The depositional history of near-surface Qatar aquifer rocks and its impact on matrix flow and storage properties

Abstract

Carbonates and evaporites of Paleogene age form the shallow-aquifer rocks mantling most of Qatar, including the Paleocene and Lower Eocene Umm er Radhuma Formation and the Lower to Middle Eocene Rus and Dammam Formations. A core-based study was carried out to improve general understanding of the stratigraphic controls on aquifer matrix properties in Qatar. A cumulative total of 377 m of 10-cm-diameter core was recovered from three boreholes in central and northern Qatar, drilled to depths of greater than 120 m. Sedimentological attributes of these rocks were investigated through core and thin-section description, X-ray diffraction-based mineralogical assessment, as well as whole-rock stable isotopic analysis and integrated with interpretation of gamma-ray logs. Stratigraphic correlation of the penetrated intervals was then undertaken using sequence stratigraphic concepts and isotope stratigraphy (δ 13 C trends) in the context of recently published regional paleomaps and structural studies. In the area of Qatar, the Umm er Radhuma Formation and the overlying Traina Member of the Rus Formation were deposited in marine settings of two different basins. These basins, which extended to the south and north of Qatar, respectively, are interpreted to have been separated by a topographic high, the location of which was controlled by the presence of high-angle normal faults. The southern basin was more restricted and was the site of extensive evaporite and clay-rich siliciclastic deposition during early stages of Rus Formation. Similar evaporites and fine siliciclastic deposits are not observed in time-equivalent strata of the northern basin. During subsequent deposition of the Al Khor Member of the Rus Formation, as well as the Dammam Formation, the basins appear to have been interconnected, and fine-grained siliciclastic deposits are interbedded with, but subordinate to, carbonate strata across Qatar. Most rocks recovered for this study are dolomitic, and dolomitic rocks free of other mineral phases tend to have significant porosity (20-50%) and permeability (10-1000 mD). Decreased connectivity, flow, and storage capacity are caused by (1) the presence of gypsum beds and nodules (only southern Qatar, upper Umm er Radhuma Formation, and Rus Formation), (2) the presence of pore-occluding clays (typically palygorskite) to varying degrees in all formations, and (3) the occurrence of diagenetic calcites, most commonly in the Dammam Formation. Aquifer quality of the near-surface rocks of Qatar is in large part a function of their depositional history and is to a degree predictable using reconstruction of basin architecture, as well as sequence stratigraphic concepts.

Early and pervasive dolomitization by near‐normal marine fluids: New lessons from an Eocene evaporative setting in Qatar

“…In an assessment of the fauna present in the UER in south-western Qatar, including both planktonic and large benthic foraminifera, Boukhary et al (2011) interpreted the UER to reflect deposition in an open marine to sheltered lagoon environment. Rivers et al (2019a) analyzed the same research cores utilized in the present study ( Fig. 2) and came to similar conclusions with regard to depositional…”

“…The southern margin of the Tethys region was covered by shallow carbonate ramps during much of the late Palaeocene and early Eocene (Sharland et al, 2001), resulting in subtidal to peritidal carbonate sequences with intercalated clays and minor evaporites that cover the Arabian Peninsula (Powers et al, 1966;Sharland et al, 2001). The UER is composed primarily of metrescale peritidal carbonate and evaporite sequences towards the eastern borders of Saudi Arabia and Kuwait (Saller et al, 2014), with more subtidal to open marine carbonate deposits in Qatar (Rivers et al, 2019a), where it lies exclusively in the subsurface and is up to 370 m thick (Powers et al, 1966;Cavelier, 1970;Eccleston et al, 1981). The UER overlies the lithologically similar Cretaceous Simsima Formation with a boundary that is biostratigraphically unconformable (Cavelier, 1970;Hewaidy, 1994).…”

“…The UER overlies the lithologically similar Cretaceous Simsima Formation with a boundary that is biostratigraphically unconformable (Cavelier, 1970;Hewaidy, 1994). The UER represents a large-scale regression, and is unconformably overlain by the lower Eocene Rus Formation, which is characterized by metrescale cycles of dolomite, gypsum and clay (Cavelier, 1970;Eccleston et al, 1981;Al-Saad, 2003;Rivers et al, 2019a). Although micropalaeontological studies are inconclusive, the lower UER is interpreted to be upper Palaeocene in age, whereas the uppermost intervals are interpreted to be lower Eocene (Powers et al, 1966;Hasson, 1985;Hewaidy, 1994;Boukhary et al, 2011;Pollitt et al, 2012).…”

“…These faults effectively separated Qatar into a south-western proximal and north-eastern distal basin, relative to the Arabian Plate, at the end of the Palaeocene. Bifurcation of the basin can explain the higher abundance of siliciclastic and evaporitic material in the south compared to the relatively pure carbonate packages in the north (Rivers et al, 2019a).…”

“…Recent work indicates that the UER in Qatar is primarily composed of subtidal deposits, with a general shallowing-upward trend towards the UER/Rus boundary (Boukhary et al, 2011;Rivers et al, 2019a). In an assessment of the fauna present in the UER in south-western Qatar, including both planktonic and large benthic foraminifera, Boukhary et al (2011) interpreted the UER to reflect deposition in an open marine to sheltered lagoon environment.…”