Stratigraphic Mistakes and Forced Models: Lessons Learned from Flood-World, Flat-World and Lowstand-World Correlations
C. Robertson Handford
Strata-Search, LLC, Austin, TX
Stratigraphic interpretations, new and old, should always be tested and subject to change as concepts evolve and new data become available. If not, the interpretations can become entrenched and blindly adhered to over time, which can lead to a stagnation of new ideas and concepts. Companies, which lack creative approaches and do not challenge old paradigms, will likely develop incorrect exploration and reservoir models. The stratigraphic framework of some of the world's most productive fields and stratigraphic units are now being reinterpreted because new data (cores, 3D seismic) cannot support the current models. Erroneous models commonly result from faulty stratigraphic concepts, which generally fall into three categories. (1) Flood world stratigraphers do not recognize sequence boundaries and they datum well-log sections on “the first shale above reservoir sand.” This is exemplified by a reservoir characterization project in which the investigators erroneously correlated a Miocene sandstone reservoir as a layercake rather than as a time-transgressive series of retrogradational parasequences overlying a sequence boundary. (2) Flat-world stratigraphers correlate and interpret Cretaceous and Jurassic carbonate successions in the US and the Middle East as layercake shelves and ramps but careful correlations tied to seismic show greater than expected depositional relief (3) Lowstand-world stratigraphers interpreted sequence boundaries at most sand-shale contacts (sharp and gradational) separating bedsets and clinoform bodies in Miocene strata. This is unreasonable in light of comparison to similar features in modern deltas where many sand-shale bedset/clinoform-boundary contacts are created by autogenic processes associated with progradation, delta-lobe switching, and flood-events.