鄂尔多斯盆地盒8段致密气藏气水分布特征及成因分析

Distribution characteristics and genetic analysis of gas and water in tight gas reservoirs of the 8th section of Shihezi Formation in Ordos Basin,China

鄂尔多斯盆地上古生界盒8段致密砂岩气藏含气饱和度分布范围为17%~87%,不同地区气水分布特征存在明显差异,成因机理不明确。通过对盒8段致密砂岩储层特征和压差、浮力作用下致密砂岩储层中天然气运聚特征的分析,阐明了不同区块盒8段气藏气水分布特征及成因机理。研究结果将盒8段气藏划分为超低含水饱和度区、束缚水区、局部自由水区3类。超低含水饱和度区位于盆地东部的子洲、清涧地区,成因为天然气饱和充注后高温天然气对束缚水进一步蒸发汽化;局部自由水区主要分布于盆地西部和盆地北缘的鄂托克前旗、鄂托克旗和伊金霍洛旗地区,成因为天然气充注量及动力不足,对地层水驱替不彻底,浮力作用下在储层物性较好或裂缝发育的构造高部位气水发生分异,含气富集受储层物性和局部构造双重控制;束缚水区主要位于乌审旗、定边、吴起和横山地区,成因为致密储层可动水基本被天然气全部驱替,束缚水主要分布于孔隙的边角处和微细毛管中,浮力不发挥作用,气水混储。

The gas saturation of tight sandstone gas reservoirs of the 8th section of Shihezi Formation of Upper Paleozoic in Ordos Basin is 17%-87%.There are significant differences in the distribution characteristics of gas and water in different regions,and the genetic mechanism is unclear.By analyzing the characteristics of tight sandstone reservoirs and the migration and accumulation characteristics of gas in tight sandstone reservoirs under pressure difference and buoyancy,the distribution characteristics and genetic mechanisms of gas and water in different blocks of the 8th section of Shihezi Formation reservoir are elucidated.The conclusion is that the 8th section of Shihezi Formation reservoir can be divided into three types:ultra-low water saturation zone,bound water zone,and local free water zone.The ultra-low water saturation area is located in the Zizhou and Qingjian areas in the eastern part of the basin,mainly due to the continuous injection of high-temperature natural gas after gas saturation,which further evaporates and vaporizes the bound water;local free water areas are mainly distributed in Otog Front Banner,Otog Banner and Ejin Horo Banner in the north edge of the basin and the west of the basin.Due to insufficient natural gas filling and power,the formation water displacement is not complete.Under the effect of buoyancy,gas and water are differentiated in the high part of the structure with good reservoir physical properties or developed fractures,and gas enrichment is controlled by both reservoir physical properties and local structures;the bound water area is mainly located in Uxin Banner,Dingbian,Wuqi and Hengshan areas,because the movable water in tight reservoirs is basically displaced by natural gas.The bound water is mainly distributed in the corners of pores and micro capillaries.The buoyancy does not play a role,and gas and water are mixed.