John B. Gordon, S. George Pemberton, Murray K. Gingras, and Kurt O. Konhauser
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 shore face sediments, a fine- to mid-medium-grained chert-rich lithareniteis 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 grainsegregation of the tracemaker is unclear, but in the Bluesky For-mation presented in this study, grain segregation has improved the reservoir quality by effectively resorting compaction- andcement-resistant chert and quartz into the burrow fill. Primaryr eservoir quality can be preserved in the presence of chert asporeoccluding 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 mechanicalcompaction and are not easily squeezed into adjacent porespaces 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 Macaronichnusand similar burrowed sediment can have elevated reservoir quality.
