ORDOVICIAN RESERVIORS OF TAHE OIL FIELD

Observation of drilling cores and casting-sections and the characters of pore structures and permeability of the matrix of the lower- Ordovician carbonate in comparing with the lower limit of a reservoir for matrix reveal that the porosity, permeability, maximum pore diameter, saturation and median throat diameter in the matrix with undeveloped fissures, holes and solution pores are far lower than those of a reservoir limit although some micro-pores are developed in the matrix which therefore has no capacity of storing and producing oil-gas. The favorable reservoir spaces in the carbonate rock could be divided into five types according to their origin, configuration and size, i.e., solution fissure, solution hole, cave, erosional fissure and structural fissure, which form five different reservoirs including cave-type reservoir, erosional fracture reservoir, structural fissure reservoir, and solution pore marginal beach grainstone reservoir. A systemic and integrated criteria is thus obtained for the Ordovician carbonates in the Tahe oilfield. It is very important for the exploration and development of oil-gas to study and clarify origin and occurrence of these favorable reservoirs.