Depositional Controls on the Distribution and Character of Organic-Rich Mudstones: A Sequence Stratigraphic Analysis of the Duvernay Formation of the Western Canadian Sedimentary Basin

Abstract

The Upper Devonian Duvernay Formation of Western Canada is a prolific source rock that in recent years has become an exploration target for shale gas and liquids. Development of the Duvernay has demonstrated the importance of robust models for rock properties such as porosity, permeability, organic richness, and fracturability. Depositional processes and conditions govern the character and distribution of mudstone lithofacies, which are directly linked to variations in rock properties. The depositional and sequence stratigraphic models presented here are based on a detailed sedimentology and stratigraphy study, combined with geochemical and petrophysical datasets. Twelve lithofacies were characterized based on lithology, sedimentary and biogenic structures, fossil type and abundance, and type and abundance of cement. Significant variation exists between organic-rich lithofacies, indicating that organic-rich mudstones were deposited in variably energetic and oxygenated environments, rather than within persistently anoxic, stagnant bottom waters. Correlation of core descriptions to a network of 759 wells with wireline logs led to the creation of a basin-scale sequence stratigraphic framework with 3 third order sequences. The sequence stratigraphic model shows a strong sea level and basin morphology control on basinal lithofacies. Transgression, and subsequent highstand normal regression in sequence 1 (oldest) resulted in significant platform construction in the northeast side of the basin. Sediments become consistently finer-grained and organic-rich away from the platform. Major transgression at the start of sequence 2 resulted in flooding of the platform and significant upslope trapping of sediments, locally so significant that no sediment reached the platform edge and large zones of non-deposition developed. Lithofacies basinward of these zones are especially clay mineral-poor and organic-rich. A sea level stillstand or lowstand at the base of sequence 3 resulted in progradation of basin-margin facies, reduction in the extent of organic-rich mudstone deposition, and an influx of clay minerals into the basin. The extent of organic-rich, siliceous mudstones is greatest during transgressive and highstand systems tracts. Sustained highstands result in increased carbonate sedimentation, which dilutes organic matter. Sea level stillstands result in progradation of argillaceous sediment, and reduction of TOC values due to dilution and oxygenation.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016