Characteristics and origin of the ultradeep Ordovician fault-karst reservoirs: An example from the Shunbei-Yuejin area, Tarim Basin

Fault-controlled reservoirs are critical hydrocarbon-bearing reservoirs in carbonate rocks. The ultradeep Ordovician fault-karst reservoirs in the Tarim Basin are representative carbonate reservoirs whose formation was controlled by tectonics. An integrated analysis of seismic, drilling, logging, and core data and the petrology and geochemistry of the area suggest that the strike-slip fault, lithology, and diagenetic fluids affected the size and formation of the fault-karst reservoirs. The meteoric water dissolution during episodes I–III in the middle Caledonian Orogeny was critical in the formation of the fault-karst reservoirs. In contrast, organic acid, magmatic-hydrothermal fluid, and Mg-rich high-salinity fluid had a negligible effect on the development of these reservoirs. The impact of meteoric water was likely limited in the strike-slip fault zone due to its short exposure time. The releasing bend, grainstone, and packstone were more conducive than the restraining bend, single fault, wackestone, and mudstone to the development of the fault core and fault damage zone in the strike-slip fault, resulting in meteoric water dissolution. The size of the fault-karst reservoir increased from south to north due to the onlap of Upper Ordovician strata in the north. The formation mechanism and development pattern of fault-karst reservoirs in the study area provide information for the analysis of carbonate reservoirs formed under similar conditions.