Towards a Comprehensive Volcanologic, Magmatic and Structural Model for Superhot Geothermal Systems, Case Study of Los Humeros Caldera Complex, Mexico
Gerardo CARRASCO-NÚÑEZ, Gianluca NORINI, Federico LUCCI, Guido GIORDANO, Javier HERNÁNDEZ, Francisco FERNÁNDEZ, Jaime CAVAZOS, Héctor CID, Pablo DÁVILA, Steven BARRIOS, Daniela PEÑA
Superhot geothermal systems (SHGS) are now important targets for geothermal exploration worldwide. Unravel how SHGS work is a challenging task that requires a solid geologic knowledge of the studied geothermal system, which includes its volcanologic, magmatic and structural evolution in order to explain how the geothermal system originated and developed through time. The geothermal field of Los Humeros volcanic complex (LHVC) is a classic example of SHGS. It is located in the central-eastern zone of Mexico, and is one of the most important geothermal fields with an installed capacity of ca. 94 MW. Several studies on LHVC exist; however, due to the complexity of the volcano-structural internal structure, there are still open questions that must be addressed to achieve a better understanding of how the magmatic plumbing system works and its relationship with the geothermal field. This paper presents a multidisciplinary set of studies including volcanologic, structural, petrologic and petrophysical approaches that together integrate a preliminary comprehensive model. The formation of the LHVC involved multiple trapdoor, and less piecemeal caldera collapse events. These events occurred over an intricate pre-caldera geologic scenario composed of a highly folded and fractured sedimentary basement filled by a thick sequence of andesitic lava flows, followed by rhyolitic volcanism. As a result, a highly complex internal structure formed at depth, that connected with fracture systems formed during a caldera resurgence stage, and that may constitute the main paths of the geothermal fluids to the surface. The local stress field caused by the injection of new magma during the resurgence stage and the occurrence of hydrothermal fluids in the geothermal reservoir result from the interplay among the heat source (inferred magma intrusion), the recent ground faulting (caldera resurgence) and intense post-calderas volcanic activity. It is possible that the main resurgence faults correspond to the reactivation of the inherited tectonic planes in the basement underlying the LHVC. A polybaric magmatic system, as obtained by inverse thermobarometry models, supports a configuration with multiple magmatic stagnation levels that fed different eruptive phases of the post-caldera stage of LHVC. Although it is clear that fractures are the main control of permeability, an important contribution is provided by primary vesiculation in the matrix (microporosity) of the volcanic rocks. Furthermore, spatial lithofacies variations of the overlying ignimbritic unit (Xáltipan ignimbrite), particularly controlled by primary welding degree and secondary mineralization, must be taken into account, since it produces strong permeability variations, and therefore irregularities on the fluid path of the geothermal reservoir. Recurrent injection of magmas until the Holocene is also associated with a diversification of the magmatic sources and their interaction with the geothermal system. These results point to a more complex geothermal system than previously believed, that requires the correct integration of these studies.This abstract presents results of the GEMex Project, funded by the the Mexican Energy Sustainability Fund CONACYT-SENER, Project 2015-04-268074, and by the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 727550.
|        Topic: Geology||Paper Number: 12032|