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Controls on Organic-Rich Mudstone Deposition: the Devonian Duvernay Formation, Alberta, Canada

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Controls on Organic-Rich Mudstone Deposition: the Devonian Duvernay Formation, Alberta, Canada Controls on organic-rich mudstone deposition: The Devonian Duvernay Formation, Alberta, Canada by Levi J. Knapp A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Earth and Atmospheric Sciences University of Alberta © Levi J. Knapp, 2016 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 Formation 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. 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 ii progradation of basin-margin facies, reduction in the extent of organic-rich mudstone deposition, and an influx of clay minerals into the basin. The geographic extent of organic-rich, siliceous mudstone deposition was greatest during transgressive and highstand systems tracts. Sustained highstands resulted in increased carbonate sedimentation, which diluted organic matter. Sea level stillstands resulted in progradation of argillaceous sediment, and reduction of TOC values due to dilution and oxygenation. PREFACE: This thesis is part of a larger study that examines the sedimentology, stratigraphy, geochemistry, petrophysics, and geomechanics of Devonian shales of Western Canada. The sedimentology and sequence stratigraphy of the Duvernay Formation are presented here. The study presented in this thesis was undertaken in order to examine the depositional processes and conditions responsible for organic-rich mudstone deposition, and to generate a predictive capability for lithofacies and rock properties by developing a sequence stratigraphic model. The need for a greater understanding of mudstone deposition and rock property variation has been recognized by the oil and gas industry as it attempts to produce hydrocarbons from fine- grained reservoirs. More generally, robust sequence stratigraphic models for mudstones allow for the prediction of the distribution of sediment bodies and lithologies, and also explain how sea level change and basin morphology affect the character of fine-grained sediments. The thesis is divided into two main chapters. Chapter 1 examines the lithologies and sedimentary facies of the Duvernay Formation and presents a depositional model that explains the processes and conditions present upon deposition of the sediments. Lithofacies defined in Chapter 1 are integrated with total organic carbon data collected by Julia McMillan to examine the factors governing organic enrichment. Examination of drill core was carried out at the AER Core Research Center and TerraTek Labs in Calgary, and at the University of Alberta in Edmonton. Sedimentology discussions with Dr. Korhan Ayranci and comparison to his results iii from the Horn River Basin were very valuable in developing interpretations. Chapter 1 will be submitted to Sedimentary Geology as Knapp, L.J., McMillan, J.M, and Harris, N.B., 2016a. Lithofacies variations in organic-rich Duvernay Formation mudstones. Chapter 2 presents a sequence stratigraphic model that explains variations in the lithologies and sedimentary facies from Chapter 1 in the context of sea level variation. Depositional units and surfaces identified in core were correlated through a network of wireline logs on GeoScout. Chapter 2 will be submitted to the AAPG Bulletin or Marine and Petroleum Geology as Knapp, L.J., McMillan, J.M, and Harris, N.B., 2016b. Sea level and basin morphology control on distribution and character of organic-rich mudstones: A sequence stratigraphic model for the Duvernay Formation, Alberta, Canada A parallel MSc. thesis was completed by Julia McMillan to analyze geochemical and petrophysical properties of the Duvernay Formation. Samples for geochemical and petrophysical analysis were taken from the same cores that were used for the sedimentology study so that multiple datasets were available for every sample. Julia sampled cores at 1 meter spacing and at points of interest so that all major lithofacies and stratigraphic intervals were represented. By integrating our data we have created more robust depositional models to explain rock property variation. Sedimentological observations combined with geochemical analysis provide the opportunity to characterize the depositional environment in greater detail. The integration of sedimentological, geochemical, and petrophysical analyses on the same set of samples allows us to comprehensively characterize the properties of the rock, and predict the nature of one data set based on the character of another. iv Table of Contents Abstract: .................................................................................................................................................... ii PREFACE: ..................................................................................................................................................... iii CHAPTER 1: LITHOFACIES VARIATIONS IN ORGANIC-RICH DUVERNAY FORMATION MUDSTONES ....... 1 1.1 Introduction: ........................................................................................................................................ 1 1.2 Geologic Setting: ................................................................................................................................ 2 1.3 Methods: ............................................................................................................................................. 5 1.4 Results: ................................................................................................................................................ 8 1.5 Discussion: ........................................................................................................................................ 30 1.5.1 Lithofacies Interpretations ......................................................................................................... 32 1.5.2 Transport and Deposition of Fine-Grained, Organic-Rich Sediment ........................................ 43 1.5.3 Depositional Model .................................................................................................................... 47 1.6 Conclusion: ....................................................................................................................................... 51 CHAPTER 2: SEA LEVEL AND BASIN MORPHOLOGY CONTROL ON DISTRIBUTION AND CHARACTER OF ORGANIC-RICH MUDSTONES: A SEQUENCE STRATIGRAPHIC MODEL FOR THE DUVERNAY FORMATION, ALBERTA, CANADA ................................................................................................................................... 53 2.1 Introduction: ...................................................................................................................................... 53 2.2 Geological Setting: ............................................................................................................................ 54 2.3 Methods: ........................................................................................................................................... 58 2.4 Results: .............................................................................................................................................. 60 2.4.1 Lithofacies Analysis ................................................................................................................... 60 2.4.2 Lithofacies Stacking Patterns and Cyclicity .............................................................................. 60 2.4.3 Geographic Variation in Lithofacies and Stratal Thickness Variations ..................................... 69 2.5 Discussion: ........................................................................................................................................ 81 2.5.1 Sequence Stratigraphic Method ................................................................................................
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