Shales make up a large proportion of the rocks in most sedimentary basins and form the seal and source rocks for many hydrocarbon reservoirs. In unconventional shale plays, shale acts as both the source rock and the reservoir. Currently, there is great interest in organic-rich shales because they represent an enormous energy resource. Wensaas et al. (2012) and Gading et al. (2012) demonstrate that the top of a source-rock interval is characterized by a drop in acoustic impedance and a strong negative reflection amplitude that dims with offset—amplitude variation with offset (AVO) class IV behavior. Organic-rich shales are often strongly anisotropic because of the partial alignment of anisotropic clay minerals and the bedding-parallel lamination of organic material within the shale (Vernik and Nur, 1992; Vernik, 1993, 1994; Vernik and Landis, 1996; Vernik and Liu, 1997; Sondergeld et al., 2000; Hornby et al., 2003; Walsh et al., 2007; Sondergeld and Rai, 2011). This leads to anisotropy at all scale lengths, and neglecting shale anisotropy may lead to incorrect estimates of rock and fluid properties derived from inversion of seismic AVO data. In this paper, the effect of anisotropy on the AVO response of organic-rich shales is investigated using a rock-physics model. This uses parameters given by Vernik and coworkers in their study of the Bakken shale.

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