Features and petroleum significance of Ordovician carbonate reservoir in Tazhong area, Talimu Basin

The analysis of exploration data shows that under the control of the tectonic evolution of Talimu Basin, mega-thickness carbonate deposit develops in Tazhong area during Ordovician period, which mainly consists of lower Ordovician dolomite series and middle—upper Ordovician limestone series; two unconformity surfaces develop in between lower Ordovician and middle—upper Ordovician and at the top of upper Ordovician separately. Under the joint action of structures, sedimentation and epigenetic transformation, two sets of carbonate reservoir rock systems develop in this area, they are lower Ordovician dolomite reservoir rock system and middle—upper Ordovician limestone reservoir rock system separately, and five types of carbonate reservoir bodies, they are inside dolomite type, weathering crust karst type, tectonic fracture type, sedimentary facies belt controlling type and deep fluid transforming type. According to the development and the distribution of reservoir rocks, it is put forward that, laterally, the oil and gas exploration activities should be enhanced in the northern slope, the west area and the northwestern pitching end of Tazhong area, and vertically, the emphasis of exploration should be on deep lower Ordovician dolomite reservoir series.