| Abstract |
The state of gas equilibria in the production wells and the natural thermal areas of Tongonan, Mahanagdong and Alto Peak geothermal fields in Leyte was analyzed. Three approaches were applied: the first one involves equilibria of H2 and Ar in gases (Giggenbach, 1980); the second one involves the breakdown of ammonia; and the third one is the breakdown of methane or the Fischer-Tropsch reaction (D 'Amore et al., 1993). The results of the studies showed that the parent fluids in Tongonan geothermal field consist mostly of liquid phase with reservoir temperature of 310-320░C. The thermal area of Kapakuhan also showed similar temperature indicating that the gases in the fumaroles were directly derived from the first boiling of the parent fluids. The fluids migrating towards the outflow in Malitbog sector have lost 5.0% of the original steam and have re-equilibrated at temperature of 240-270░C. In Bao-Banat-i thermal areas, which mark the outflow of the reservoir, the fluids were derived from liquid reservoir with temperature of about 180░C. The present production wells of Tongonan-1 showed two gas trends. Those wells lying proximate to the upflow area in Upper Mahiao have gained steam which was possibly induced by pressure drawdown. On the other hand, the production wells lying close to the reinjection sink showed steam loss of about 2.0 to 5.0% due to mixing of degassed reinjected fluids. In Mahanagdong, the parent fluids have temperature of 31o"C, which is reflected in the H2/Ar geothermometer of Mahanagdong fumarole. The two-phase fluids in the upflow zone have gained vapors by as much as 5.0%. The fluids in the outflow sector have lost 7.0% of the original vapor and reequilibrated in reservoir temperature of 240-260oC. The acid wells in the north also showed vapor gain by as much as 50% which is explained by incomplete equilibration of the original magmatic gases with the reservoir rocks. On the other hand, the gain of 1.0% vapor in a cold well in the west can be attributed to the inflow of cooler steam condensates. In Alto Peak, the Danglog solfatara indicates temperature of greater than 370░C or that of a magmatic environment. All of the wells showed vapor gain @om 1.0 to 50.0% attesting to the vapor-rich nature of the Alto Peak fluids. This can be an inherent characteristic of a magmatic System presently evolving into a hydrothermal Jystem. |