Tight gas sandstone reservoirs in China: characteristics and recognition criteria

Tight gas sandstone (tight gas) reservoirs have been widely discovered in a number of basins in China including the Sichuan, Ordos, Turpan-Hami (Tuha), Songliao, Junggar, Tarim, Chuxiong and East China Sea basins. Genetically they are of either primary or secondary accumulations. The tight gas accumulations are mostly associated with coal strata. The reservoir rocks are mainly lithic arkosic sandstones and feldspathic lithic arenite. They are characterized by low compositional maturities, low cement contents and moderate petrological textural maturities. The diagenetic evolution of the reservoirs is generally at a high level, reaching the mid-late diagenesis stages. Reservoir porosities are dominated by secondary intragranular dissolution porosity and intergranular dissolution porosity with subordinate primary residual intergranular porosity. The pore throats are sheet like or bending-flake like in geometry and generally have poor connectivity. The tight gas reservoirs have poor petrophysical properties and show strong heterogeneities. Locally, some moderately porous and low permeable or low porous and moderately permeable sandstone reservoirs are present. Compaction and cementation appear to be key factors contributing to the formation of the tight gas sandstone reservoirs. Based on the synthesis of a vast data set obtained from some known tight sandstone gas accumulations from (1) the upper Palaeozoic sandstones in the Ordos Basin, (2) the Triassic Xujiahe Formation in the Sichuan Basin, (3) the Jurassic sandstones from the foothill belt in the Tuha Basin and (4) the Cretaceous deep reservoirs in the Songliao Basin, the criteria for recognizing tight gas sandstone reservoirs in China can be established as an in situ permeability of less than or equal to 0.1 mD and with no natural commercial production.