The Influences of Bioturbation on Reservoir Facies Quality

Biogenically modified sedimentary flow media can occur as slightly contrasting permeability fields (i.e., dual-porosity networks), or as well-defined, highly contrasting permeability fields (i.e., dual-permeability networks). Dual porosity presents a complex reservoir-modeling problem because, in the presence of more than one fluid phase, flow is promoted along tortuous (worm-burrow-form) permeability pathways, but unburrowed matrix also contributes to the volumetric fluid delivery. Flow in dual-permeability flow media is restricted to the transmissive-burrow conduits, and fluid resources may be immobile or absent in the tighter (unburrowed) rock. Notably, bioturbate fabrics can dominate permeability fabrics in very tight flow media. This is especially true of deep-basin-hosted shallow-sand deposits wherein minor variations in permeability (dual-porosity burrow networks) may provide the only keys that will open the permeability jails described by Shanley in earlier work.

As with fractured media, secondary recovery methods (solvent extraction or waterflood) in burrowed media can isolate large parts of the active flow network. Single-fluid-phase numerical and laboratory experiments demonstrate this effect and show that the main factors that influence the quality and behavior of burrowed flow media are (proposed in their order of importance) burrow occurrence density, burrow connectivity, burrow/matrix permeability contrast, burrow surface area, and burrow architecture. Some of these parameters are dependant on each other.

With respect to permeability burrowed permeability fabrics, 3-D imaging techniques are an essential component of the reservoir facies analysis. Computer Tomography (CT) scans, Micro-CTscans, and MRI techniques have the most potential in burrow-reservoir analysis. These techniques can be used collaboratively to fully assess the nature of burrow-modified flow media.