When all else fails, try planar lamination - Possible insight into the Montney and other similarly fine-grained reservoirs

Speaker: Bill Arnott | Department of Earth and Environmental Sciences University of Ottawa

October 16, 2018 |  11:30 am doors open 
Marriot Hotel, Kensington Ballroom | 110 9 Ave SE, Calgary AB T2G 5A6

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ABSTRACT 
Planar-laminated strata are ubiquitous in modern and ancient sedimentary environments that range from the continental to deep marine, and include economically important units like the Montney Formation and other similarly fine-grained units in the Western Canada Sedimentary Basin and worldwide. Planar lamination indicates that bed-load sediment transport is more or less spatially uniform and irrespective of flow type (unidirectional, oscillatory, combined) a planar bed surface is the stable bed state, which commonly, and most intuitively, coincides with high-energy transport conditions. Under low energy conditions, on the other hand, and beginning at threshold transport, bed surface sediment transport typically becomes spatially non-uniform and the bed surface ornamented by an array of bed forms that build above and below the general bed level. Accordingly, the commonality of planar lamination, especially in shallow- and deep-marine settings, would suggest that a significant part of the sedimentary record was deposited under high-energy transport conditions. Puzzling then, is the equally, if not more common occurrence of planar-laminated strata that grade continuously upward to mud(stone) with no intervening unit suggestive of low-energy bed-load transport. 

Based on experiments using sediment-propelled density currents passing through a medical-quality CT scanner, it is argued that the absence of these low-energy structures is not related to incompatible flow speed, transport bypass or erosion, but instead to near-bed sediment concentration conditions that discourage spatially non-uniform sediment transport and the consequent initiation and amplification of bed-surface defects that otherwise would evolve into distinctively internally cross-stratified bed forms. As expected these conditions are common in both coarse- and fine-grained, high-energy density currents. Of particular note, then, is that these conditions also form in low (depth averaged) sediment concentration fine-grained flows, which because of low sediment concentration also have low flow speed. It is these latter currents that are being increasingly recognized as the principal physical mechanism responsible for mobilizing and depositing much of the sediment in distal shallow-marine and deep-marine environments, including units like the Montney Formation and other non-conventional reservoir units around the world. It is in these settings where spatially uniform bed-surface transport should dominate, and accordingly, planar-laminated strata should, and does, dominate the sedimentary record. Notably also, these planar-laminated strata exhibit a distinctive alternation of centimetre- to sub-millimetre-thick, well-sorted clay-rich and clay-poor laminae, which may have important implication on overall stratal rheology, and accordingly, fracture development and propagation.  

BIOGRAPHY

Bill Arnott is a Professor and current Chair of the Department of Earth and Environmental Sciences at the University of Ottawa. His research and that of his merry band of undergraduate, graduate and postdoctoral researchers is outcrop and laboratory based and focuses on sedimentary environments that range from the continental to the deep marine, sediment transport and depositional processes that range from unidirectional to oscillatory to combined, open channel to density currents, and increasingly the geochemistry of Neoproterozoic Earth.