How Can Deep Geothermal Projects Provide Information on the Temperature Distribution in the Upper Rhine Graben? The Example of the Soultz-Sous-Forêts-Enhanced Geothermal System

Abstract

The Upper Rhine Graben (URG) hosts thermal anomalies that account for the development of oil fields. Recently, a geothermal power plant has been installed in this area. Data obtained in this framework provide an insight into the temperature distribution in the URG. The present thermal gradient at Soultz-sous-Forêts is not linear: nearly 90 °C/km down to 1400 m depth, then about 12 °C/km from that depth down to 5000 m. The combination of temperature conditions and natural fluid circulation in fracture networks has led to the hydrothermal alteration of the granite into mineral assemblages such as those including illite, quartz and calcite. Illite is locally impregnated with organic matter of two kinds: a mature type derived from oil source rocks and a less mature type derived from surficial sedimentary layers indicating the km-scale of transfer. Newly formed crystals of quartz and calcite from around 2000 m depth record a fluid temperature range of 130 to 170 °C, consistent with modelling and the temperatures measured at present in the drill-holes at this depth. In such hydrothermally altered zones, local variations of temperature are encountered indicating current fluid flows that are being sought for geothermal purposes.

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

“…Illitic minerals are well known to be indicators of fluid circulation as well as paleocirculation systems [93]. [19,94] show that illite crystallization episodes can occur, for example, in a temperature range of 120 to 160 • C, corresponding to the argillic alteration facies. The illitization process mainly develops in plagioclase and biotite.…”

“…According to [104], the average calcite content of a fresh granite is 0.252 wt.%, and does not exceed 1.8 wt.%. As a consequence, measurements over this last value can be regarded as a calcite anomaly, and so are representative of a granite affected by argillic alteration, which may be a sign of paleo fluid flow [94]. Ten samples out of fifteen from the NH have a calcite content higher than 1.8 wt.%.…”

Scaling in a Geothermal Heat Exchanger at Soultz-Sous-Forêts (Upper Rhine Graben, France): A XRD and SEM-EDS Characterization of Sulfide Precipitates

“…Both flows are totally independent of one another and never mix. The SHEx was installed as bypass on the reinjection line [17]. It was tested over three months (late January to April 2019) in the presence of inhibitors, after which it was dismantled to allow scaling and corrosion studies.…”

“…Each water box is closed by a flange (Figure 2D). [17]; (D): location of samples o the east water box. Note that only one tube is represented in (B) for each temperatur for exact front representation of the location of tubes, after [17]).…”

“…The temperature gradient recorded at Soultz (red dashed line) and key figures are indicated on the left. Figure modified after [17,18]; size of the deep reservoir from [19].…”