Abstract: Upper Paleozoic Benxi Formation, Shanxi Formation and Shihezi Formation in Yichuan-Huanglong Area are the chief target formations for exploration and development, and the natural gas enrichment and accumulation are principally controlled by the sedimentary facies and reservoir physical properties. After the evolution of marine-marine-continental transition facies-continental sedimentary system in the Late Paleozoic, the sedimentary facies types are different and the change law of reservoir physical properties is not clear yet. Therefore, the characteristics of Upper Paleozoic reservoirs in Yichuan-Huanglong Area were comprehensively studied with sedimentary evolution analysis, field outcrops, drilling cores, micro reservoir analysis, logging curves and other data. The result shows: Benxi Formation is mainly developed with tidal-flat facies, Shanxi Formation and He₈ Member of Shihezi Formation mostly developed with meandering river-braided delta facies. Tidal channel of Benxi Formation and underwater distributary channel of Shanxi Formation and He₈ Member are the most favorable reservoir facies zones. The sand bodies of the tidal channel of Benxi Formation are lenticular in shape and limited in distribution; the sand bodies of the underwater distributary channel at the front edge of the meandering river delta in Shanxi Formation are migrated and superimposed in multiple periods, with a certain scale; the sand bodies of the underwater distributary channel at the front edge of the braided river delta in He₈ Member are superimposed vertically and connected horizontally, with a blanket-shaped distribution. The reservoir of Benxi Formation is dominated by quartz sandstone, with main pore types of primary intergranular pores and secondary dissolution pores; the reservoirs of Shanxi Formation and He₈ Member are mainly composed of lithic quartz sandstone, with main pore type of lithic dissolution pore. The lithology and pore structure of the Upper Paleozoic sandstone reservoir are the main factors affecting the hydrocarbon showing. The reservoir as a whole is characterized by extra-low porosity and ultra-low permeability, but the content of quartz in the rock gradually decreases from bottom to top, the content of rock chip and fillings gradually increases, and the lithology and pore structure of the reservoirs gradually become worse from Benxi Formation to Shanxi Formation and He₈ Member. The sand bodies of the thick tidal channel of Benxi Formation and the continuously superimposed distributary channel of Shanxi Formation and He₈ Member are lithologically pure and coarse-grained, with good physical properties and high gas abundance, and they are the dominant reservoirs, will the high-quality pores developed in Benxi Formation, which is easy to accumulate natural gas. There is much for reference of the study results to the exploration, development and reserve enhancement of Yichuan-Huanglong Area.

Key words: Upper Paleozoic, sedimentary evolution, reservoir characteristics, controlling factors of reservoir formation, Yichuan-Huanglong Area, Ordos Basin