Research On The Characteristics Of Upper Paleozoic Reservoirs Of Yanchang Gas Field,Ordos Basin

The Ordos basin is one of the most potential basins of gas prospecting in China. The northern region of the YanChang gas field of upper Neopaleozoic is rich of nature resources. For the low understanding of this area, a comprehensive research based on regional geological data is urgent need.Based on core observation and logging data, with casting thin sections, SEM, high pressure injection, rate-controlled mercury penetration, NMR, micro-pore textures of reservoirs in Upper Paleozoic are studied. Remanent magnetism is analysised, original orientation can be obtained, fracture strike in the study area is statistically analyzed, characteristics of reservoir are explained from macro by magnetic susceptibility anisotropy of rock.The area of Yan'an-Zichang of upper Neopaleozoic as object in this research,with geologic correlation, the intended interval was divided by Benxi formation?C₂b?,Shan 1 group?P₁s₁?,Shan 2 group?P₁s₂?,He 8 group?P₂h₈?, in the bottom of C₂b exists large plane of unconformity.Transitional facies developed in the study area, tidal flat facies tide zone facies was developed in C₂b,delta front was developed in P₁s_,anastomosed stream delta front was developed in P₂h_g. The rocks are from the northern source, and the marine-continental transitional facies was developed in the Upper Paleozoic strata of the area, which reflects the trend of uplifting of the northern strata and shallowing of the seawater.In Yan'an-Zichang area, C₂b and P₁s₂ of the Upper Paleozoic are dominated by quartz sandstone, followed by lithic quartz sandstone and lithic sandstone. P₁s₁ and P₂h₈ is mainly lithic sandstones. The sandstones in the reservoirs are dominated by clay minerals, mainly kaolinite, illite, siliceous and carbonate. The reservoirs that have undergone strong cementation were denser and lots of porosity were lost. The maturity of C₂b sandstone structure is high, gradually lower layer maturity, In this area, sandstone mostly in late diagenetic stage A, stage B, a few areas got into the late diagenetic stage C.Yan'an-Zichang area porosity distribute in range of 3%-8%; with no significant differences among the layers; the average permeability varied greatly among different layers. The porosity type of the P₂h₈ of the block was the largest among the remaining intergranular pores, the second was the feldspar dissolved pores, the remaining one is the intergranular pore and the heterogeneous base dissolution in the P₁s₁ and P₁s₂. Throat was developed by five types:pore shrinkage type, necking type, sheet or flake-shaped, bundle-like throat. The results of mercury intrusion and nuclear magnetic resonance ?NMR? show that the pore is characterized by small pore throat structure, large pore-throat structure variation, medium displacement pressure and medium pressure variation. With the results of constant velocity mercury penetration, small pores of the characteristics of the P₁s_, throat samples are small and medium-sized pores, C₂b samples showing a small throat, the characteristics of large pores. Pressure mercury test and nuclear magnetic resonance results show that the P₂h₈ with a large throat, small pore characteristics, Pis of samples of the throat is large and small, pore medium, C₂b samples showing small throat, the characteristics of large pores. The saturation of the movable fluid is more controlled by the throat, which restricts the gas saturation of the reservoir.Core sampling has been done to mark the line, try to restore the underground position with the ancient geomagnetic method, can get more intuitive information from the core. Under the orientation of the thin section, it is found that there is a certain degree of development in this area. With the paleomagnetic method to restore the core underground position, abundant information can be obtained from the core. Directional lamellae were observed microscopically, form the development and statistics of micro-cracks in this area, the results of the study on the anisotropy magnetic susceptibility ?AMS? coincide well with the sedimentary facies maps. The superiority orientation of the reservoir physical properties is well consistent.For the first time, Mercury and nuclear magnetic resonance experiments were performed on the target layer in the study area, and the method of the ancient geomagnetic field played an enlightening role in the study of the reservoir. From the point of view of comprehensive analysis, it is considered that the deposition range of particle size and spatial distribution of decision sandstone body, the diagenesis of the reservoir to alteration, pore throat structure is an important factor restricting reservoir performance. On the whole, P₁s₂ reservoir physical property of Yan'an-Zichang area is better, and it can be the main gas producing layer.