Good-bye to the Upper Cretaceous Seaway – Models for Lewis, Fox Hills, and Lance Strata in the Greater Green River and Wind River Basins

Mike Hendricks, Hendricks and Associates, Inc, 6950 South Holly Circle, Suite 101, Centennial, CO 80112, phone: 303-843-9890, [email protected]

Rapid filling of Rocky Mountain basins occurred near the end of the Upper Cretaceous (Maestrichtian). Depositional environments associated with this final regression include shelf, slope, and turbidite-basin deposits of the Lewis Formation, shoreface deposits of the Fox Hills Formation, and coastal plain and fluvial plain deposits of the Lance Formation. This talk will emphasize the Fox Hills and Lance strata and their reservoir potential.

The Fox Hills was deposited in near shore and shoreface environments. Mesotidal deposits occur along the western edge of the Great Divide and Washakie basins and deltaic deposits are common in the central and eastern portions of these basins. Reservoir potential exists in shoreface environments where lobes of sand are encased in shale or pinch out into shale. Slope channels, related to low stand or bypassed distributary channels, are also potential reservoirs.

Lance deposits in the Jonah area and throughout the Wind River Basin are fluvial plain sediments that were deposited in single to multi-story, braided channels and low to highly sinuous meandering channels. A west to east depositional change from fluvial plain to coastal plain environments occurs in the Lower Lance across the Rock Springs Uplift. Productive channel sandstones within the coastal plain are locally interbedded with coals. East of the Rock Springs Uplift, intertonguing of Fox Hills shoreface environments occurs within coastal plain deposits. Potential reservoirs exist in these marginal marine and marine sandstones.

In the Wind River Basin, Lower Meeteetse coastal plain strata are time equivalent to the Lower Lance in the Great Divide and Washakie basins, but channels systems are poorly developed and only marginally productive.