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Seismic Characterization of Thin Interbeds Guided by Seismic Geomorphology and Waveform Analysis in Wheeler (Stratal-Slice) Domain

Abstract

Prediction of seismically thinly interbedded reservoirs has long been a challenge. Although the net thickness can be estimated from composite amplitude, the number of interbeds and their localities are believed to be impossible to determine. Major difficulties include the facts that (1) the top and base of a thin bed are not resolved by seismic data, and (2) the length of the seismic response window is significantly larger than the thin-bed thickness. As a result, a 3D seismic image of a thinly interbedded section is a mixture of individual interbed responses and interferences of multiple interbeds, which is at most a half-true representation of the reservoirs. I introduce a seismic sedimentology-based workflow that would shed light on dealing with the difficult problem of improving resolution of stratigraphic analysis of thin beds. The workflow starts with creating stratal slices for amplitude display of thin interbeds in Wheeler domain. The thin-bed images can be interpreted in terms of seismic geomorphology for their depositional (facies) meaning. However, the superimposed facies patterns of neighboring units on stratal slices prevent a clear interpretation of individual systems and their spatial relationships (stacking pattern). A waveform analysis of each facies (depositional element) along a relative geologic-time axis can help restore waveform for each of the individual facies. The center of a thin bed can be picked at the maximum amplitude (for 90-degree data) or zero-crossing (zero-phase data). The systematic travel-time differences between individual thin-bed waveforms reveal possible depositional history (stacking pattern). A geologically realistic thin interbed reservoir model was created for this study. The procedure was tested in a field data set collected in a complex Cretaceous fluvial system, which demonstrates the utility of the concept and methodology in mapping thin-bedded reservoirs and in helping studies of seismic geomorphology.