Keywords: 
• Qaidam basin /
• mixed deposit /
• reservoir property /
•   /
• initial porosity

Abstract: The lower part of Xiaganchaigou Formation is an important prospecting object in the central areas of northern Qaidam basin. To find out the factors controlling the reservoir property is the basis of prospecting. Based on the normal thin sections, cast thin sections and drilling data, the rocks characteristics between Lenghuqihao and Nanbaxian were correlated. The mixed deposits were found very common in the central part of northern Qaidam basin，but the cementaion type and the cement composition vary obviously in different sedimentary facies. Because of the high salinity of Lenghuqihao area which located in littoral facies，the cements contains a lot of anhydrite and the sandstones are mainly basal cemented. The salinity of Nanbaxian located in delta front facies is lower because of the great mounts of water input by the rivers, so there is little anhydrite in the sandstones which are porous cemented to basal cemented. The initial porosity in Nanbaxian is much higher than Lenghuqihao, and the sandstones were much more easily dissolved. The abundance of petroleum in two aeras is very different as the great difference in initial porosity and porosity evolution model.