Abstract: The Middle Permian Qixia Formation in NW Sichuan Basin has become the important and hot exploration field in Sichuan Basin. To clear the reservoir characteristics and main controlling factors of Qixia Formation in this area, the field outcrops and drilling core observation, rock thin section examination and reservoir physical property testing were used to analyze the characteristics of petrology, reservoir space and physical properties, three-dimensional seismic data were used to recover the ancient geomorphology, and then the main controlling factors of the reservoir were discussed. The result shows that the reservoir rocks are mainly crystalline dolomite, dolomitic leopard-spot limestone/dolomite and a small amount of grainstone. A variety of reservoir spaces are developed in Qixia Formation, and different reservoir rocks develop different reservoir space types. Intercrystalline pores, intercrystalline dissolved pores, and nonselective dissolved pores and cavities are mainly developed in the dolomite reservoir. A small amount of intragranular and intergranular dissolved pores are developed in the part of grainstone. Fractures can be developed in all types of reservoir rocks. The reservoir physical properties show the characteristics of middle-low porosity and permeability, and medium-macro-crystalline dolomite has the best reservoir property. The reservoir development is controlled by multiple factors, and the high-energy shoals in the platform-margin and intra-platform are the material foundation for the formation of reservoirs. Paleogeomorphology of sedimentary period of Qixia Formation was restored by the method of the residual thickness. The result shows that the sedimentary highland not only controls the high-energy facies belt distribution but also the main development area of the late dolomitization and dissolution, thus controlling the reservoir distribution of Qixia Formation. The Zhongba-Shuangyushi structure belt (Jiange-Jiangyou region)is in the highland of Qixia Formation paleogeomorphology, and develops platform-margin shoal facies dolomite reservoir with superior physical properties and a bigger reservoir thickness and a wider area, which shows a good exploration prospect. The important viewpoint put forward for the first time that the paleogeomorphology controls the distribution of high quality reservoir provides a good way of thinking of reservoir seismic prediction.

Key words: fuzzy recognition, principal component analysis, lithology identification, well logging curve, Sulige gasfilelds