羌塘盆地侏罗系低渗透碎屑岩储集层特征

Characteristics of the Jurassic low-permeable clastic reservoirs in the Qiangtang Basin

羌塘盆地侏罗系碎屑岩主要包括三角洲相、滨海相和河流相砂体,岩性以细砂岩和粉砂岩为主。储层经历了较强的压实作用和胶结作用,颗粒间线—凹凸接触,胶结物主要为钙质和硅质,储集空间以次生溶孔为主,包括颗粒溶孔、粒间溶孔等,溶孔以中孔和小孔为主,喉道为细喉和微喉,构造裂缝的发育对改善储层的渗透性有重要作用。物性统计表明:孔隙度大多小于5%,渗透率小于1mD,为低孔、低渗储层。结合低渗储层分类标准,将碎屑岩储集层分为3类进行了评价,认为上侏罗统雪山组储层最好,其次为中侏罗统雀莫错组,而夏里组相对较差;最有利储集层发育区主要位于北羌塘坳陷西部中央隆起带北缘和盆地东部雀莫错地区,较有利储层发育区位于北羌塘坳陷中部和东部地区,南羌塘坳陷储集层较差。压实作用和胶结作用较强,后期溶蚀作用相对较弱,是造成目前低孔低渗储层的主要因素。

The Jurassic clastic reservoirs in the Qiangtang Basin consisted of fine sandstones and sihstones with depositional delta, littoral and fluvial facies. Strong compaction and cementation during the burial history resulted in line-concave contact pattern between grains with calcite and siliceous cements. The main pores in the reservoirs were secondary ones including intragranular and intergranular dissolved pores. Most of the dissolved pores were middle to small sized ones with thin or tiny throats. Furthermore, the development of structural fractures played a key role in determining the permeability of the reservoirs. Laboratory experiments classified the reservoirs to be low porous（ with a general porosity of lower than 5% ）and low permeable （ with a permeability of less than 1 mD） ones. According to classifying criteria of low permeable reservoirs,the clastic reservoirs in the studied area could be divided into three categories. Among them, the Xueshan Formation in age of Late Jurassic was the best one, the Quemocuo Formation and Xiali Formation in age of Middle Jurassic took the second and the third positions respectively. The best reservoirs were located in the northern part of the Central Uplift and the better ones lied in the central and northeastern part of the North Depression, while the reservoir qualities in the South Depression were fairly poor. Strong compaction and cementation, and weak dissolution process were main reasons for the development of the low porosity and low permeable reservoirs in this area.