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The Shunbei ultra-deep carbonate strike-slip fault-controlled reservoir in Tarim Basin is rich in reserves. The strike-slip faults and the natural structural fractures are the main storage space and flow channels of hydrocarbon resources. Therefore, studying the natural fracture development characteristics in this kind of reservoir is of great significance. The lithology of the Middle and Lower Ordovician strata in the Shunbei area is limestone and dolomite, including packstone, wackestone, grainstone, boundstone, lime mudstone, silicified limestone and silty-fine crystalline dolomite, medium-coarse crystalline dolomite. Based on the statistics of fracture density and brittleness index of core samples, it is found that in limestone, lime mudstone has the highest brittleness index, while boundstone has the lowest; in dolomite, the brittleness index of medium-coarse crystalline dolomite is higher than that of silty-fine crystalline dolomite. The natural structural fracture density has an obvious positive correlation with the rock brittleness index. The brittleness index of dolomite is generally higher than that of limestone. When the brittleness index is the same, dolomite has superior porosity and permeability, whereas limestone has a stronger capacity to fracture. Those kinds of highly brittle layers in carbonate reservoirs are more likely to be geological sweet spots in the Shunbei area. [ABSTRACT FROM AUTHOR]