Ichnologic Suites and their Controls on Permeability Distributions in the Lower Cretaceous Viking Formation, South-Central Alberta, Canada
La Croix, Andrew D.; Ayranci, Korhan; MacEachern, James A.; Hsieh, Amy; Allen, Diana M.; Dashtgard, Shahin E.
Bioturbation disrupts primary depositional fabrics, altering sorting and creating textural heterogeneities. Biogenic modification, however, does not necessarily lead to porosity and permeability reduction. Sand-filled burrows, in particular, can act as porous and permeable flow pathways through otherwise low-permeability media. The relationships between porosity - permeability distributions and trace fossil suites are, however, understudied and poorly understood. In this study, we evaluate the ichnology of offshore and lower shoreface deposits in the Upper Albian Viking Formation, to assess their impacts on textural-controlled permeability heterogeneity.
Thoroughly bioturbated (BI 4-6) upper offshore sandy mudstones display trace fossil suites dominated by deposit-feeding structures (e.g., Helminthopsis, Planolites, Phycosiphon, Asterosoma), with uncommon dwellings of suspension-feeders (e.g., Cylindrichnus, Skolithos). Permeability differs by two orders of magnitude between mud-dominated lamina-sets and beds, and sandy intervals consisting of beds and laminae as well as sand-filled burrows. The predominantly horizontal orientations of the sand-filled burrows produce planiform textural heterogeneities with only limited vertical connectivity between high permeability layers, leading to an overall anisotropic permeability distribution in the vertical direction. Moderately bioturbated (BI 3-4) lower shoreface muddy sandstones display trace-fossil suites with subequal proportions of deposit-feeding structures (e.g., Asterosoma, Planolites, Chondrites, Phycosiphon) and dwellings of suspension-feeders (e.g., Ophiomorpha, Skolithos, Cylindrichnus). The permeabilities of mudstone beds and lamina-sets versus sandier layers generally vary only by one order of magnitude. As the burrows are not constrained to a particular orientation, textural heterogeneities are more isotropic than in the offshore.
Heterogeneous, dual-porosity flow networks have the potential to promote significant mobility of subsurface fluids, contributing to the storage and deliverability of hydrocarbons in bioturbated reservoirs. Fluid recovery from offshore deposits, where trace fossil suites lead to horizontal, anisotropic permeability, should utilize hydraulic fracturing to vertically connect pay sands. Trace suites within lower shoreface units are more conducive to conventional completion techniques, as sand-filled burrows only subtly enhance the permeability but overall increase isotropy.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013