Abstract

Polygonal fault systems are common structural features of intracratonic continental margins. The map-view geometry of these faults became apparent with the use of powerful fault-imaging seismic attributes, such as coherence and curvature. However, these attributes lack the amplitude information necessary for lithological evaluation. We developed a 3D diffraction volume that not only imaged faults but also contained amplitude information. From the unmigrated stack volume, we extracted diffractions that were transformed into amplitude envelope and root-mean-square amplitude volumes. These attributes, together with clay volume (Vclay) data, were extracted along interpreted horizons and fault planes. Crossplots between seismic attributes and Vclay enabled linear relationships between the attributes and Vclay, which were used to infer lithological composition within fault zones. Our results found that, although the fault zones were clay filled, some subvertically inclined clay-poor zones that could serve as permeable pathways were present along the fault planes. In map view, images from diffraction volume were comparable with those obtained from coherence and curvature attributes; however, diffraction images appeared to be busy because of the huge number of diffracted waves embedded in the data. In addition, we found that, although Vclay increases with increasing diffraction energy, no systematic relationship exists between Vclay and curvature, or between Vclay and coherence. As such, curvature and coherence cannot be used to predict lithological distribution within fault zones. Furthermore, we observed that the higher the diffraction energies, the higher the fluid saturation, suggesting higher impedance contrast at the diffraction points. Therefore, we determined that by analyzing diffraction data, it was possible to infer likely sediment variations that largely control permeability within fault zones.

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