Probabilistic AVO inversion of transversely isotropic medium for better characterization of North Sea Oxfordian turbidite reservoir

Sedimentary rocks often obey vertical transvers isotropy due to the nature of the sedimentation process or the inherent orientation of rock grain minerals (e.g. clay platelets). Despite several documented cases in the literature, the effect of anisotropy is often neglected in seismic AVO inversion. We study the effect of transverse isotropy in shale encasing isotropic sand using probabilistic AVO inversion. The inversion algorithm is inverting seismic pre-stack data to lithology and fluid probabilities. A synthetic case demonstrates how anisotropy in shale may lead to wrong interpretation of fluid content in underlying sand. A field case in the northern North Sea shows that accounting for anisotropy in shale may improve the discrimination between good and poor sand of an Oxfordian turbidite reservoir. The results are encouraging and consistent with observations of sandy intervals in the wells and the depositional system of the study area. Our results clearly demonstrate that transverse anisotropy in sediments may give a significant contribution to the AVO gradient that otherwise could be misinterpreted. In the study area it becomes especially important, since there is a lack of contrast in acoustic impedance between the encasing shale and the target sand.