Overview of Carbonate Rock Types in the Middle East

Abstract

The complexity and diversity of carbonate reservoirs necessitate having a consistent approach in defining rock types. Core data on a rock-type basis are required as input for modelling reservoir performance. The objectives of this paper are to provide detailed understanding of the controlling factors in rock typing and to develop a unique rock-typing approach that can apply to carbonate reservoirs in the Middle East region. More than 1,000 core plugs were studied from seven different carbonate reservoirs. The plugs were carefully selected from 15 wells to represent the primary property variations along the cored intervals. The data set available included laboratory-measured helium porosity, gas permeability, thin-sections, and high-pressure mercury injection capillary pressure (MICP). Plug-scale X-ray computed tomography (CT) imaging was acquired to help ensure the samples were free of induced fractures and other anomalies that can affect the permeability measurements. Rock textures were analyzed in the thin-section photomicrographs and were classified based on their content as grainy, muddy, and mixed. Special attention was given to the diagenesis effects, mainly compaction, cementation, and dissolution. Porosity was defined as interparticle, intercrystalline, moldic, intraparticle, or vuggy. The texture information was plotted in the porosity-permeability (poro-perm) domain and was determined to produce three distinct poro-perm relationships. Each texture provided an unique poro-perm trend. Rock types were defined on the poro-perm trends and showed a strong link to diagenesis and capillary pressure (Pc). For each poro-perm trend, the rock types were distinguished by detailed Dunham textures and different degrees of diagenesis. The detailed textures and diagenesis were correlated reasonably well with the poro-perm data and MICP, hence providing robust rock types. A new rock-typing approach was successfully applied in different carbonate reservoirs. The poro-perm cloud from the different reservoirs was resolved into trends and ranges based on textures and diagenesis. Common controlling factors were observed, which can help produce data analogues along more reservoirs in the region. The texture-diagenesis-based rock types provided more insight into the effects of geology on fluid flow and saturation. Geological textures were derived along the cored intervals and provided upscaling options for permeability and rock types in the reservoirs.