Overview of naturally permeable fractured reservoirs in the central and southern Upper Rhine Graben: Insights from geothermal wells

Identifying fluid circulation in fracture zones (FZs) is a key challenge in the extraction of deep geothermal heat from natural reservoirs in the Upper Rhine Graben. This study focuses on permeable FZs present within the granitic basement penetrated by deep geothermal well GPK-1 at Soultz and GRT-1 and GRT-2 at Rittershoffen (France). The various temperature (T) log datasets acquired from these wells during production and at equilibrium, with the associated flow logs, allow for the unique opportunity to interpret fluid circulation at the borehole scale. All permeable FZs identified by permeability indicators measured during drilling operations and from image logs spatially coincide with positive or negative T anomalies observed in the T logs during production and/or at equilibrium. However, within the FZs, partially open fractures act as narrower paths for circulation at different temperatures. These temperatures can even be estimated with confidence if the associated flow log is available. The polarity of the T anomalies correlates with the state of equilibrium of the well and thus can change over the well history. During production, the temperature of the water inflow through the fractures can be estimated relative to the mixture of water circulating below the fractures. At thermal equilibrium, the water temperature is estimated with respect to the temperature of the surrounding rock formation. Because temperature fluxes and geothermal fluids are intimately linked, T logs are a useful, reliable, and very sensitive tool to localize the inflow of geothermal water through FZs.

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The complete lithostratigraphic section of the geothermal wells in Rittershoffen (Upper Rhine Graben, eastern France): a key for future geothermal wells

BSGF - Earth Sciences Bulletin ◽

10.1051/bsgf/2019012 ◽

2019 ◽

Vol 190 ◽

pp. 13 ◽

Cited By ~ 3

Author(s):

Philippe Duringer ◽

Coralie Aichholzer ◽

Sergio Orciani ◽

Albert Genter

Keyword(s):

Middle Jurassic ◽

Gamma Ray ◽

Upper Jurassic ◽

Upper Permian ◽

Upper Rhine Graben ◽

Rhine Graben ◽

Geothermal Wells ◽

The Subject ◽

Upper Rhine

Between 2012 and 2014, in Rittershoffen, in the Upper Rhine Graben, two geothermal boreholes (GRT-1 and GRT-2) reached the granitic basement at a depth of around 2150 m. The wells crossed about 1160 m of Cenozoic and 1050 m of Mesozoic. In the Cenozoic, these wells crossed the greatest part of the Eocene, the lower part of the Oligocene and a thinner Pliocene. The Quaternary is poorly represented (less than 10 m). In the Mesozoic, the wells crossed the Lower Dogger, the entire Lias and the entire Triassic. A reduced thickness of about 50 m of Upper Permian terminates the sedimentary column before entering into the granitic basement. A major erosional unconformity separates the Middle Jurassic from the Tertiary units (the Upper Jurassic, entire Cretaceous and Paleocene are absent). The Rittershoffen drilled doublets were the subject of particular attention concerning the acquisition of a very precise stratigraphic profile. In this paper, we give the recognition criteria for the fifty-seven formations crossed by the GRT-1 well and their upper and lower boundaries as well as their specific gamma ray signatures. The data are presented in four figures: a general complete log displaying the main sets and three detailed, precise logs showing the sedimentary formations overhanging the granitic basement: the Tertiary, the Jurassic, the Triassic, the Permian and the basement.

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

Geofluids ◽

10.1155/2019/4849860 ◽

2019 ◽

Vol 2019 ◽

pp. 1-30 ◽

Cited By ~ 3

Author(s):

Chrystel Dezayes ◽

Catherine Lerouge

Keyword(s):

Sedimentary Cover ◽

Upper Rhine Graben ◽

Fluid Circulation ◽

Tectonic Events ◽

Rhine Graben ◽

Tectonic Environment ◽

Convective Circulation ◽

Geothermal Wells ◽

Upper Rhine ◽

Fault Network

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.

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