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Colorado School of Mines, Golden, Colorado

Abstract: An Outcrop-Based Sequence Stratigraphic Framework for the Upper Cretaceous Lewis Shale and Fox Hills Sandstone, South-Central Wyoming

Although the Upper Cretaceous Lewis Shale and Fox Hills Sandstone of southcentral Wyoming contain an estimated total resource of 10.7 Tcfg, only 0.6 Tcfg has been exploited because reservoirs are difficult to target and produce. This is probably because of wide variations in sandstone geometries and distributions, at least in part. Well-exposed outcrops of these formations along the flanks of the Great Divide and Washakie basins, and a wealth of subsurface data in the basins, provide an excellent opportunity to develop a predictive tool to determine sandstone geometry and distribution.

A fourth-order sequence stratigraphic framework has been developed for the Lewis Shale and Fox Hills Sandstone from outcrops, well logs, and 2-D seismic data. Five fourth-order depositional sequences were recognized and mapped. The sequences were deposited in transitional shelf, slope, and basin environments. Outcrop investigations suggest Lewis reservoir sands were deposited as laterally discontinuous channel deposits and laterally continuous sheets by turbidity currents in a relatively deep-water setting (below storm wave base). The sequence stratigraphic framework shows that thin-bedded sheet deposits usually occur on the upper slope, channel deposits occur on the middle and lower slopes, and thick-bedded sheet deposits occur on the lower slope and basin. Comparison of outcrop gamma-ray signatures of laterally discontinuous channel deposits and laterally continuous sheet deposits integrated with core has provided criteria to predict sandstone geometry from well-log response and position within a depositional sequence.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana