Conceptual Hydrogeological Model of the Rangas Sector, Bacman Geothermal Field, Philippines

Abstract

Recent deep drilling in the Rangas sector of the Bacman Geothermal Field (BGF), Philippines provided new information to be able to delineate the extension of the existing resource block towards the southeast. The subsurface stratigraphy and hydrothermal alteration mineralogy intersected by the three new wells, together with geological structures and their associated permeabilities were evaluated to come up with an updated conceptual hydrogeological model for the Bacman 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). An older sedimentary unit beneath the GSF, characterized by quartz-rich sandstone was first encountered in one Rangas well. This was dated Late Miocene using nannoplankton fossil analysis. The shift in the clastic framework composition from dominantly andesitic in GSF to quartzose in the underlying unit indicates a change in the provenance of sediments, and possibly tectonic setting from a volcanic terrain to a plutonic-metamorphic basement. The term Rangas Sedimentary Formation (RSF), is proposed for this new rock unit. An in-depth correlation of the various rock units within the PV, GSF, and RSF were done for a better understanding of their lithostratigraphy and reservoir characteristics. Both the PV and GSF are intruded by the Cawayan Intrusive Complex, and late magmatic dikes which are deemed to provide the heat source for the Bacman geothermal system. On the other hand, hydrothermal alteration mineralogical assemblages were used as geothermometers to be able to draw their corresponding isothermal contours across the Rangas sector. The secondary biotite alteration zone at deep levels suggest temperatures of more than 300°C beneath one well in Rangas, indicating the persistence of hot fluids and the probable extension of the resource block in the southeastern sector of Bacman.