Azimuthal anisotropy for fractured reservoir characterization in the presence of structural dips

Abstract

In this work, the effect of natural fractures on the azimuthal response from structured reservoirs encased within carbonate rocks is investigated through forward modeling. Two fractured carbonate reservoirs are examined: the first one is composed by sediments nearly flat; the second one is in an area with reflectors having dip up to twenty degrees. When modeling fracture behavior, even if fractures are sub-vertical, it is usual to suppose that they are vertical. For dipping reflectors, even if fractures are supposed to be vertical, they are not normal to vertical versus the dipping reflectors. Thus, the reflections from a dipping reflector above vertical fractures are quite similar to reflections from a flat reflector with inclined fractures below. We compare synthetic seismic data from two different models having respectively vertical fractures and sub-vertical fractures (with an inclination of 70º). Fracture inclination perturbations give azimuthal variations especially on results of converted waves. Indeed, reflections from dipping interfaces present azimuthal variations of amplitudes even for isotropic media. For a model composed by several dipping layers, we investigate the combined effects of fractures and dipping reflectors. In particular, we discuss Amplitudes versus Offset and Azimuth (AVOA) of converted waves that, at present, have been used just for their splitting and we show how AVOA can help fracture characterization.