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 (δ13C 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.
Highlights
The state of Qatar forms a peninsula protruding northward from Saudi Arabia into the Arabian Gulf, a shallow foreland basin of the Zagros Mountains (Fig. 1)
Integrated descriptions for cores from boreholes 1–3 are shown in Figs. 5, 6, and 7, respectively, with accompanying whole-rock-isotope and mineralogical data
Fossils could only be identified in dolomitized rocks where crystal sizes were small (< 20 μm) and where dolomite textures mimicked that of the original limestone
Summary
The state of Qatar forms a peninsula protruding northward from Saudi Arabia into the Arabian Gulf, a shallow foreland basin of the Zagros Mountains (Fig. 1). 2005), biostratigraphy (Boukhary et al 2011), and structure (Rivers and Larson 2018) Building on these studies, an effort was undertaken to update the understanding of the near-surface stratigraphy of Qatar in light of both the Rivers and Larson (2018) structural study, as well as recently published regional maps of Eocene depositional environments (Tai et al 2016). Detailed rock textural classification and fossil identification, as well as measurements of mineralogical composition by X-ray diffraction (XRD), were carried out on these cores. These datasets were used for classification of depositional facies, as well as lithostratigraphic interpretation. The purpose of this study was to improve the general understanding of Qatar aquifer rocks and the depositional controls on aquifer quality with respect to rock matrix flow and storage capacity
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