Detecting and Overcoming Misleading Seismic Amplitude Anomalies
in the D Sandstone, Denver Basin
Diversified Operating Corporation, Golden, CO
Fairfield Industries, Lakewood,
The “D” is a Denver Basin upper Cretaceous siltstone and sandstone that produces hydrocarbons from both shoreline and incised valley-fill deposits. The non-productive “D” facies is a regional siltstone and is usually thinner and much lower in porosity and permeability than the shoreline and valley-fill “D” reservoir deposits. Thickness changes in the “D” will typically generate sub-resolution amplitude changes in P-wave seismic data and, therefore, conventional seismic-stratigraphic imaging can help discriminate between productive and non-productive “D” deposits.
However, some high-amplitude anomalies have been drilled which do not result in the penetration of thickened “D” interval. One of the complicating issues affecting the “D” reflection is listric-faulting in the overlying Niobrara formation. Reflections and diffractions from high-impedance Niobrara formation may interfere with the “D” reflection, and conventional seismic processing often fails to recover the proper “D” amplitudes.
Another issue affecting “D” reflection amplitudes is amplitude variation with source-receiver offset (AVO). Misleading amplitudes in the “D” reflection stack occur where seismic acquisition limitations create highly variable trace offset distribution between CMP bins. These changes are particularly insidious with the gradual reduction in offset range around the perimeter of 3-D surveys and in under-shoot regions within the survey.
Pre-stack time migration is helpful in reducing the Niobrara-D_SS interference anomalies, and more uniform offset distribution in stacking is fundamentally important in addressing AVO-induced amplitude anomalies. Also, coherent-noise reduction treatments applied before stack will help seismic gain-recovery schemes to meet model assumptions and improve amplitude normalization.
AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming