Biogenically enhanced permeability: A petrographic analysis of Macaronichnus segregatus in the Lower Cretaceous Bluesky Formation, Alberta, Canada

Abstract

A detailed petrographic analysis was conducted on several core samples from the Cretaceous-aged Bluesky Formation in the La Glace area in Alberta, Canada. Within the gas-producing zone of these upper shoreface sediments, a fine- to mid-medium-grained chert-rich litharenite is intensely bioturbated with Macaronichnus segregatus. Petrographic analysis showed that the burrow mantle is generally lined with dark-colored, iron-rich (mainly chert, shale clasts, and organic grains) fragments, whereas the burrow fill contained mainly quartz and light-colored chert fragments. The reason for the dark-colored grain segregation of the tracemaker is unclear, but in the Bluesky Formation presented in this study, grain segregation has improved the reservoir quality by effectively resorting compaction- and cement-resistant chert and quartz into the burrow fill. Primary reservoir quality can be preserved in the presence of chert as pore-occluding quartz overgrowths do not form on chert fragments as they do on monocrystalline quartz, thus leaving open, well-connected primary pores and hence elevated permeability. Chert fragments are resistant to the effects of mechanical compaction and are not easily squeezed into adjacent pore spaces as are ductile rock fragments. Further research is needed to test the chemical constituents of the grains that modern tracemakers ingest. This may lead to a better understanding of why sands burrowed with Macaronichnus and similar burrowed sediment can have elevated reservoir quality.