ABSTRACT

The scarcity of large-diameter cores in frontier wells has revived interest in grain-size analyses as an alternative tool for establishing depositional environments. Unlike sedimentary structures, which require diamond core for identification, size-frequency distributions can be computed from thin sections of sidewall cores and even cuttings. Samples of known environmental facies from widely separated areas were selected and analyzed to re-evaluate this approach.

Grain-size distributions of ten sandstone-body types (braided stream, point bar, beach, tidal flat, dune, stream-mouth bar, distributary channel, interdistributary beach, density and debris flow) are analyzed by three different techniques: 1) moment measure cross-plots; 2) factor analysis, and 3) graphic plots. The first two techniques fail to provide reliable facies identification. Graphic log probability plots, which are more sensitive to subtle variations at the tails of the grain-size spectrum, provide meaningful results, particularly when used in conjunction with other well data. Characteristic curves are presented for each environment and are discussed in terms of fundamental sedimentary processes.

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