Abstract: The Ordovician reservoir in Tahe Oilfield is one of the ancient mantle rock reservoirs.It was rebuilt by multiphase secondary structure-karst effect.The distribution of the reservoir in plane and portrait of reserving body are of strong heterogeneity.In the evolution process of multistage reservoir-forming,the fluid distribution in dimension is complicated and difficult to be described and estimated effectively with the groovy research method of reservoir description and reserve assessment,because of the gradient injection of fluid.Four models for description of small stratum,structure,fracture-karst and fluids were developed,and a geological reservoir model was proposed,on the basis of detailed research on the reservoir characteristics.The conformation of reservoir body in dimension was obtained,according to the class of layer cell per 60 meters.The orderliness of reservoir body distribution in plane was depicted,and the reservoir parameters were researched.The partition of different calculated cells was made out,and the diversification of reserves abundance of every belt and reserves of different reserving bodies were obtained.A set of methods for reservoir description and reserve assessment of Tahe Ordovician carbonate fracture-cave type pool was developed,with combination of karst reserving formation analysis,seismic-logging joint inversion technique,numericalmodeling,reservoir-forming mechanism,reservoir rebuilding and behavior analysis of production testing.These methods have been applied broadly in the development of the Tahe Ordovician pools effectively.The rate of gaining output increases by 5 percent to 8 percent.