AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Spatial and Temporal Variations of a High Net-to-Gross Fluvial System: Middle Wasatch Formation, Three Canyons, Utah – An Outcrop Study of Fluvial Cyclicity
(1) Blue Stone Oil & Gas, Denver, CO.
(2) Chevron Center of Research Excellence, Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO.
(3) Denver Museum of Nature & Science, Denver, CO.
(4) Chevron Energy Technology Company, Houston, TX.
(5) Enhanced Oil Recovery Institute, University of Wyoming, Laramie, WY.
(6) Department of Geology, University of Colorado, Denver, CO.
The Eocene middle Wasatch Formation of the Uinta basin comprises high net-to-gross fluvial strata deposited in a foreland basin. At Three Canyons the entire middle Wasatch (355 m) is exposed on multiple canyon walls, enabling 3D characterization. This study documents spatial and temporal changes in architectural elements, facies and stratigraphic stacking patterns providing an outcrop analog for other high net-to-gross fluvial systems.
The middle Wasatch spans ~1.5 m.y. and contains three orders of stratigraphic cyclicity. One long-term cycle 355 m thick has a net-to-gross ratio up to 85%. Regionally extensive paleosols interbedded with muddy strata bound the cycle. The paleosols are laterally continuous in a strike direction for over 110 km and in a dip direction for ~12.5 km before continuing into the subsurface. These paleosols are interpreted to record a system-scale cessation of sand input, resulting in regional retrogradation along the southern part of the basin.
Four meso-term cycles are present in the study area. They average 85 m in thickness and can be traced in outcrop for 9.6 km and 6.4 km in a strike and dip direction respectively. The cycles are vertically asymmetric and contain the following from bottom to top: 1)a low net-to-gross interval of overbank mudstone and sandy/silty splay deposits that thicken upward and are interbedded with paleosols; 2)an erosional based, high net-to-gross interval dominated by spatially clustered, vertically amalgamated, single-story channels; 3)mudstone-filled channels that erosionally cut the sand-rich amalgamated channels below; 4)sand-rich laterally accreting channels; and 5)a low net-to-gross interval of overbank fines grading into a paleosols that begin the next cycle. The meso-term cycles show overall progradation into the basin and are interpreted to represent system-scale avulsion events caused by outside forcing functions such as tectonics and/or climate.
Fifteen short-term cycles are present with an average thickness of 20 m. They can be traced in strike and dip directions for 10s-100s of meters. Erosive based, single-story channels (sand or mudstone-filled) bounded by paleosols below and overbank fines above typify these cycles. Within each meso-term cycle they show an overall decrease in thickness vertically. These short-term cycles are interpreted to document auto-cyclic avulsion events at the local scale resulting in changes channel elements, channel reactivation and/or in hydrodynamic energy.