Developmental pattern and distribution rule of the fracture-cavity system of Ordovician carbonate reservoirs in the Tahe Oilfield

Ordovician carbonate fracture-cavity reservoirs in the Tahe Oilfield are a special type of reservoirs,in which fractures and cavities constitute the main reservoir space.In the Tahe area,the development of ancient karst is closely related with the regional tectonic evolution because Ordovician carbonate reservoirs in the Tahe Oilfield experienced karstification in the middle Caledonian and early Hercynian,and the karstification of the long-term exposed weathering crust during the early Hercynian played a major role in controlling the development of these karst reservoirs.Based on the geological survey of palaeokarst outcrops of the Yijianfang,Liuhuanggou,Sandaoban and Wudaoban regions in the northern margin of the Tarim Basin,developmental characteristics of reservoirs in the Tahe Oilfield revealed that there were three developmental patterns of the fracture-cavity system in Ordovician carbonate reservoirs: i.e.underground river system,karst caves,and corroded pores and fissures,of which the former two were the most dominant types for the reservoirs.The fracture-cavity system developed well in the vicinity of fault belts,and the horizontal distribution of the fracture-cavity system was controlled by palaeo-geomorphology.For instance,karst caves were extremely well-developed in peak-cluster of a karst platform,and then in knob-and-kettle topography on gentle karst slopes and in rugged country of valleys.Vertically,the development of karst cavities was apparently of zoning and superficial karst belts were the most important segment for karst development.