Petrology Of Philippine Geothermal Systems And The Application Of Alteration Mineralogy To Their Assessment

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Journal Article: Petrology Of Philippine Geothermal Systems And The Application Of Alteration Mineralogy To Their Assessment

Abstract
Philippine geothermal systems occur in the vicinity of large Holocene calc-alkaline volcanic complexes. Wells drilled in these areas encountered multiple intrusions; the latest dikes are the subsurface manifestations of the youngest heat source. Commonly, at least two hydrothermal regimes are juxtaposed in a single area, with the latest being in equilibrium with the present temperature and chemical regime. Alteration by neutral-pH water is pervasive and abundant. A contact-metamorphic aureole also occurs near intrusives. Alteration due to acid-sulfate fluids is generally confined to permeable structures. Neutral-pH alteration is divided into four zones on the basis of key clay minerals, and two subzones are defined by calc-silicates. These are the smectite (ambient to 180°C), transition (180-230°C), illite (230-320°C) and biotite (270-340°C) zones. Subzones are defined by epidote (250-340°C) and amphibole (280-340°C). The four main zones of acid alteration are: kaolinite (ambient to 120°C), dickite ± kaolinite (120-200°C), dickite ± pyrophyllite (200-250°C), and pyrophyllite ± illite (230-320°C). Where relict high-temperature alteration reaches the surface, the area being drilled is usually the outflow zone of the present system. These hydrothermal mineral assemblages are used: (1) as geothermometers; (2) to assist in determining the depth at which the production casing will be set during drilling; (3) to estimate fluid pH and other chemical parameters; (4) to predict possible corrosion and scaling tendencies of the fluids; (5) as a measure of permeability and possible cold water influx into wells; (6) as a guide to field hydrology; and (7) to estimate roughly the thickness of the eroded overburden.

Author 
Agnes G. Reyes








Published Journal 
Journal of Volcanology and Geothermal Research, 1990





DOI 
Not Provided
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Citation

Agnes G. Reyes. 1990. Petrology Of Philippine Geothermal Systems And The Application Of Alteration Mineralogy To Their Assessment. Journal of Volcanology and Geothermal Research. (!) .