Reconstructing Paleofluid Circulation at the Hercynian Basement/Mesozoic Sedimentary Cover Interface in the Upper Rhine Graben

Abstract

In this paper, we focus on paleocirculation at the Hercynian basement/sedimentary cover interface in the tectonic environment of the Upper Rhine graben. The goal is to increase our understanding of the behavior of the fracture-fault network and the origin of the hydrothermal fluids. We studied orientations, mineral fillings, and fluid origins of fractures that crosscut the Hercynian granitic basement and the Permo-Triassic formations in relation to the major tectonic events. Because the Mesozoic formations and the Hercynian basement on the graben flanks and inside the graben do not have the same evolution after uplift, our study includes 20 outcrops on both graben flanks and cores of the Soultz-sous-Forêts geothermal wells located inside the graben. The Hercynian granitic basement and Permo-Triassic formations were affected by several brittle phases associated with fluid circulation pulses related to graben formation during the Tertiary. We distinguished at least four stages: (1) reactivation of Hercynian structures associated with pre-rift tectonics during the early Eocene and descending meteoric waters, characterized by shearing/cataclasis textures and precipitation of illite and microquartz; (2) initiation of convective circulation of deep hot brines mixed with descending meteoric waters at the Hercynian basement/sedimentary cover interface during this first stage of Eocene rifting, characterized by dolomite and barite fillings in reactivated Hercynian fractures; (3) N-S tension fractures associated with rift tectonics just prior to uplift of the graben shoulders during Oligocene extension and descending meteoric waters, characterized by cataclastic textures and precipitation of quartz, illite, hematite, and barite; and (4) current convective circulation of deep hot brines mixed with descending meteoric waters at the Hercynian basement/sedimentary cover interface, characterized by calcite and barite fillings within the graben. This convective circulation is today present in deep geothermal wells in the western part of the Rhine graben.

Evolution of diagenetic conditions and burial history in Buntsandstein Gp. fractured sandstones (Upper Rhine Graben) from in-situ δ18O of quartz and 40Ar/39Ar geochronology of K-feldspar overgrowths

“…In the meantime, several hydrothermal events are recorded regionally (Fig. 2B), with several pulses recorded within the underlying basement and with fault zones affecting the sedimentary deposits since the Triassic (Boiron et al 2011;Loges et al 2012;Walter et al 2016Walter et al , 2019Dezayes and Lerouge 2019). Quartz, siderite, and barite mineralise in the fracture network linked to fault systems that were active during rift opening.…”

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

“…They lead to geochemical, mineralogical and petrophysical (porosity and permeability) modifications of the rocks [6]. In granitic rocks, fluid circulations usually occur through the fracture network at different scales [6,7] involving a pervasive alteration which may influence up to cubic kilometers of rock [8]. Thus, an intense fluid/rock interaction [9] can significantly change the mineralogy, the chemistry and the texture of the bulk-rock [7] among which the common formation of clay minerals, including illite.…”

“…In granitic rocks, fluid circulations usually occur through the fracture network at different scales [6,7] involving a pervasive alteration which may influence up to cubic kilometers of rock [8]. Thus, an intense fluid/rock interaction [9] can significantly change the mineralogy, the chemistry and the texture of the bulk-rock [7] among which the common formation of clay minerals, including illite. Exploited geothermal reservoirs are located at depth, and the only and limited available data come from drillings (cores, cuttings) and seismic surveys.…”

How Do Secondary Minerals in Granite Help Distinguish Paleo- from Present-Day Permeable Fracture Zones? Joint Interpretation of SWIR Spectroscopy and Geophysical Logs in the Geothermal Wells of Northern Alsace

“…The mid-Carboniferous granitic basement of the Upper Rhine Graben (URG) has been affected by several extensional and compressional tectonic phases that developed a multiscale fracture network during the Tertiary [1,2]. Today, the network hosts hydrothermal circulations and acts as the main pathway through which the natural brine sustains wide convection cells [3][4][5]. More than 30 years of geothermal research in the pilot geothermal project of Soultz-sous-Forêts (Soultz) (Alsace, France) led to a 3D knowledge of those multiscale fault and fracture networks [6,7].…”

“…Fault and fracture networks will be designated by the general term "FZ" in this paper. Their internal complexity in terms of architecture and associated permeability is a key question that must be answered when granitic rocks are used as hydrogeological reservoirs, for nuclear waste disposal, or as deep geothermal systems ( Figure 1) [4,[8][9][10][11][12]. In the URG, the current targets for geothermal projects are hydrothermal fractured reservoirs in granitic rocks lying under a thick sedimentary cover [13].…”