Vitor Abreu: Simplified Guide to Sequence Stratigraphy

A Simplified Guide to Sequence Stratigraphy - Case Studies and Examples
In the past four decades the sequence stratigraphy jargon has proliferated, resulting in multiple definitions of the same surface or new surfaces and units based on drawings of deposition in response to relative changes in sea level. The close association between base-level changes, the formation of surfaces, and specific stratal stacking that define systems tracts are at the heart of the confusion.
This presentation proposes a back-to-basics approach, emphasizing key observations that can be made from any geologic data: lithofacies, lithofacies association, vertical stacking, stratal geometries, and stratal terminations. Parasequence stacking is a consequence of the interplay of accommodation and sediment supply, which are controlled by eustasy, tectonics, climate, and initial depositional profile.
Sequence stratigraphic surfaces are defined based on shoreline position, and systems tracts are defined based on shoreline trajectory; therefore, shoreline position and trajectory are in fact the key observations to be made in a sequence stratigraphic interpretation. Terms like lowstand, falling stage, transgressive and highstand should be replaced by observation-based terms like “progradation-aggradation”, “degradation”, “retrogradation” and “aggradation-progradation”. Finally, research remains to be done on the relations between stratal stacking and various controls, and on the formation and chronostratigraphic significance of surfaces that demarcate changes in stacking.
The interaction of tectonics and eustasy controlled the relative changes in sea level in the geologic past and is often hard to determine which of them is the leading control on a case-by-case basis. More importantly for the interpreter, the resulting stratigraphic architecture controlled by these relative changes in sea level are the result of the changes in the rate of creation and destruction of accommodation and sedimentation rate through time. The important part of this discussion is that the relative changes in seal level are not recorded directly in the geologic data. However, the changes in the accommodation creation and sedimentation rates are and can be directly measured from the geologic data.
Therefore, we support an interpretation method to build sequence stratigraphic frameworks based on the direct observation of the changes in accommodation and sedimentation rate through time. With this approach, it is possible to build internally consistent frameworks based on the physical relationship of strata, independent of time duration and magnitude of events, not tied to sea level curves and calibrated to age control. Another added advantage is that the same methodology can be applied to build stratigraphic frameworks in lakes or continental strata, as well as in deep marine or carbonate systems. Case studies will be shown from exploration to production to emphasize the importance of applying this simplified approach to achieve significant business results.