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

Key words: 
• mixed sedimentary rock /
• reservoir /
• Jiuquan Basin /
• Lower Cretaceous

Abstract: 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.