Environmental Variability and Autogenic Stratigraphy of the Frontier Formation, Powder River Basin, Wyoming, USA

Abstract

The Upper Cretaceous Wall Creek member of the Frontier Formation in Wyoming's Powder River Basin contains actively produced unconventional hydrocarbon reservoirs. Continued success of development of this unit partly depends on its detailed characterization. We have collected available cores and well logs from the southwestern part of the basin. Additionally, eight full stratigraphic sections and two partial stratigraphic sections were measured along a north-south traverse at the western margin of the basin. These data offer insight into local-scale and basin-scale variability of depositional environments represented by the lithofacies of the Wall Creek member. Measured sections exhibit 1–4 shallowing upward parasequences that grade up from shales, to thinly interbedded sands and shales, to amalgamated sandstone packages. Overall, there is an increase in the sandstone to shale ratio from south to north. However, the total thickness of the Wall Creek Member increases to the south. Sandstone packages range in thickness from 8 to 26 meters and contain a range of environmental and sediment transport indicators. Paleocurrent measurements from cross-stratification imply a diverse but southerly paleo-transport direction overall. Bioturbation via vertical and horizontal burrows of the thinly interbedded facies is common but not universally present. The degree of bioturbation, sandstone to shale ratio, and ichnofacies assemblages vary between and within outcrops. Some outcrops contain only Cruziana ichnofacies, whereas others contain a mixture of Cruziana and Skolithos ichnofacies. These observations support the hypothesis that these strata represent a range of depositional environments, from mixed river-wave-dominated deltaic to near shore. It is clear that there was greater accommodation in the south but that the fluvial sediment source was located in the north near Kaycee, Wyoming. Coupled with southerly longshore transport, the system preserved an asymmetrical delta and a variety of near shore lithofacies. We interpret the non-systematic variability of stacking patterns and sedimentary structures between outcrops as the result of autogenic processes in this complex depositional system.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015