Talk Title: Fluvial Reservoir Architecture – Examples from the Mesaverde Group, Colorado and Wyoming
Speakers: Kurt Rudolf and Penny Patterson, ExxonMobil Exploration and ExxonMobil Upstream Research Company
Talk Date: February 17, 2006
Publication: The Outcrop, February 2006, p. 4
The Upper Cretaceous Mesaverde Group of northwestern Colorado (Williams Fork Formation) and south central Wyoming (Ericson Formation) contains up to 4,000 feet of interbedded fluvial sandstones and floodplain mudstones. Based on outcrop and subsurface studies, systems tracts are expressed in this non-marine succession by variations in net to gross. Lowstand Systems Tracts are characterized by low preservation of overbank mudstones, due to interpreted accommodation controls. This results in a vertically amalgamated, high net/gross succession of fluvial sandstones. Transgressive and Highstand Systems Tracts are low net/gross and poorly amalgamated, due to significant fine-grained floodplain deposits.
These alluvial successions reveal two differing styles of fluvial deposition and stratal architecture within the low net-to-gross stratigraphic intervals. One reservoir lithofacies association consists of fining-upward successions separated by well-developed lateral accretion surfaces. Hence, these fluvial deposits are interpreted to represent the classic architecture of point-bar deposits of meandering streams. These point-bar deposits are typically interbedded with coal beds and carbonaceous floodplain mudstones. A second lithofacies association contains fluvial sandstones with laterally and vertically amalgamated macroforms. Paleocurrent data and interpreted stratal geometries indicate that these macroforms accreted oblique and downstream within the channel scours. These channel fills are interpreted to have developed within low-sinuosity braided streams. Flashy discharge is indicated by thin current-rippled caps on thick trough cross-bedded intervals. Because both the meandering and low-sinuosity braided streams are developed with mudstone-prone floodplain deposits, the variation in style of fluvial deposition is most likely related to the presence/absence of vegetation (i.e. coals) and associated bank stability.