Water-rock interaction and hydrothermal mineral equilibria in the Tendaho geothermal system

The Tendaho geothermal system occurs within a NW-SE-trending rift basin fil led with Quaternary volcanics (mainly basalts) and fluvio-lacustrine sedime nts. Three deep (TD-1, TD-2 and TD-3) and one shallow (TD-4) geothermal wel ls have been drilled. The waters of productive wells TD-1, TD-2 and TD-3 ar e typical Na-CI geothermal waters with reservoir temperature ranging betwee n 220 and 270 degrees C. Chemical analysis of core samples (altered basalts ) shows increased Ca, Fe, Mg, Al content towing to the dissolution of plagi oclase and femics and the precipitation of wairakite, laumontite, epidote, garnet, calcite and clay minerals) and decreased Na, K, Si and Ti towing mo stly to the dissolution of glass matrix). Petrographic study of cuttings an d core samples shows: (1) evidence of an early stage of calcite, zeolite (w airakite or laumontite) and quartz crystallization, while calcite underwent different stages of dissolution/precipitation, possibly due to abrupt chan ges in pH and CO2 partial pressure; (2) that epidote, garnet, prehnite, pyr oxene and amphibole crystallization occurred after wairakite or laumontite. Chlorite is the main layered silicate in the basaltic rocks in wells TD-I and TD-2, temperatures beyond the stability of smectite and chlorite/smecti te (C/S) interlayers must have been reached. The smectites and C/S interlay ers in association with chlorite in well TD-3 indicate that this well has h ad a more complex thermal history, with variable temperatures. The smectite s occur at temperatures above 120 degrees C, which is considered to be the threshold for the transformation of smectite to illite. No evidence of dise quilibrium conditions of smectites has been found at reservoir temperatures currently present at Tendaho. Fluid inclusion data indicate heating in the well TD-1, and thermal conditions similar to the ones measured in the deep est part of well TD-2, while the uppermost part of this latter has undergon e cooling. Intense cooling has affected well TD-3, drilled far from the upf low zone of the field, probably in an area characterized by self-sealing, c ooling and very low permeability. (C) 1998 Elsevier Science B.V. All rights reserved.