The influence of faults on the development of canbonate karst reservoir in main area of tahe oilfield and its significance in petroleum geology

The Ordovician karst fracture-cave reservoir in Tahe oilfield has strong heterogeneity, and palaeogeomorphology, fault and fracture play important roles in the development of the complexity of the fracture-cave reservoir. Based on rich geological and geophysical data, the influence of faults on the karst reservoir development in carbonatite under different palaeogeomorphic conditions are analyzed through the interpretation of faults and the activity of internal faults in carbonate rocks in different periods, combined with palaeokarst environment and karst products. 
The results show that there are not only strike-slip faults but also an NNE-thrust fault in the sixth and seventh districts of Tahe Oilfield, which control the direction of long-axis anticline in the center of the study area. The anticline becomes an important watershed and most of the surface gullies develop along suitable faults from the top of the anticline to lower areas. According to the karst geomorphology, water system and fracture-cave distribution, landforms are divided into three types: hoodoo-upland, karst depression and karst basin. In the hoodoo-upland, the fracture networks around the faults are dissolved and small and medium-sized fractures develop, and the reservoirs have low filling degree and good performance. In the karst depression, the landforms are transformed by strong water erosion and karst dissolution. The underground rivers and the palaeogeomorphic gullies controlled by high-angle strike-slip faults are relatively straight, while the others controlled by low-angle faults are tortuous. Unfilled caves and intergranular pores in cave fillings are the main reservoir spaces. In karst basin, the Ordovician soluble limestone is covered by stucco deposits, which greatly weakens the karstification. The fractures and caves can develop only along the faults and fractures at a very deep depth. The spatial structure, connectivity, porosity and permeability are complicated. The main reservoir types are fractures, fracture-cave and isolated caves. The filling types are fault karst breccia, giant crystal chemical filling or no filling. Therefore, faults affect the development of reservoir types and fillings under different geomorphology and karst water conditions, which has important guiding significance for the accurate exploration and development of carbonate fracture-cave reservoirs.