Fossil vs. Active Geothermal Systems: A Field and Laboratory Method to Disclose the Relationships between Geothermal Fluid Flow and Geological Structures at Depth (original) (raw)
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2009
At Spiaggia Barbarossa on the Italian island of Elba, a hydraulic cataclastic shear zone produced by hydrothermal fluid overpressure was investigated in order to study anisotropic structures and fracture networks from macroscopic to microscopic scale. Fluid filled fractures and the degree of fracture networks that functioned as fluid pathways in a fossil hydrothermal system were also of interest. Tourmaline dykes and veins crosscut the oldest Tuscan units, the Calamita schists, parallel and oblique to the foliation and the Porto Azzurro pluton. Both metasomatic and hydrothermal tourmaline were identified in these veins. Metasomatic tourmaline exhibits reverse zoning with schoerl-rich cores and dravite-rich rims that indicate progressive replacement of biotite. Hydrothermal tourmaline forms the matrix fluid of cataclastic shear zones embedding quartz clasts within the range of ~ 100 – 2000 μm size. Fractal geometry analyses of the shear zones indicate the concentration of deformation...
In the framework of geothermal exploration campaigns 3D structural-geological modeling plays an important role in the understanding of geothermal systems. The focus for the Brady's geothermal system located in the Basin and Range province is on the identification of structural controls on fluid flow and permeability anisotropy. In addition to 3D structural-geological modeling, the applied exploration strategy also includes stress field analysis and surface geochemical surveys. We have used 1) detailed geological maps, 2) borehole data, 3) processed 2D seismic and gravity data, and 4) a digital elevation model as input parameters of the 3D model. Based on these data, four representative cross sections have been developed as a major input for a preliminary 3D geological model. Well logs are used to verify the stratigraphic structure between the cross sections. The major strike direction of the faults is NNE. Normal faulting is the dominant stress regime. Dip angles range from 45° to 80°. The 3D model consists of eight geological units. The Mesozoic basement consists of granites and metamorphic rocks. Above, a sequence of Tertiary ash-flow tuffs, lacustrine sediments, and lava flows of different composition has been encountered. 3D structural models populated with geomechanical and stress data can help to delineate between dilational and shear zone both being prone for channeling fluids. In a later stage, stress data derived from fault plane analysis shall be integrated into the 3D structural-geological model applying the slip and dilation tendency technique to estimate hydraulically active fault zones. These results shall be verified by surface gas measurements to understand the impact of individual faults on fluid flow.
Cap rock characterization of geothermal systems is often neglected despite fracturing may reduce its efficiency and favours fluid migration. We investigated the siliciclastic cap rock of Rosario de La Frontera geothermal system (NW Argentina) in order to assess its quality as a function of fracture patterns and related thermal alteration. Paleothermal investigations (XRD on fine-grained fraction of sediments, organic matter optical analysis and fluid inclusions on veins) and 1D thermal modelling allowed us to distinguish the thermal fingerprint associated to sedimentary burial from that related to fluid migration. The geothermal system is hosted in a Neogene N-S anticline dissected by high angle NNW-and ENE-striking faults. Its cap rock can be grouped into two quality categories: • rocks acting as good insulators, deformed by NNW–SSE and E–W shear fractures, NNE-SSW gypsum-and N-S-striking calcite-filled veins that developed during the initial stage of anticline growth. Maximum paleo-temperatures (b 60 °C) were experienced during deposition to folding phases; • rocks acting as bad insulators, deformed by NNW-SSE fault planes and NNW-and WNW-striking sets of fractures associated to late transpressive kinematics. Maximum paleo-temperatures higher than about 115 °C are linked to fluid migration from the reservoir to surface (with a reservoir top at maximum depths of 2.5 km) along fault damage zones. This multi-method approach turned out to be particularly useful to trace the main pathways of hot fluids and can be applied in blind geothermal systems where either subsurface data are scarce or surface thermal anomalies are lacking.
Geothermics, 2024
The Vall`es-Pened`es Fault is a Neogene normal fault marked by the presence of two established geothermal systems at La Garriga-Samalús and Caldes de Montbuí, within the Catalan Coastal Ranges (NE Spain). The analysis and collation of existing and new geological and geophysical datasets provide the basis for the development of an improved conceptual model that explains the presence and localization of hot geothermal fluid systems at relatively shallow depths (e.g., 60 ◦C at surface and 90 ◦C at 1 km). Geothermal flow is concentrated within Paleozoic granodiorites of the immediate footwall of the V-P fault, host rocks that are susceptible to faultrelated fracturing, and the generation of both extension and hybrid fracture systems in association with active fault displacements. Flow localization is enhanced further by the presence of fault-related structural complexities, with both systems marked by 300 m wide steps in the main fault trace. These are attributed to relay development and breaching characterizing host rocks by high fracture intensities and fault rock development on a fault that locally has a vertical displacement of over 1.5 km. Accentuated fracturing and deformation are consistent with strain localization predicted by existing models for the development of fault zones along normal faults. The plumbing of the geothermal systems is attributed to up-fault flow in combination with lateral flow controlled by the intersection of the V-P fault with a low-angled Paleogene thrust defining the base of the hostrock granodiorites, with the geothermal systems localized at the distal end of the thrust. Sustained geothermal flow is attributed to groundwater flow circulation associated with seismic pumping and valving of warmer and deeper fluids, and the ingress of groundwater along faults and within fractured basement rocks.
Influence of Geological Structures on Fluid and Heat Flow Fields
Geothermal exploration is commonly performed in the absence of 3-D seismics or other high quality methods constraining the geometry of the underlying geological system. Therefore, uncertainty resides in the structural model itself. This is in addition to the usual assumption of stochasticity in the petrophysical properties. We show here that small changes in the geometrical description of the geology can produce large changes in the character of heat transport. Combining these sensitivities with the inherent uncertainties in geological model characterization, we suggest that the careful consideration of the systematics of geothermal heat transport is warranted. At present, these sensitivities are not widely recognized because the infrastructure to explore these links has not previously existed to the best of our knowledge. We have therefore constructed an integrated workflow that combines geological modeling directly with fluid and heat flow simulation. This allows us to calculate t...