Porosity and Permeability in Bioturbated Sediments

Owing to the textural contrast that commonly exists between matrix and trace fossils, biogenic flow media are common in the rock record. Broadly speaking, the permeability contrast between the matrix and the trace-fossil-affected zones constitutes the most important parameter for characterizing biogenically influenced flow media. Biogenic permeability is separated into two categories: (1) highly contrasting permeability fields (dual-permeability networks) and (2) comparably diffuse and lowly contrasting permeability fields (dual-porosity networks). Dual-permeability flow media normally display poor reservoir characteristics in that only the permeable conduits (i.e., trace fossils) effectively transmit fluids, and resources may be absent in the tighter matrix. Also, a large number of tortuous, tubular flow paths constitute the flow medium. Dual porosity may also reduce the resource quality of a sedimentary rock by introducing nearly unpredictable heterogeneities and often presenting a gradient of permeability fields between the burrowed and matrix end-members. The assessment of bulk permeability, which in practical terms is the upscaled permeability from trace-fossil versus matrix-scale to bed- and bedset scales, is in need of research and refinement. At the present, a few studies have shown that the bulk permeability of strata containing isolated burrows dominantly follows the harmonic mean of burrow/matrix permeabilities. As burrow connectivity increases, the geometric and the arithmetic means of permeability can be applied. A gradation exists between the three methods that we are not yet able to characterize. Recent research has shown that the geometric and harmonic means can be applied to media that are > 20% bioturbated. Factors other than connectivity influence bulk permeability, including burrow diameter and burrow architecture. The influence of these parameters is not yet quantified.