Developmental model and filling response characteristics of paleokarst conduits in the Middle-Lower Ordovician strata of Well 094 area, Tahe Oilfield

Abstract

The Ordovician strata in the Tahe Oilfield of the Akekule Low Uplift, Tarim Basin, host diverse palaeokarst fracture–cavity reservoirs. Among these reservoirs, the large-scale epignic karst conduit system that formed during the early Hercynian represents one of the most significant reservoir spaces. The Well 094 well block, located in the Carboniferous-covered western slope area of the Tahe Oilfield, contains typical Ordovician palaeokarst conduit-type reservoirs. On the basis of 3D seismic and well logging data, in this study, the palaeokarst hydrogeomorphic conditions of the Ordovician strata in this area are reconstructed; the spatial distribution, developmental conditions, and filling characteristics of large-scale branchwork karst conduits are analysed; and their controlling factors are investigated. The results indicate that the Ordovician strata in the Well 094 well block were situated in the early Hercynian karstification zone, characterized by a typical binary flow field structure of a karst watershed. The karst landform exhibits medium-to-high relief, with well-developed asymmetrical branchwork surface drainage systems converging from north to south into a karst canyon. The karst conduits are predominantly developed approximately 120 ms below the top of the Lower-Middle Ordovician series. The development of these conduits was jointly controlled by the steep topographic gradient of the Ordovician strata, the syncline-confined groundwater convergence zone between secondary anticlines, and the water-conducting effects of NNE- and NNW-trending strike-slip faults. The karst conduits display distinct east‒west differential evolution. The eastern conduits experienced intense vertical erosion and roof collapse, forming a subsurface fluvial window geomorphology with severe clastic infilling, whereas the western conduits maintained relatively stable configurations. This finding suggests that conduit evolution is influenced by tectonic uplift and the hydraulic gradient, with the degree of infilling closely related to the watershed area, fault activity, and evolutionary stage. Highly evolved conduit segments tend to exhibit more severe infilling.The complete filling of the cave not only reduces porosity but also significantly compromises the reservoir's effective storage capacity. These findings provide a theoretical basis for evaluating the effectiveness of similar karst fracture–cavity reservoirs worldwide.