鄂尔多斯盆地定边油田长7 致密砂岩储层成岩作用及孔隙演化规律

鄂尔多斯盆地定边油田延长组长7储层石油地质储量丰富,但砂体致密、非均质性强等因素严重制约了油气的有效勘探开发。综合运用铸体薄片、扫描电镜等资料,深入分析研究区储层的岩石组分、孔隙类型、物性特征及成岩作用,将成岩演化事件与地质响应有机匹配建立研究区孔隙演化模型,厘清储层成岩演化及孔隙演化规律,同时对比不同深度成岩作用对孔隙演化规律的影响程度。结果表明,研究区长7储层以长石砂岩为主,主要发育黏土、碳酸盐等胶结物,有效孔隙类型以残余粒间孔、次生溶孔及晶间微孔为主,为典型的特低孔、超低渗透致密砂岩储层。成岩演化过程先后经历快速压实—早期胶结、胶结—早期溶蚀、溶蚀增孔、压实—晚期胶结等阶段,目前处于中成岩A期。压实作用是导致孔隙减小、储层致密的主控因素,平均减孔量为20.40%;胶结作用平均减孔量为14.02%,其中晚期胶结是储层致密的关键因素;溶蚀作用平均增孔量为5.87%,可延缓储层致密过程。长7^1油层段底部与长7^2油层段中部储层压实、胶结作用减孔量相对较弱,溶蚀增孔量相对较大,可作为研究区勘探开发的首要目标。

Diagenesis characteristics and evolution of porosity of Chang7 tight sandstone reservoir in Dingbian Oilfield,Ordos Basin

Chang7 reservoir in Dingbian Oilfield of Ordos Basin has abundant oil reserves,but its tightness and anisotropy restrict the effective exploration and development.Based on thin section,scanning electron microscope(SEM)analysis,the rock compositions,pore types,physical properties,and diagenesis of Chang7 tight reservoir are studied.The porosity evolu tion model of tight sandstone reservoir are established at Chang7 by using the diagenetic evolution events and geological comprehensive response,to clarify the evolution of diagenesis and porosity.Meanwhile,influences of diagenesis at different depths on the porosity are compared,which provides a theoretical basis to explore the relatively dominant reservoirs in the study area.The results show that:The clastic rocks of Chang7 tight reservoirs in the study area are mainly feldspar sand stone and feldspar lithic sandstone,mainly containing clay minerals,carbonate,and other cement.The effective pore types are mainly secondary dissolved pores and residual intergranular pores,indicating a typical tight sandstone reservoir with ul tra-low porosity and ultra-low permeability.The diagenetic evolution of Chang7 reservoir in the study area has gone through stages of rapid compaction-early cementation,cementation-early dissolution,mainly dissolution-pore increase,compaction-late cementation,etc.This suggests that Chang7 oil reservoir in the research area is currently in the middle dia genetic stage A.Compaction is the main controlling factor leading to pore decrease and tight reservoir formation,with an av erage pore reduction of 20.4%.The average pore reduction by cementation is 14.02%,and the cementation in the late stage is the key to form tight reservoirs.The dissolution increases the porosity by 5.87%on average,which delays the densifica tion.The sandstone compaction and cementation in the bottom of Chang7^1 and middle of Chang7^2 are relatively weak,and the dissolution increasing pore volume is relatively strong,which indicates a preferred reservoir for exploration and develop ment in this area.