| Abstract |
A shallow acid assemblage of sulfur and kaolinite 2 alunite, dickite 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. This shallow acid zone formed at temperatures 5 160oC. Below 0 mRL down to-600 mRL, a deep acid zone of diaspore and pyrophyllite f alunite and dickite 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 which are consistent with measured stable temperatures. The dominance of mid alteration in the shallow portions of Cawayan indicates its surcial origin from the oxidation of H2S. This fluid descended through highly permeable structures to levels as deep as -1000 mRL. 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. |