Comprehensive evaluation of fracture-cave units in karst carbonates in Tahe Oilfield, Tarim Basin

Karstified carbonate is a complicated reservoir rock. Some fracture-cave units are connected and occur as oil reservoirs. Based on the studies of karst zones in the weathered crust of Ordovician carbonates, the genetic construction of fracture-caves and the filling materials in fracture-caves, a four-step method to identify and evaluate the fracture-cave units in karstified Ordovician carbonates in the Tahe Oilfield was proposed. Firstly, the genetic construction and boundaries of fracture-cave units were defined. Secondly, the genetic types and distribution of fillings in the caves were identified. Thirdly, the porosity and permeability of the fillings in the caves and the carbonates of cave walls were predicted. Finally, the connectivity between wells in the fracture-cave units were dynamically estimated. The four-step identification of fracture-cave units is a comprehensive evaluation. In addition, oil production potential is predicted for the fracture-cave units. This method was applied to several fracture-cave units in the Tahe Oilfield, which resulted in the re-recognition of reservoir rocks with karst fracture-caves and their contents.