This study investigates microporosity in an outcrop analogue of the Upper Jurassic (Kimmeridgian) Arab-D carbonate reservoir in central Saudi Arabia, integrating outcrop facies analysis, petrographic and SEM data and statistical analyses. At the study location in Wadi Nisah, the outcropping succession includes the uppermost Jubaila Formation and the entire Arab-D Member of the Arab Formation which together comprise the Arab-D reservoir interval. The succession is composed of eight lithofacies which can be grouped into three lithofacies associations based on their depositional environments. The stromatoporoid lithofacies association includes dolomitic mudstones, dolomitic wackestones and stromatoporoid wackestones and packstones; the skeletal bank lithofacies association includes burrowed fossiliferous wackestones and peloidal fossiliferous grainstones; and the tidal flat lithofacies association comprises laminated mudstones, wave-rippled sandy grainstones, and supra- and intertidal muds with rip-up clasts. The lithofacies were classified into mud-dominated, grain-dominated and dolostone textural groups.
Microporosity and associated permeability in the analysed samples (n = 125 for porosity and n = 61 for permeability) range from 0.11% to 4.8 % and 0.36 to 4.35 mD, respectively. Three types of microporosity were observed: (i) between macro- and micro-sparry calcite crystals; (ii) between micrites of varying morphologies; and (iii) within macro-sized dolomite crystals. Microporosity distribution was controlled by sparry calcite cement, micrite crystal size, sorting and shape, and the presence of dolomite crystals.
Statistical analyses of microporosity and associated permeability show non-normal distributions for both variables. Coefficients of variation indicated high variability for porosity and permeability, which may be attributed to the high degree of heterogeneity in the pore system. In general there was a poor correlation between microporosity and permeability, but the correlation improved when visualized for individual textural groups.