Geothermal Plumbing System of Puhagan, Southern Negros Geothermal Field, Philippines

Abstract

Productivity in the Puhagan area of the southern Negros geothermal field comes from four sources; (a) intraformational permeability in the Southern Negros Formation consisting of volcanics and volcaniclastics, and in the Okoy Sedimentary Formation; (b) lithological contact between the two formations: (c) fracture permeability induced by intrusive bodies; and (d) faults. This productivity now supports the steam requirements of a 3 x 37.5 MWe power plant designated Palinpinion I. Faults that transect Puhagen are believed to be the major contributors to its production capability because they serve as the main trucklines of a geothermal plumbing system that conveys the hot fluids to the other sources of productivity that act as distributaries. The fault system was worked out by determining the attitude of individual faults form well intersections, using graphical solutions. Eleven faults constitute the system - four of these are major faults, and the remaining seven are splays of three of the majors. The majors are compressional while the splays are tensional in character. This model will be useful in interpreting the results of tracer and interference tests, and the siting of later replacement wells.