Outcrop-Based Reservoir Characterization of a Fluvial Reservoir System: John Henry Member, Straight Cliffs Formation (Cretaceous, Utah)

Characteristic features of fluvial deposits, including rapid lateral facies changes, numerous erosion surfaces, and frequent absence of regional marker beds, cause significant limitations to geologic interpretations of fluvial reservoirs based on subsurface data. Outcrop-based studies of analogous deposits provide useful references for producing detailed interpretations of subsurface fluvial deposits.

The John Henry Member of the Upper Cretaceous Straight Cliffs Formation, located in the Kaiparowits Plateau of southern Utah, is an ideal location to study the sedimentologic and stratigrapic characteristics of fluvial deposits due to the abundance and quality of outcrops located throughout the plateau. Because the transition from fluvial to paralic and shallow marine strata of the Western Interior Seaway is present within the outcrops of the plateau, sequence-based interpretations of fluvial architecture can be directly compared to previous interpretations originated in the marginal marine strata.

A detailed study of the fluvial strata located in Bull Canyon was conducted, which included a series of measured sections, hundreds of paleocurrents, and high-resolution panoramic photographs, used to map vertical and horizontal facies distributions. Changes in the style of deposition, ranging from proximal fluvial to deltaic, occurring along the profile of the paleo-river system were investigated by correlating strata present at Bull Canyon, this study, with locations both updip and downdip, which have been the subjects of previous studies.

Trends observed vertically within the section included the transitions between: tidally-influenced channel belts; laterally extensive, laterally accreting channel belts; isolated channel belts embedded within thick floodplain muds; clusters of laterally restricted, laterally accreting channel belts and isolated channel belts; laterally extensive, locally amalgamated, downstream accreting channel belts; and highly amalgamated, downstream accreting channel belts.

Reservoir characterization based in on grain size, average porosity, net to gross estimates, channel heterogeneity, and channel belt size and connectivity showed that fining upward and coarsening upward trends in fluvial architecture correspond to upward decreasing and upward increasing trends in reservoir analog quality. This relationship demonstrates that depositional models of alluvial architecture may be used to predict reservoir quality.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California