Exploration of medium - and low - enthalpy geothermal resources in the sub - Andean area for the sustainable development of the cities of Salta province (northwestern Argentina)

Abstract

Exploration methodologies commonly applied in oil and gas research can provide important information also in the perspective of exploitation and reservoir management of medium- and high-enthalpy geothermal systems. The purpose of this study has been to evaluate the possibility of exploitation of the Rosario de La Frontera geothermal system through a multidisciplinary approach based on the integration of different methodologies: structural analyses at the outcrop scale, a reservoir fracture modelling performed with the software MoveTM (Midland Valley Exploration Ltd.), and paleo-thermal analyses (organic matter optical analysis - vitrinite reflectance: Ro%, clay mineralogy by means of X-ray analyses, fluid inclusions analyses on veins and mono-dimensional thermal and burial modelling with the aid of Basin Mod 1-D software). The geothermal system of Rosario de La Frontera belongs to the Sierra de La Candelaria anticline, one of the positively inverted structures cropping out in the Sub-Andean foreland of the northwestern Argentina. Despite of the occurrence of different thermal sites along the frontal structures of the Andean retro-wedge, the high temperatures of the thermal fluids of this system makes it as the most promising site for potential exploitation. Such a development could contribute to improve the sustainable growth of the cities of the Salta province. This active geothermal system is characterized by several hot springs that mainly occur near the town of Rosario de La Frontera, at the northern edge of the regional Sierra de La Candelaria anticline, with temperatures ranging between 24° and 90°C. Fractured sandstones of the Cretaceous syn-rift deposits (Pirgua Subgroup) provide the reservoir of the geothermal system. Differently, the low permeable post-rift and the syn-orogenic deposits (respectively, Balbuena and Santa Bárbara subgroup, and Metán subgroup) mainly act as cap-rock. These deposits are deformed in the N-S trending hanging wall anticline (Sierra de La Candelaria) and dissected by subsequent strike-slip and normal fault planes. The anticline and the associated structures (faults and fractures) represent the structural context of the geothermal system. The current study employs a multidisciplinary approach in order to assess the quality of the reservoir and cap-rock of the geothermal system. Special regard is devoted to explore how faults and fractures affect fluids circulation at depth, and control their natural upwelling to the surface. The adopted methodological approach provide new data on the evolution of this Sub-Andean foreland structure, previously poorly investigated, constraining also the burial and thermal history experienced by the studied sedimentary succession from Barremian to Quaternary ages. Moreover, the integration with published hydrogeological, geochemical and geophysical (audiomagnetotellurics) data (CUIA project 2010 – International University Cooperation Italy-Argentina) addressed to the elaboration of a conceptual model of thermal fluids circulation, identifying the best area for drilling-purposes in perspective of undirect heat uses (electricity production). Structural and stratigraphic analyses conducted at the outcrop scale allowed to reconstruct the tectonic evolution of the Sierra de La Candelaria anticline (CHAPTER 3) highlighting the type and intensity of the deformation that affected the anticline in the different structural positions. Two main deformation phases were identified: an extension happened during the Cretaceous rifting and a positive inversion and folding occurred since Upper Miocene times. Accordingly, the regional anticline formed at the hanging wall of a N-S Cretaceous normal fault. This high-angle normal fault was reactivated as an oblique reverse fault since it had an oblique trend with respect to the WNW-ESE Andean shortening direction. The event of positive inversion can be divided into an early, intermediate and mature sub-stages of deformation. At the outcrop scale, tectonic stylolites and shears fractures mainly testify the early develop of the regional anticline. Progressively, in the intermediate stage, an oblique NNW-SSE fault plane enhanced the development of the minor Termas and Balboa anticlines. In correspondence of these anticlines, fault planes, with the associated Riedel shears, shear fractures, calcite- and gypsum-filling veins and deformation bands record both the strike-slip kinematics and the folding process related to inversion. E-W and ENE-WSW normal faults and extensional fractures are mainly associated to the mature stage of the deformation, recording the final stage of the folding process. The performed three-dimensional model of the anticline together with the reservoir fracture modelling (CHAPTER 4) addressed to reservoir evaluation: i.e. lateral extent, thickness and volume, tectonic features, porosity/permeability patterns, possible reservoir compartments. The results show a reservoir volume of 53 Km3 with a mean thickness of 450 m, confined to the east by the main high angle thrust fault that borders the anticline on the eastern side and to the west by a continuous, almost N-S oriented, lineament well visible on remote sensing images. To the north, it is delimited by a normal fault plane trending E-W and dipping to the north. Whereas, to the south, the reservoir is confined by its outcrops and by a NE-SW strike-slip fault occurring in the Ceibal area. In the northern sector of the anticline, at the northern edge of Termas and Balboa anticlines, the reservoir deposits reach the maximum depths of ~2,100 m and ~2,400 m, respectively. Focusing on this northern portion of the regional anticline considered as the most interesting for different heat uses since the thermal fluids reach the highest temperatures, a reservoir volume of 28 Km3 with a fracture portion of 0.001 Km3 were computed. Furthermore, 1.89*1017 J of extractable geothermal heat was calculated from the reservoir portion where temperature exceeds 60°C, at least for direct heat uses. The reservoir fracture modelling confirm the interest for this area because of the high values of the computed secondary permeability and porosity (4.2E-04 and until ~20mD, respectively) as a result of the intensive brittle deformation that affect this portion of the anticline. The modelling also shows the highest secondary permeability values in the direction parallel to the strike of the main NNW-SSE fault plane. These results allow considering this fault plane as the main structure controlling the migration path of hot fluids at depth and their upwelling from the reservoir to the surface, as also demonstrated by associated calcite-filled veins. At the same time, the NNW-SSE fault plane may act as a barrier to fluid flow, producing a reservoir compartimentalization. Paleo-thermal analyses conducted on the cap-rock of the geothermal system (CHAPTER 5), well exposed in this northern portion of the regional anticline, suggest that it acts as a good insulator. Clay mineralogy analyses indicate a low thermal maturity of these deposits with low amounts of illite in I-S mixed layers, comparable with the thermal maturity detected on the same deposits exposed along the western limb of the anticline, far away from the influence of the hot springs. Thermal and burial models reconstructed in the two investigated areas of the anticline, far away and close to the area of the hot springs, demonstrate that this thermal maturity represents the result of the burial history experienced by the cap-rock under a geothermal gradient of about 40 °C/Km stabilised after the rifting event. Furthermore, as demonstrated by vitrinite reflectance data computed for the post-rift Yacoraite Formation (0.6-0.7%), the rift succession experienced a thermal evolution compatible with late diagenetic conditions in the early stages of hydrocarbon generation during Upper Miocene times. A high degree of the cap-rock thermal alteration (80% of illite in I-S mixed layers) occurs in correspondence of its most fractured portions. The R3 structures do not fit the performed purely conductive 1D thermal models. These results together with fluid inclusion analyses, that indicate an entrapment temperature (Th) of 115°C on the same fractured outcrops, suggest that hot fluids circulation may have played a key role in the thermal alteration of the cap rock only where it was intensely fractured. In this framework, deformation due to strike-slip kinematics rather than to folding process may have focused hot fluids and driven localized thermal alteration of the cap-rock. Coupling these results together with the recently published geochemical, hydrogeological and audiomagnetotelluric data (Barcelona et al., 2013; Invernizzi et al., 2014; Chiodi et al., 2015) (CHAPTER 6), I suggest that the most favourable site for a potential future exploitation of the thermal fluids is located along the northern buried plunge of the Balboa anticline. The high temperatures reached by the thermal fluids in this area (~120°C) suggest that an undirect heat use for electric power generation could be feasible. Furthermore, the compartimentalization ruled by the NNW-SSE fault plane may guarantee, at the same time, the sustainability of the geothermal resource exploited at present by the Hotel Termas, in correspondence of the Termas anticline.