Abstract: The discovery of Block C of the Tahe Oil Field, Xinjiang marked a recent breakthrough in the oil and gas exploration in northwestern China. The source and reservoir rocks in the oil field consist dominantly of the Lower Ordovician strata, that may be assigned to the buried hill-type carbonate reservoir rocks. The reservoir rocks are nonhomogeneous and undersaturated, and characterized by low porosity and constantly changing permeability. The primary recoveries of the Ordovician hydrocarbon reservoirs in Block C of the studied oil field have been determined with the aid of empirical formula, tables and analogy analysis on the basis of geology, reservoir type and available information.