鄂尔多斯盆地代家坪地区延长组8段低孔渗砂岩成岩作用及成岩相

上三叠统延长组长8段砂岩是鄂尔多斯盆地代家坪地区重要的勘探目的层，以长石岩屑细砂岩和岩屑长石细砂岩为主，目前处于中成岩A期，属典型低孔特低渗储层。强烈的成岩作用是储层致密化的主要原因，其中压实作用和碳酸盐岩胶结作用造成储层原生孔隙大量丧失，绿泥石胶结和长石溶蚀对储层物性改善起到了积极作用，碱-酸-碱交替的成岩介质环境变化控制了粘土矿物形成、碳酸盐胶结物沉淀以及长石溶蚀作用。综合多项定性和定量参数，将长8段砂岩划分为5种成岩相类型，定量计算不同成岩相埋藏过程中孔隙演化表明，由沉积作用形成的不同成岩相中砂岩原始组分差异是导致胶结类型和溶蚀程度差异的物质基础，不同成岩相中成岩作用类型及强度是造成砂岩孔隙演化及物性差异的主要原因。优质砂岩储层分布与成岩相密切相关，其中分布于水下分流河道砂体中的弱压实绿泥石胶结相和中压实次生溶蚀相砂岩物性最好，是研究区长8段砂岩储层中最有利的成岩相带。

Diagenesis and diagenetic facies of low porosity and permeability sandstone in Member 8 of the Yanchang Formation in Daijiaping area,Ordos Basin

Member 8 sandstone of the Upper Triassic Yanchang Formation is the major exploration target in Daijiaping area, Ordos Basin.Member 8 sandstone of the Yanchang Formation mainly consists of feldspathic litharenite and lithic arkose, currently is at the A phase of middle diagenesis stage, and is categorized into low-porosity and extremely low-permeability reservoir.The intensive diagenesis is main cause of reservoir densification.Compaction and carbonate cementation largely reduce the primary porosity, while chlorite cementation and feldspar dissolution improve the reservoir quality, and the alkaline-acidic-alkaline change of digenetic environment control the formation of clay mineral, the precipitation of carbonate cements, and the dissolution of feldspar.Five types of diagenetic facies are identified in the Member 8 sandstone of the Yanchang Formation according to multiple qualitative and quantitative parameters.Quantitative calculation of porosity evolution in different digenesis facies shows that the differences of primitive compositions among different diagenetic facies as a result of sedimentation is the material basis of the cementation types and the dissolution strength, while the differences of diagenesis types and intensity is the main cause of the differences in porosity evolution and reservoir quality.The distribution of high quality reservoirs is closely related to the diagenesis facies.The weakly-compacted chlorite cement facies and modera-tely-compacted secondary dissolution facies in subaqueous distributary channel sandstone show the best reservoir quality, thus are the most favorable diagenesis facies in the Member 8 sandstone of the Yanchang Formation reservoir.