The fluid-rock interaction history of the Tongonan geothermal field, Leyte, Philippines

Abstract

The Tongonan geothermal field, west Leyte, has been penetrated by 52 wells as deep as 2.6 km. The field presently produces an average of 60 MWe from the Mahiao (western) and Sambaloran (central) sectors of the field. Exploitation of the Malitbog (eastern) sector is under consideration. The well pressure profiles show that upwelling of geothermal fluids occurs in the Sambaloran sector. The measured temperature profiles demonstrate that the regional ground-water flow is from the northeast. Cores and cuttings recovered during drilling have allowed a detailed petrographic, geochemical and isotopic study of the reservoir rocks which has led to the development of a model for the water-rock interaction history of the field. Chronological development of the field's evolution is believed to be as follows. The extensive tholeiitic basalt and andesite volcanism and plutonism which occurred along a major structural feature in the Philippines, the Philippine Fault, in the Tongonan region in the mid- to late-Tertiary produced the Bao Volcanic Formation and the Mahiao Plutonic Complex. The Bao Volcanic Formation is composed of basalt and andesite lavas and volcaniclastic sediments interbedded with up to three thin horizons of shales and limestones containing Miocene fossils. Chemical and mineralogical evidence indicates that the Bao Volcanic Formation is the same composition as the gabbros and monzodiorites in the Mahiao Plutonic Complex. When the magmas first intruded, they contact metamorphosed the overlying volcanics and plutonic basement rocks. The evidence for contact metamorphism is provided by electron microprobe analyses of biotite and actinolite in diorite cores from well 408; these have similar TiOC2 contents (3-5%) to the biotites, and AI2O3 contents (3-5%) to the actinolites present in contact metamorphosed rocks elsewhere. The geothermal system, which is inferred to have developed in the Miocene, was a low temperature (200-250ºC), alkali chloride fluid type. This conclusion is based on (1) geothermometry of chlorites, epidotes, and illites, which formed at least 100ºC below present-day temperatures; and (2) the presence of these minerals in cores from well MN1 located 2 km outside the present field boundary. In the Pliocene, the Philippine fault reactivated as a result of a westerly change in the direction of the Pacific plate. Uplift occurred at Tongonan while block faulting in the Philippine fault zone south in the Bao Valley coincided with intrusion of calc-alkaline granodiorite into the Malitbog sector. These magmas fractionated approximately equal proportions (30-40%) of plagioclase and hornblende, with lesser amounts of magnetite and apatite. Granodiorite magma did not reach the surface, but hornblende-dacite dykes in the Bao Volcanic Formation are subsurface manifestations of this magma. As the granodiorite intruded and crystallised, large (~2 mole%) amounts of gas (mainly CO2, but also H2S) in geothermal steam dissociated to form a corrosive acidic condensate which intensely altered the host rocks and deposited kaolinite, alunite, pyrophyllite and diaspore to depths of about 500 m below sea level. Evidence for past gassy boiling fluids in the western (Malitbog) sector is provided by the extensive relict acid alteration there, the absence of zeolites, and fluid inclusion observations (coexisting liquid- and vapour-rich inclusions and positive internal pressures identified during crushing) . Epidote, illite, chlorite, quartz, anhydrite and garnet deposited from boiling, neutral pH, alkali-chloride, geothermal waters in the deep reservoir. These exchanged oxygen isotopes with the reservoir rock. The salinity of this fluid has not changed significantly because the freezing point depression measurements made on fluids trapped in fluid inclusions (mainly in anhydrite) are approximately constant (~2 weight % NaCl equiv.) and the same as the present-day deep fluid concentration. Comparison of homogenisation temperatures on fluid inclusions with measured temperatures suggests that the entire field has cooled by an average of about 30ºC since the Pliocene. In the Quaternary, calc-alkaline granites intruded the basement rocks in the Mahiao sector. The gas released from this magma may account for the higher (=0.3 mole% CO2) gas concentrations in the Mahiao relative to the Malitbog fluids. Steam condensation also caused acid alteration in the Mahiao sector but this is less extensive here than in the Malitbog. The deep neutral-pH fluids deposited quartz and lesser albite, adularia, wairakite, apatite, fluorite, talc, anhydrite, calcite, illite, chlorite, prehnite, epidote, pyrite, and base metal sulphides, and exchanged oxygen isotopes with the rock probably during the Pleistocene. The present-day deep fluid discharging from wells has deuterium and oxygen isotope compositions of -35 %o and -1 %o respectively, and is not in isotopic equilibrium with the reservoir rock (+3 %o for O2) at 330º in the centre of the field. Na (present as albite) enrichment of the basement rock occurred at high temperatures (>250ºC) The chemistry of Holocene fluids (eg. 10,000 mg/kg) was much the same as the chemistry of the Pleistocene except that the CO2 content of the gas discharging through the Tongonan geothermal system is lower (<0.4 mole%) now.