Natural Reservoir Evolution in the Tolhuaca Geothermal Field, Southern Chile

Abstract

The Tolhuaca geothermal reservoir formed as a liquid-dominated hydrothermal system in a topographically-elevated young volcanic rift zone on the flank of Tolhuaca stratovolcano. Early shallow upflow established temperatures near boiling in a roughly horizontal liquid reservoir zone at 100 to 200 m depth. The shallow high temperatures eventually supported hydrothermal brecciation – boiling events that penetrated into a deeper reservoir to at least 950 m depth. Boiling was followed by steam-heated water invasion that cooled the reservoir. The latest stage has involved reservoir reheating and boiling to create a system with deep hot brine below a separate shallow steam zone. An upflow conduit that links and supports these two zones has been interpreted based on the presence of a resistive zone related to relatively high temperature mineralization. Well temperatures nearby suggest that the upflow is currently steam-dominated down to 600 m depth. The reservoir zones are overlain by steam-heated water. The top of the shallow steam zone appears to be controlled by quenching by steam-heated water. This model is supported by surface mapping, fluid geochemistry, geophysics, core and cuttings mineralogy, rock isotopes, fluid inclusions, local hydrology, well temperatures and well testing. These data provide an unusually detailed model of reservoir evolution in a field with just two slim wells and a deep well in progress.