Origin and Evolution of Ordovician Karst Cave Reservoir in Tahe Oilfield

Karst caves,holes and cracks are important oil conductivity to Ordovician fractured reservoir and control the distribution of oil and gas in Tahe oilfield.Karst system of Ordovician reservoir in Tahe oilfield developed from Caledonian to Early Hercynian.Based on the karst origin,evolution and growth mechanism of Tahe oilfield,it is considered that the north area of pinch O3s is the result of karst in Caledonian and Early Hercynian,and the south area of pinch O3s is the result of karst in Caledonian.Utilization of core,drilling,logging and seismic data,the carbonated caved-fracture reservoir caves are divided into 4 types: Excavation or dissolution by aggressive fluids;Deposition of allochthonous(externally derived) sediments;Deposition of autochthonous(internal derivation and precipitation) sediments;Collapse at near-surface and in subsurface.Finally,the article establishes the distribution pattern of carbonated caved-fracture reservoir.