Azimuthal Offset-Dependent Attributes (AVO And FVO) Applied To Fracture Detection

Abstract

Using the amplitude versus offset (AVO) and the frequency versus offset (FVO) information, the diagnostic ability of P-wave seismic data in fracture detection is investigated. The offset-dependent attributes (AVO and FVO) are estimated by using an eigenvector based estimation technique, the multiple signal classification frequency estimator. These attributes are applied to the determination of principal orientation of fractures in carbonate fractured reservoirs located in the Maporal field in the Barinas basin of southwestern Venezuela. Our studies show that, in the crack normal direction, P-wave reflectivity is characterized by a large increase of amplitude with offset (large positive AVO gradient) and a large frequency decay with offset (large negative FVO gradient). In the crack strike direction, P-wave reflectivity is characterized by a wide range of AVO gradients but a small variation of FVO gradients. The analyses of inverted offset-dependent velocities and theoretical calculations show that the lateral heterogeneity in the reservoir zone can lead to large variations of AVO signatures. The offset-dependent frequency attribute can help lessen the ambiguity in fracture detection. The combination of the offset-dependent frequency attribute is more beneficial than using the offset-dependent amplitude attribute alone.