Struktur Geologi dan Potensi Zona Permeabel di Prospek Geothermal Montelago, Pulau Mindoro, Filipina
Exploration drilling for geothermal power plant had been planned in Montelago prospect, Mindoro Island, Philippines for purposes of electricity supply for local consumer there. Generally, geothermal exploration drilling pointed to find target as permeabel zone which could have relationship with geological structure or lithological contact. Therefore, need to conduct detailed geological and structural mapping for this drilling program taking place well and successfully.
The methods that used in conducting this study were literature review, remote sensing, field geological mapping, data processing in laboratory and studio (petrographic, X-Ray Diffraction dan stereographic analysis) as well as data integration and interpretation for concluding the goal of study.
According to field observation, acquired outcrop data of fourty one observation points. Generally, the lithologies in study area were consisted of Quartenary volcanic rocks which dominated by andesite, tuff, basalt and volcanic breccia. The study area could be divided into eight rocks unit from older to younger, they were: Bambang Tuff-Andesite, Buloc Andesite, Montelago Tuff, Pungao Volcanic Breccia-Andesite, Matabang Bundok Basalt, Lugta Volcanic Breccia, Bungao Volcanic Breccia-Andesite and Bagaygay Andesite Units. From this field geological mapping, also found nine surface manifestations that dominated by warm spring.
According to alteration identification through X-Ray Diffraction and petrographic analysis of rocks sample of twenty one observation points, found alteration types of argillic, advanced argillic and phyllic. This alteration could indicate that altered zone used to be permeable zones so that hydrothermal fluid could interact with surrounding rocks.
According to field measurement of geological structure, acquired 1802 fractures that consisted of 1472 shear fractures, 175 extension fractures and 155 joints. Shear fractures had dominant strike orientation in NNE-SSW dan WSW-ENE direction with dip >40o. Extension fractures had strike orientation in NE-SW direction with dip >40o. Joints had strike orientation scattering in almost all azimuth quadrants with dip >20o. This data indicated that principal stress in study area directing NE-SW. This tectonic stress probably controlled geological structure directing NE-SW as permeable zone.
There were nineteen major faults that determinated. These major faults were strike-slip fault and normal fault. Most of fault determinated in the study area were confirmed with lineament trend of high or low residual Bouguer anomaly. Faults belonging to ranking 1A-1C, that are fault F8, fault F13 and fault F18, and ranking 2A-2C, that are fault F10, fault F9 and fault F12 at Pungao-Lugta were important permeable faults in the study area because these faults had NE-SW orientation with field evidences of fault planes and fractures, associated with surface manifestation or alteration, and associated with lineament from satellite imagery or residual Bouguer anomaly. These faults were main drilling target in Montelago geothermal prospect.
Dr. Eng Suryantini ST. Dipl. Geothermal Tech., M.Sc