1. Research Institute of Exploration & Development, Qinghai Oilfield, PetroChina, Dunhuang 736200, Gansu, China;
2. Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China;
3. Well Testing Company, Qinghai Oilfield, PetroChina, Dunhuang 736200, Gansu, China

The tight oil reservoirs in western Qaidam Basin are characterized by complicated inner structure and strong reservoir heterogeneity. In order to develop the tight oil more efficiently, the characteristics of tight oil reservoirs of upper Ganchaigou Formation in Zhahaquan area were studied through observation of thin sections, XRD, SEM and CT scan. The results show that:the upper Ganchaigou Formation is a typical tight reservoir, which mainly consists of siltstone-fine sandstone with high interstitial material. Primary porosity and micron-size pore are the most important reservoir space, and the average porosity of effective reservoir is 5.9%,average permeability is 0.43 mD. The pore throat radius varies from nano-size to micron-size, the micron-size pore shows sheet distribution with good connectivity, while the nano-size pore shows isolated distribution. The property of the reservoir is mainly controlled by deposition and diagenesis. The bar sand is with high sand content, high purity and good physical property. Compaction and carbonate cementation are the major factors that reduced the reservoir quality, while the anhydrite cementation has dual influences. The reservoir quality was improved significantly by dissolution during the late diagenetic period. Effective reservoir of tight oil can be classified into three types, of which the ClassⅠis "sweet spot" of the studied area, with best reservoir conditions and industrial oil flow can be obtained in the conventional oil test. Class Ⅱ and Ⅲ reservoirs are with poor reservoir conditions, which need fracturing to get industrial oil flow.