BIOGENIC-CONTROLLED PERMEABILITY IN THE FERRON SANDSTONE MEMBER OF THE MANCOS SHALE, UTAH

Generally, it has been assumed that all bioturbation has a negative effect on permeability. This is because bioturbation has been demonstrated to disrupt the sorting of laminated sediment. However, recent studies demonstrate that bioturbation also has the potential to significantly increase permeability. Permeability enhancement in bioturbated media has been recognized in five interrelated scenarios: 1) Surface-constrained textural heterogeneities; 2) Non-constrained textural heterogeneities; 3) Weakly defined textural heterogeneities; 4) Cryptic bioturbation; and 5) Diagenetic textural heterogeneities. This study analyzes the effect of bioturbation textures on permeability using samples from the Ferron Sandstone Member of the Mancos Shale, an Upper Cretaceous (Turonian) clastic wedge that prograded westward into the Western Interior Seaway, in Utah. Three samples of similar grain sizes were chosen from the fluvial-dominated Cottonwood, Muddy Creek and Farham sites as well as the wave-delta dominated Notom site. Trace fossil identification, permeability measurements and thin section analysis of these samples prove that biogenic sediment reworking can impact permeability either negatively or positively. Overall, biogenic structures have a significant effect on fluid flow dynamics. Burrows may provide flow conduits, however their irregular geometries commonly produce dead ends and cut-offs. The recognition of how the biogenic structures impact on permeability is required in order to properly identify potential speed zones and baffles during reservoir characterization for subsequent modeling input. An understanding of how burrow-associated heterogeneities control fluid flow within sedimentary units is necessary if production from bioturbated reservoirs is to be optimized.