Hydrochemical and isotopic (δ18O, δ2H, 87Sr/86Sr, δ37Cl and δ81Br) evidence for the origin of saline formation water in a gas reservoir

- Origin of produced formation waters (PFW) is evaporated seawater.
- Comprehensive hydrochemistry and isotope methods are used.
- Intergranular saturated brine of the gas reservoir is the salinity source of PFWs
- Li/Cl vs. Br/Cl variation diagram is proposed to distinguish waters with different origins.

The Permo-Triassic Kangan gasfield in southern Iran is composed of an aquifer, the Kangan Aquifer (KA), and an overlying gas reservoir. It is located in the Kangan and Dalan Formations and consists predominantly of limestone and dolomite. The Kangan gasfield is exploited from 36 wells at depths from 2300 to 2860 m. The quality of the produced formation waters (PFW) varies from fresh to saline, with salinities of up to 60,000 mg/L. The present study aims to identify the origin of the salinity of the PFWs. The concentration of major ions, Li, Sr and Br, as well as δ18O, δD, 87Sr/86Sr, δ37Cl and δ81Br isotope compositions of the KA water and PFW samples, a fresh and a brine spring, and the Persian Gulf water were all measured. The suite of isotopic (δ37Cl and δ81Br) and hydrochemical data (Li, Cl and Br concentrations and CF index) in this study indicates that the salinity origin of the PFWs is evaporated seawater, which could be either the KA water or the intergranular saturated brine in the Kangan Gas Reservoir (KGR), both derived from Permo-Triassic evaporated seawater. δ18O and 87Sr/86Sr data exclude the KA water as the source of the saline PFW, leaving the intergranular brine of the KGR as the most likely source although further isotopic and geochemical tests are needed to verify this. This work also proposes, for the first time, the use of a Li/Cl versus Br/Cl diagram to distinguish between evaporate formation water, water modified by halite dissolution and fresh water. It is recommended to apply these comprehensive methods to deep brine aquifers, especially related to gasfields.