| Abstract |
Located in the Southwestern part of Bolivia, in the Altiplano area (Andean Mountains), the Sol de Mañana/Apacheta geothermal field, well-known since the 1970’s as the Laguna Colorada geothermal project, appears as a target for geothermal development. To that end, it seems important to know as precisely as possible the shape and the properties of the geothermal reservoir components, i.e. local geology and more particularly the structural pattern, the distribution of different lithologies and their associated transfer properties. Indeed, such a knowledge will affect the choice of future drilling sites. For this, the aim of this study is focused on (1) structural analysis: lineaments on DEM, field data and scanline surveys, and (2) matrix characterization: petrological and petrophysical investigations. The geothermal field is composed of Miocene to Pleistocene volcanic rocks: andesites highlight the well-marked volcanoes and lava flows whereas ignimbrites display a more or less flat topography. At the outcrops-scale, andesites and ignimbrites display evidence of magmatic flow structures, marked in particular by the preferential orientation of minerals, vacuoles and volcanic clasts. Highly hydrothermalized rocks are visible on outcrop, notably close to active fumarolic areas, as in Sol de Mañana, but also in areas without current surface manifestations. Field and DEM investigations show brittle structures organized according to four main orientations, at different scales: NW/SE, NE/SW, N/S and E/W, leading to an interpreted local compartmentalization of the geothermal reservoir. At the local-scale, a swarm of NW-oriented normal faults with sub-vertical attitude displays a pattern of horsts and grabens system. At the outcrop-scale, numerous non-tectonic brittle structures are also identified and marked by complex curve shapes, with free apertures reaching few centimeters, average extensions of several meters long and with no specific orientation. Locally, hot fluids circulations caused the formation of secondary mineralogical assemblages corresponding to zeolite-facies conditions. In this case, the mineralogical transformations observed in the rocks show that the fluids have circulated parallel to sub-horizontal magmatic flow structures or along subvertical fracture zones. At depth in the reservoir, higher temperature conditions are confirmed by greenschist-facies mineral assemblages observed in drill core, as well in ignimbrites as in andesites. Such temperature estimate (above 300°C) highly suggests that steep faults evolve at depth into schistose structures which could form more or less horizontal draining zones for the fluids. Matrix properties are analyzed through petrological and petrophysical studies. Porosity, bulk density, permeability, thermal conductivity and P-wave velocity were measured on andesitic and ignimbritic samples, with two facies: fresh and hydrothermalized rocks. Structural, petrological and petrophysical data were used to build a 3D geological model of the Sol de Mañana/Apacheta geothermal field. In this new model, structural blocks are limited by main faults whose dips are interpreted to reduce with depth. Convective cells are interpreted to be confined within these separate blocks, resulting in them being a major driver of field geometry and boundaries. |