Identification and characterization of hydrothermally altered zones in granite by combining synthetic clay content logs with magnetic mineralogical investigations of drilled rock cuttings

Is There a Link Between Mineralogy, Petrophysics, and the Hydraulic and Seismic Behaviors of the Soultz‐sous‐Forêts Granite During Stimulation? A Review and Reinterpretation of Petro‐Hydromechanical Data Toward a Better Understanding of Induced Seismicity and Fluid Flow

“…Logs of the GPK1 borehole indicating lithology, synthetic clay content log (SCCL; Meller, Kontny, et al, : SCCL1: no clay, SCCL5: highest clay content, normalized to the highest density of clay‐filled fractures; see paper for details), fracture orientation and dip, spectral gamma ray (K, Th, and U), Young's modulus, calcite content (new data), and magnetic susceptibility (Meller, Kontny, et al, ). FGMV: fresh granite maximum value (maximum value of calcite content in a fresh granite, in wt %).…”

“…In GPK2, high calcite contents up to nearly 12 wt % and high clay contents are linked with high fluid conductivity, while low calcite and low clay contents are found in low permeability zones (Figure ). On the opposite, in GPK3, high calcite content up to nearly 13 wt % and high clay content as well as secondary magnetite formation as an indicator of high hydrothermal alteration grade derived from magnetic susceptibility measurements (Meller, Kontny, et al, ) correlate with low fluid conductivity, and highly permeable zones show low calcite content. In GPK4, the main fracture zone shows a low calcite content and the overall behavior of the borehole is rather similar to that of GPK3 (Figure ).…”

Possible natural fluid pathways from gravity pseudo-tomography in the geothermal fields of Northern Alsace (Upper Rhine Graben)

“…There is the positive magnetic anomaly coinciding with the AN1-AN3 gravity anomalies indicating a granitic intrusion of the so-called magnetite series (Ishihara 1977) with about 1 wt.% of primary magnetite as the main carrier of susceptibility (Just et al 2004) and a mean density of about 2,660 kg m −3 (Rummel 1991). It may be speculated that the increasing extension of AN3 with increasing low cut-off wavelengths might be related to an increasing contribution of the underlying two-mica granite revealing typically lower susceptibility (Meller et al 2014), but differences between the two in susceptibility are not very high (between about 1 · 10 −3 and 7 · 10 −3 m 3 kg). Density measurements in the two granites show that there is no significant change between them throughout the well lengths in Soultz (Grecksch et al 2003).…”

The significance of hydrothermal alteration zones for the mechanical behavior of a geothermal reservoir

“…Increased clay content is seen at the bottom of the wells below 4,600 m at the transition between the porphyritic and the two-mica granite. For details of this neural network method, the SCCLs, and the calibration of the logs by magnetic mineralogical investigations refer to Meller et al (2014a;2014b) (Figure 3).…”