Characterizing deep groundwater using evidence from oil and gas exploration wells in the Lower Kutai Basin of Indonesia

Groundwater at depths exceeding 500 m can be an important source of freshwater. However, the characteristics of deep groundwater in many regions of the world, including Indonesia’s sedimentary basins, remain vaguely defined. This study investigates the pressure regimes, hydraulic head distributions, salinity, and hydrochemical facies of deep groundwater using available evidence from oil and gas exploration wells in the Lower Kutai Basin of Indonesia. Pressure measurements and wireline log data reveal three pressure regimes within the basin: hydrostatic, overpressure, and underpressure. The top of the overpressure varies, from close to the surface onshore to depths of ~4.5 and ~3.8 km in the Mahakam Delta and offshore, respectively. Computed hydraulic heads at the top of the overpressure range from ~191 m above sea level onshore to ~71 m below sea level offshore, and are indicative of regional groundwater flow. The observed salinity of deep groundwater within the basin indicates predominantly brackish or saline conditions. Fresh (total dissolved solids < 1 g/L) groundwater to a depth of ~2 km is found at a small minority of wells onshore and in the delta; no fresh groundwater is found offshore. Four hydrochemical facies are observed: Na⁺/Cl^–, Ca²⁺/Cl^–, Na⁺/HCO₃^–, and Na⁺–Ca²⁺/HCO₃^–. This study indicates that deep fresh groundwater in the Lower Kutai Basin is of localized occurrence. Recharge from meteoric water may replenish deep fresh groundwater within the hydrostatic zone and sustain water supplies, whether brackish or fresh. Water produced from clay diagenesis is also cited as a possible process of freshening deep groundwater.