Comprehensive Evaluation and Reservoir Classification in the Taiyuan Formation in the Daniudi Gas Field Da17 Well Area, Ordos Basin

The Taiyuan Formation in the Daniudi Gas Field of the Ordos Basin was formed in a tidal flat mixed sedimentary environment. It represents a typical tight sandstone reservoir with significant exploration and development potential. However, current research on the reservoir evaluation of the Taiyuan Formation is relatively limited. This study focuses on the Taiyuan Formation in the Da 17 well area of the Daniudi Gas Field, employing methods such as petrographic analysis, X-ray diffraction, porosity/permeability measurements, mercury injection capillary pressure, and scanning electron microscopy to conduct a detailed investigation of sand dams in the tidal flat environment. By combining reservoir characteristics with detailed sedimentary microfacies classification, and on the basis of discussing the relationship between reservoir heterogeneity and natural gas production capacity, this study performs reservoir classification and evaluation. The characteristics and distribution patterns of relatively high-quality reservoirs are clarified. Through the analysis of lithological characteristics and sedimentary markers, it is determined that the Taiyuan Formation in the Da 17 well area represents a typical tidal flat sedimentary environment, with sand dams serving as the primary development sites for sand bodies. The Taiyuan Formation sandstone is mainly composed of medium- to fine-grained lithic quartz sandstone and lithic sandstone. The predominant pore types include primary pores, secondary pores, and fractures. The porosity of the reservoir ranges from 0.3% to 14.2%, while the permeability varies from 0.006 to 29.7 × 10−3 μm2, classifying it as a typical tight sandstone reservoir. Using the gas testing method, the lower limit of the physical properties of the Taiyuan Formation reservoir has been determined to be 3.9%. Considering the heterogeneity characteristics and productivity information of the sand layers, the reservoirs of the Taiyuan Formation are classified into three types: Type I reservoirs are predominantly composed of pebbly coarse sandstone and coarse sandstone, with porosity > 10%, permeability > 0.8 × 10−3 μm2, mainly developed in the center of sand dams, exhibiting medium heterogeneity; Type II reservoirs consist of coarse sandstone with porosity ranging from 4% to 10% and permeability from 0.1 to 0.8 × 10−3 μm2, with the main body of the sand dams belonging to this reservoir type, also showing medium heterogeneity; Type III reservoirs are composed of medium to fine sandstone, with porosity < 4% and permeability < 0.1 × 10−3 μm2, mainly developed at the edges of sand dams, exhibiting strong heterogeneity. By integrating the distribution characteristics of sedimentary facies sand bodies, the planar distribution characteristics of the various reservoir types are clarified.