Origin of Formation Water from the Unayzah and Khuff Petroleum Reservoirs, Saudi Arabia

Chemical and isotopic fingerprinting techniques provide support for tracing the origin and evolution of formation waters taken during downhole and drill stem tests from exploration wells in Saudi Arabian oil and gas fields. Low molar Cl/Br ratios (85-93) and a δ18O reverse loop suggests an evaporative seawater origin for hypersaline water from the Upper Permian-Lower Triassic Khuff Formation. In contrast, formation water from the Upper Carboniferous-Lower Permian Unayzah Group shows a heterogeneous trend between low salinity/low δ18O freshwater and hypersaline/intermediate δ18O brine, the latter being a product of evaporated seawater. Unayzah water salinity increases linearly with depth at the rate of 77 to 98 mg/L per meter, allowing quantitative prediction of water resistivity (Rw) at specific reservoir depths. The increase in Ca and depletion of Mg and HCO3 with Unayzah depth suggests the impact of temperature-triggered ion exchange processes that alter the primary host rock and formation water composition.