MICRO- AND MESOSCALE RESERVOIR COMPARTMENTALIZATION OF A SANDY BRAIDED STREAM; LOWER JURASSIC KAYENTA FORMATION

Conceptual models of stacking of architectural elements provide the necessary geometric and lithofacies associations to produce predictive reservoir models for efficient discovery and production of hydrocarbon fields. These models, however, can be more useful if they are based upon depositional processes and not limited to geometric descriptions.

Sandy braided streams 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 streams, 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 sandstone 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. Thus, lithofacies C is interpreted to represent local slack-water deposits, forming either in protected parts (e.g. behind bars) when the channel was flowing, or flood incursions into otherwise abandoned parts of the braid plain.

Lithofacies A-C are distributed in the Kayenta Formation so that zones of relatively permeable sedimentary rock (lithofacies A) are bounded by zones of relatively less permeable units (lithofacies B and C), producing distinct mesoscale compartmentalization. We suggest 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.