Previous studies of grain size as an indicator of sandstone depositional environments have had mixed results; for that reason, the method is seldom if ever used as an exploration tool. The problem with these previous studies is that they concentrate on grain-size parameters for individual samples, and fail to recognize that various depositional environments are characterized by a number of discrete facies, each with its own characteristic sedimentary structures and grain-size populations. For that reason, vertical progressions in grain-size data are far more diagnostic of depositional environment than are scatter diagrams in which one grain-size parameter is plotted against another.
Grain-size progressions for ancient sequences of the Upper Cretaceous Point Lookout formation and Eocene Queen City Formation demonstrate the value of the technique. The data show that distinctions can be made between fluvial, estuarine/tidal distributary, flood-tidal delta, foreshore, and shoreface sandstones. More importantly, our method relies on analysis of samples collected at random intervals (generally 2 ft or .6 m), so that it is applicable where sidewall cores are available. Diagenetic complications (other than those resulting from silica cementation) do not appear to threaten the sensitivity of the method. Using automated settling analysis, these data are obtained rapidly and can aid in subsurface correlation as well as determining the depositional environments of sandstones.