A laboratory approach for the calibration of seismic data in the western part of the Swiss Molasse Basin: the case history of well Humilly-2 (France) in the Geneva area

Abstract

Abstract A collection of 81 plugs were obtained from the Humilly-2 borehole (France), that reached the Permo-Carboniferous sediments at a depth of 3051 m. Experimental measurements of physical parameters and mineralogical analysis were performed to explore the links between sedimentary facies and seismic characteristics and provide a key tool in the interpretation of seismic field data in terms of geological formations. The plugs, cylinders of 22.5 mm in diameter and $$\sim$$ ∼ 30 mm in length were collected parallel and perpendicular to the bedding in order to explore their anisotropy. Ultrasound wave propagation was measured at increasing confining pressure conditions up to 260 MPa, a pressure where all micro-fractures are considered closed. The derivatives of velocities with pressure were established, allowing the simulation of lithological transitions at in-situ conditions. At room conditions, measured grain densities (kg/m3) range from 2630 to 2948 and velocities vary from 4339 to 6771 and 2460 to 3975 $${\text {m/s}}$$ m/s for P- and S-waves propagation modes, respectively. The largest seismic-reflections coefficients were calculated for the interface between the evaporitic facies of the Keuper (Lettenkohle) and the underlying Muschelkalk carbonates (R $$_{\text {c}}=$$ c = 0.3). The effective porosity has the range of 0.23% to 16.65%, while the maximum fluid permeability $$\left( \text {m}^{\text {2}}\right)$$ m 2 is 9.1 $$\times \text {10}^{-{\text {16}}}$$ × 10 - 16 . A positive correlation between porosity and ultrasound velocity has been observed for P- and S-waves. The link between velocities and modal content of quartz, dolomite, calcite, and micas has been explored. This paper presents a unique set of seismic parameters potentially useful for the calibration of seismic data in the Geneva Molasse Basin.

Keywords

• Ultrasound seismic velocities
• Geothermy
• Borehole rock samples
• Reflectivity
• Swiss Molasse