3D Rock Physics Template for Anisotropic Formations - Application to the Study of Shale

Rock Physics Templates (RPT) have been introduced to help non-experts in rock physics for lithology and pore fluid interpretation of sonic log data and elastic inversion results. Here we propose an extension of the approach in order to deal with anisotropy. For this we start with the classical RPT chart, namely crossplot of acoustic impedance Zp and P-wave over S-wave velocity ratio. A third dimension, here an anisotropy coefficient, is reported as isolines. On this initial 3D RPT chart we superimpose as color-coded points interpretation parameters, such as porosity or shaliness. We use this representation for studying chemically compacted shales, idealized as thinly stratified random mix of transversely–isotropic smectite and isotropic illite, our purpose being more to illustrate the value of the representation than to demonstrate relevancy of the model. The interpretation parameters are illite proportion Villite and the Orientation Distribution Function coefficients W200 and W400 of the clay minerals in smectite. Among other results, it clearly appears from the charts, as expected, that increasing illitization tends to stiffen the rock, and as a consequence to increase Zp. Increasing alignment of the clay minerals (mainly by increasing W200) surprisingly tends to decrease the P-wave time processing anisotropy parameter Eta.