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Abstract

A detailed study of outcrops, cores, and well logs in the Upper Cretaceous Lewis Shale and Fox Hills Sandstone of south-central Wyoming indicates that the formations were deposited in a transitional shelf-slope-basin setting during a time of rapid relative changes in sea level and sediment supply. The relative changes in sea level and sediment supply affected the location and style of sand and mud deposition. Sequence analysis from outcrops, cores, and in well log correlations was used to interpret a high frequency sequence stratigraphic framework along a regional, 65 km (40 mi.) long, depositional dip-oriented cross section.

High frequency lowstand, transgressive, and highstand systems tracts are interpreted in the sequence stratigraphic framework. Lowstand systems tracts can be divided into the basin floor fan, slope fan, and prograding complex. The basin floor fans comprise continuous thick-bedded sandstones and thin-bedded mudstones that bi-directionally downlap the basin floor in the well log cross section. Slope fans comprise channelized, discontinuous sandstones, thin-bedded continuous sandstones, and continuous mudstones that downlap the basin floor fan and onlap the slope in the well log cross section. Prograding complexes comprise thin-bedded, continuous sandstones and mudstones that downlap the slope fan and basin floor fan and onlap the upper slope and shelf in the well log cross section. All the lowstand systems tract deposits are good candidates for reservoirs and stratigraphic traps when deposited beyond the shelf-slope break.

Transgressive systems tracts comprise highly continuous, organic rich shales when deposited below storm wave base and continuous interbedded sandstones and mudstones when deposited above storm wave base. They onlap the coastline and downlap the basin floor in the well log cross section. They are potential source rocks and seals.

Highstand systems tracts comprise continuous, thick-bedded sandstones and thin-bedded mudstones when deposited above storm wave base and continuous thin-bedded sandstones and mudstones when deposited below storm wave base. They onlap above the coastline and downlap on the shelf and upper slope in the well log cross section.

Development strategies in Lewis fields should integrate a regional perspective with time-based correlations, which allows for the interpretation of the different types of lowstand systems tract deposits, their associated depositional environments, net-to-gross, lateral continuity, and vertical connectivity.

The Lewis Shale and Fox Hills Sandstone can be used as an outcrop and subsurface stratigraphic analog to mudrich turbidite reservoirs, most notably some Gulf of Mexico and offshore South Africa turbidite reservoirs.

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