Carbonate reservoirs from producing oil and gas fields have extreme ranges of porosity and permeability, both locally, within a single reservoir zone, and in terms of average values for entire reservoir zones.1 This study describes the latter type of variation for two major resevoir formations in the Middle East and lists the factors that seem likely to account for the striking overall differences between these units.
Compilation of average porosity and permeability data for petroleum reservoirs in the Permian/Triassic Khuff Formation and the Jurassic Arab Formation confirms and quantifies the major differences known to exist between these units. These data were provided by IHS Energy (Energy Data Information Navigator Database), as reported previously by 1. The average reservoir data are supported by a compilation of published porosity-permeability data from specific fields, where core and wireline log data provide insight into geologic controls. The geographic distribution of the combined data are shown in Figure 1.
Most Khuff reservoirs have average porosity <10%, while most Arab reservoirs have average porosity of 15–27% (Fig. 2). Higher average porosity shows broad correlation with shallower top-reservoir depth in both Khuff and Arab data (Fig. 3). There is also broad correlation of higher average porosity with higher average permeability in both Khuff and Arab data (Fig. 4).
The above differences reflect a combination of depositional, diagenetic, and economic factors (Fig. 5). Khuff strata were deposited on an extensive, poorly-circulated, very low-relief shelf and consist in large part of interbedded mudstone and grainstones having relatively fine grain size, with major amounts of depositional calcium sulphate present. Arab reservoirs were deposited under better-circulated conditions nearer to margins facing deep intracratonic basins and thus have coarser, more grain-dominated fabrics and lesser content of chemically precipitated grains.