Biogenically Enhanced Permeability: a Petrographic Analysis of Macaronichnus Segregatus Burrowed Bluesky and Cardium Formation Sandstones

John B. Gordon¹, S. G. Pemberton², Murray K. Gingras², and Kurt O. Konhauser^2
1Petro Canada Oil and Gas, Calgary, AB, Canada
²Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada

Early diagenesis of siliciclastic sands in marginal-marine environments begins immediately upon deposition. Moreover, some marginal-marine settings support large populations of burrowing macrofauna animals, which can have a significant effect on early diagenetic processes. Present day reservoir properties such as porosity and permeability can become enhanced or destroyed as a result these early biogenically mediated diagenetic processes.

This study describes the interaction and early diagenesis of Macaronichnus burrowed upper shoreface deposits of the Lower Albian aged Bluesky at La Glace and the Turonian aged Cardium upper shoreface deposits at Wapiti, to test for the effect of biogenically modified reservoir quality using petrographic techniques. It was found that producing gas zones in these deposits corresponds to a chert-rich sand intensely bioturbated with Macaronichnus segregatus. The burrows are generally light coloured, with dark coloured grains forming a mantle near the burrow walls.

Energy dispersive x-ray (EDX) analysis show that the burrow walls are generally lined with dark coloured, iron-rich (mainly chert) fragments while the burrow fill contains mainly quartz and light coloured chert fragments. The highly resistant dark coloured chert appears to act as a “compaction shelter” for the sediment within the burrow. The light coloured chert in the fill also resists mechanical compaction, but does not precipitate pore occluding quartz overgrowths that destroy reservoir quality. The reason for the dark grain segregation of the tracemaker is unknown, but may suggest that the tracemaker simply avoids iron-rich grains; alternatively, the bacteria that the burrowing animal seeks do not inhabit iron-rich grains.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas