Summary Applying the sequence stratigraphy principles, the upper part of Zakum Formation in Field "B", Onshore Abu Dhabi, is divided into two parasequence sets. The lower part comprises of a progradational interval overlain by a retrogradational package, with the boundary between each package marking a stillstand. These two parasequence sets are believed to represent highstand deposition with punctuated sea level rise creating alternately progradational and retrogradational packages. This reservoir is one of the major gas-producing zone and comprises of five lithofacies; bioclastic peloidal grainstone, algal packstone/floatstone, bioclastic peloidal packstone, algal wackestone/floatstone and bioclastic peloidal wackestone/ packstone. These sediments are believed to have been deposited on a homoclinal carbonate ramp that dipped very gently seaward; with inner ramp deposits making up the reservoir zone and middle ramp deposits the dense interval. Porosity and permeability are well developed in the reservoir sections due to a lack of pore-filling cements. Pore types include primary intergranular, intercrystalline micropores, primary organic and secondary dissolution. Reservoir quality is for the most part facies controlled, although with a significant diagenetic overprint due to widespread dissolution. The best reservoir quality is associated with the bioclastic peloidal grainstone and Lithocodium packstone/floatstone lithofacies, which can be demonstrated to show an order of magnitude, increase in permeability compared with adjacent lithofacies. Analysis of both thin section descriptions and high pressure mercury injection have led to the identification of five distinctive rock types (RT I to RT V) within the upper part of Zakum reservoir. A rock type is defined as a reservoir rock within which the geological and petrophysical properties that effect fluid flow are consistent and predictable. Accordingly, each reservoir rock type has a certain effective pore throat size distribution, which will produce a particular family of capillary pressure curves and controls porosity, permeability and water saturation. RT I occurs in grainstone lithofacies and characterized by well-connected interparticle porous network that will promote good to very good permeability. RT II is represented by packstone and Lithocodium packstone/floatsone lithofacies. The main pore types are enhanced intercrystalline mesopores associated with microporosity in the mud matrix and primary intraparticle macropores associated with the Lithocodium. However, the secondary pore types are more common than the primary pores. RT III is similar to RT II as lithofacies, however, the primary pore types are more common than the secondary pores. RT IV occurs in wackestone and Lithocodium wackestone/ floatsone lithofacies where the dominant pore types are secondary intercrystalline mesopores. RT V is represented by the wackestone/packstone lithofacies with intercrystalline micropores associated with the lime mud matrix. Methods of Study Detailed geological description was made of the slabbed cores to characterise the lithofacies present and to interpret the environments of deposition. Particular attention was paid to depositional texture (lithology, grain size and bioclastic content), sedimentary structures, diagenetic features (including stylolites and fractures), hydrocarbon staining and the type and nature of porosity. Details of microfacies and diagenetic variation were greatly aided by examination of approximately 850 thin-sections prepared from the core plug trims or core chips.

It's been long years of trials worldwide to find the relation between permeability and any other reservoir property or log and/or relation with combination of them. There have been even neural network, fuzzy logic and equation solver software modules used to enable extracting and finding probable sophisticated equation for permeability determination from log data, in a way replicating Archie's Equation for water saturation determination, however; it's important to keep in mind that core analysis were unable to provide proper reservoir water saturation due to coring operation deficiency in preserving core fluid from the time of coring until reaching surface and labs unless highly cost pressure coring was conducted. While core plug permeability measurements provide solid reference of permeability; in the other hand, if water saturation determination from logs is wrong no one can easily challenge it and proves opposite. Here, in this paper a new analysis approach is innovated and detailed, as the analysis did not look for one relationship between permeability across all geological layers and specific log data like previous approaches; instead, the new approach is finding multi relations between permeability variations across each layer and several log data types until finding the suitable log or logs that are providing adequate relation. As we found that each reservoir layer permeability range responds to different log data types more sound and obvious than other log data types, in a way reflecting the domination reasons for permeability ranges whether they are due to fracture or matrix or any other structural, stratigraphic, deposition or digenetic effects. This technique is utilized successfully in three different reservoirs; fractured limestone, non-fractured heterogeneous limestone and dolomitized-limestone.

Oil industry's basic tool for in-situ determination of hydrocarbon saturation is resistivity log. Since 1942, Archie's empirical relations have been used to calculate oil (and water) saturations, which are vital for field development. Sw (Water Saturation) and So (Oil Saturation) values are needed in several phases of reservoir life; exploration, development, monitoring, management and long term plans. In one of major Abu Dhabi offshore fields, a study to develop optimum methodology for determining Archie parameters that represent the corresponding rock properties is initiated. This is to replace the use of one constant value of by variable curve that respond to different rock properties. In this paper, Archie's equation parameters determination techniques have been reviewed and analyzed. Cost effective, high confidence rock & Log type integration methodology were innovated to determine variable values of two parameters; Cementation and Saturation Exponent. Before this study, Special Core Analysis measurements were the main techniques to get the two parameters. In this study, thorough investigation on utilizing Neutron, Density, Resistivity & Dielectric logging measurements as replacement of core analysis have showed for valuable opportunity to establish new water saturation log interpretation standards in carbonate rock.