Study on the characteristics of Upper Paleozoic reservoirs in Ordos Basin: a case study of He 8 reservoir in Zichangdong area

The complex mineral composition and diverse electrical characteristics of the Upper Paleozoic reservoir in the Ordos Basin hinder gas–water layer identification, affect the optimal selection of development layers and seriously restricts the efficient development of production capacity. This article takes the He 8 reservoir in the Zichangdong area of the Ordos Basin as the research object, and the characteristics of microscopic pore throat are revealed by means of casting thin sections and high pressure mercury injection. The gas and water layer identification chart is established for the first time, and the fluid electrical response law and the spatial distribution of gas saturation were elucidated. The results show that the sandstone types of the He 8 reservoir are mainly lithic sandstone, lithic quartz sandstone and feldspar lithic quartz sandstone, which belong to low porosity-ultra-low porosity and tight reservoir. The average porosity is 5.25% and the average permeability is 0.153 mD. The gas-bearing property of the reservoir is good, and the subsequent key development well area is defined. The resistivity of the water layer is below 39Ω·m, and the threshold value can guide the optimization of the development layer. The research results can be used to distinguish the gas and water layers and predict the favorable development areas of the reservoirs in the area. The research method provides reference for the study of reservoir characteristics of other gas reservoirs.