3D ichnofabrics in shale gas reservoirs

Bednarz, Małgorzata (2014) 3D ichnofabrics in shale gas reservoirs. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This PhD research project uses three-dimensional ichnology to address issues of shalehydrocarbon reservoir quality and provides new tools for ichnofabric analysis and ichnotaxonomic considerations. The study presents deterministic (devoid of conceptual simplifications and interpretations) visualizations of the true spatial geometry of the aff. Chondrites, aff. Phycosiphon and Nereites trace fossils and models the three-dimensional arrangements of the burrow components. The volumetric reconstructions of the real geometry of the trace fossils allowed for their comparison with the previously established visualizations and for reconsideration of pre-existing palaeobiological models. To date three-dimensional understanding of the majority of trace fossils is presented as conceptual drawings available only on two-dimensional media. Such reconstructions are extrapolated mainly from observations of cross sections of burrows from core and outcrop and do not allow for realistic volumetric quantification and full elucidation of complex trace fossil geometries in the context of the host-sediment. The new methodology based on precise serial grinding and volume-visualization presented herein addresses this gap in ichnological knowledge, and is especially useful for examination of the ichnofabric contained in mudstones and muddy siltstones where the application or non-destructive methods of 3D reconstructions as CT scanning or MRI is impossible owing to the rock petrological characteristics (e.g., low burrow-matrix density difference). Volumetric calculations formulated in this study allowed for quantitative characterization of the fundamental attributes of the trace fossils and ichnofabric. The quantitative analytical methods of three-dimensional ichnology presented herein considerably improve our understanding of the petrophysical characteristics of the bioturbated mudstone and therefore they significantly inform the quality of shale gas reservoirs. Five bioturbated samples of organic-rich mudstones collected from shale-gas reservoir type facies of different ages (from Yorkshire [UK], Northumberland [UK], Baja California [Mexico] and Muddy Creek Canyon [Utah]) were reconstructed in 3D at a 1:1 scale. Visualization and volumetric analysis of the spatial distribution and architecture of burrows in reconstructed phycosiphoniform and aff. Chondrites ichnofabrics provides insights into the effects of these taxa on the rheological and petrophysical characteristics of mudstones. It has been demonstrated in the course of this thesis that, in addition to creating significant volumes of silty (clay-poor) zones of enhanced porosity and permeability, trace fossils propagate in all directions infiltrating substantial spatial volume of “tight” matrix and generate horizontally and vertically connected frameworks of densely packed quartzose strips, thereby improving permeability isotropy (kh≈kv) and increasing stress isotropy. It is illustrated herein that shale ichnofabrics can create extensive fracture-prone planes of weakness in sediments that are of importance to hydraulic fracturing methods. Burrows similar to Phycosiphon and Chondrites significantly increase the surface area of the interface between the organic-rich matrix and silty burrow fills, thereby increasing the potential for diffusive transport of hydrocarbon molecules from the “tight” matrix to wellbore-connected volumes. By creating dense, highly interconnected brittle boxworks, ichnofabrics also have the potential to improve the fracturability of reservoir mudstones by affecting fracturespacing and fracture connectivity. The burrow spacing approach developed and employed in this study may form the basis for future modeling of fracture spacing and assessment of fracture complexity in stimulated hydrocarbon-charged shale intervals with bioturbation.

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