Print this pageInsights from Stratigraphic Modelling Applied to Subsurface Well-Log Interpretation
SHANLEY, KEITH W., and J. MICHAEL BOYLES
Understanding the detailed architecture of shallow-marine and nonmarine strata is dependent on a high resolution chronostratigraphic framework based on careful correlation and mapping of stratal surfaces. In a succession of siliciclastic deposits in which lateral variations in sediment supply are likely to have occurred, the most robust, regionally persistent, time-significant surfaces are associated with sequence-boundary unconformities. We show the results of forward stratigraphic models which indicate that despite pronounced changes in sediment flux, the physical expression and chronostratigraphic significance of sequence boundaries remains unaffected. Our models also indicate that the occurrence, number, and timing of condensed-section deposits are directly affected by changes in either the timing of sediment delivery, or the phase of sediment supply relative to changes in accommodation.
These models clearly illustrate the time-transgressive nature of sequence boundary development and provide insights to ongoing debates including the development of "forced regression" deposits and the identification of "downward stepping" progradation. Further consideration of these models in terms of progradation/aggradation ratios also provides insights into the development/destruction of accommodation space that can be applied to the interpretation of outcrops, cores, and well-logs. Examination of parasequence stacking patterns in terms of these progradation/aggradation ratios allows the placement of sequence boundaries to be more reliably interpreted and/or anticipated. We show examples from the Cretaceous of the Western Interior, the Carboniferous of the Midcontinent. and the Miocene of the Gulf of Mexico that illustrate specific subsurface applications of these concepts.