Depositional and Diagenetic Controls of Flow Properties in Early to Middle Eocene Aquifer Rocks of Qatar

Abstract

Paleogene-age carbonates and evaporites form the shallow aquifer rocks that mantle most of Qatar, including the Paleocene to Lower Eocene Umm er Radhuma (UER), 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 >120 m. These rocks have been characterized through core and thin-section description, XRD, as well as core-plug porosity, permeability, and MICP measurements. A 40-m section of exposed Dammam Formation from a cave in central Qatar was also described. Depositionally the rocks from central Qatar can be separated into four intervals: 1) m-scale fining-upward cycles of fossiliferous open-marine deposits with clay-rich caps (UER), 2) fine-grained stromatolite-bearing, restricted shallow-marine deposits (uppermost UER), 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). In spite of the lack of any evidence of significant burial, the diagenesis of these rocks is complex. The UER and the Rus carbonates are almost completely dolomitized. The Dammam in central Qatar is only partially dolomitized, with the amount of dolomitization varying both laterally and vertically. 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.

Aquifer quality in these rocks is a function on of both depositional and diagenetic features. 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 UER). Finer and clay-bearing dolomitized rocks have storage but lower permeability (upper UER). 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). Sediment tentatively interpreted as cave fill has been observed in this core at depths of >60m below the surface.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018