Abstract

In 2001, a wide-azimuth 2C/3D OBS survey was acquired over an offshore carbonate field in the Middle East, and processed with techniques specifically designed to preserve and extract the azimuthal effects in the data. The results showed significant azimuthal anisotropy in amplitude, with apparent azimuthal variation in the P-wave AVO slope of as much as 100% at selected target horizons. The azimuth of the most positive slope was generally NW/SE, in agreement with the regional tectonic trend. The magnitude of the anisotropy varied markedly, with patches of strong anisotropy, with a granularity of 10's to 100's of metres, and a regional trend on the 2km scale. Vertical variation of these patches supports the hypothesis that the effects are due to subsurface anisotropy, rather than acquisition artifacts.

Introduction

A well-established technique used on surface reflection seismic data is Amplitude-Variation-with-Offset (AVO). AVO uses the amplitude of seismic reflection at a given horizon, as a function of increasing source-receiver offset distances, to infer lithological and fluid properties at that horizon. AVO analysis may also examine raypaths of varying source-receiver azimuths, in what is known as Amplitude- Variation-with-Offset-and-Azimuth (AVOA). With additional assumptions, AVOA allows the determination of fracture strike direction and fracture density. Such information may be interpreted and integrated with reservoir models to infer the localized stress field, tensor permeabilities, and fluid-flow directions. In contrast to methods (such as seismic coherency) that determine large-scale faults, AVOA analysis determines media properties much smaller than the seismic wavelength. These can be key to understanding a reservoir.

AVOA was first reported by Lynn and Thomsen1 for land data, and by Mallick and Frazer2 and Lefeuvre3 for marine data, with theory first reported by Thomsen4. For conventional 3D marine seismic surveying techniques, the narrow distribution of source-receiver azimuths precludes detailed azimuthal studies on P-wave data. Only recently, through the use of Ocean Bottom Seismic (OBS) surveys, has marine data possessed azimuthal distribution appropriate for AVOA analysis.

A modern 2C/3D OBS seismic survey over a carbonate field in the Middle East was acquired in 2001 with a full distribution of offsets and azimuths. In addition to the conventional processing flow, designed for proper imaging of the field, special techniques were applied to preserve and extract the azimuthal information for physical characterizationof the subsurface. The results yield the only small-scale picture of the fracturing pattern that is possible away from well control, and are therefore likely be extremely important in future reservoir management.

Theory

AVOA exists because fractures and other small features in a formation cause seismic properties (such as reflectivity and velocity) to vary with azimuth, in what is known as azimuthal anisotropy. By contrast, polar anisotropy is due to a fabric or pattern in a rockmass such that the elastic properties vary with polar angle (the angle from the vertical) only; this is the simplest form of anisotropy. However, when the horizontal azimuths are not all equivalent, as (for example) in the case of vertical parallel fractures, seismic properties will vary with azimuth (compass direction). In a simple case, the equations which govern the seismic waves are simply those of polar anisotropy, but with the polar axis horizontal, instead of vertical. In a more realistic case (i.e. the case of shales or thin-bedded sequences with a set of vertical fractures), the equations are those of orthorhombic symmetry (cf Tsvankin5).

Azimuthal anisotropy is typically attributed to fractures, cracks, and microcracks, aligned by tectonic paleostress and current stress in the reservoir (Crampin6), although deposition history and style may be influential (Sayers7). As anisotropy is controlled by sub-wavelength properties of the rockmass, an understanding of azimuthal anisotropy is a window to the patterns of fractures and cracks pervasive within a formation.

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