The Role of Endobenthic Organisms In Enhancing Porosity and Permeability of Sedimentary Aquifers and Reservoirs

Bioturbation is the mixing of sediments by the burrowing activities of animals, and ichnologists have long recognized the importance of bioturbation in modifying primary textures of sedimentary deposits. When bioturbation and trace fossils generated by one or more endobenthic organisms are prominent within a sedimentary deposit, a distinctive and commonly characteristic ichnofabric results. Recently, in addition to study of ichnofabrics, increased attention has been directed to the role of endobenthic organisms in enhancing (or sometimes reducing) the porosity and permeability of sedimentary media.

In shallow-marine environments, dominant bioturbators include decapod crustaceans, polychaetes and other worms, and meiofauna – the diverse interstitial community that inhabits pore spaces of sediments. Burrowing activity of meiofauna results in cryptobioturbation, a subtle alteration of original textures generally resulting in greater homogeneity. Callianassid shrimp commonly dominate the deep-tier fauna in siliciclastic and carbonate, sandy shallow-marine settings. Their thickly lined, pelleted burrows are assigned to the ichnogenus Ophiomorpha. Occurrences of robust, densely packed shafts and tunnels of Ophiomorpha can result in highly permeable macroporous zones, as is the case with parts of the Pleistocene limestone of the Biscayne aquifer of southeastern Florida. Burrows of the ichnogenus Thalassinoides, variously attributed to decapods, can dominate firmground layers, with fill material in the tubular burrows having significantly greater porosity and permeability than surrounding matrix. The burrows can serve as transport pathways to reservoir horizons, as has been proposed for parts of the famous Arab-D reservoir of the Ghawar Field, Saudi Arabia.

New numerical and geophysical methods are emerging to enable better study and quantification of ichnogenic porosity and permeability as applied to the understanding of sedimentary aquifers and reservoirs. These analytical methods include lattice Boltzmann methods, geophysical borehole logging, tracer-test analyses, and 3-D imaging techniques such as digital optical borehole imaging, X-ray computed tomography, and nuclear magnetic resonance imaging.