Outcrops as guides to subsurface natural fractures: Example from the Nikanassin Formation tight-gas sandstone, Grande Cache, Alberta foothills, Canada

摘要

Outcrop studies are essential for understanding subsurface fractures, because some key attributes such as length and height distribution cannot be measured using wellbores. Discrepancies between outcrop and subsurface fracture attributes arise because loading paths due to burial, exhumation, and thermal paths differ. Consequently, even where rock type and structural setting are close matches, differences may exist in mechanical properties and fracture growth. Here we investigate petrologic criteria and disseminated arrays of quartz-filled microfractures as tools for comparing and contrasting fractures in outcrop with those as sampled by core for tight gas sandstones. Fracture cements are key diagnostic criteria for proving correspondence between outcrop and core. We offer guidelines on selecting suitable outcrop analogs that differ markedly from current practice. Our illustration uses outcrops and core samples of Late Jurassic to Early Cretaceous Nikanassin Formation, a tight gas sandstone, but aspects of the outcrop vetting procedure can be applied to other rock types. In some core and outcrops, Nikanassin Formation fractures contain quartz and ankerite cement assemblages, crack-seal and rind textures, and aperture size distributions that show that the outcrop and subsurface localities we examined share the same fracture stratigraphy and similar diagenesis. These outcrops can thus be used to measure attributes including those that are unlikely to be captured in core, such as fracture height and length distributions, relative and absolute fracture intensity with structural position, and degree of fill by synkinematic cement. In our example, absolute fracture intensity and distribution of postkinematic carbonate cements are site specific and differ between outcrop and core, but in ways that can be quantified.