Shallow Aquifer upper ∼120 m Rocks of Qatar: Depositional and Diagenetic Controls on Water Storage Capacity and Hydraulic Conductivity

Carbonates and evaporites of Paleogene-age form the shallow-aquifer rocks that mantle most of Qatar, including the Paleocene and Lower Eocene Umm er Radhuma, and the Middle Eocene Rus and Dammam Formations. Nearly complete 10-cm-diameter rock cores have been recovered from boreholes in central and northern Qatar to depths of greater than 120 m. A 40-m section of exposed Rus and Dammam Formation from a cave in central Qatar (Misfer Cave) was also described. Assessment of aquifer quality in these rocks was undertaken through core and thin-section description, quantitative mineralogical determination (X-ray diffraction), as well as core-plug porosity, permeability, and pore-throat (mercury-injection capillary pressure) measurements. Our work shows that the rocks from central Qatar can be separated into four depositional intervals: 1) m-scale fining-upward cycles of fossiliferous open-marine deposits with clay-rich caps (Umm er Radhuma), 2) fine-grained stromatolite-bearing, restricted shallow-marine deposits (uppermost Umm er Radhuma), 3) m-scale bedded marginal-marine gypsum deposits intercalated with thin shallow marine carbonate and clay deposits and capped by rooted and microcodium-bearing surfaces (Rus Formation), and 4) open-marine carbonates overprinted by karst processes (Dammam). Aquifer storage capacity and hydraulic conductivity are mostly a function of diagenetic features, and in spite of the lack of any evidence of significant burial, the diagenesis of these rocks is complex. The Umm er Radhuma and the Rus carbonates are almost completely dolomitized, whereas the Dammam is only partially dolomitized, with the amount of dolomitization varying both laterally and vertically. Best aquifer quality in central Qatar, based both on core plug data and well spinner tests, can be found in coarsely-dolomitized intervals that lack clay (lower Umm er Radhuma). Finer and clay-bearing dolomitized rocks have storage but lower hydraulic conductivity (upper Umm er Radhuma). Both gypsum-rich rocks and depositional limestones (which are generally mud-bearing) have lower porosity and permeability (Rus and Dammam). In central Qatar the effect of karst overprinting is variable, generally leading to lower matrix porosity and permeability due to clay translocation from above, but large vugs are also observed at exposure surfaces. The borehole in northern Qatar was located within a karst collapse feature (doline) that is characterized by internal drainage. Based on satellite images, the collapse feature, likely reflecting the presence of deeper karst is approximately 1 km in width. Sediment tentatively interpreted as a cave deposit, and composed mainly of dolomite silt, has been observed in this core at depths of greater than 60 m below the surface. This fine-grained material intercalates with courser breccias that are interpreted as collapsed country rock. The impact of karstification on country rock matrix properties (porosity and permeability) is still to be determined. Whereas the rocks from northern Qatar are still under investigation, they are distinct from the central Qatar equivalents in that they lack any bedded evaporites. This leads to the prediction that the shallow aquifer in northern Qatar will be less compartmentalized than in central Qatar. The elemental makeup of waters from different stratigraphic intervals in both boreholes are being compared to help understand how the central and north shallow Qatar aquifers contrast.