川中地区须家河组致密砂岩气藏气水分布形成机理

Mechanisms of gas-water distribution in tight sandstone gas reservoirs of Xujiahe Formation,central Sichuan Basin

川中地区须家河组致密砂岩气藏因气源供应不足形成了气水过渡带;因成藏充注较早导致了区域大面积含气;因砂岩储层致密造成了气水混杂分布;由局部构造幅度控制了气水分异程度;由储层裂缝提供了气水渗流通道。该区须家河组气藏气水分布的形成机理可归纳为:晚三叠世末至晚侏罗世的未熟-低熟天然气在局部构造控制下于构造闭合高点处形成纯气顶;早白垩世—古近纪大量生成的天然气因砂岩储层致密而无法完全驱替出孔隙水,形成气水混杂分布格局;古近纪以来的喜马拉雅运动形成大量裂缝,改善了储层物性,在裂缝发育带聚集成藏。

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 Creta-ceous 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 dur -ing the Himalayan movement since the Paleogene .All these conditions worked together to form the present tight gas reser-voirs in the formations of the basin .