Analysis of water–rock interaction in Middle-Lower Ordovician strike-slip fault in T-SH Oilfield, Tarim Basin, NW China

Strike-slip faults developed in the middle lower Ordovician carbonate formation in the T-SH area of Tarim Basin play an important role in controlling the migration and accumulation of hydrocarbon. The reservoir in the study area is reformed by multi-period fluid, and the influence of different component fluid on different positions of fault is of great significance to define the reservoir development model and determine the reservoir quality. Based on core samples observation, the characteristics of water–rock interaction along the fault in the study area are analyzed by scanning electron microscope and electron probe. The results show that the strike-slip fault in T-SH area is affected by two periods of fluid. The first period fluid flows from top to bottom. Its CaCO3 is supersaturated, alkaline environment and rich in clay elements (Al, Si, K and Fe). The clay elements in the fluid come from the upper Qiaerbake Formation. In the second period, the composition of the fluid is obviously single, the CaCO3 is under saturated. The fluid is in an acidic environment and contains a small amount of Al and Si elements. The fault core is only affected by the first period fluid and is dominated by cataclasis-cementation system. The broken breccia at the top of Yijianfang Formation is filled with clay and asphalt and cemented with bright calcite, and no open space is found. Fractures are well developed in the damage zone, which are affected by two periods of fluid. In the damage zone, not only the cataclasis-cementation system, but also the cataclasis-corrosion phenomenon can be seen. The wall rock is only affected by the first period fluid, cementation and no corrosion. The internal structure of strike-slip fault determines the difference of fluid permeability in different parts of the fault, and affects the later fluid migration and transformation. The fault structure formed by cataclasis is transformed and shaped by fluid, thus forming the present fractured-vuggy reservoir.