| The development characteristics and the main controlling factors of Ordovician fractured-cave carbonate reservoirs in Lungu Oilfield of Tarim Basin are always a major factor affecting the success rate of drilling, and the underground river widely developed is a key to solve this problem. Based on the geological theory about modern karst and ancient karst, 22 underground river systems of Ordovician carbonate buried-hill of Lungu Oilfield were characterized by the field geological modeling and well log and seismic data analysis. The results show that the development of underground river is mainly controlled by tectonics, paleogeomorphology, fracture and surface river. The rock types of underground rivers are mainly limestone of lower Ordovician Yijianfang Formation and Yingshan Formation, including calcarenite, bioclastic limestone and micrite. The reservoir space is dominated by large caves and fractures developed in the areas along the main channels and distributary channels and in the ceiling of the cavity. Due to the complex internal structure of the underground river, the tubulose- hall-caves, tiankengs, and other reservoirs which are related to the main channel, are the most abundant areas of oil-gas accumulation. In the early stage of formation, the underground river is in a high connectivity state, while in the late stage, because of the existence of blockage points and the filling at the lower parts of underground river, it only connects some independent blocks locally. |
