Characteristics and origin of over-dissolution residual fault-karst reservoirs in the northern Tahe oilfield, Tarim Basin

This study aims to investigate the characteristics of the Early Hercynian paleokarst caves in the extensively eroded area of the northern Tahe oilfield in the Tarim Basin.Using methods such as paleogeomorphic reconstruction, karst water system analysis, logging-core observation-based cave identification, and seismic attribute characterization, we identify the cave types and styles in well block YQ5, characterize fracture-cave structures in this area, and explore the origin of the caves. The results show that the well block YQ5, located in the region with flat and gentle terrain to the north of Nos.Ⅱ and Ⅲ paleokarst platforms of the Tahe oilfield, generally exhibits residual landforms after modification by multi-phase superimposed karstification. Karst peak-depression, karst hill-depression, and karst hillplain zones of low relief are primarily observed in the well block, with peak cluster-ridge/valley zones extending in the NE direction in the south. Different from the main body and slope area of the Tahe oilfield, the well block YQ5 displays a flow direction of the underground and surface water systems inconsistent with the geomorphic trend. Furthermore, the disrupted karst water system therein by tectonism, leads to an incomplete karst water cycle of recharge, runoff, and discharge. Caves of underground river type and over-dissolution residual fault-karst reservoirs are primarily found in the well block. The caves of underground river type are characterized by significant filling, which compromises the effectiveness of reservoir spaces, thus adversely affecting oil and gas exploitation. In contrast, for the over-dissolution residual fault-karst reservoirs, the vertical erosion along strike-slip faults, resulting from adecline in regional erosion base level due to the tectonic uplift of the karst platforms, is beneficial to the continuous development and preservation of these reservoirs, thus contributing to effective oil and gas exploitation. Overall, the dominant factors for the over-dissolution residual fault-karst reservoirs encompass the development of strike-slip faults, the denudation intensity of strata, and negative landforms. Consistent with the paleokarst platforms observed in the Tahe oilfield, karst in the well block YQ5 also experienced three evolutionary stages: the deeply incised meandering river stage, the karstification-induced modification stage, and the infiltrated fault-karst stage. During these stages, caves of underground river type underwent continuous transformation and destruction, while the fault-karst reservoirs experienced ongoing construction.