Petrological and Isotopic Characterization of the Tikolob Geothermal Reservoir, Bacon-Manito, Philippines

Abstract

Tikolob is a potential geothermal expansion area located along the main outflow, northwest of the existing production field in the Bacon-Manito (Bacman) Geothermal Project, Philippines. This study integrates new data from petrology and mineral isotopic chemistry with existing geophysical and reservoir models to provide a glimpse of the subsurface hydrogeologic conditions of the Tikolob resource in relation to the rest of the Bacman geothermal resource. Two-dimensional inverse modeling of magnetotelluric data from past survey campaigns and 3D numerical reservoir simulations provided evidence for a possible heat source beneath Mount Tikolob. Subsurface data from exploration drilling in nearby Mount Kayabon yielded an initial formation temperature estimate of ~240-260 °C based on the alteration mineral assemblage of epidote ± illite, tremolite/actinolite, and wairakite/laumontite from wellbore samples. These results were corroborated by epidote stable isotope chemistry, where calculated temperatures from D/H and 18O isotopic fractionation curves reflect similar temperature ranges of alteration mineral formation. However, evidence of retrograde alteration and mineral paragenesis indicate discordant reservoir conditions, lowering the estimated downhole temperature to ~200-220 °C. Further, isotopic data shows highly distinct signatures of epidotes in the sector (higher δD and lower δ18O) compared with epidotes from other wellbores in Bacman. Together, these datasets point to the existence of a heat source in Tikolob that has cooled down by ~60 °C and has undergone a separate physiochemical evolution from the main Bacman resource.