Tight sandstone gas reservoirs of the Xujiahe Formation in central Sichuan Basin were observed to have many features.They contain gas-water transition zones that may be due to inadequate supply of gas from source rocks

pervasive gas distribution possibly as a result of early gas charging during gas accumulation

mixed gas/water distribution because of the tightness of sandstone reservoirs

control of local structural amplitude on gas-water differentiation and fractures acting as seepage channels for gas and water.The gas-water distribution pattern in the reservoirs may be explained as follows:immature-low mature gas that were generated during the Late Triassic to Late Jurassic accumulated and formed the pure gas cap on structural high under the control of local structure;the large amount of gas generated during the Early Cretaceous to Paleogene could not completely displace pore water due to the tightness of sandstone reservoir

resulting in mixed gas/water distribution;the physical properties of the reservoirs werer greatly improved by numerous fracutures formed during the Himalayan movement since the Paleogene.All these conditions worked together to form the present tight gas reservoirs in the formations of the basin.