Mohamed Al-Senafy

The dissolution of fugitive gases in saline to hypersaline groundwater in arid regions is largely unclear. Groundwater samples from three major oil field aquifers in northern (NK), southeastern (SEK), and western Kuwait (WK) were investigated to determine the concentration, sources, evolution, and distribution of dissolved CH₄. Saline NK wells demonstrated the highest concentration of CH₄ at 688 µg/L, followed by WK at 419 µg/L and SEK at 102 µg/L. Elevated CH₄ concentrations were detected in Na-Cl–SO₄ and Na-Cl type groundwater. CH₄ hotspots were located in regions with increased lineament intersections and proximity to older wells near the hydrocarbon reservoirs, exhibiting an inverse correlation with conductivity and H₂S levels. Microbes in the SEK region utilize carbon substrates, leading to reduced CH₄ levels in comparison to NK and WK. δ¹³C_CH₄ values showed relatively depleted signatures for WK (− 51.2‰), while NK displayed enriched values alongside decreasing CH₄ levels, indicating thermogenic sources. Enriched δ¹³C_CH₄ values in SEK (+ 9.5‰) are associated with elevated CH₄ concentrations. The δ²H_CH₄ values range from − 479‰ to − 140‰, while δ²H_H₂O values range from + 4 to − 36‰, indicating the presence of mixed methanogenic sources across the regions. In NK, the thermogenic methane was observed with variation in isotope signatures due to complex processes. In SEK, although reducing environments prevailed, the carbon isotope fractionation is exceeded by the hydrogen isotope due to the significant impact of microbial oxidation during the diffusive migration process. In the west of Kuwait, the isotopic signatures of methane are depleted, mainly due to the oxidation of organic matter or through Sulfate-Driven Anaerobic Oxidation of Methane (SD-AOM) and carbonate dissolution. Four primary sources of CH₄ are microbial, methyl-substrates, carbonate substrates, and mixed sources, which are identified alongside two main processes: biogenic and thermogenic.

Kuwait depends on two major aquifers for the groundwater resources, the younger Kuwait aquifer and the older Dammam formation. The groundwater is brackish to saline in Kuwait aquifers with a probability of contamination adjoining the oil field region. In this context, groundwater samples were collected from three major oil fields in Kuwait (North, Southeast, and West). Sulfate and nitrate were dominant in all three regions. The geochemical signatures indicate the predominance of Na-Cl-SO₄ water type in the North (NK), Na-Ca–Cl in the Southeast (SEK) and Na-Ca–Mg–Cl-SO₄ type in the West (WK). The dolomite and gypsum saturation index dominated the sulfate and carbonate minerals, irrespective of the regions. The log pCO₂ of the groundwater samples ranged from − 1.8 to − 6.2, reflecting the mineral dissolution and microbial process, which mainly contained lower values and governed the carbonate mineral saturation. The common ion effect of Ca has influenced the saturation of carbonate and sulfate minerals. Correlation between EC, pH, and a few major ions was identified, while CH₄ and C₂H₆ were correlated with these parameters only in NK. The principal component analysis indicates that the dissolution of ions from the aquifer matrix leads to brackish groundwater, followed by silicate weathering in NK and hydrogenation in SEK and WK. Further, the study also identified the dissolution of carbonate minerals and fluoride in the NK, sulfate minerals in the SEK, and silicate minerals in the WK. Hence, the study infers that geochemical variation is due to lithology, groundwater salinity, and the physicochemical conditions prevailing in the aquifer.