Indirect estimation of the clay content of clay-rocks using acoustic measurements: New insights from the Montiers-sur-Saulx deep borehole (Meuse, France)

• Published in: Marine and petroleum geology Vol. 53; pp. 117 - 132
• Main Authors: Cosenza, P., Robinet, J.C., Prêt, D., Huret, E., Fleury, M., Géraud, Y., Lebon, P., Villiéras, F., Zamora, M.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-05-2014; Elsevier
• Subjects: Callovo-Oxfordian claystone; Clay minerals; Clay-rocks; Continental interfaces, environment; Elastic-wave velocities; Sciences of the Universe; Wireline logging tools; Clay minerals; Elastic-wave velocities; Callovo-Oxfordian claystone; Clay-rocks; Wireline logging tools
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2013.07.004

This work benefited from a comprehensive set of logging and mineralogical data that was acquired from the 2-km-deep EST433 borehole of Montiers-sur-Saulx (Meuse, France). A multiple linear regression analysis was performed on three sets of data: (a) sonic data (P- and S-wave velocities), (b) total combinable magnetic resonance (TCMR) porosity data, and (c) X-ray diffraction (XRD) data. The analysis was performed to estimate the clay content in the Callovo-Oxfordian (COx) claystone and other clay-rocks of the stratigraphic column. The inferred statistical relationships were compared to results obtained from a conventional spectral gamma ray (GR) approach.
The results indicate that simple linear regressions using only elastic wave velocities (a) accounted for more than 81% of the vertical variability of the clay content in the COx formation (i.e., coefficient regression R2 values greater than 81%) and (b) enable the estimation of clay content with the same accuracy as the spectral GR approach. Both approaches, our acoustic regressions and spectral GR methodology, provide estimate errors that are within the range of uncertainties associated with the XRD analysis applied. Moreover, using the whole set of XRD data showed that the elastic wave velocities and the TCMR porosity as explanatory variables are better predictors of total clay content than swelling clay mineral content.
Blind estimations of clay content were performed using data acquired in clay-rocks from the same stratigraphic column and in the COx formation located a few kilometers distant in the EST423 borehole. The results demonstrated that the established regressions must be used with caution when vertically and horizontally extrapolating the estimated clay content. This difficulty is partly explained by the influence of the microstructure of the studied clay-rocks, which was insufficiently taken into account via the petrophysical variables considered in this work.
•An acoustic estimation of the clay content of clay-rocks is proposed.•Our approach is based on simple linear regressions of petrophysical data.•These petrophysical data are P- and S-wave velocities, TCMR porosity and XRD data.•The accuracy of regressions is similar to that of the spectral GR approach.•Coefficient regression R2 values greater than 81% were obtained.