Diffraction imaging of lithology within polygonal fault zones in the Austin Chalk and Eagle Ford Shale, Maverick Basin, South Texas
Polygonal fault systems (PFS) are common structural features of intracratonic continental margins. The map-view geometry of these faults becomes 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. Although seismic diffractions contain amplitude information, they are not commonly used in interpretation because they are not available to interpreters. In this presentation, we extract a 3D diffraction volume from an unmigrated data set and transform it into an amplitude envelope to image faults and examine lithological composition in fault zones within the Austin Chalk and Eagle Ford Shale in South Texas. Cross plots between diffraction amplitude envelope and VCLAY extracted within the fault zones show that diffraction energy increases with increasing VCLAY. The analysis further reveals that fluid-saturated zones are associated with high diffraction energy, suggesting higher impedance contrast at diffraction points. These observations are not peculiar to the Eagle Ford Shale or Austin Chalk, but have also been observed in the Bazhen Formation and the Orenburg Region in Russia. Therefore, we conclude that by analyzing seismic diffractivity, it is possible to infer fault zones that would likely be clay-rich or be fluid-saturated within the Eagle Ford Shale and Austin Chalk and other areas with similar geology.
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