The nanoscale porosities in shales have been widely studied by industry with emphasis on organo-porosity and its significant to migration (and generation) of hydrocarbons. However, not much attention is paid to the next scale up, which would include burrows to provide potential migration pathways. The original intent of the work was to evaluate whether burrows were sufficiently abundant and interconnected to provide permeability pathways at the scale of the burrows.

The Woodford Shale in the Anadarko basin, Oklahoma has been one of the major unconventional plays in the United States for nearly a decade. This research has fully utilized ultra-high-resolution (625 micron increment per slice) and advanced 3-D Micro-CT scan technology to quantitatively describe and analyze the ichnofacies and microfacies in selected Woodford cores. The ichnofacies in the Woodford shale cores have been categorized into short and long Chondrites, Paleodictyon, Thalassinoides and Planolites. The 3-D geometries, abundance, and diversity of ichnofacies have been quantitatively described. Bioturbation Index (BI) was calculated from the sum of the abundance of all ichnofacies in each stratigraphic interval. The BI was then compared with XRF, XRD and geochemistry data to relate ichnofacies, paleo-redox environment, and sediment provenance. The stratigraphic distributions of these properties were found to be related to sea-level fluctuation, biostratigraphy (from Conodonts), and sequence stratigraphy.

Our results indicate that bioturbation and bio-activities are more common in Woodford shale than previously thought. They are not sufficiently abundant and vertically interconnected to provide permeability pathways at the scale of the burrows. However, in some core sections, the horizontal burrows are sufficiently connected within thin laminae. There is not much in the way of vertical connectivity of burrows, but along what appear to be some bedding planes there are enough touching burrows to make a permeability pathway. Although burrows frequently develop in highstand systems tracts, they also occur in presumably anoxic environments conducive to preservation of high TOC content and biogenic quartz. These relationships can aid in targeting the best horizontal landing zones in the Woodford shale.

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