Sequence Stratigraphy in Fine-Grained Rocks: Beyond the Correlative Conformity
Kevin M. Bohacs, Jack E. Neal, George J. Grabowski, Jr., 2002. "Sequence Stratigraphy in Fine-Grained Rocks: Beyond the Correlative Conformity", Sequence Stratigraphic Models for Exploration and Production: Evolving Methodology, Emerging Models and Application Histories, John M. Armentrout, Norman C. Rosen
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Mudrocks provide the source and seal of hydrocarbons and are key elements in reservoir models as baffles and barriers. Sequence stratigraphy provides an excellent framework within which one can integrate the many scales of physical, chemical, and biological observations necessary to understand these rocks across the spectrum of depositional settings. Although flooding surfaces and depositional-sequence boundaries may be subtly expressed in mudrocks, they can be recognized through distinct changes observed in core, outcrop, well-logs, and on seismic data. Beyond the chronostratigraphic utility of the correlative conformity, abundant paleoenvironmental information is recorded in fine-grained strata—depositional sequences do not just fade away into obscurity in distal reaches, but have objective attributes that allow extension of stratigraphic frameworks and play-element predictions over very large areas.
Flooding surfaces fundamentally record a critical increase in accommodation relative to sediment supply, commonly recorded in mudrocks by laterally extensive accumulations of authigenic and pelagic components, along with evidence of sediment starvation and low bottom-energy levels. Even in mudrocks, some may record minor erosion, reworking, and lag formation due to low sediment supply, but all are marked by a significant decrease in advected clastic input—contrasting with sequence boundaries.
Depositional sequence boundaries record a critical decrease in accommodation relative to sediment supply, commonly accompanied by an increase in depositional energy or a significant change in sediment supply, or both, over hundreds to thousands of square kilometers in both fine- and coarse-grained lithologies. This is recorded even in fine-grained lithofacies by regional erosional truncation associated with subsequent onlap, exposure, reworked fossils, decreased continuity at lamina to bedset scale, along with increased accumulations of advected clastics and fossils or secular changes in biogenic lithology. All of these attributes (except subaerial exposure) are observed in physically correlative distal reaches of unconformities across their correlative conformities.
Interactions of sediment supply and accommodation with pre-existing topography control the expression of depositional sequences. Marine environments tend to have the most widespread, gradually varying facies tracts, whereas paralic facies tracts tend to be most localized and abruptly changing. Lacustrine sequences vary according to lake-basin type and range from very similar to shallow-marine siliciclastic sequences to very dissimilar.