Since the inception of complex seismic trace analysis, seismic attributes have proven useful in reservoir characterization. Amplitude envelope (AE), also known as reflection strength, has been particularly successful in delineating faults and porous zones in hydrothermal dolomite (HTD) environments. Recent volume-based 3D seismic attributes and neural network studies show that the sites of highest porosity are found to be associated with very low AE values. The reason that low AE values are recorded in fault zones is poorly understood. In this paper, we construct a numerical model to investigate the occurrence of low AE values recorded in fault and porous zones. Our results show that fault zones, particularly zones bounded by parallel, vertical to subvertical faults have low AE values because common depth point (CDP) gathers from within, or in proximity to, the fault zones have a low stacking fold. Such a low fold occurs because of reflection phenomena at the fault boundaries.
The importance of amplitude envelope (AE), also known as reflection strength in reservoir characterization and hydrocarbon detection, was discussed by several authors (e.g., Taner et al., 1979; Tebo and Hart, 2005; Anstey, 2005). Taner (2001) noted that AE represents the total instantaneous energy of the input seismic traces, and relates directly to the acoustic impedance contrasts. It is computed as the square root of the sum of squares of real and imaginary parts of the complex seismic trace. It combines with instantaneous frequency to produce “sweetness,” an effective attribute in channel delineation (Radovich and Oliveros, 1998). The effectiveness of AE at predicting porous zones and fault zones in a hydrothermal dolomite environment was described by Hart (2008), and Ogiesoba (2007). These authors reported that fault zones are associated with low AE values. On AE time slices, because AE values are always positive, geologic features such as faults and porous zones with low AE values are easily distinguishable from surrounding rocks with high AE values.
Although AE attribute is very useful as a porous and fault zones discriminator, the reason these geologic features exhibit low AE values is not very well understood. Taner attributed the cause to the difference in acoustic impedance between rocks. Although this difference may exist, it does not account for the low AE values observed at fault boundaries and fault zones especially zones between vertical or subvertical faults. Al Darosar et al. (2004) showed that weaker amplitudes and hence lower AE values are recorded in fracture zones than in areas distal from the fractures. They attributed this difference to azimuthal anisotropy. In this extended abstract, we construct a numerical model to investigate reflection phenomena in fault zones and to account for the low AE values associated with fault zones seen in Rochester field, Ontario, Canada.
Briefly, the area of study lies in the St Lawrence Lowlands, which are underlain by Grenville basement rocks of Precambrian age. These are unconformably overlain by the Cambrian clastics. Overlying the clastic rocks is the Black River Group of Middle Ordovician age.