Diffraction events are recorded along with reflection data during seismic acquisition. However, after processing, final migrated stack data are devoid of diffraction events, which have been collapsed to discrete points, smoothed out, and overshadowed by reflection events. Thus, diffraction events that ought to be available for analysis of the rock are lost.
In this presentation, we segregate diffractions, and then build a 3D diffraction volume that not only images faults but also contains amplitude information used to examine lithological composition in fault zones within the Austin Chalk and Eagle Ford Shale in South Texas. The interpretation reveals that zones with moderate to high diffraction energy correspond to zones that are moderately to strongly faulted, whereas zones with little or no diffraction energy correspond to zones with very minor or no faults.
We then transform the diffraction data into amplitude envelope volume. This seismic attribute, together with clay volume (VCLAY) data, is extracted along interpreted horizons and fault planes. Cross plots between seismic attributes and VCLAY show that VCLAY increases with increasing diffraction energy. In addition, we observe that the higher the diffraction energies, the higher the fluid saturation, suggesting higher impedance contrast at diffraction points. Furthermore, cross plots between instantaneous dominant frequencies extracted from the diffraction-image volume and amplitude envelope show that within the fluid- and hydrocarbon-saturated zones, the dominant frequency is approximately constant and in the low-frequency range between 25 and 33 Hz. However, outside the hydrocarbon zones, the dominant frequency increases as the amplitude envelope decreases. Maps of instantaneous dominant frequency extracted along the top and base of Eagle Ford Shale horizons reveal that areal distribution of low dominant frequency zones are coincident with high diffraction energy. Based on the foregoing, we conclude that by analyzing diffraction data, it is possible to infer likely sediment variation and fluid/hydrocarbon sweet spots within the Eagle Ford Shale and other shale resource plays.