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University of Bristol (2022)

Hydrochemistry and environmental isotopes in a mixed carbonate-evaporite system aquifer, Qatar

Thirathititham, Rattana

Titre : Hydrochemistry and environmental isotopes in a mixed carbonate-evaporite system aquifer, Qatar

Auteur Thirathititham, Rattana

Université de soutenance : University of Bristol

Grade : Doctor of Philosophy (PhD) 2022

The Qatar Peninsula comprises a gentle N-S oriented pericline, up to 65 km wide, that extends some 180 km from the northern coast of Saudi Arabia northward into the Arabian Gulf. The modern climate is very arid, with high rates of evaporation and a mean annual rainfall of up to 100 mm/year. Recharge is facilitated by fractures and karst features developed on rocks that have been exposured over the last c.30 Ma. Two major hydrogeological provinces have been recognised, reflecting the distribution of facies within the Rus Formation. In the south of the country, the Rus is subdivided by a unit of gypsum and clays which is up to 50 m thick. This confines the underlying Middle to Lower Eocene Rus and Lower Eocene to Upper Paleocene Umm er Radhuma (UER) aquifers, which are fed by recharge from outcrop areas to the south and west within Saudi Arabia. To the north of Qatar, gypsum was either not deposited within the Rus formation or has since been largely dissolved, and the Rus and UER aquifers are in hydraulic continuity. Knowledge of the hydrochemistry of groundwater can provide an understanding of water-rock interaction, with implications for the distribution of reservoir quality in mixed carbonateevaporite rocks hosting oil and gas reserves in the region. To examine these diagenetic processes, rather than a traditional petrography this study uses water chemistry as an inexpensive tool to understand water-rock interaction at a range of spatial scales. A specific challenge in Qatar relates to sulfateenrichment of groundwater. This study is the first to employ sulfur-isotopes in the Rus and UER aquifers to identify diagenetic processes and identify interactions with the Eocene gypsum in Qatar. Hydrochemistry and isotopic data for groundwaters across Qatar are used to characterise water-rock interaction in the Eocene aquifers. The results suggest gypsum dissolution drives dedolomitisation in the Rus aquifer, resulting in enrichment of calcium, sulfate and magnesium in the groundwaters. H₂S oxidation is also implicated in driving dedolomitisation due to the high pCO₂ in Rus groundwaters. Stable isotope (δ¹⁸O, δ²H) analyses of groundwaters suggest water mixing between the UER aquifer and Rus aquifer in the northern area. Sulfur isotope data suggest that the source of sulfate in groundwater is gypsum within the Rus Formation. However, this is also the main source of sulfate for ii surface detention water, as it is pumped to the surface by production wells in the farming areas. Desert dust storms are another important source of sulfate in meteoric water, surface detention water and groundwater, resulting in common δ³⁴S in these water types


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