Title: Borehole geology of Well HLS-EX Hululais geothermal field, Sumatra Island, Indonesia

Abstract

This study characterizes the stratigraphy and hydrothermal alteration of the 3,280 m deep
HLS-EX well, in the Hululais geothermal field, Sumatra Island, Indonesia. Rock cuttings
and cores were examined through stereo- and optical microscope, clay minerals were
determined by X-ray diffractometry (XRD) and whole-rock compositions were studied by
inductively coupled plasma-optical emission spectrometry (ICP-OES) and loss on ignition
(LOI). These obtained data were combined with the results of drilling logs.
Eight rock types were distinguished in the borehole, including altered andesitic and dacitic
lavas, intrusions of basaltic diorite, diorite, and granodiorite, andesitic and dacitic tuff, and
volcanic breccia. Andesitic rocks dominate (~70%) in the borehole. Magmatic
differentiation process was observed in the weakly altered rocks. The negative correlation
between MgO and Na2O, K2O, SiO2, and Zr indicates the incompatibility of these
components in the andesitic magma from which most of the samples are derived. On the
other hand, CaO decreases with MgO due to the crystal fractionation of plagioclase, FeO
concentration decreases during the crystallization of Fe-Mg silicates (pyroxene and
amphibole), and TiO2 decreases due to the crystallization of Fe-Ti oxides.
The alteration intensity observed in the samples shows a positive correlation with the loss
on ignition measurements, suggesting most of the samples are highly altered, especially in
the pyroclastic tuff and volcanic breccia. Hydrothermal alteration zones are categorized as
the smectite zone, epidote-illite zone, mixed-layer clay zone, and secondary biotite zone.
Although it is slightly lower than the geothermometry profile, the measured borehole
temperature trend is similar, representing a high-temperature geothermal well. Hydrothermal
alteration processes have resulted in the geochemical modification of the magmatic rocks.
Major and trace components were intensively mobilized during the alteration processes, and
MgO was found to be the least mobile major component. The following processes were
observed: silicification that increases the SiO2, illitization results in gain of K2O,
chloritization, pyritization, and oxidation, which causes an increase in total iron, enrichment
of Na2O during albitization in some samples, whereas, in the other samples, kaolinization of
plagioclase decreased the Na2O concentration. Plagioclase breakdown leads to the
mobilization of CaO and the leaching of calcium in the hydrothermally altered rocks.