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.