Reservoir forming conditions and main controlling factors of tight sandstone gas in Ordos Basin

Abstract

The upper Paleozoic gas reservoir in Ordos Basin is an important tight gas reservoir energy base. Four tight gas fields with proven reserves of more than 100 billion cubic meters have been built. The analysis of tight gas source rocks, reservoirs, and other reservoir forming conditions in the basin guides for the discovery of tight gas reservoirs in other areas of the basin. The research shows that: (1) the coal seams of Benxi Formation Shanxi Formation are the main source rocks, with high abundance and maturity of organic matter, mainly high over maturity, which provides sufficient material basis for natural gas accumulation in the basin; (2) The lithology is mainly lithic quartz sandstone and quartz sandstone. The reservoir has the characteristics of “small pores, fine throat, undeveloped micro fractures, and poor pore throat connectivity”; (3) The reservoir evolved into a tight sandstone reservoir in the early Middle Jurassic, and then a large amount of natural gas was injected to form a gas reservoir in the late Jurassic Early Cretaceous, which is typical “tight first and then reservoir forming”; (4) The distribution of tight gas in Ordos Basin is mainly controlled by sedimentary microfacies and reservoir's physical properties, in which reservoir's physical properties are the key controlling factor, and fractures have an important impact on the improvement of reservoir physical properties.