Impact of authigenic iron oxides, clay content and grain size on the aquifer quality properties of the Cambrian-Ordovician Wajid Sandstone, southwest Saudi Arabia

Abstract

The Wajid Sandstone is one of the most prolific aquifer sequences in southwestern Saudi Arabia. The petrophysical properties of the Wajid Sandstone in Gebel Al-Habala, including bulk and grain densities (ρb, ρg), water and helium porosities (∅w, ∅He), gas permeability (k), true formation resistivity factor (FRT), and the Archie's parameters (a, m) were measured. The aquifer quality parameters were also estimated including the aquifer assessment index (AAI), the flow zone indicator (FZI), the aquifer potentiality index (AQPI), and discrete rock type (DRT). The Wajid Sandstone can be split into two units: the lower red unit (LRU) and upper grey unit (UGU). These are composed of four rock types (RRTs) and six hydraulic flow units (HFUs). The most prolific rock type is coarse-grained slightly ferruginated and calcareous sandstones ‘RRT1’ (average ∅He = 21.72%, ∅W = 12.58%, k = 88.9 md, R35 = 4.86 μm, AAI = 0.59 μm, FZI = 2.10 μm, AQPI = 1.5, DRT = 12.0, and average FRT = 262.0) which form the top of the LRU (HFU-2 unit) and UGU (HFU-6 unit). HFU-2 unit is characterized by the best aquifer quality parameters and is the thickest unit (27.5 m). It forms approximately 72.3% of the flow capacity of the entire Wajid Sandstone sequence and is the most prolific flow unit. The aquifer quality properties of the Wajid Formation are mainly controlled by grain size, which varies from coarse-grained sandstone (RRT1) to siltstone (RRT4). The authigenic clay and iron oxide content are additional diagenetic factors that greatly reduce the aquifer quality. The lowest quality is assigned to HFU-1 unit which is composed of highly ferruginated and compacted siltstone RRT4 (average ∅He = 13.77%, ∅W = 8.44%, k = 0.02 md, R35 = 0.058 μm, AAI = 0.012 μm, FZI = 0.08 μm, AQPI = 0.0, DRT = 6.0, and average FRT = 202.9). The free and forced-regression porosity exponent (m, m') was found to increase with aquifer quality in the studied samples.