Characteristics and genetic model of karst reservoirs of Ordovician YingshanYijianfang Formation in Halahatang area, northern Tarim Basin

The most important exploration target of the Ordovician strata in Halahatang area of the Tarim Basin is carbonate karst reservoir in Yingshan- Yijianfang Formation, and a breakthrough of oil- gas exploration was made in this area in recent years.Therefore, a further study of karst reservoir for marine petroleum exploration will be very important. In this paper, based on the analysis of regional geological background, in combination with drilling, core, logging and sesmic data, the authors studied in detail the characteristics, main controlling factors and genetic evolution model of karst reservoirs in Yingshan-Yijianfang Formation. The results indicate that the porosity and permeability of the matrix in Yingshan- Yijianfang Formation are poor, micropores which mainly consist of solution cave and fracture cannot serve as favorable reservoir space. The main types of karstification are penecontemporaneous or contemporaneous karstification, buried karstification, and weathering karstification. Superimposed reformation of polyphase karstification has made a great contribution to karst reservoirs. Based on synthetic analysis of the formation process of karst reservoirs, the authors have arrived at several conclusions: The distribution of karst reservoirs is controlled by tectonic uplift, polyphase karstification is the main factor for reservoir development, and faults and fractures formed during the tectonic evolution have facilitated the development of karst reservoirs and improved the reservoir property.