Organism–sediment interactions in shale-hydrocarbon reservoir facies — Three-dimensional reconstruction of complex ichnofabric geometries and pore-networks

• Deterministic 3D reconstructions of ichnofabrics in shales are illustrated.
• Volumetric analysis of the porous and brittle ichnofabric material is presented.
• Burrow spacing influences fracture spacing and complexity.
• Burrows create interconnected permeable paths improving fluid flow.
• Burrows partition the shale matrix improving diffusive efficiency of hydrocarbon flow.

The lithological and mineralogical characteristics of mudstones and siltstones—and their stress–strain behavior at the meter to nanometer scale—can play a critical role in the exploitation of unconventional shale reservoirs. Shale fabrics that result from bioturbation can produce extensive interconnected networks of biologically redistributed sediment grains within reservoir mudstone facies. The presence of biologically generated heterogeneities may substantially affect reservoir stimulation and thus production from shale facies. This study presents volumetric evaluation of ichnofabrics dominated by Phycosiphon-like and aff. Chondrites, and provides insights into the impact of trace fossils on the rheological and petrophysical characteristics of mudstones. It is calculated that, in addition to creating significant volumes of silty (clay-poor) zones of enhanced porosity and permeability, trace fossils create interpenetrating frameworks of brittle material that reduce communication distances from the low-permeability matrix to the higher permeability silt-rich burrows. Reducing communication distances to less than 1 cm increases the potential for diffusive transport of hydrocarbon molecules from the “tight” matrix to the wellbore-connected volumes. This is because shale ichnofabrics create abundant fracture-prone planes of weakness, and increase the surface area of the interface between the hydrocarbon-rich matrix and porous burrow fills, thereby promoting fluid exchange. Understanding of the three-dimensional characteristics of ichnofabrics may form the basis of future modeling of fracture spacing and complexity that is critical to shale gas reservoir characterization.