Abstract: The 8^th Member of the Shihezi Formation and the 1^st Member of the Shanxi Formation of the Upper Paleozoic in Sulige gasfield are mainly of fluvial facies, and the crosscutting and superimposition of multiple-stage channels resulted in a large "braided river system" in geologic history.According to different sedimentary environments and sedimentation characteristics, the braided river system can be divided into superimposed zone, transitional zone and inter-channel zone.This study deepens and develops the concept and deposit connotation of the braided river system, establishes a sedimentary hierarchy of sedimentary facies-braided river system-braided river system zone, and proposes multiple-parameter quantitative classification standards for Sulige's braided river system.Through integrated analysis of core, logging and drilling data, it is concluded that braided river system zone is the key geological factor for sedimentation and reservoir characteristics of Sulige gasfield, as it strongly controls the sedimentary microfacies distribution and scale, sand bodies' superimposition style and effective sand bodies' type and centralization.The superimposed zones of the braided river system feature in highly develo-ped and large channel bars, superimposition of multiple stage sand bodies and clustering of effective sand bodies, thus is the favorable facies zone for gas production.In transitional zone, channel filling is more developed and effective sand bodies are mainly distributed in isolation in space.In contrast, the inter-channel zone is characterized by scattered and sparse sand bodies, thus its exploitation potential is poor.In addition, the criteria of geological target selection for horizontal wells are set up for the superimposed zone and transitional zone of braided river system respectively, and horizontal well trajectory is optimized, providing a more reliable basis for geo-steering while drilling of horizontal wells.

Key words: effective sand body, sedimentary microfacies, braided river sedimentary system, tight sand gas, Sulige gasfield, Ordos Basin