The layered model of the Copahue geothermal reservoir, Argentina

D. YAGUPSKY; M. AGUSTO; H. BARCELONA

Geothermal Energy

Springer Open

Año: 2019 vol. 7 p. 1 - 22

An exhaustive review of the available data related to the reservoir of the volcanohostedCopahue geothermal system is presented. Reservoir interpretation to date suggestsa vapor-dominated nature. However, the review provided reveals the limitationsof this model, regarding the transition from shallow argillic to deep propylitic alterationzones, gas geothermometers calibrated to near-neutral pH environment, productiontests, and low recharge of the feed zone, among the other features. We propose thata layered model with a shallow vapor zone above a fluid pressurized reservoir betteraccounts for the known features of the geothermal reservoir. Temperature profilesdefine the 800-m-thick upper thermal boundary that includes the impermeable claycap. An argillic alteration layer (smectite + chlorite + illite + kaolinite) with low electricresistivity response, consistent with the presence of smectite, composes the clay cap.A shallow steam cap develops immediately below the impermeable layer. This vaporzone has 200?215 °C, low seismic activity, and high electrical resistivity, and is highlightedby the isothermal segment of the well logs. The bottom part of the reservoiris subjected to controversy because of the lack of direct measurements. The deepliquid-dominated reservoir has a temperature of ~ 280 °C, develops below 1500 mdepth under near-neutral pH condition, and probably has a pervasive propylitic alteration, and wairakite + laumontite + chlorite + epidote + prehnite + actinolite is the likely mineral assemblage. Pressure collapse might trigger the development of the shallow steam cap during deflation?inflation volcanic cycles or Pleistocene-to-recent tectonic deformation. This work points out the main weak points of the previous conceptual model, providing an alternative one based on the first comprehensive compilation of data of the Copahue reservoir. Further research, including a deep exploration drilling stage, is required to establish the physicochemical reservoir state precisely and to validate the proposed layered model.