Abstract

Several petrological methods (core, cutting and thin section examination, X-ray diffraction, fluid inclusion geothermometry and electron microprobe analyses) were applied to core and cuttings samples from wells ULB-01 (vertical) and ULB-02 (deviated). Data from the analyses were used to interpret subsurface conditions in the Ulumbu geothermal field.

In both wells the top 800 m consist of volcanic breccias with interbedded andesite lavas and tuffs. From 800 m to 1887 m, the lithologies encountered are limestone and volcaniclastic sediments, with intercalations of basaltic lava and tuff.

Permeable zones are recognized from drilling records and the presence of quartz, albite and adularia. These zones are present in well ULB-01 at 650-800 m, 900-1000 m and at 1500 m, and in well ULB-02 at 610 m. The temperatures predicted from the occurrence of minerals and from the fluid inclusion study (heating) are similar to the measured borehole temperatures. In Ulumbu, epidote occurs at 230 °C, wairakite is stable at 210-220 °C, laumontite and prehnite are found at 195 °C and 220 °C, respectively.

The deep geothermal fluid is possibly a neutral pH chloride water, as indicated by the stability of chlorite, albite, adularia, wairakite, epidote, illite and smectite at depth. The presence of epidote, wairakite and prehnite suggests that the fluid has low dissolved CO₂ content. So, although the reservoir contains limestone, no major calcite scaling problems are expected during the production stage.