Specifying recharge zones and mechanisms of the transitional geothermal field through hydrogen and oxygen isotope analyses with consideration of water-rock interaction

Recharge mechanism and water-rock interaction (WRI) along the recharge flows in a transitional geothermal system that characterize the chemical and physical properties of vapor and liquid phases, has not yet been fully understood due to complexities of fluid origin and geologic structure. To clarify the fluid origin and WRI processes in the system, this study applied hydrogen and oxygen isotope, B, Cl, and rare alkali metals analyses by considering fractionation characteristics of O-18 in rocks and liquid phases, and correction of delta H-2 and delta O-18 values in the steam composition. The investigation were done by using 20 samples collected from liquid-dominated, vapor-dominated, and two-phase wells in one of transitional reservoir fields in West Java, Indonesia. The isotopic fractionation factors calculated from a single-step steam separation clearly divided the samples into four zones: boiling parent fluid, vapor, condensate fluid, and diluted steam-heated fluid. The parent fluid that has initial Cl- concentration of 10,000 mg/kg and low water-rock ratio of W/R <= 0.2 compositions was found to be the most essential part of the recharge system. Recharge mechanism involves meteoric water from the elevation of 1200 m -1300 m a.s.l. infiltrates deeply through NE-SW regional faults, and becomes the parent fluid in the reservoir. The residual fluid after boiling of the parent fluid remains in the liquid reservoir with W/R <= 0.2, while the vapor phase forms a parasitic steam cap above the liquid reservoir with 0.2 <= W/R <= 0.7.