CHARACTERISTICS AND ORIGINS OF ORDOVICIAN KARST RESERVOIR IN HALAHATANG AREA,NORTHERN TARIM BASIN

Recently, big progresses have been made in oil and gas exploration and development in the Ordovician carbonate reservoir in the Halahatang area, northern Tarim Basin. Integration of the data from outcrops, drilling cores and seismic profiles indicates that there are three kinds of karst reservoirs in the Halahatang area, i.e. large Karst caves, dissolution pores and dissolution fractures. With the pinchout line of the Sangtamu Formation as the boundary, the study area is divided into two parts:the northern denudated area and the southern covered area.Seismic data suggest that karst reservoir in the northern denudated area is mainly affected by weathering crust,the coverage and intensity of the weathering process affect the scale of the karst reservoir.On the other hand, the distribution pattern of fractures is another factor which controls karst development. The superimposed relationship between circulation loss and fault system suggests that the fault system and its effect on the interlayer karstification is the main control factor over the karst reservoir developed in the southern area of the pinchout line of the Sangtamu Formation.There are three development stages responding toⅠ,Ⅱand Ⅲepisodes of the middle Caledonian respectively in this region. The ground and underground rivers under the control of faults played significant role on development of the karst reservoirs in the study area.