Building Anisotropic Models for Large Scale TTI Imaging in Areas with Limited Well Control - Kwanza Basin Case Study

Abstract

Depth imaging with anisotropic models has been shown to deliver more geologically plausible models and accurate images. Deriving parameters that describe the anisotropic properties of the subsurface requires incorporating well information. There are, however, vast exploration areas around the world with very limited to no well control that do require high-quality anisotropic imaging to allow adequate interpretation of deeper targets below complex structures. We present a general workflow for building tilted transversely isotropic (TTI) models in areas of very limited well control. We incorporate published knowledge of the area and analysis of data anellipticity, with a derivation of Thomsen's δ in wells from adjacent areas. We use single regional Thomsen's parameter trends hung from the water bottom. Finally, we present a case study that applies this workflow to seismic data from the Kwanza basin offshore Angola where TTI models were built over more than 12000 km2 in two adjacent areas. By accounting for TTI anisotropy, we have produced geologically plausible and interpretable images from relatively old narrow-azimuth streamer data with only moderate offsets of 4.8 km.

Building Anisotropic Models for Depth Imaging: Comparing Different Approaches

“…Figure 2 shows case study where a TTI model was built for a block in the Kwanza basin offshore Angola of greater than 8000 km 2 . Figure 3 shows generalized anisotropic model building workflow described by Zdraveva and Cogan (2011) that uses all methods and techniques described above. Figure 3 shows generalized anisotropic model building workflow described by Zdraveva and Cogan (2011) that uses all methods and techniques described above.…”

“…These profiles were then used together with a V P0 field from the second tomography iteration of the reference VTI model and a structural framework consisting of the water bottom and two major horizons to construct a TTI model using a multistep process described in details by Zdraveva et al (2010). As a reference we will use an existing, simple VTI model built using uncalibrated ε and δ trends hung from the water bottom and V P0 updated with three iterations of multiazimuth multiscale tomography.…”

“…For all of the examples, we build anisotropic models using variations of the generalized workflow described by Zdraveva and Cogan (2011) and compare the results against images and models from previous imaging efforts with isotropic or simple regional VTI models. Because many anisotropic models will fit a single surface-seismic data set, we evaluate the final model correctness not only on image focusing, reduction of residual curvature, and ties to well data, but also on the geological, rockphysics and geomechanical plausibility of the model and image.…”

Anisotropic model building in complex media: Comparing three successful strategies in deep water Gulf of Mexico

“…We closely follow this practice in all three case studies included in this paper. Figure 1 shows a schematic of the generalized anisotropic model building workflow described by Zdraveva and Cogan (2011) that allows geophysicists to build anisotropic models of any complexity and use all additional nonseismic information available for a given project. It has five main steps: (1) Evaluation of anellipticity over the full project area; (2) Derivation of Thomsen's δ and ε at well locations;…”

“…The three case studies included in this paper illustrate the importance of having a broad portfolio of tools and techniques that allow the design of fit-for-purpose model building strategies. For all of the studies, we build anisotropic models using variations of the generalized workflow described by Zdraveva and Cogan (2011) and evaluate the final model correctness not only on image focusing, reduction of residual curvature, and ties to well data, but also on the geological and geomechanical plausibility of the resulting model and image.…”