| The Middle-Upper Cretaceous bioclastic limestone reservoirs can be classified into two types, one with high-porosity and high-permeability and the other with high-porosity and low-permeability, in Ahdeb Oil Field located in the central and southern parts of Iraq. These reservoirs are different in sedimentary environments, reservoir spaces and pore textures as well as porosity and permeability. The high-porosity and high-permeability reservoirs usually develop in grainstone in intra-platform shoal subfacies, in which the pore spaces are commonly composed of mouldic pores, non-fabric selective dissolution pores, intragranular dissolved pores and intergranular pores. This type of reservoirs belongs to the best reservoirs with good configuration of pores and throats, and high porosity and permeability. The high porosity and low permeability reservoirs mainly develop in the low-energy micrite limestone, in which the pore spaces are commonly composed of intrafossil pores and intercrystal pores. This type of reservoirs is of poor configuration of pores and throats and very low permeability. It is shown that the reservoirs are controlled by sedimentary facies and constructive diagenesis so that the intra-platform shoal is the most favorable depositional facies and the dissolution, neomorphism and dolomitization lead to the majority of secondary porosity. Besides, the syngenetic dissolution and supergene dissolution are the keys of forming favorable reservoirs. |
