Petrographic Analyzes of Fractured Granites Used as an Analog of the Soultz-sous-Forêts Geothermal Reservoir: Noble Hills, CA, USA

Abstract

The work presented here is part of the European MEET project (Multidisciplinary and multi-context demonstration of EGS exploration and Exploitation Techniques and potentials), which aims to enhance demonstration of geothermal energy production throughout Europe. Petrographic analyzes of a geothermal reservoir are key issues for the understanding of fluid circulation in granitic rocks. Through field work, microscopical observations (optical microscope, SEM) and XRD analyzes, we aim at characterize a fractured and altered granitic body found in the Noble Hills (NH, Southern Death Valley, CA, USA) chosen as an analogue of the Soultz-sous-Forêts (SsF) geothermal reservoir (Upper Rhine Graben, Alsace, France). Our current state of knowledge reveals that the NH range is composed of a fractured and altered Mesozoic granitic body intruding Proterozoic sedimentary series. The composition of the granite and its alteration processes present similarities with the ones at SsF: oligoclase is transformed into kaolinite (tosudite at SsF)/ illite/calcite and biotite into chlorite/illite. The whole range suffers a substantial amount of shear deformation linked to the activity of the Southern Death Valley Fault Zone (SDVFZ) possibly much higher compare to SsF. A gradient of deformation can be followed through the range and along which samples were collected. Those ones reveal that the shear deformation can either reactivate mineralized fractures and create new drains for fluid circulation at low strain or promote the plugging of fractures due to the development of a high amount of clay minerals at high strain. Through analogue studies we here aim at 1) better understand the geometry of fluid circulation paths in the granitic fractured reservoirs and 2) gain insights in fluid rock interaction processes as these can drastically modify fractures properties through time.