Micro- and Mesoscale Reservoir Compartmentalization of a Sandy Braided River; Lower Jurassic Kayenta Formation

Ranie M. Lynds
University of Wyoming, Department of Geology and Geophysics
Laramie, WY
lynds@uwyo.edu

Sandy braided rivers and their associated bars, channels, channel belts, and floodplains, behave in a generally predictable manner and thus develop fluvial architectural patterns that, while not as obvious as meandering or avulsive rivers, are recognizable in outcrop. The Kayenta Formation (Lower Jurassic; southern Utah and western Colorado) is a sandy braided stream deposit that displays micro- and mesoscale reservoir compartmentalization, defined by the spatial distribution of relatively permeable and impermeable lithofacies.

Three lithofacies are discernable at the microscale (centimeter to meter scale). Lithofacies A is medium to coarse-grained sand interpreted as barforms primarily composed of dune and planar bedforms. Lithofacies B is siltstone and fine-grained sandstone that interfingers with the toes of the accretionary sets of lithofacies A, representing either barform migration into local slack water areas or channel-thread abandonment. Lithofacies C is siltstone and very fine-grained sandstone expressed as low-energy ripples. These ripples lie abruptly atop barform deposits, suggesting deposition in low flow zones along the active river. Lithofacies C is interpreted to represent local slack-water deposits, forming in protected parts (e.g. behind bars) in the active channel, or flood incursions into abandoned parts of the braid plain.

Lithofacies A-C are distributed in the Kayenta Formation so that relatively permeable zones (lithofacies A) are bounded by zones of relatively less permeable units (lithofacies B and C), producing distinct mesoscale compartmentalization. This suggests that the primary control on braided stream reservoir continuity is mesoscale zonation that can impact reservoir permeability at the scale of several to tens of meters.