Time-Rock Correlation: Essential Element in Depositional Models

Wayne M. Ahr

Predicting the trend of a potential reservoir facies in modern clastic or carbonate deposits requires a knowledge of the interplay between sediment source, physics of the depositional environment, and topography of the depositional surface. Detrital "thicks" indicate lows on the depositional surface and "thins" indicate highs. However, carbonates may build up as thicks over highs. Clastics may accumulate in shallow water depocenters linked to coastal progradation (the Gulf Coast). Sands updip and muds downdip indicate updip retention and winnowing of imported allochthonous clastics. On the other hand, carbonates can accumulate as lime sands updip on ramps (Smackover), or as lime sands or reefs downdip on slope breaks (parts of the Cotton Valley, the Guadalupian, and the S uart City trend of Texas). As long as there is a time reference, the distinction between ramps and shelves is relatively simple. Confusion can result with three-dimensional sequences where time surfaces--the third dimension--are not known and correlations are based on rock type or seismic reflection surfaces alone. Clastics and carbonates do not form in the same ways, they may not be retained in equivalent places on similar but separate platforms and, as has been demonstrated recently, they react differently to sea level changes. It follows that facies prediction and basin analysis must take into account the differences between clastic and carbonate platforms and define ramp and shelf-type platforms in two ways: (1) in the sedimentological sense where deposition takes place on a single t me surface, and (2) in the time-stratigraphic sense where geometry and juxtaposition of facies in the final, time-stacked entity may vary with changes in the rate of sediment input, dispersal patterns, and in both the site and rate of sediment retention over time. In the latter case, distinctions between ramps and shelves and locations of reservoir facies may be accomplished only to the extent that time-rock relationships are understood.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.