Fluid-Rock Interactions in a Paleo-Geothermal Reservoir (Noble Hills Granite, California, USA). Part 1: Granite Pervasive Alteration Processes away from Fracture Zones

Abstract

Only few data from geothermal exploited reservoirs are available due to the restricted accessibility by drilling, which limits the understanding of the entire reservoir. Thus, analogue investigations are needed and were performed in the framework of the H2020 MEET project. The Noble Hills range, located along the southern branch of the Death Valley pull-apart (CA, USA), has been selected as a possible granitic paleo-reservoir. The aim is to characterize the pervasive alteration processes affecting this granite, away from the influence of the faults, in terms of mineralogical, petrophysical and chemical changes. Various methods were used as petrographic, geochemical and petrophysical analyses. Mineral changes, clay mineralogy, bulk rock chemical composition, calcite content and porosity were determined on different granite samples, collected in the Noble Hills granite, far from the faults and in the Owlshead Mountains, north of the Noble Hills, considered as its protolith. In order to complete the granite characterization, the metamorphic grade has been studied through the Noble Hills granite body. This complete characterization has allowed distinguishing the occurrence of three stages of alteration: (1) a pervasive propylitic alteration characterized by calcite-corrensite-epidote-K-white mica assemblage, (2) a more local one, only present in the Noble Hills granite, producing illite, kaolinite, illite/smectite, calcite and oxides, characteristic of the argillic alteration, which overprints the propylitic alteration and (3) weathering evidenced by the presence of montmorillonite in the Owlshead Mountains, which is considered as negligible in both granites. Alteration was also outlined by the correlation of the loss on ignition, representing the hydration rate, to porosity, calcite content and chemical composition. Moreover, the Kübler Index calculated from illite crystals allowed to identify a NW-SE temperature gradient in the Noble Hills.

Variability of Radionuclide Sorption Efficiency on Muscovite Cleavage Planes

“…Due to slow reaction kinetics, crystal surfaces are not always available as "healed" equilibrium surfaces after such processes, and nanotopography as a controlling parameter cannot be neglected. [ 71 ]…”

“…Due to slow reaction kinetics, crystal surfaces are not always available as "healed" equilibrium surfaces after such processes, and nanotopography as a controlling parameter cannot be neglected. [71] This study provides a first approach toward a mechanistic understanding of heterogeneous sorption on mineral surfaces. The current approach is simplified in comparison to what can be expected in natural rock systems, e.g., no large mineral surfaces are available for the calculation of sorption energies, no explicit water molecules are included and no cations are presorbed on the surface.…”

Water–Rock Interaction Processes in Tíscar and Larva Active Faults (Betic Cordillera, SE Spain)

“…Highsalinity waters in restricted environments can accelerate mineral authigenesis processes [58]. The flow of fluids controlled by tectonics can produce high-salinity microenvironments facilitating changes in the mica octahedral layer charge that favor its transformation and the neoformation of clay minerals [59][60][61]. Abad et al [54] described smectite neoformation during a low-temperature stage in the Padul fault.…”

Editorial of Special Issue “Enhanced Geothermal Systems and Other Deep Geothermal Applications throughout Europe: The MEET Project”

“…This is due to the instability of the primary mineralogical assemblages under the new physicochemical conditions, which leads to the formation of new mineral phases. Hydrothermal alteration was described for outcrops far from the faults [6] and close to them [7]. The structural characterization of the fracture network that conducted the fluids responsible for the hydrothermal alterations is presented in [8,9].…”

“…During the four years of the project, two main types of exploitations were investigated: Enhanced Geothermal Systems (EGS) and oil-to-geothermal conversion or co-production. The following topics were addressed: the upscaling of thermal power production and optimized operation of EGS plants (papers [1][2][3][4][5]); variscan geothermal reservoirs in granitic and metamorphic rocks (papers [6][7][8][9][10][11][12][13][14][15][16][17]); and technical, economic and environmental assessment for oil-to-geothermal fields and EGS integration into energy systems (papers [18][19][20]). These 20 papers give an overview of some of the work performed in the MEET project, but they are not exhaustive of all the results obtained in this frame.…”