21 January 2016

Seismic anisotropy, that is to say the directional dependence of seismic velocity, is quite common in sedimentary formations and is often linked to the presence of shale. If not correctly taken into account it can strongly affect surface seismic data interpretation, seismic to well tie and azimuth versus offset analysis. From the analysis of two large databases of up to 800 ‘shaly’ samples in a broad sense, including shales but also mudshale, clayshale, siltstone, argillite, claystone, siltshale, mudstone, we demonstrate that seismic anisotropy in such formations is to a large extent determined by factors other than compaction processes, such as depositional environment, chemical composition of fluid, silt fraction, etc. Furthermore, the alignment of the individual clay platelets, main constituents of shales, can explain most of the anisotropy measurements of the databases. Assuming the elastic properties of the individual clay platelets, we propose simple plots for straightforwardly quantifying the Legendre Orientation Distribution Function coefficients and of the clay platelet alignment from the measurement of seismic anisotropy parameters.

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