Characteristics and accumulation mechanism of tight sandstone gas reservoirs in the Upper Paleozoic, northern Ordos Basin, China

The Ordos Basin is a significant petroliferous basin in the central part of China. The Carboniferous and Permian deposits of transitional and continental facies are the main gas-bearing layers in the north part of the basin. The Carboniferous and Permian natural gas reservoirs in the northern Ordos Basin are mainly tight sandstone reservoirs with low porosity and low permeability, developing lots of “sweet spots” with comparatively high porosity and permeability. The tight sandstones in the study area are gas-bearing, and the sweet spots are rich in gas. Sweet spots and tight sandstones are connected rather than being separated by an interface seal. Sweet spot sand bodies are vertically and horizontally overlapped, forming a large gas reservoir group. In fact, a reservoir formed by a single sweet spot sand body is an open gas accumulation. In the gentle dipping geological setting and with the source rocks directly beneath the tight reservoirs over a large area, the balance between gas charging into tight reservoirs from source rocks and gas loss from tight reservoirs through caprock is the key of gas accumulation in tight sandstones. Both the non-Darcy flow charging driven by source-reservoir excess pressure difference and the diffusion flow charging driven by source-reservoir gas concentration difference play an important role in gas accumulation. The results of mathematical modeling indicate that the gas accumulation cannot be formed by just one of the above mechanisms. The diffusion of gas from source rocks to reservoirs is a significant mechanism of tight sandstone gas accumulation.