Abstract: The Chang-3 oil reservoirs in the Longdong region are recently selected as the important oil reservoirs in the Ordos basin. The study of the authigenic minerals is significant to the study of the densification mechanism of the oil reservoirs. The Chang-3 oil reservoirs consist of lithic arkose and feldspathic lithoarenite, and are interpreted to be the low-porosity and low-permeability reservoirs with an average porosity of 13.35% and average permeability of 4.90×10^-3 μm², belonging to the A stage of the middle diagenesis. The contents, types, distribution and diagenetic sequences are discussed in the present paper on the basis of cast section, SEM, CL images and X-ray diffraction analysis. The early chlorite coatings facilitate the preservation of the primary porosity, and late pore filling may reduce the porosity and permeability of the sandstones. The mineral illite is prone to block the pores and throats, resulting in poor reservoir properties. The precipitation of kaolinite is often associated with the dissolution of feldspar, and partial strong dissolution can be seen under the microscope. The iron-bearing carbonate cementation is developed in the later stages, and destructive to the development of the sandstone reservoirs. The quartz overgrowths and authigenic grains may take up much of the pore spaces, and in turn, strengthen the pressure resistance. On the whole, the cementation of the authigenic minerals in the Chang-3 oil reservoirs has the negative effects on the reservoir properties, and also is one of the controlling factors of densification mechanism of the oil reservoirs.