Application of Sequence Stratigraphy and Petrography in Preparation of Reservoir Rock Typing Scheme in One of Thamama Gas Reservoirs of Onshore Abu Dhabi
Authors: Al-Habshi Ali , Darwish Abdel Rahman , Hamdy Thanaa , Shebl Hesham 
Published: December 2007

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.