity and diagenesis, while microscale physical properties and pore-throat structure of the reservoir also have a significant impact on the distribution. The Shan 1 gas reservoir is adjacent to the Taiyuan source rock and has a high hydrocarbon charge potential, which in turn drives out the formation water to form the main gas-bearing formation. In addition, areas with significant diagenetic imprints, such as strong dissolution and weak cementation, are also favorable locations for gas accumulation due to favorable physical properties and pore connectivity. Burial history and hydrocarbon generation and expulsion history showed that there are two periods of gas accumulation (175-200Ma; 105-140Ma), source rocks had reached a midhigh maturity phase, and a large amount of kerogen was generated and expelled, with natural gas transported and charged through the transport system during the second period. Hydrocarbon generation intensity essentially determined the volume of gas accumulated in the reservoir; longer continuous charging is more conducive to the formation with high gas content. This study identifies and discusses some of the controlling variables for gas-water distribution and predicts potential exploration targets for tight gas sandstone reservoirs.