The geochemistry of the Mahanagdong sector, Tongonan geothermal field, Philippines

Abstract

In this report water geochemistry is used to evaluate the properties of waters from five wells and nine springs in the Mahanagdong sector of the Tongonan geothermal field. The spring waters can be classified into two groups, the alkali chloride waters of Bao, Banati 1 and the wells, and the steam-heated waters of Paril and Hanipolong springs. Based on quartz and Na-K geothermometer temperatures, which are within ±25°C of measured downhole temperatures, the highest subsurface temperature of 290-325°C is predicted in well MG-3D. A slight cooling is experienced by fluids from well MG-5D, located to the southeast. The subsurface temperature estimate for this well is about 267°C. Mixing of the deep hot water with cold groundwater is suggested by the enthalpy-chloride mixing model, to give rise to the Bao and Banati 1 spring waters with 70 to 77% hot water component and subsurface temperature estimates in the range of 218-233°C. Boiling of fluids of the same origin as that of the well waters is also indicated. There is strong evidence that the upflow is near well MG-3D, which is located in the northeast part of the Mahanagdong sector. Further study, however, must be carried out to verify whether Bao and Banati 1 spring waters represent the outflow from the field.