Cave，solution pore and fracture are the primary reservoir spaces in Lower Ordovician，the 4th block of Tahe Oilfield.
Cave is the most effective reservoir，while fracture takes the second place. It is difficult to form effective reservoir for matrix，
owing to its poor porosity and permeability. Effected by the tectonic movement of early hercynian，the fracture was mainly
distributed in the direction of NNE，NNW and NE. Integrated with description of the karst feature of core，electronic probe
element analysis and isotope analysis，we found that there are four periods of karstification in this area，of which the periods
of Middle-Late Caledonian and early hercynian played an important role in controlling the formation of the karst cave. Karst
controlled the distribution of fracture-cave reservoir，and the ancestral water system located in different karst areas had impact
on the shape of cave-fracture body. Fracture and its accompanying crack not only increased the contact area but also improved
the solution range of the surface and ground water with the carbonate reservoir，which caused 90% wells with drilling empty
and mud loss distributed near the fault zone，where karst developed very well.
