Semi-Quantitative Analysis of Bioturbation – Implications for Core Studies

In low-permeability (unconventional) reservoirs trace fossils may provide the primary hydrocarbon flow pathways. Therein, the degree of bioturbation, along with the size and morphology of ichnofossils, can strongly influence reservoir properties such as porosity and permeability and, thereby, storativity and deliverability. In sub-surface studies of core it is difficult to assess the degree to which sediment has been bioturbated. Schemes have been proposed to make semi-quantitative observations of bioturbation. The two most commonly used are the “Ichnofabric Index” and “Bioturbation Index”. These classification systems attempt to classify bioturbate texture based upon the level of original sedimentary fabric that is disturbed.

Modeling of simple burrow fabrics shows that it is problematic to estimate the volume of media that is burrowed based upon a cross-sectional view of core. Therefore, expressing bioturbation as a percentage may be invalid as the visible burrowed area can differ significantly from the volume occupied by the same burrows (both those visible and those not cross-cut by the slabbed core). Four types of burrows were modeled based on their idealized geometries – Planolites, Skolithos, Phycosiphon and Zoophycos. In addition, idealized burrow suites with varying proportions of each of the four burrow types were flow modeled to gain an understanding of burrow interconnectedness.

Where horizontal burrows are dominant, the cross-sectional assessment of burrow area (i.e. % area burrowed) closely approximates the percent volume occupied in the third dimension. Counter intuitively, when a suite of dominantly vertical burrows is considered, 2-D views greatly over estimate the burrow volume (approximately 2 times). Thus, caution should be used when applying the Bioturbation Index or Ichnofabric Index to characterize bioturbate texture, especially at higher intensities. Particularly for permeability studies, we recommend the use of a modified bioturbation index, which presents the volume occupied by burrows from a 2-D view.