鄂尔多斯盆地盐池—姬源地区三叠系长4+5砂岩成岩演化特征与优质储层分布

Characteristics of Diagenetic Evolution of Chang 4+5 Sandstones (Upper Triassic)in Yanchi-Jiyuan Area,Ordos Basin and Distribution of High-Quality Reservoir

盐池—姬塬地区延长组长4+5砂岩由台型和坡型三角洲前缘砂体组成,以岩屑长石砂岩和长石砂岩为主,其中石英、长石和岩屑平均含量分别为24.7%、43.2%和10.0%,岩屑也以变质岩屑和火成岩屑为主,占岩屑总量的88.0%。砂岩目前处于早成岩阶段B期的晚期,Ro为0.8%,温度在70～90℃,主要受酸性成岩环境控制,因此次生溶孔发育。在对砂岩各种组分含量统计的基础上,定量计算表明机械压实作用是造成研究区原生孔隙消亡的最主要原因,其次为胶结作用,而由溶蚀作用增加的孔隙占初始孔隙度比例较小(<10%)。结合沉积相分析成果,在研究区可划分出3种沉积—成岩相带,即坡型三角洲前缘—绿泥石粘土膜胶结原生孔相、坡型三角洲前缘—晚期铁方解石胶结次生溶孔相和台型三角洲前缘—晚期铁方解石胶结相,其中有利的高孔高渗储层区主要分布在前两种有利沉积—成岩相带结合部位。

Chang 4 ＋ 5 sandstones of Yanchang Formation in Yanchi-Jiyuan area~ are mainly composed of platformtype and slope-type delta front sandbodies. Lithic feldspathic and feldspathic sandstones are chief rock types, of which the average content of quartz, feldspar and rock fragments are 24.7% , 43.2% and 10.0% , respectively, and metamorphic and igneous fragments are dominant constitutes, accounting for 88.0% of total fragment content. At present, these sandstones are in the late period of the Early diagenetic B stage（Ro =0.8% and T =70-90℃ ） , so they are un- der the control of acidic diagenetic environment, being of abundant secondary dissolution porosity. On the basis of the statistics of sandstone compositions, it is indicated by the quantitative calculation that compaction is the dominant reason for the loss of the primary porosity in the study area, and next cementation, whereas the increasing ratio of porosity caused by the dissolution is relatively small （ 〈 10% ）. In combination with the results of sedimentary facies, three types of sedimentary-diagenetic facies zone could be divided, i.e. slope-type delta front-chlorite film primary porosity facies , slope-type delta front-late ferrous calcite cemented secondary porosity facies and platform delta front late ferrous calcite cemented facies, and the favorable reservoir facies is mainly distributed in the joint area of the former two facies zones.