Previously undocumented natural fractures are present in buried Cambrian sedimentary rocks of southwestern Ontario. Those rocks are candidate repositories for carbon capture and storage, and fractures would affect the movement on CO2 plumes from injector wells. This paper illustrates examples of natural fractures observed in cores from that area and qualitatively describes and interprets them in a manner meant to be accessible to a general geoscience and engineering readership. Extension fractures are approximately vertical, and shear fractures are inclined at an acute angle to the vertical. Deformation bands, a type of shear fracture formed by cataclastic deformation, are present in porous sandstones. Some shear fractures form conjugate sets. All fractures are either mineralized (principally by calcite) or, in the case of deformation bands, have had porosity and permeability destroyed by cataclastic deformation. As such, they would act as barriers or baffles to the movement of CO2 in otherwise permeable sandstones. Fracture orientations suggest the principal compressive stress was vertical at the time of their formation, but their strike direction can generally not be established. This and other interpretation limitations arise because of the nature of the available core (unoriented thin slabs) and lack of supplemental datasets (e.g., borehole image logs) that could assist with fracture characterization. Targeted data-collection programs, combined with more quantitative analyses of the fractures documented herein, will be needed to provide the information necessary to incorporate the effects of natural fractures into dynamic models that predict the movement of injected CO2 in southwestern Ontario.
Bruce Hart (Tue,) studied this question.