Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Controls on Fluvial Reservoir Architecture: Insights from the Blackhawk Formation Outcrop Belt, Wasatch Plateau, Utah
(1) Earth Science & Engineering, Imperial College London, London, United Kingdom.
(2) Earth and Environmental Sciences, University of New Orleans, New Orleans, LA.
We are characterizing fluvial and coastal-plain facies architecture at multiple spatial scales, from inter-well to play fairway, in an outcrop analogue of exceptional scale and continuity: the Cretaceous Blackhawk Formation in a thick (200-300 m), laterally continuous (100 km) outcrop belt along the eastern Wasatch Plateau, central Utah. These strata record an upward transition from high-accommodation, lower coastal plain to low-accommodation, upper coastal plain settings, and are coeval with shoreline deposits in the contiguous Book Cliffs outcrop belt. The resulting dataset allows conceptual models of fluvial stratigraphy to be rigorously tested.
The detailed internal architectures of selected fluvial sandbodies have been documented in several “windows” of the outcrop belt. Sandbodies at all stratigraphic levels in the Link Canyon “window” were deposited by geomorphic channels of low-to-moderate sinuosity that exhibited variable degrees of braiding. Individual channel storeys, which represent the migration of a geomorphic channel segment and adjacent bar(s), are stacked laterally and vertically to form channel-belt and channel-complex sandbodies. The geometry and dimensions of the resulting sandbodies principally reflect channel migration history rather than channel geomorphology. Storey dimensions and stacking patterns within each sandbody are highly non-uniform, such that (1) there are no systematic trends shared by the sandbodies, (2) sandbodies are generally not confined within paleo-valleys, and (3) variability within each sandbody is more pronounced than variability between sandbodies. These results suggest that local avulsion history is the principal control on fluvial sandbody architecture and dimensions, and that avulsion history is not governed by proximity to the coeval shoreline.
At the scale of the entire outcrop belt, the Blackhawk Formation exhibits trends of increasing size, abundance and interconnectedness of major fluvial sandbodies from (1) its base to its top, and (2) south to north. Extrapolating from our interpretations of the Link Canyon “window”, these trends can be attributed respectively to (1) a progressive decrease in the rate of tectonic subsidence, and (2) increasing proximity to a long-lived (structurally controlled?) sediment entry point near the more rapidly subsiding northern part of the outcrop belt. Variations from these trends are subtle, and cannot easily be related to the migration history of the coeval shoreline.