鄂尔多斯盆地旬宜地区三叠系延长组7段致密油储层孔隙发育特征及其主控因素

Pore development characteristics and main controlling factors of tight oil reservoir in the seventh member of Triassic Yanchang Formation,Xunyi area,Ordos Basin

鄂尔多斯盆地旬宜地区三叠系延长组7段为典型的致密油储层,具有低孔、低渗、非均质性强等特征,明确储层孔隙发育特征及其主控因素,有利于致密油的勘探与开发。通过岩石薄片、物性、扫描电镜、X衍射分析、压汞等实验,深入研究了致密油储层的岩石学特征、储集空间、成岩演化等储层发育特征,并揭示其主控因素。鄂尔多斯盆地旬宜地区延长组7段储层埋深在500~1250 m,岩性以岩屑长石砂岩为主,其次为长石岩屑砂岩,填隙物以方解石、白云石和泥质杂基为主;研究区储层砂体厚度大,成分成熟度高,骨架颗粒中抗风化能力较强的石英和长石等刚性矿物含量高,石英含量在30%~77%之间,平均为44.97%,长石含量在4%~52%之间,平均为31.61%;孔隙类型以粒间溶孔和颗粒溶孔为主,其次为残余粒间孔,少数为微裂缝,平均孔隙度为7.3%,平均渗透率为0.4×10-3μm^(2)。储层成岩阶段处于中成岩A期,在白垩纪完成了致密化,较差的抗压实能力、碳酸盐胶结作用、伊/蒙混层、自生石英为储层致密化的主要原因;较高的石英和长石含量、早期油气充注保存了大量原生孔隙,中成岩期持续的溶蚀作用和破碎作用为次生孔隙发育的主要原因。

The seventh member of the Triassic Yanchang Formation(Chang 7 Member)in the Xunyi area of the Ordos Basin is a typical tight oil reservoir characterized by low porosity,low permeability,and strong heterogeneity.Elucidating the pore development characteristics and primary controlling factors of the reservoir is beneficial for tight oil exploration and development.Through thin-section analysis,physical property tests,scanning electron microscopy(SEM),X-ray diffraction(XRD)analysis,and mercury intrusion porosimetry,this study investigated the petrological characteristics,reservoir space,and diagenetic evolution of the tight oil reservoir,revealing its main controlling factors.The reservoir depth in Chang 7 Member of the Xunyi area of the Ordos Basin ranged from 500 to 1250 m.The lithology was primarily composed of lithic arkose sandstone,followed by feldspar lithic sandstone,with the interstitial materials mainly consisting of calcite,dolomite,and mud.The sand bodies in the reservoir were thick,with high compositional maturity.Rigid minerals,such as quartz and feldspar,which are highly resistant to weathering,made up a large portion of the framework grains.The quartz content ranged from 30%to 77%,with an average of 44.97%,while the feldspar content ranged from 4%to 52%,with an average of 31.61%.The pore types were mostly intergranular dissolved pores and intragranular dissolved pores,followed by residual intergranular pores and a few microfractures.The average porosity was 7.3%,and the average permeability was 0.4×10-3μm^(2).The reservoir is in the middle diagenetic stage A and has completed tight compaction during the Cretaceous.The primary factors contributing to reservoir densification included its poor resistance to compac-tion,carbonate cementation,illite/smectite mixed layers,and authigenic quartz.The quartz and feldspar content and early oil and gas charging preserved a significant quantity of primary pores.The ongoing dissolution and frag-mentation during the middle diagenetic stage were the primary causes for the development of secondary pores.