| Abstract |
Recent deep drilling in the Bacman Geothermal Field (BGF), Philippines, provided enough additional information on subsurface stratigraphy and hydrothermal alteration mineralogy to come up with an updated hydrogeological model for the field. Two major rock units underlie the Bacman Field: Late Pliocene-Late Pleistocene Pocdol Volcanics (PV) composed of andesitic lavas, tuffs, and breccias; and the underlying carbonate-volcano-clastic sequence of the Late Miocene-Early Pliocene Gayong Sedimentary Formation (GSF). Both the PV and GSF are intruded by dikes of the Cawayan Intrusive Complex which serve as the heat source for the geothermal system. A field wide correlation of the PV and GSF was done for a better understanding of their distribution and petrologic characteristics. Based on added subsurface information from drilling, hydrothermal alteration zones and their corresponding isothermal contours, based on mineral geothermometers and fluid inclusion homogenization temperatures, were drawn across the field. The secondary biotite alteration zone at deep levels indicates upflowing fluids of >300°C beneath the Palayang Bayan and Botong sectors. These fluids preferentially outflow towards the Manito lowlands to the northwest, Cawayan to the southwest, and Rangas to the southeast. |