Connectivity and residual oil distribution patterns of fractured-vuggy karst reservoirs in strike-slip fault zone: A case study of the Tarim Basin

For ultradeep, fractured-vuggy carbonate karst reservoirs located in the strike-slip fault zone within the Tarim Basin, their three-dimensional architecture is identified as a primary factor influencing their connectivity and residual oil distribution. This study focuses on the geologic outcrop in the Yijianfang area within the basin that exhibits the characteristics of typical fractured-vuggy reservoirs. Using unmanned aerial vehicle (UAV)-based scanning of the outcrop, analysis of ground-penetrating radar (GPR) data, and the field measurements, we determine the parameters of the faults, fractures, and karst caves. By examining the distribution characteristics of the large and small karst caves, fault cores, and fracture zones exposed, we establish the developmental model of fractured-vuggy reservoirs in the outcrop area in the fault zone. Based on the mechanisms of Riedel shear structures and the stress ellipse theory, we analyze the structural characteristics of the strike-slip faults. Afterward, we conduct fine interpretations of the strike-slip fault, main body of fractured-vuggy reservoirs, and fracture zones in the Yueman well block of the Fuman oilfield based on 3D seismic data, while combining conventional interpretations with intelligent identification. With the help of interference tests of a well group, we identify the major factors controlling the connectivity of the fractured-vuggy reservoirs. Accordingly, we divide the reservoir units based on production performance including single-well productivity, water breakthrough cycle, changes in tubing pressure, and static pressure gradients of the wellbore. Additionally, we explore the impacts of reservoir architecture on residual oil distribution. The results indicate that the strike of the fault plane represents a primary factor governing the development of fractured-vuggy reservoirs. These fractured-vuggy karst reservoirs have evolved through three stages: the single fault-plane fault-fracture combination without karstification, single fault-plane fracture-karst cave combination with weak karstification, and single fault-plane fracture-karst cave combination with strong karstification. These evolutionary stages correspond to three combination patterns of fractured-vuggy karst reservoirs, namely the single, parallel, and complex fault-plane patterns. Data on production performance can serve as a basis for determining reservoir connectivity since single-well production, tubing pressure, and stratigraphic static pressure reflect the degree of reservoir connectivity. Fault-fractured-vuggy karst reservoirs with varying fault-plane combinations manifest three reservoir connectivity types: strong connectivity along the fault zones of the same strike, moderate connectivity via intersecting fault zones with different strikes, and lateral weak connectivity via non-intersecting fault zones with different strikes. The fractured-vuggy reservoirs with the oblique fault-fracture plane combination, manifest slightly weak connectivity, with fractures and reservoirs particularly well-developed at fault intersections. In contrast, the fractured-vuggy reservoirs with the single fault of the same strike exhibit strong internal connectivity, indicating great potential for oil exploitation.