Abstract: Kuqa Depression in Tarim Basin is one of the major gas sources for the west-east national gas transmission project.The Lower CretaceousBashijiqike Formation in the Keshen gas field is the major pay zone with a burial depth exceeding 6 000 m and low matrix porosity and permeability (<0.1×10^-3 μm²).Fractures can tremendously improve the permeability of reservoirs.Therefore, a thorough analysis and quantitative description of fracture growth and accurate prediction of the spatial distribution pattern of fracture permeability of the Formation are critical to natural gas development in the study area.Methods including drilling and coring, FMI imaging, carbon oxygen isotopic chronological analyses, fracture modeling through holograhic laser scanning of outcropwere combined with experimental analysis procedures such as CT scanning, SEM, cathode luminescence, laser scanning confocal microscope, high pressure mercury injection and electron microprobe to study both qualitativelyand quantitativelythe enhancement ofreservoir propertiesof the Formation by fractures.The fractures in the gas field aredominated by half-filled shear fractures with an effective opening ranging between 0.1 and 1.5 nm.There arethree stages of structural fractures developed in the Formation, and the fractures of each stage line up in different patterns at different structural locationsunder the control of structural stress.The fractures have a general porosity of less than 0.1%, but they caneffectively improve the permeability of the reservoirs by 1 to 3 orders of magnitude and result inthe formation of high-permeability reservoirs 150 m below the unconformity face.Fractures formed during early and middle stages served as pathways of fluids for tight reservoir diagenesis and cementation, unfavorable for the preservation of connective pores and throats.Those formed during later stage were accompanied by oil and gas charging processes and enlarged pore throats in reservoir matrix through dissolution.Micro-fractures had limited effect upon the connectivity of pores and throats in the reservoirs (only effectively connecting pore-throats with aperture between 10 to 100 times).In summary, fracture network formed during the late stage is the key to high productivity of the gas field.

Key words: ultra-deep layer, fracture, reservoir, tight sandstone, Kuqa Depression, Tarim Basin