Delineation of Sandstone Reservoir Flow Zones in the Central Bredasdorp Basin, South Africa, Using Core Samples

This study aims to introduce a conceptual petrophysical flow zonation scheme for Valanginian age gas-bearing reservoirs in the Bredasdorp Basin, offshore of South Africa. The workflow presented here establishes a framework in a petrophysical context that allows for the classification of facies. Facies 4 is a medium-grained sandstone, facies 3 is a medium- to coarse-grained sandstone, facies 2 is a fine- to very fine-grained sandstone, and facies 1 is an interbedded bioturbated sandstone. The facies are integrated with three unique zonation methods to evaluate the reservoirs. The three zonation methods (Winland r35 pore throat, hydraulic flow unit, and stratigraphic modified Lorenz plot) are evaluated and integrated with fluid saturation results. The results reveal five flow zones: tight, very low, low, moderate, and high. The high-flow zone is the best reservoir quality rock composed of megaporous rock type and is associated with facies 4. The tight flow zone contains the lowest rock quality and is associated with the water interval located at the base of the studied wells, which extends laterally to all the wells. The gas column is within the low- to high-flow zones. Smaller pores retain free water, while larger pores retain gas. Results from X-ray diffraction (XRD) reveal that ankerite mineral has a significant effect on flow zones. Samples with high ankerite content, > 5%, tend to have low permeability associated with very low- to low-flow units, while samples with low ankerite content, < 5%, are associated with moderate- and high-flow zones. The pressure gradient results indicate vertical hydraulic connectivity with average gas pressure gradient of 0.07 psi/ft (1 psi = 6.89476 kPa, 1 ft = 0.3048 m) and water pressure gradient of 0.43 psi/ft.