Clay Alteration Study from Wells of Tompaso Geothermal Field, North Sulawesi, Indonesia

Abstract

Tompaso geothermal field is situated on the northern arm of Sulawesi Island, Indonesia. It is one of PT. Pertamina Geothermal Energy (PGE) project in the eastern part of Indonesia archipelago. Petrology analysis from cuttings (general lithology description, XRD & petrography data) and borehole data (rate of penetration, circulation losses, and PT measurements) of wells LHD-27 and LHD-30 from Tompaso Geothermal field have been used for this study. It is mainly a detailed study of clay mineralogy. Clay minerals are known to be sensitive to temperature and chemical changes. Therefore clays are very useful for geothermal exploration as a geothermometer as well as assessing the subsurface fluid pH. The clay mineralogy occuring in the Tompaso geothermal system consists of: kaolinite, smectite, chlorite-smectite (corrensite) from shallow to intermediate level (T: 50-130oC); illite at intermediate level as a transition zone (T: 180-250oC); chlorite is predominant in deeper levels and acts as a clay marker for high temperature ( more than 250oC) and permeability zone when it is associated with epidote and secondary quartz. Most clay minerals forming temperatures are in agreement with the current stable borehole temperatures. The degree of crystallinity of these clays increases with depth as shown by their decreases in Kubler indices. It also shows that the crystallinity of clay is temperature dependent.