Abstract: The fine sedimentary rock of semi-deep lake subfacies in Xiagou Formation of Qingxi Depression, Jiuquan Basin is composed of thick argillaceous dolomite and dolomitic mudstone with development of a number of laminations. Optical microscopy, cathodoluminescence microscopy and fluorescence microscopy are used to observe the color, morphology and distribution of laminations and analyze their mineral components. Field-emission scanning electron microscopy is used to inspect the types, sizes, shapes and distribution of reservoir space in the laminations. The pore throat structural characteristics of different laminations are analyzed by means of N2 absorption method and nano-CT technique. There are mainly four types of laminations, such as carbonate lamination (mainly dolomite), siliceous lamination (mainly feldspar and quatz), argillaceous lamination and organic matter lamination, with a thickness of 0.03-0.5 mm. Micro-fractures developed in carbonate lamination and siliceous lamination, usually filled with organic matter. Siliceous lamination contains comparatively coarse mineral grains and is often deformed and disconnected. Argillaceous lamination is in dark color with development of clay mineral interlayer pores, which are filled with organic matter too. Therefore, argillaceous lamination is high in organic matter abundance. Organic matter lamination is distributed in a state of continuity, discontinuity and dissemination. It is an important source rock with a small quantity of fractures distributed on the periphery and in the internal part. From the semi-deep lake subfacies to peripheral fan delta subfacies, the laminations change gradually from the lamination couplets dominated by carbonate components to the lamination couplets dominated by siliceous components with deformations and coarser mineral grains. Formation of laminations is related to climate, property of water body, terrigenous input volume and bioactivity. The paleo-sedimentary environment and formation mechanism of the laminations can be deduced according to the types, shapes and thickness of laminations. The laminations usually lead to different source-reservoir symbiotic relations. The carbonate lamination and siliceous lamination are good in reservoir capability, while the argillaceous lamination and organic matter lamination are associated, thus having a good potential for hydrocarbon sourcing.