Lowstands from the Rise?

Sherry L. Becker
ExxonMobil Exploration Company, Houston, TX

The concept of “dislocated sequences” is commonly invoked to interpret carbonate sequences deposited during sea-level fluctuations of high amplitude. These comprise lowstands (if any) deposited in basins with corresponding transgressive and highstand systems tracts along platform margins and tops. Principal to such interpretations is the notion that sea-level rise dominates carbonate deposition, i.e., carbonates tend to “catch-up and keep-up” with sea-level rise.

On the contrary, this study illustrates that carbonate lowstands can occur both in basinal positions and on carbonate platform tops. Deposition occurs on platform tops during sea-level falls of higher order and also during relative sea-level lows. This study further recognizes “complete sequences”, which form both in the basin and on the platform top or margin. The governing control behind these carbonate geometries seems to be the existence of an “optimal accommodation zone”. That is to say, carbonate deposition can occur at any "point" where relative sea-level intersects the carbonate platform in time and space (accounting for lag time).

Forward sediment modeling shows that systems tracts may indeed be deposited as complete basinal or platform-top sequences, and many platform-top successions can have robust, thick, progradational, lowstand deposits with corresponding thin, transgressive to highstand deposits. Any expectation for diagenesis of platform-top lowstands or sequences is markedly different than current paradigms of subaerial exposure, and thus has significant implications for the prediction of reservoir quality.

This method provides a tool to decipher the juxtaposition of carbonate stratal geometries and facies (misclassification and thus miscorrelations) and to troubleshoot reservoir productivity.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas