Abstract: Fracture is the key factor affecting the development of basement rock reservoir. Using core, thin section and imaging logging data, the characteristics of structural fractures in bedrock reservoir of Jianbei Gas Field is analyzed, and the improvement effect of fractures on reservoir properties is addressed. The structural fractures are mainly vertical-high angle and semi-unfilled, with open fractures accounting for 63.3%, mainly showing near EW, NW–SE, NE–SW and near SN orientations. The dominant orientation of fractures in the upper layer of basement rock is near EW (accounting for 80%), with high fracture density and large aperture; while most fractures in the lower layer are NW–SE and NE–SW, with heterogeneous distribution and generally small aperture; On the plane, the fractures in the high part of the anticline have the greatest liner density and aperture, followed by the those in the structural slope and saddle, and the effective fractures in the fault control area have relatively low liner density and small aperture. Structural fractures promote weathering and leaching of basement rock, and effectively communicate secondary pores, improve reservoir permeability by 1~2 orders of magnitude. Fracture development results in the formation of high-quality reservoir with a thickness of about 40~150 m below the unconformity surface. The comprehensive analysis shows that the highquality reservoir in the high part of the structure has a large thickness, which controls the high productivity of gas wells and is favorable for exploration and development deployment. The 40 m thick reservoir section under the basement unconformity has experienced strong weathering and leaching and has good properties, which is a favorable target for horizontal well drilling.

Key words: Qaidam Basin, Jianbei Gas Field, basement rock reservoir, fracture, weathering and leaching