title: Unraveling the behavior of amagmatic geothermal systems through thermal–hydraulic simulations: insights from the Agua Blanca Fault, Baja California, Mexico
Creators: name: Carbajal Martínez, Daniel
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 550 Earth sciences &amp; geology

description: In view of the ongoing global challenges such as climate change, it is crucial to explore and develop energy sources that are sustainable. Amagmatic orogenic geothermal systems, situated within crystalline rocks along fault zones in regions that lack recent magmatic activity, represent promising renewable energy sources for heat exploitation and power generation. The reservoir temperatures in these systems can reach as high as 250 °C, even in the absence of a magmatic heat source. This remarkable thermal potential is due to heat transport by water that circulates deeply through fault networks. Previous studies have provided valuable insights into the behavior of such systems, but better understanding is required to enable effective exploration and assessment of their energy potential.
The main goal of this thesis is to provide a detailed description of the key parameters controlling the behavior of amagmatic orogenic geothermal systems, and thereby contribute to broader efforts to develop sustainable energy resources. To achieve this goal, multidisciplinary research techniques were employed, including geological, geochemical, and geophysical analyses, all integrated via thermal–hydraulic numerical simulations. The thesis is divided into three main parts: (i) introduction (Chapters 1 and 2), (ii) research papers (Chapters 3-6), and (iii) concluding remarks (Chapter 7). The research papers form the core of the thesis and cover three main topics:
•Topic I. Characterization of geothermal systems along the Agua Blanca Fault (ABF) inBaja California, Mexico (Chapter 3):
In the initial phase, thermal waters and dissolved gases were sampled from seven hot spring sites strung along a 90 km segment of the ABF. A combination of geochemical, petrophysical, and seismic hypocenter data with previous geodetic studies resulted in a conceptual model that explains the governing processes controlling these amagmatic geothermal systems. Hydraulic head gradients caused by the surface topography drive meteoric water deep into the fault system, where it is heated by the background geothermal gradient. Where the ABF crosscuts the Pacific coast, thermal springs are mixtures of infiltrated meteoric water and seawater. Geochemical analyses show that all the discharging thermal waters equilibrated with quartz at 100–220 °C, deep within the host fault. Accordingly, meteoric water has been deduced to have infiltrated to depths of at least 5–11 km into the brittle crust. Subsequently, the infiltrated water has ascended along preferentially permeable zones of the host fault system, eventually discharging in thermal springs at temperatures ranging from 37 °C at inland sites to 102 °C on the Pacific coast. Notably, higher spring temperatures correlate positively with the degree of extensional displacement along the fault system, which serves as a proxy for fault permeability. All systems have a mantle He contribution ranging from 1–11% based on 3He measurements,confirming its amagmatic origin. Correlations between hydraulic head gradients, residence times, and 3He/Hetotal ratios of the thermal waters show that the hydraulic headgradient controls the length and depth of the flow paths. These findings reveal that thepermeability of the ABF and its hydraulic head gradients are the key factors controlling the behavior of amagmatic orogenic geothermal systems.
•Topic II. 3D simulations of the La Jolla Beach system (Chapters 4 and 5):
The hottest subaerial geothermal system along the ABF is at La Jolla Beach. This system discharges thermal water up to 94 °C at the coastline, with a seawater content between 15 and 50%, and a mantle He contribution of less than 3%. The high discharge temperature at the beach, which to our knowledge is the hottest recorded in amagmatic systems worldwide, is attributed to a combination of factors, including the location of the spring within the highest permeability segment along the ABF and its location near a mountainous region of the coastline (up to 1 km in elevation). To gain deeper insights into the behavior of the La Jolla Beach system, three-dimensional (3D) thermal–hydraulic simulations using the software TOUGHREACT were performed for this case study. These simulations were conducted on a regional scale (up to 34 × 12 × 11.5 km), including the topography and bathymetry of the study area to assess their role in controlling regional water circulation within the ABF. To constrain the simulations, the location, temperature, and salinity of the hot springs, and fault extension observed at La Jolla Beach were compared with the corresponding simulation results. Two distinct numerical simulation scenarios were explored:
(i)The first scenario considers the presence of a preferential upflow zone right beneathLa Jolla Beach, within the ABF. (ii) The second scenario considers a more extended dataset, including the topography of the crystalline basement and the sediment distribution on the seafloor, based on previous studies. Moreover, this scenario accounts for not only the pressure of the seawater column but also the density and viscosity of seawater, both of which are temperature-dependent.
The results of the first scenario show that assuming the presence of a highly permeable upflow zone beneath La Jolla Beach is sufficient to match the observed discharge temperature at the right location. This demonstrates the importance of permeability in controlling the discharge temperature of amagmatic geothermal systems.
The second simulation scenario shows that the discharge temperature observed at La Jolla Beach can also be matched at the right location without having to define a highly permeable upflow zone. To do so, the permeability of the entire ABF, as well as the permeability of the seafloor sediments that control the infiltration of seawater into the brittle continental crust, need to be taken into account. This scenario thus highlights the crucial role of dense seawater, which acts as a hydraulic barrier that forces thermal discharge right at the coast. In conclusion, the record high discharge temperature observed at La Jolla Beach likely results from the presence of a structural (i.e., permeability) anomaly within the host fault precisely at the coast where the hydraulic head is at its minimum. The combination of high hydraulic head gradients and elevated permeability leads to very high upflow rates and discharge temperatures of almost 100 °C despite the absence of any magmatic activity.
•Topic III: Functioning of global coastal geothermal systems (chapter 6):
The intriguing observations and numerical results of La Jolla Beach provided the motivation to explore whether other coastal geothermal systems worldwide behave in the same way. The results of generalized 3D thermal-hydraulic simulations using TOUGHREACT show that seawater incursion consistently blocks the seaward flow of groundwater through faults that intersect a coastline. This feature, combined with the hydraulic effects of fine-grained, low-permeability sediments that typically blanket the fault trace along the seafloor, systematically leads to discharges of thermal water at the coastline.
In conclusion, this doctoral thesis enhances our understanding of amagmatic orogenic geothermal systems, emphasizing the importance of factors like fault permeability, hydraulic head gradients, and the presence of permeability anomalies, but also highlights that they could form substantial thermal anomalies within the rocks surrounding the upflow zones. The findings suggest that sustainable energy exploration should prioritize valley floors and coastal zones intersected by regional fault zones. Under such ideal conditions with high hydraulic head gradients and elevated permeabilities, amagmatic geothermal systems lead to thermal anomalies within which the critical temperature threshold for power production (120 °C) is reached at shallow depths (<2 km). Exploitation of this largely petrothermal heat could constitute a significant sustainable energy source.