湖平面升降对混积咸化湖盆碳酸盐岩储集层的控制:以柴达木盆地英西地区古近系下干柴沟组上段为例*

以柴达木盆地英西地区古近系下干柴沟组上段(E_2-3xg²)为例,基于岩心、薄片、扫描电镜、测录井、X衍射及物性分析,探讨湖平面升降对混积型碳酸盐储集层形成及其分布的控制。英西地区E_2-3xg²整体为咸化背景下的湖相混合沉积,内部由多种结构类型的湖平面升降沉积旋回在纵向叠置而成,并伴随岩石结构产状及矿物组分的周期性变化。区内储集层类型多样,以混积型碳酸盐岩为主,各类储集层的纵向演化及物性分布与单个湖平面升降旋回具有良好的耦合关系,自下而上构成原生晶间微孔-溶扩晶间微孔-充填残余溶孔的孔隙组合,储集层物性具有向上变好的趋势。各类储集层的形成机理与空间分布受湖平面升降旋回控制,白云石晶间微孔型储集层形成于湖退早-中期,与准同生白云石化作用相关,物性一般,分布广泛;溶扩晶间微孔型和溶孔型储集层形成于湖退晚期,与地貌高部位的短暂暴露溶蚀相关,储渗性能优异,局部发育。

Influence of lake-level fluctuation on the mixed saline lacustrine carbonate reservoir: A case study from the Upper Member of Paleogene Lower Ganchaigou Formation in the Yingxi area of Qaidam Basin

Based on analyses of cores,thin sections,scanning electron microscope,well-logging,X-ray diffraction and physical properties of the Upper Member of the Paleogene Lower Ganchaigou Formation(i.e. E_2-3xg²)in the Yingxi area of Qaidam Basin,this paper aims to investigate the influence and control of lake-level fluctuations on the formation and distribution of mixed saline lacustrine carbonate reservoir. Deposited in a saline lacustrine basin,the carbonate reservoir of the E_2-3xg² in the Yingxi area contains mixed lithologies and stacking sedimentary cycles resulting from multiple sets of lake-level fluctuations. The rock structure and mineral composition of the E_2-3xg² show similar periodical changes associated with the lake-level fluctuation cycles. Multiple reservoir types,dominated by mixed saline lacustrine carbonates,are present in this area. The vertical evolution and distribution of physical properties of various reservoirs have a good coupling relationship with a single lake-level fluctuation cycle. From bottom to top,the pore assemblage is characterized by primary intercrystalline micropores,slightly dissolved micropores,and dissolved pores. Accordingly,reservoir physical properties tend to get better upward. The formation mechanism and spatial distribution of various reservoirs are mainly controlled by the fluctuation of lake level. The reservoir dominated by dolomite intercrystalline micropores is developed during the early-to-mid regression when strong evaporation and concentration of water prompt penecontemporaneous dolomitization. This reservoir type is characterized by moderate reservoir property but wide distribution. Reservoirs dominated by slightly dissolved and expanded micropores and dissolved pores,however,are formed during the late regression. They are mainly developed in the ancient landform highlands,which are more likely to be exposed and altered by meteoric freshwater dissolution. Therefore,these reservoirs are characterized by good reservoir property but local distribution.