Reservoir Characteristics And Main Controlling Factors Of The Upper Paleozoic Box 8-Mountain 2 In Ordos Yanqi 2 Well Area

The Yanqi2 Well of southeast Ordos Basin boasts great exploration potential. The predominant payzone lies in the Shihezi Formation and Shanxi Formation in the Lower Triassic. Problems resulting in the knowledge of the exploration and development still exist due to the complexity of this reservior. Elaborate stratigraphic divisions and correlations about the He8 Formation and Shan2 Formation in this well block were made based on essential geographic materials, such as thin slices, cores, well logging, drilling and completion data in this paper. Moreover, the sedimentary facies, reservior characteristics,heterogeneity and the distribution rules of high-quality reservoirs are also systematically discussed in this paper. And the following conclusions and knowledge are obtained through studies:1. Strata of the He8 Formation and Shan2 Formation demonstate a west-inclining monoclinal structure as a whole, with an angle of inclination less than 1 degree. The strata feature good development, continuity and consistant thickness, the variation of which varies locally, which indicates weak tectonic activities.2.He8 Formation and Shan2 Formation are delta-front subfacies deposition, where two depositional microfacies, namely distributary channels and inter-distributary bays developed and the former occupies a dominant position. These two depositional microfacies in the He8 Formation and Shan2 Formation go with inheritance. The sandbodies of the underwater distributary channels present multiple fine upward overlaps, extending along the south and north direction and containing the development of inter-distributary sedimentations among sandbodies.3.He8 Formation and Shan2 Formation are tight sandstone reservoirs of ultra-low porosity and ultra-low permeability, with the frame debris predominantly being quartz.The He8 Formation features offwhite?grey fine-grained quartz-sandstone; the Shan2 member consists largely of grey middle-coarse grained sandstone and detrited quartz sandstone. After diagenesis processes like:compression, chemical dissolution, cementation, micro-fissuration and erosion, the diagenetic process features the Stage A of the late diagenism. Primary intergranular pores, intergranular dissolved pores, intergrannular dissoluted hole expansions, cave-filling caverns, intercrystal pores of the clay minerals as well as the structural fissures were developed, with the dissolution porosity accounting for a large portion and strong heterogeneity in the throat.4.Sedimentary environment controls the distribution of the sandbody, clastic components and textural characteristics of the reservoir rocks. High porosity maturity in the sedimentary subfacies of the distributary channels of these two formations are quite similar. The microfacies of underground distributary channels feature relatively stronger hydrodynamic force, resulting in well-sorted coaser-grained sandstone, rich in quartz and sparse in shale content. Diagenesis controls the types of porosity and the physical properties of the reservior, while dissolution, microfissurization and formation of clay material crustings can increase the porosity and the permeability of the as well as the the quality of the reservoir. Mechanical compaction, pressure dissolution and and the formation of quartz overgrowth, kaolinite, illite and carbonate are diagenesis that destroy the pososity and permeability of reservoirs. Tectonic activities contribute little to the oil and gas reserve there.5.Strata of He8 Formation and Shan2 Formation can be divided into four categories: premium (?), good (?), medium (?) and poor (?), with the ? and ? being the predominant ones. There is good positive correlation between porosity and gas zone (gas-bearing zone): the greater the porosity, the thicker the corresponding gas-bearing intercals and the higher yield zones are basically from the premium reservoirs. The porosity in the sedimentary subfacies of distributary channels are well developed, therefore the gas output is remarkable, which further indicates that the quality and distribution of the sandstone in the target formations are majorly subjected to the control of sedimentary phases.