Distribution of petrophysical parameters in the cambro-ordovician dibsiyah member of the wajid sandstone, Sw Saudi Arabia

The Cambrian-Ordovician Dibsiyah Member is the lowermost unit of the Wajid Sandstone, a major aquifer in SW Saudi Arabia. The Dibsiyah is present throughout most of south-central Arabia including the Rub'Al-Khali Basin where it is a potential exploration target and a reservoir for gas. The Member can be divided into Upper and Lower Units based on depositional facies. The Lower Unit is interpreted to have been deposited in a fluvial setting, whereas the Upper Unit, which displays abundant bioturbation, was deposited in shallow-marine conditions. Patterns of porosity and permeability distribution in the Dibsiyah Member were analysed statistically. In general, the Member exhibits a limited range of porosity values and a wide range of permeability values. Histograms show that the porosity distribution of both Lower and Upper Units are approximately normal, and that their corresponding vertical and horizontal permeability distributions are positively skewed. The coefficient of variation for porosity in both the Lower and Upper Units is less than 0.5 indicating that the porosity range is small. However, the horizontal and vertical permeability values are more variable due to the spatial distribution of architectural elements and sedimentary structures within both units. The dissolution of unstable minerals, together with inhomogenous grain packing and poor connectivity, has resulted in a variable permeability distribution.