Research of karst reservoirs characteristics and paleokarst development pattern of the Ordovician Yingshan formation north-slope of Tazhong, Tarim basin area

The study area is situated in the northern slopes of Tazhong paleo-uplift, Tarim basin, where paleokarstic features are massively developed in the carbonate rocks of Yingshan Formation of the Ordovician, with abundant oil and gas resources and a broad prospect for the exploration and development of the natural resources. The development of karst reservoir and the patterns of solution cavities and fissures were studied, based on the data derived from rock chemical analysis, core drilling, and down-hole seismic and imagining logs. The results show that thick-layered pure limestone of the top two members of Yingshan Formation belongs to strong karst suite, where corrosion hole, karst fissure and cave are developed. Post-tectonic karstification has made the region become the main place for oil-gas accumulation. The results also show that vertical karst caves and solution fissures mainly occur in the depth ranges of 0 ~ 50 m and 80 ~180 m below the Yingshan Formation, of which the karst caves substantially constitute the oil-gas reservoir in this area. This was confirmed by the venting and leaking phenomena during the well drilling. In combination with the characteristics of paleo-geomorphology, we established the development patterns of paleokarstic cave system in the area between wells ZG18 and ZG22,in order to provide a concrete geological basis for karst reservoir prediction and well exploration.