Rock physics of organic shales

Abstract

Several publications over the last 20 years have established that black organic shales are generally characterized by strong velocity anisotropy, low velocity in the bedding-normal direction, and relatively low density and porosity (e.g., Vernik and Nur, 1992; Vernik and Liu, 1997). These rock properties are of interest in seismic attribute studies, but even more so in geomechanical applications related to reservoir characterization and hydraulic fracture stimulation. Organic shales typically, but not necessarily, express strong vertical transverse isotropy (VTI), with axis of symmetry perpendicular to the bedding-parallel lamination and clay-particle-preferred orientation. High-resolution SEM imaging (Figure 1) immediately reveals all these textural features in addition to silt grains and the lenticular distribution of the solid organic matter (kerogen), which in turn is characterized by significant intraparticle porosity in mature shales. Any TI medium is described by the five independent elastic constants ([Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]) and detailed multidirectional ultrasonic core measurements can provide the most accurate means for their adequate evaluation. Single-well sonic- and density-log data fall short of the full TI tensor description resulting in major ambiguities related to empirical correlations.