Abstract: The Lower-Middle Ordovician carbonates in the Yubei Area, Tarim Basin, are considered promising exploration targets.However, the complexity of the controlling factors and distribution patterns of the reservoirs hindered the exploration process in the area.By using petrologic, physical and geochemical methods, we classified the reservoirs into five types:cavern type, dissolution pore type, fracture-vug type, vug-fracture type and fracture type.The main controlling factors of the reservoirs were determined to be dissolution, dolomitization and fracturing.It is suggested that the relative low stand sea level during the Early-Middle Ordovician caused the relative paleogeomorphic highs being repeatedly exposed to penecontemporaneous dissolution, which resulted in the formation of centimeter-scale caves, millimeter-scale mold pores, along-bedding dissolution and geopetal cements that are not cotrolled by single uniform exposure surface.The dissolution degree in the Penglaiba Formation was overly stronger than that in the Yingshan Formation.The dolomitization processes that mainly occurred in penecontemporaneous and shallow burial stages strengthened the resistance of carbonates to compaction and pressure dissolution, thus provided favorable condition for the preservation of early dissolution pores and caves and contributed indirectly to the forming of reservoirs in the area.The fractures formed through multiple tectonic stages.The fractures formed before the Late Hercynian epoch were filled up by calcite, dolomite and silica and made less contribution to the formation of reservoirs.However, the fractures formed during the Late Hercynian and Himalayan epoch were effective in contributing to the formation of reservoirs due to weak diagenetic processes.

Key words: dolomitization, reservoir, carbonate, Lower-Middle Ordovician, Yubei area, Tarim Basin