Cenozoic fine-grained sedimentary rocks in the southwestern Qaidam Basin, as typical salinized lacustrine sediments, are significantly different from those in other freshwater lakes under the impact of the high salinity in the lake. Many fine-grained sedimentary rock samples were selected from the lower member of Neogene (N1) in the Gaskule Oilfield, Qaidam Basin, northwest China, with the aim to analyze and test their petrology, pore structure, and organic geochemical characteristics and determine its genesis and geological significance. Based on previous results, a new genesis of mixed sedimentary rocks is proposed. The Cenozoic lacustrine fine-grained sedimentary rocks in SW Qaidam Basin belong to mixed sedimentary rocks, which can be divided into three types, including carbonate and terrigenous clastic-bearing mudstone, carbonate-bearing to terrigenous clastic mixed sedimentary rocks, and carbonate-terrigenous clastic mixed sedimentary rocks. The geneses of mixing are quite different from that of mixed marine rocks. According to the sedimentary characteristics of the salinized lakes, three new geneses of mixed deposits are proposed: syn-sedimentary mixing, varve interbedded mixing, and biogenic mixing. The reservoir spaces of the Cenozoic lacustrine fine-grained sedimentary rocks in SW Qaidam Basin include residual intergranular pores, intercrystalline pores, microfractures, and minor dissolved pores. The reservoirs have super-low porosity and ultra-low permeability, with a porosity of 1.82%–21.04%, averaging 7.71%, and permeability of (0.0028–254.86) × 10?3 μm2, averaging 2.82 × 10?3 μm2. The test results of total organic carbon (TOC) content show that the highest content of organic carbon reached 1.41%, and those of vitrinite reflectance show that Ro ranges from 0.525%–0.824%, with an average of 0.806%. The organic geochemical characteristics show that the fine-grained mixed sedimentary rocks of SW Qaidam Basin have the potential for self-generation and self-storage. As an unconventional reservoir, it has potential industrial value. This is the first study of fine-grained sedimentary rocks in the Qaidam Basin, which offers certain theoretical and practical values.