Andrew D. La Croix

Chapter 27 – 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.

Computer modeling bioturbation: The creation of porous and permeable fluid-flow pathways

Computer modeling of trace fossils (Skolithos, Thalassinoides, Planolites, Zoophycos, and Phycosiphon) and ichnofacies (Skolithos, Cruziana, and Zoophycos ichnofacies) is undertaken to assess the impact of bioturbation on porosity and permeability trends in sedimentary media. Model volumes are randomly populated with the digitally modeled trace fossils to test for connectivity between burrows. The probability of vertical and lateral interconnections is compared with bioturbation intensity. The results of the simulations indicate that biogenic flow networks develop at low bioturbation intensity, between 10 and 27.5% bioturbation (BI-2). However, the efficiency of connectivity is controlled by the architecture of the burrows. For all trace-fossil and ichnofacies models, regardless of trace-fossil orientation, continuous horizontal and vertical connectivity across the sediment volume is achieved within a 0 to 10% range in bioturbation. In subsurface aquifers and petroleum reservoirs, the presence of bioturbation can significantly influence fluid flow. In particular, for marine sedimentary rocks, where burrows are more permeable than the surrounding matrix, a greater degree of three-dimensional burrow connectivity can produce preferred fluid-flow pathways through the rock. Recognizing these flow conduits may enable optimization of resource exploitation or may contribute to increasing reserve estimates from previously interpreted nonreservoir rock.

Associating X-Ray Microtomography with Permeability Contrasts in Bioturbated Media

The use of X-ray microtomography (micro-CT) to evaluate the contrasts in permeability associated with bioturbation in two- and three-dimensions (2D and 3D) is investigated in this article. Core analysis, spot-permeametry measurements, and petrographic thin-sections of the six different datasets from Western Canada (Debolt Formation, Wabamun Group, and Medicine Hat Member) and offshore Norway (Ula Formation, Lysing Formation, and Nise Formation) show that the trace fossils commonly modify the permeability and porosity of the reservoir through sediment reworking and diagenesis of the sediment. In this study, micro-CT techniques are used to analyze the X-ray attenuation associated with bioturbation in siliciclastics and carbonates. Because X-ray attenuation represents the transmissivity or absorptivity of a sample to the incident X-rays, each of which is related to sediment composition and sediment densities, micro-CT images are integrated with sedimentological data and petrographic data to identify the trace fossils and matrix in 2D and 3D. Due to the difference in sediment grain size and sorting (i.e., porosity) within the burrows and matrix, analysis of the X-ray attenuations allows for their delineation in 2D cross-sectional images. When processed as 3D volumes, the micro-CT images show the complexity in burrow connectivity and burrow orientations within a sample. More importantly, the 3D volumes help show the distribution of porosity that can be linked to measured permeability values within a sample. Quantification of the heterogeneous burrow fabric on reservoir and resource quality is made from these processed micro-CT images.

Sedimentological trends across the tidal–fluvial transition, Fraser River, Canada

The tidal–fluvial transition (TFT) is a complex depositional zone in rivers, where tidal- and river-flow interact. However, subdivision of the TFT into discrete zones is not well established. Studies of the lower Fraser River, Canada, reveal criteria for subdividing the TFT into zones that experience similar hydrodynamic and salinity variations from year-to-year, and three zones are identified: (1) mixed tidal–fluvial with persistent brackish-water zone (mixed tidal–fluvial); (2) fluvially dominated and tide influenced, freshwater to brackish water transition zone (turbidity maximum); and, (3) fluvially dominated and tidally modulated, freshwater zone (tidal backwater). Of the four main datasets (sedimentological, ichnological, palynological, and geochemical) evaluated across the TFT of the Fraser River, only sedimentological and ichnological datasets can be used reliably to distinguish between depositional zone within the TFT. Mud volume is highest in the turbidity maximum zone and decreases in both the landward and seaward direction. Rhythmic (tidal) bedding is common in the mixed tidal-fluvial zone, and sand–mud interbedding occurs in both the turbidity maximum and mixed tidal–fluvial zones. Significant sand–mud interbedding is not expected in the tidal backwater, nor landward of the tidal backwater. Higher salinity and longer residence times of saline water at the bed are manifested ichnologically in larger diameter burrows, higher bioturbation intensities, and a more diverse trace assemblage. Variations in river discharge result in a heterogeneous distribution of burrows, where burrows are mainly concentrated in mud beds. Increased fluctuations in depositional energy and increasing sedimentation rate reduce trace density. The depositional trends presented from the lower Fraser River are intended as a basis for comparison to trends defined from the TFTs of other major river systems. This work demonstrates that it is possible to predict relative depositional position across the TFT, and that a comprehensive model for TFT deposits can be constructed.