Mineral Alteration in Acid and Neutral-pH Hydrothermal Reservoir: The Case of Mahanagdong Geothermal Field, Leyte

Abstract

The silicate and surfide mineral equilibria for the Mahanagdong Geothermal Field production wells were investigated using mineral activity diagrams. Evaluation of silicates focused on K-, Na- and Ca-bearing minerals which are the most widely distributed alteration minerals in the reservoir.

The stabiliy of these minerals depends on three factors, namely, temperature, concentration of the cation species, and pH of the fluids. Among the silicate minerals, the following are in equilibrium with the Mahanagdong jluids: K-mica (illite), wairakite, kaolinite, epidote and K-feldspar. Geologic logging of the cuttings has confirmed the presence of these minerals in the open hole section of both neutral and acid wells in Mahanagdong. It is aident, therefore, that no silicate minerals can be used to distinguish the acidfiom the neutral-pH fluids in this field. The stability or activity of the surfide minerals are governed by two factors. These are the pH and the redox state (expressed in terms of 0, fugacity) of the fluids. Surfide minerals in equilibrium with both neutral and acidfluidrr are pyrite, chalcopyrite and bornite. Among the dissolved species of suvur, H a is likewise in equilibrium with both fluid types. Together these iridicate that the fluids in Mahanagdong have already evolved from a magmatic to a hydrothermal system.

Aside from silicate and sulfide mineral equilibria, calcite and anhydrite saturation for each Mahanagdong well were also evaluated. Wells which showed anhydrite supersaturation are the acid wells situated in the northern part of the field. This condition is attributed to their inherent high SO, concentration. Wells which exhibited calcite saturation and supersaturation at reservoir condition are the neutral-pH wells in the outfrow zone of the sector.