鄂尔多斯盆地临兴气田盒八段封闭体系下富长石砂岩成岩滞留现象及效应

Diagenetic detention phenomenon and effects of feldspar-rich sandstone in closed system of H8 Member,Linxing gas field in Ordos basin

鄂尔多斯盆地临兴气田盒八段天然气地质储量占比大但储层物性较差,储层致密机理不清,制约着该层储量的有效动用。利用岩心分析资料,通过物性、成岩矿物、包裹体及孔隙演化等分析,对盒八段储层特征及致密成因开展了研究。研究表明:①临兴气田盒八段为辫状河三角洲平原分流河道砂岩,岩性具有中—粗粒、富长石和岩屑的特征,孔隙类型以次生溶孔和晶间微孔为主,平均孔隙度7.1%,平均渗透率0.38 mD,属特低孔超低渗储层;②盒八段储层具有较强压实、不充分溶解、普遍蚀变和较强自生黏土矿物、碳酸盐矿物胶结等特殊的成岩作用特征,孔隙演化计算显示早晚两期胶结减孔量可达5.7%,而溶蚀增孔量仅为4.5%;③盒八段砂岩在沉积后发生了封闭环境下的成岩滞留作用,主要表现为不充分溶解、较强蚀变和胶结,物性响应表现为“调孔降渗”,成岩滞留作用是盒八段储层致密化的主要地质因素。

The H8 Member of Linxing gas field in Ordos basin accounts for a large proportion in natural gas geological reserves,but its poor reservoir physical properties and unclear compaction mechanism restrict the effective production of reserves in this layer.Based on the drilling and core test data,the reservoir characteristics and tight genesis of H8 Member are studied through the analysis of physical properties,diagenetic minerals,inclusions and pore evolution.The research results show that:①The H8 Member of Linxing gas field is braided river delta plain distributary channel sandstone,which has the characteristics of medium-coarse grain,rich feldspar and rock debris.The pore types are mainly secondary dissolved pores and intercrystal micropores,with an average porosity of 7.1%and an average permeability of 0.38 mD,belonging to ultra-low porosity and ultra-low permeability reservoir;②The reservoir of H8 Member has special diagenetic characteristics such as strong compaction,insufficient dissolution,general alteration and strong authigenic clay mineral and carbonate mineral cementation.The pore evolution calculation shows that the cementation in the early and late stages can reduce the pore volume by 5.7%,while the dissolution can only increase the pore volume by 4.5%;③The sandstone of H8 Member has undergone diagenetic detention in a closed environment after sedimentation,which is mainly manifested by insufficient dissolution,strong alteration and cementation,and the physical property response is“pore adjustment and permeability reduction”.Diagenetic detention is the main geological factor for the reservoir densification of H8 Member.