ABSTRACT

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. Isopach thicks of detrital sediments indicate lows on the depositional surface and isopach thins indicate highs. Carbonates, because they include rocks which may form by detrital, non-detrital, biogenic, or chemical processes at the site of deposition, may build up as thicks over antecedent topographic highs. Siliciclastics 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. Lime sands can also accumulate in updip locations on ramps (Smackover), or in downdip locations at slope breaks (south Florida and the Permian Capitan facies, for example). As long as there is a time reference, ramps and shelves are easy to distinguish. Confusion may occur when three-dimensional sequences (time surfaces = the third dimension) are encountered, the shape of the depositional surface is not known, and stratigraphic correlations are based only on rock type or seismic reflection surfaces. Detrital and non-detrital rocks do not form in the same ways, they may not be retained in equivalent places on platforms and they react differently to sea level changes. It follows that facies prediction and basin analysis must take into account the differences between detrital and non-detrital sedimentation and that ramps and shelves can be defined in two ways: (1) in the sedimentological sense where deposition occurs on a bedding plane (a single time-surface); and (2) in the time-stratigraphic sense where juxtaposition of facies in the final, time-stacked depositional body will vary as a function of the rate of sediment input, sediment dispersal, and sediment retention over time. In the latter case, distinctions between ramps and shelves and the locations of attendant reservoir facies will be accomplished only to the extent that time-rock correlations are known and used.