混积岩储层特征--以酒泉盆地青西凹陷下白垩统混积岩为例

混积岩是指陆源碎屑与碳酸盐组分经混合沉积作用而形成的岩石,广义混积岩包括由陆源碎屑与碳酸盐组分在空间上构成交替互层或夹层的混合。本文在大量资料调研基础上,结合酒泉盆地青西凹陷下白垩统混积岩储层薄片观察、场发射扫描电镜观察、全岩及黏土矿物分析、压汞测试、纳米CT等研究工作,分析了混积岩储层岩性组合、储层特征等。宏观上,混积岩岩性复杂多样,由白云质泥岩、泥质白云岩、灰质泥岩、灰质白云岩、白云质灰岩等组成,剖面上呈现大段厚层式、厚夹薄层式、近等厚互层式三种组合样式;微观上,黏土矿物、碳酸盐矿物及有机质等呈纹层状或块状分布;储集空间主要为微-纳米级的粒间孔、粒内孔以及微裂缝,矿物组成及其含量是影响储集空间类型的一个主要因素;多数混积岩储层孔隙度、渗透率低,为致密储层。

Characteristics of Mixed Sedimentary Reservoir: Taking the Lower Cretaceous mixed sedimentary rock of Qingxi depression in Jiuquan Basin as an example

Mixed sedimentary rock is referred to the rock composed of terrigenous clastics and carbonate components under mixed sedimentation. In a broad sense, it also includes the spatially alternating layers or intercalations of terrigenious clastic and carbonate components. On the basis of the research on a lot of materials and study on the lower Cretaceous mixed sedimentary reservoir in Qingxi depression of Jiuquan Basin, using methods such as thin-section observation, field-emission scanning electron microscope observation, bulk minerals and clay minerals analysis, mercury injection test and nano-CT, analysis on lithologic associations and reservoir characteristics has been carried out on the mixed sedimentary reservoir. Macroscopically, the mixed sedimentary rock is lithologically complex, composed of dolomitic mudstone, argillaceous dolomite, lime mudstone, lime dolomite and dolomitic limestone, and so on, and presenting three types of vertical profile associations as great suites of thick layers, thick layers intercalated with thin layers and interbedded layers of similar thickness. Microscopically, clay minerals, carbonate minerals and organic matters present laminated or massive distribution form. The mixed sedimentary reservoir develops micro-nanometer scale interparticle pores, intraparticle pores and microfractures. The composition and content of minerals are one of the main factors affecting reservoir space types. Most mixed sedimentary reservoir has low permeability and low porosity, being regarded as tight reservoir.