Complex relationship between porosity and permeability of carbonate reservoirs and its controlling factors: A case study of platform facies in Pre-Caspian Basin

Based on a large amount of core analysis data in eastern Pre-Caspian Basin, the relationship between permeability and porosity and its influencing factors are studied. The sedimentary environments of the Carboniferous System in eastern Pre-Caspian Basin include open platform, restricted platform and evaporate platform. For dolomite reservoirs there are three main combination patterns of pores, namely, inter-crystalline and solution pores, inter-crystalline micro-pores, inter-crystalline and intra-crystalline pores, among which the first combination are highest in porosity and permeability. For limestone reservoirs, the main combination patterns of pores are inter-particle pores, inter-particle and intra-particle pores and moldic pores. Lacking connecting pore throats, moldic pores are poor in permeability. In the dolomite reservoirs, fractures and vugs are well-developed and increase the permeability significantly. Under similar porosity, the permeability is obviously determined by the size of pore throat and the percentage of middle to large size throats in different types of reservoirs. In limestone reservoirs, fractures are principally micro-fractures, the percentage of connecting pores to total pores is positively correlated with permeability; while in dolomite reservoirs, there is not such an obvious relationship between the percentage of connected pores to total pores and permeability. Sedimentary environment, controlling the distribution features of reservoir types, is the cause of the complex relationship between permeability and porosity.