Building Anisotropic Models for Depth Imaging: Comparing Different Approaches

Abstract

Seismic images are only as good as the velocity models used to produce them. As we move from "easy oil" to "difficult oil" targets in sub-salt, sub-basalt and deep complex areas, we can no longer build the simple isotropic models of the past. To fully leverage the potential of new data types (e.g. wide azimuth and long offsets), we have to move to anisotropic imaging with vertical or tilted transversely isotropic (VTI or TTI) models in all geological provinces. Incorporating anisotropy increases our ability both to focus the seismic data and to accurately position the reflectors for drilling decisions. While these goals are achievable with anisotropic models, they are only met when geology information and data from boreholes are intimately incorporated into velocity model building from the very start. We discuss several different approaches for anisotropic model parameter estimation and we illustrate some of the possible strategies with examples from the Gulf of Mexico and West Africa.

FEW CONSIDERATIONS, WARNINGS AND BENEFITS FOR THE E&P INDUSTRY WHEN INCORPORATING STRATIFICATIONS INSIDE SALT SECTIONS

“…The quality of seismic images depends on the acquisition parametrization as well as the processing strategy, especially in terms of the applied migration process (Yilmaz, 2001). This same author has emphasized that considering the structural complexity when building the seismic image has a tremendous influence in reproducing a model faithfully.…”

“…The anisotropy consideration when building velocity models in order to generate more realistic seismic images have become increasingly obvious (Cogan et al, 2011;Zdraveva et al, 2011;Cooke et al, 2012). Anisotropic aspects involved in velocity model building depart from well information when comparing expected and observed results.…”

Improving Pre-Salt Reservoirs Seismic Images When Considering the Stratified Evaporites Insertion in the Initial Model for the Velocity Updating Processes Prior to the Seismic Migration

“…Ji et al (2011) defend the idea that homogenous velocity for the evaporitic section affects the images quality under the salt section. They cite the weakness and discontinuity in seismic reflection, diffractions-like events and misinterpretation as the main problems when choosing those homogeneous compressional velocity models over more heterogeneous ones.…”

“…Raymer et al (1999Raymer et al ( , 2000 state that the salt extrinsic anisotropy could be over than 7% in the preferred direction of evaporation. Landrø et al (2011) analyze salt-mine outcrops and infer a moderate degree of anisotropy in the order of 5% difference between horizontal and vertical velocities. Several authors consider that anisotropy for any geological layer is mandatory in order to generate more realistic seismic images (Cogan et al, 2011;Zdraveva et al, 2011;Cooke et al, 2012).…”

Evaporitic Velocity Modeling Uncertainties and Variabilities: Implications for Pre-Salt Projects in the Santos Basin, Brazil

“…Most halokinetic models used to reconstruct the basin's structural styles consider the salt section as almost homogeneous, with predominance of halite (Demercian et al, 1993;Ings et al, 2004;Guerra & Szatmari, 2009;Guerra & Underhill, 2012). The concept of salt flooding is a common approach in seismic processing, and involves the assignment of a constant velocity to the evaporitic section prior to the tomographic inversion process to update it (Guo & Fagin, 2002;Zdraveva et al, 2011). Ji et al (2011) defend the idea that heterogeneous salt velocity models improve the seismic imaging.…”