Characteristics and origin of high porosity and low permeability carbonate reservoirs in the Sa'di Formation, Halfaya Oil Field, Iraq

The high-porosity and low-permeability reservoir in the Sa'di Formation of the Halfaya oil field in Iraq makes this a special oil and gas reservoir in the Middle East. Reservoir characteristics and the origin of high porosity and low permeability were studied by core observation, cast thin sections, physical property testing and mercury injection experiments. Rocks in the reservoir include grained limestones (oolitic limestones and limestones), packstones, wackestones and micritic limestones. The Sa'di Formation is characterized by ramp facies, and develops inner ramp and outer ramp subfacies. The lower part of the formation has better physical properties than the upper part. Pores are mainly intrafossil and intragranular. Mercury injection experiments showed a high driving pressure, small throat radius and poor flowability. The reservoir is mainly controlled by sedimentary facies, and the reservoirs of different facies showed different properties. Diagenetic transformation has constructive effects on the formation of secondary pores, resulting in poor cementation. High porosity and low permeability of the reservoir can be explained by the following factors:(1) Intrafossil pores developed, yet were poorly connected. (2) Sedimentary environment was featured by poor hydrodynamic conditions, and was poorly reformed. (3) The reservoir was controlled by sedimentary process, and the digenesis impact was very weak. (4) Pore types varied, but had low contribution to permeability.