Comparing Fluvial Sand-Body Stacking in Paleocene-Eocene Willwood (Bighorn Basin, Wyoming, U.S.A.) and Wasatch (Piceance Basin, Colorado, U.S.A.) Formations

Abstract

Analysing the distribution, geometry and sedimentary character of multi-storey sand bodies (MSBs) provides insight into the local and regional scale morphological processes that influence deposition in fluvial systems and control the character, distribution, and connectivity of fluvial hydrocarbon reservoirs. Previous studies have observed relationships between channel and floodplain deposits in outcrops in order to understand the role of allogenic and autogenic controls on basin architecture. Basin-filling models have also been used to evaluate the effect of sediment flux, river discharge, aggradation rate, floodplain sedimentation, and subsidence on fluvial stratigraphy. However, reconciling predictions from models with field data can be difficult due to scale differences between model results and outcrop exposures. In this study, we measured the scale and distribution of MSBs using photo panels and digital outcrop models of two Paleocene-Eocene fluvial deposits in the western U.S: the Willwood Formation (Bighorn Basin, Wyoming, USA) and the Wasatch Formation (Piceance Basin, Colorado, USA). These units were deposited during the Paleocene-Eocene Thermal Maximum (PETM) climate change and show a range of concomitant channel, floodplain, and avulsion changes. Using a scaled 2D basin-filling model, we evaluated MSB stratigraphic architecture under a variety of channel, floodplain, and sediment supply conditions that were possible under PETM climate change. We compare model results to outcrop datasets in order improve our understanding of how basin-scale architecture is influenced by changes in sediment supply, river migration and channel avulsion. Preliminary results suggest that Willwood rivers were relatively insensitive to climate forcing during the PETM, while Wasatch rivers changed significantly. MSB stacking patterns in the Willwood formation may show increased connectivity due to a possible increase in relative avulsion frequency, but might also be consistent with large-scale autogenic avulsion clustering. Significant fluvial changes across the PETM in the Wasatch Formation led to distinct changes in MSB stacking during basin filling. The differences in response of these two systems indicates that the sensitivity of river systems to changes in sediment flux, floodplain topography and cohesion, channel aggradation rate, and discharge variability can significantly influence reservoir connectivity.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018