Tight Gas Reservoir Accumulation Conditions and Distribution Patterns during the Upper Palaeozoic of the East-Central Ordos Basin, Central China

Abstract

The Upper Palaeozoic sandstones in the east-central Ordos Basin have abundant tight gas reserves. Studies of the accumulation conditions and distribution patterns of sandstone tight gas reservoirs in this region might provide important guidance for future exploration. The current study found that this region has satisfactory gas reservoir accumulation conditions, based on a systematic observation and analysis of rock core (lithic) images and well logging, source rock geochemistry, and reservoir bed physical property data. The primary source rocks are widely dispersed and have generally entered the mature or over-mature stages of gas generation. The reservoir beds have poor physical properties and strong heterogeneity. The mudstones in the regional seal (i.e., the upper Shihezi Formation) are generally undercompacted and have sufficient sealing capability. Due to the presence of mudstone interlayers, the gas-bearing capacities of these gas reservoirs show a gradual decrease from the P1s2 of Shanxi Formation toward both sides in a vertical direction. The horizontal distribution of the gas reservoirs is primarily concentrated in the sand bodies controlled by barrier island sand dams and subaqueous distributary river channel sediment microfacies. Tight sandstone reservoir beds usually contain gas, and “sweet spots” are the major high-yield regions in the area.