川西坳陷沙溪庙组砂体特征与连通性评价——以平落坝构造为例

Analysis of sandstone characteristics and connectivity of Shaximiao Formation in Western Sichuan Depression——Taking Pingluoba Structure as an example

川西坳陷位于四川盆地中西部,是印支期以来龙门山挤压推覆所形成的平行于龙门山造山带的西陡东缓不对称箕状形态的前陆盆地。中侏罗统沙溪庙组为川西坳陷重要的气藏层段,砂体连通性是控制气藏成藏的主要因素。平落坝构造处于川西坳陷南部前陆逆冲推覆带与前陆盆地之间的区域,为呈北东走向西南端略窄而东北端稍宽的长轴背斜。应用平落坝构造沙溪庙组的砂地比、沉积相、物性和含油气性等参数,开展储层砂体特征及连通性的分析和空间分布评价,探讨砂体连通性对天然气运聚的影响。川西坳陷沙溪庙组发育河湖三角洲分流河道和河口坝沉积的多层薄砂体,纵横向分布不稳定;平落坝构造沙溪庙组连通性砂体的评价参数砂地比≥20%且孔隙度≥4%,钻井测试为气层或含气显示;连通性砂体构成了浅层输导格架,影响天然气的运聚和调整。

The Western Sichuan depression,located in the central and western part of the Sichuan Basin,is a foreland basin with an asymmetrical dustpan-shape in the west and east which is formed by the compressional nappe of Longmenshan and is parallel to the Longmenshan orogenic belt since the Indosinian period.Shaximiao Formation of middle Jurassic is an important gas reservoir in the Western Sichuan Depression and sand body connectivity is the main factor controlling gas reservoir formation.The Pingluoba structure is located in the area between the foreland thrust nappe belt and the foreland basin in the southern part of the Western Sichuan Depression.It is a northeast-trending long-axis anticline with a slightly narrower southwest end and a slightly wider northeast end.The Shaximiao Formation in the Western Sichuan Depression has developed multi-layered of thin sand bodies deposited by river-lake delta distributary channel and mouth bar,and the vertical and horizontal distribution is unstable.Therefore,the spatial distribution range of connected sand body is evaluated comprehensively.The connected sand body constitutes the shallow transmission grid,which affects the transport and accumulation and adjustment of natural gas.Through segmental statistics and analysis of parameters like sand-strata ratio,porosity and gas-bearing,the minimum threshold of the evaluation parameters for the connected sand bodies of the Shaximiao Formation in the Pingluoba structure was determined to be sand-strata ratio≥20%and porosity≥4%,and drilling test is gas layer or gas display layer.Based on this,comprehensive evaluation of the spatial distribution range of the connected sand bodies was performed.The connected sand bodies constitute the shallow transmission grid,which affects the transport,accumulation and adjustment of natural gas.