MUDROCK MICROSTRUCTURE: INSIGHTS INTO SEDIMENTARY HISTORY AND MECHANICS

The multi-scale quality of mudrocks poses a challenge to understanding their role as hydrocarbon source and reservoir, cap rock (including in carbon capture and storage efforts), and recorder of geologic history. To meet these challenges silt grain boundaries were digitized from a collection of SEM images and compositional X-ray maps from samples covering a range of basin settings and burial depths in Pearsall, Barnett, and Marcellus Formations. A characteristic distribution of grain size and orientation was found in all samples, with asymptotically vanishing amounts of >10 micron-diameter grains and well-defined bedding-parallel grain-shape fabrics. A ductile compaction history is suspected in many samples because of lenticular microtextures defined by organic material, and trends in grain-contact abundance with depth. To explore this, triaxial deformation experiments were carried out resulting in well-defined yield stresses. However, loading and failure curves varied from highly linear to slightly non-linear, and exhibited varying strength and anisotropy depending on silica concentrations. Importantly, an observed porosity increase after deformation at the ~10-100 nanometer scale likely is due to deformation of the organic and clay materials; upcoming microscopy will determine the relative importance of microfracturing. We interpret that silt microstructure in mudrocks is a result of sediment transport of the fine-silt fraction away from the primary sites of sediment influx to a position where continual reworking by near-bottom currents occurs without further sorting. The ensuing compaction generates a silt fabric that becomes insensitive to diagenesis immediately following burial. This silt fabric has been suggested to form “stress bridges” that protect porosity, but ductile deformation, and potentially microfracturing (as-yet undocumented), could still impact porosity either via geological processes or during injection.