Subsurface geology and hydrothermal alteration of Cachaços-Lombadas sector, Ribeira Grande geothermal field, São Miguel island, Azores

Abstract

The present report focuses on the high temperature geothermal area of CachaçosLombadas in the 245°C two-phase liquid dominated Ribeira Grande geothermal system in São Miguel Island. Subsurface geology and hydrothermal alteration are studied, using data from four of the eight existing wells. Emphasis is laid on the determination of the lithostratigraphy and identification of possible correlations between the geological units and the permeability encountered in the wells, including the analysis of tectonic structures inferred in the area. The present work reveals that pyroclastic rocks dominate the upper part of the subsurface geology, locally interrupted by trachyte lava flows and, at deeper levels, basalt lavas are the predominant lithological unit. Furthermore, the results show that north-northwestern trending structures probably play a very important role in the distribution of permeability in the Cachaços-Lombadas area.

Temperature distribution in the area was also investigated and the results are consistent with the main conclusions from previous studies, showing a clear boundary of the geothermal system in the southern part of the area and suggesting that the location of the upflow zone of the geothermal system is likely to be in the east to northeastern parts of the Cachaços-Lombadas area. The alteration mineralogy allowed determining the alteration zoning pattern in the Cachaços-Lombadas area, identifying two zones of hydrothermal alteration, smectite zone and chlorite zone, beneath a shallow zone of relatively unaltered rocks. The hydrothermal alteration investigations were complemented with the study of fluid inclusions, allowing determination of the thermal history of the geothermal system. The results reveal that the geothermal system was hotter at some point in the past. However, since cooling has occurred, the system has been in equilibrium, with evidence of some fluctuations of the fluid levels within the geothermal reservoir.