Karst development characteristics and reservoir controlling factors of Ordovician carbonate rocks in T738 well area of Tahe oilfield

T738 well area of Tahe oilfield is located in the slope zone of transition from Karst highland to karst basin, which is a shallow covered area of upper Ordovician with large-scale karst fractured-vuggy reservoirs. On the basis of Paleogeomorphology restoration, combined with paleontology, seismic, drilling and geochemical data, this paper analyzes the karst development characteristics of Ordovician carbonate rocks and the main controlling factors of reservoirs in this area. The results show that the fractured-vuggy reservoirs of Yijianfang and Yingshan formations of Ordovician in the well area are mainly the result of the superimposition and transformation of karst process in the middle Caledonian and the Early Hercynian. The ancient water system undergoes layer by layer dissolution along the pinch-out line of Lianglitage and Chalbac formations, and vertically infiltrates and horizontally erodes along the deep fractures. Vertically, two erosion zones, vertical infiltration and horizontal runoff, are developed. At the same time, water is discharged along the through deep fractures, forming a complete supply-runoff-discharge system. Regionally, the widely developed faults have obvious control over the reservoir. According to the interaction between three groups of faults with summarized, among which the northwest fault zone is the most favorable reservoir development location.