The depositional framework and stratigraphic architecture of the Lower Triassic Montney Formation, northeastern British Columbia
Creators: Gonzalez Hernandez, Patricia
Publisher: name: University of Alberta Library
Year: 2022

<p>The Lower Triassic Montney Formation contains some of the largest unconventional hydrocarbon plays in the world, and over the last decades has become critical to Canada’s main energy resources. These unconventional plays consist mainly of fine- to coarse-grained siltstone beds which are difficult to characterize due to their subtle grain-size variations, diminutive biogenic structures, thin interbedding with bioclastic-rich intervals, and very subtle facies changes across sequence stratigraphic surfaces. Because of this, correlation and understanding of the stratigraphic architecture within the Montney Formation in northeastern British Columbia is still problematic, and a better control on these fabric-dependant heterogeneities is necessary. When working with fine-grained successions that are difficult to characterize, integration of high-resolution data sets may help enhance the understanding of the depositional framework and stratigraphic relations of the unit. Based on detailed sedimentologic, paleontologic, and ichnologic analysis, the Montney Formation in northeastern British Columbia consists of eleven lithofacies and three recurring facies associations interpreted to represent deposition along a storm-dominated mixed siliciclastic-carbonate ramp, in offshore, offshore transition and lower shoreface settings, intermittently influenced by ephemeral rivers. Trace fossil assemblages correspond to a stressed expression of the Cruziana Ichnofacies. Size-diversity index values show an abrupt increase across the Smithian-Spathian boundary, likely associated with improved oxygenation conditions during the Spathian. Glossifungites-demarcated surfaces were identified and are associated with both autogenic and allogenic processes. Allogenic Glossifungites Ichnofacies corresponds to a sequence boundary at the contact between the Montney Formation and overlying Sunset Prairie Formation. Mineralogical analyses were integrated to help understand palaeodepositional changes, including: 1) the overall prograding trend throughout the Montney, with increasing quartz and feldspar concentrations from bottom to top; 2) fluctuating sedimentation rates in carbonate-rich intervals; and 3) the relative rise in sea level and fluctuating redox conditions associated with the phosphate-rich, lowermost Spathian interval. Furthermore, high-resolution chemostratigraphic analyses were performed to help interpret paleoredox conditions and sequence stratigraphic surfaces that are difficult to identify through core analysis and well logs alone. From the integration of all data sets (sedimentologic, ichnologic, mineralogic, chemostratigraphic), the sequence stratigraphic framework of the Montney Formation is presented, and is interpreted to be associated with continuous positive accommodation, without intervening stages of negative accommodation. This study highlights the significance of using detailed, high-resolution, and objective data in the understanding of the stratigraphic architecture of fine-grained sequences that do not follow the predictions of standard sequence stratigraphic models.</p>