ORIGIN OF ACID FLUIDS IN THE CAWAYAN SECTOR BACMAN GEOTHERMAL PRODUCTION FIELD

Abstract

A shallow acid assemblage of sulfur and kaolinite +/- alunite, dlctite and pyrophyllite dominates the hydrothermal alteration in the seven wells in
Cawayan from surface down to 0 mRL. These zones range in thickness from 20 to 220 m. Thts shallow acid zone formed at temperatures < 160?C. Below 0 mRL down to -600 mRL, a deep acid zone of dlaspore and pyrophyllite +/- alunite and dlcktite appear in thin, discrete zones which coincide with measured temperatures between 180-250?C.
Neutral-pH alteration of epidote, calcite, chlorite and illite prevail at deeper levels. Actinolite veins and biotite at -1300 mRL suggest temperatures of about 280?C whtch are consistent with measured stable
temperatures

The dominance of acid alteration in the shallow portions of Cawayan indlcate its surfkial origin from the oxidation of H2S. These fluid descended
through highly permeable structures to levels as deep as -1OOOmRL. Its incursion resulted in mineral blockages and casing corrosion in CN-1. CN-2D and CN-3D. In contrast, the prevalence of neutral-pH
alteration at deeper levels of the Cawayan reservoir shows that a neutral type of fluid circulates at these depths. This brine upflows beneath CN-3D and CN-4D and flows laterally towards the CN-2D/
2RD/3RD sector.

As a consequence of this study, future production wells should be drilled in the CN-3D/4D sector to tap the upflowing, hot, neutral brine. Structures
which channel acid fluids should be avoided or intersected above the production casing shoe which should be set below -600 mRL.