姬塬西部地区长8_1致密储层特征及孔隙度演化分析

Tight reservoir characteristics and porosity evolution of Chang 8_1 in western Jiyuan area

储层孔隙度演化分析是恢复储层致密化过程、明确储层致密化原因的有效手段。利用钻井岩心、铸体薄片、扫描电镜和粒度分析等资料,在查明鄂尔多斯盆地姬塬西部地区长8_1致密储层特征的基础上,定量分析了其孔隙度演化过程。姬塬西部地区长8_1储层岩性以细粒岩屑长石砂岩和长石岩屑砂岩为主,储集空间以粒间孔、长石溶孔为主,孔隙结构较差,属于典型的低孔、低渗透致密储层。以薄片观察为基础,采用计点统计的方法恢复了成岩作用过程中孔隙度的演化,并定量求取了各成岩作用对孔隙度演化的贡献值。结果表明:姬塬西部地区长8_1储层砂岩初始孔隙度为41.35%,进入埋藏成岩阶段后,压实作用导致了孔隙度的不可逆降低,造成的孔隙度损失率为57.10%,胶结、交代作用造成的孔隙度损失率为32.55%,而后期溶蚀作用带来的孔隙度增加率为6.48%,微裂缝作用带来的孔隙度增加率为0.15%。计算所得长8_1储层砂岩孔隙度为7.02%,而实测孔隙度为7.12%,相对误差为1.40%,在合理范围之内。在姬塬西部地区,压实作用和胶结作用造成的长8_1储层初始孔隙度损失率高达89.65%,因此,强压实、强胶结作用是导致该区长8_1储层致密化的主要原因。

Analysis of porosity evolution is an effective way to restore the densification process and clear the reasons of reservoir densification. The data of core drilling observation, cast sections, SEM and grain size analysis were used to analyze the characteristics and porosity evolution of Chang 8_1 tight reservoir in western Jiyuan area, Ordos Basin.The results show that the Chang 8_1 reservoir rock types are mainly fine-grained lithic arkose and feldspathic litharenite,the pore types are mainly intergranular pores and dissolved pores of feldspar, and the pore structure is poor. The clastic rocks experienced compaction, cementation and dissolution, thus producing tight reservoirs with low porosity and low permeability. Based on rock thin section observation, the porosity evolution during diagenesis was recovered by statistical analysis method, and the contribution value of each diagenesis on porosity evolution was quantitatively obtained. The porosity evolution parameters show that the primary porosity of Chang 8_1 reservoir is 41.35%, the porosity loss rates during the process of mechanic compaction and the process of cementation and metasomasis are 57.10%and 32.55%, respectively. The porosity increase rates during the process of dissolution and the fracturing are 6.48%and 0.15%, respectively. The porosity of Chang 8_1 reservoir calculated is 7.02%, and the porosity analyzed in the laboratory is 7.12%, so the error rate in the quantitative study of porosity is 1.40%. The Chang 8_1 reservoir in Jiyuan area has a relatively high primary porosity, but the porosity loss rate is 89.65% resulted by mechanic compaction and cementation. In fact, although sedimentary condition is the basis for the formation of original compositions and the rock texture, the strong compaction, strong cementation and weak dissolution are the dominant diagenesis types which destroyed the reservoir physical properties to become tight reservoir.