Characteristics and controlling factors of dolomite reservoirs of Lower Ordovician Penglaiba Formation in Yubei area

Penglaiba Formation of Lower Ordovician in Yubei area becomes the target stratum for oil and gas exploration of marine carbonate rocks in Tarim Basin, and it developed large amounts of dolomite reservoirs. Due to the deep burial and low degree of exploration of dolomite, the cognition of its origin is not perfect and need further studying. Based on field geological inspection, core observation, identification of thin section, physical properties analysis, combined with SEM and the results of isotope geochemistry of carbon, oxygen and strontium, this paper analyzed the characteristics and controlling factors of dolomite reservoirs of Penglaiba Formation in Yubei area. The dolomites of Penglaiba Formation consist of replacement dolomite and recrystallization dolomite, which can be further subdivided into three categories, silty crystalline dolomite, medium grained dolomite and coarse crystalline dolomite in terms of grain size,and the most important reservoir rock is medium grained dolomite. The reservoir space in dolomites is subject to intercrystal pores, intercrystal dissolved pores, intracrystal dissolve pores, dissolved pores and fractures as migration channels. Dissolved pores and fractures are the most important reservoir space. The most common reservoir space is mainly the overlapped complex of cracks and cavities. Fracturing and its associated fractures and karstification are the keys for the development of dolomite reservoir of Lower Ordovician Penglaiba Formation. Fault activities control the development of associated fractures, which makes fresh surface water or deep hydrothermal infiltrate or upwell along faults and fractures, and then dissolution occurs. Atmospheric fresh water karstification effectively improves the dolomite reservoir properties at the top of Penglaiba Formation, while the hydrothermal karstification results in poor development of the lower reservoir of Penglaiba Formation, owing to filling with silica in the most holes and crakes.