Characteristics of karst reservoirs in the Ordovician buried hills of the Halahatang area，northern Tarim basin

Crack-cavity type karst reservoirs, which lie in the Ordovician buried hills, are the primary production horizons of the Halahatang oil field northern Tarim basin. Based on karst geology background and tectonic evolution of this area，using integrated data of drilling-logging wells，thin slices and seismic surveys, this work analyzes the characteristics, spatial distribution, and controlling factors of the karst reservoirs in this area. The results show that these reservoirs are largely present in the Middle Ordovician Yijianfang formation and the Lower-Middle Ordovician Yingshan formation. In vertical direction, there are two sets of reservoirs in verti in the buried hills, which are the upper superficial dissolution belt and lower runoff dissolution belt. The superficial belt contains cavity type and crack-cavity type reservoirs, which are controlled by karstification at the ends of the deposition of both the Yijianfang and Sangtamu formations. The runoff type of reservoirs are of cavity and crack-cavity types, which are dominated by karst channel systems, determined by multiple episodes of karstification including the end Lianglitage period and end of the Sangtamu formation sedimentation. Many factors, such as the layers of karst, episodes of karstification, ancient landforms, ancient drainages and faults, jointly controlled the distribution of karst reservoirs, resulting in complicated crack-cavity systems in this area.