Seismic anisotropy of shales: What determines the sign of Thomsen's delta parameter?

摘要

Shales are a major component of sedimentary basins, andrnare anisotropic because of the partial alignment of plate-likernclay minerals. Shale anisotropy needs to be quantifiedrnto reliably extract reservoir fluid, lithology and porernpressure from seismic data, and to understand time-to-depthrnconversion errors and non-hyperbolic moveout. A singlernanisotropy parameter, δ ,is sufficient to explain therndifference between the small-offset normal moveoutrnvelocity and vertical velocity, and to interpret the small-offsetrnAVO response of P-waves (Thomsen, 1986). Thernsign of this parameter is poorly understood, with bothrnpositive and negative values having been reported in thernliterature. The sign of δ can be explained by consideringrnthe properties of the regions between clay particles and therndisorder in the orientation distribution of clay particles.