A fine detail physico-chemical depositional model for Devonian organic-rich mudstones: A petrographic study of the Hare Indian and Canol Formations, Central Mackenzie Valley, Northwest Territories

Abstract

The Hare Indian and Canol Formations, which are part of the Horn River Group in the Northwest Territories, Canada, primarily consist of organic-rich mudstones deposited during the Middle to Late Devonian. The formations were previously considered to represent marine basin fill accumulated in an oxygen-starved distal shelf setting, evidenced by the organic-rich character, pyrite content, and lack of macro-scale bioturbation. The depositional model, paleo‑oxygenation interpretations, and methods of organic carbon preservation presented in this study are in contrast to previous assumptions of the Horn River Group mudstones. Detailed petrographic, sedimentological, and ichnological analyses were carried out on thin sections taken from several cored Horn River Group intervals. These organic-rich mudstone units contain eight distinct microfacies, representing four main sedimentation processes acting on an interpreted distal shelf setting: (1) pelagic suspension settling, (2) plug-like sediment-gravity flows, (3) surge and surge-like low-density turbidity currents, and (4) debrites. Pelagic suspension-settling dominated in distal, quiet waters out of the reach of persistent storm influence. Debrites, plug-like flows, and low-density turbidite processes represent a continuum, wherein storm influence was the dominant driver in sediment delivery. Several morphologically distinct, microscopic biogenic-sedimentary structures (i.e. ichnofossils) have been identified throughout the mudstone intervals, indicative of sediment pore waters that were at least periodically partially oxygenated. Evaluation of total organic carbon content against bioturbation and microfacies interpretation suggest that persistent anoxia was not the dominant factor in organic carbon preservation, but rather a result of a combination of heightened sedimentation and burial rates and possible amplified rates of primary productivity. The results of this study may be applied to evaluations of other organic rich mudstones to enhance paleo-depositional interpretations.

Introduction

Physico-chemical depositional conditions of ancient marine organic-rich fine-grained sedimentary successions have, in the last several decades, become a popular topic for detailed petrographic analyses, owing to the various roles they play in petroleum systems (i.e. variously serving as sources and/or unconventional reservoirs) (e.g., Macquaker et al., 2007, Macquaker et al., 2010a; Aplin and Macquaker, 2011; Ghadeer and Macquaker, 2011; Schieber, 2011; Plint et al., 2012; Egenhoff and Fishman, 2013). This stems from their inherent, small-scale (millimeter to sub-millimeter) variability associated with being thin bedded, laminated, and altered by bioturbation and diagenesis (Macquaker et al., 2014). Despite their importance in the petroleum system, mudrock facies are still relatively poorly understood. Conventional interpretations are typically limited to slow hemipelagic suspension settling in oxygen-depleted bottom waters (Aplin and Macquaker, 2011). Further studies have identified small-scale (petrographic) primary sedimentary structures and features in mudstone beds that appear plane parallel laminated in hand sample and outcrop, including low-angle ripple foresets (Schieber et al., 2007), intercalated clay-dominated and silt-dominated ripple features (Yawar and Schieber, 2017), intrabasinal rip-up clasts (Schieber et al., 2010), sub-millimeter-thick scour and lag deposits (Schieber, 1998), wave-enhanced sediment-gravity flows (Macquaker et al., 2010a), and normally graded sub-centimeter beds (Ghadeer and Macquaker, 2011). Such findings have revealed that fine-grained deposits are more depositionally dynamic than previously understood. Microbioturbation (sub-millimeter-sized features interpreted as biogenic sedimentary structures, i.e., microscopic trace fossils) has also been identified in many organic-rich mudstone units (Savrda and Bottjer, 1991; Macquaker and Taylor, 1996; Schieber, 2003; Egenhoff and Fishman, 2013) that were previously thought to preclude endobenthic animals. Kabanov and Jiang (2020) have used micropaleontologic studies of sponge spicules to provide compelling evidence that previously interpreted persistent anoxic bottom waters were likely punctuated by periods of weak re‑oxygenation; and Dashtgard et al. (2015) and Dashtgard and MacEachern (2016) found a lack of macrobenthic organisms in modern-day shelfal muds with only slightly reduced oxygenation. Together these findings indicate more complex paleoredox conditions than persistent and pervasive anoxia for the deposition of seemingly unbioturbated organic-rich mudstone units.

The identification of bioturbation in mudrock facies is exceedingly important. In marine settings, penetrative bioturbation is only produced by macrofaunal and meiofauna (< 500 μm) organisms. While there are other organisms that can move on top of sediment, such as motile protists (Matz et al., 2008), they lack the musculature to move through sediment. Bioturbation can be taken as direct evidence of the presence of dissolved oxygen in the bottom waters. In other words, the presence or absence of bioturbation and the size of the trace makers provide a very useful proxy for identifying the lowermost limits of oxygenation at the sea floor. A significant problem in identifying bioturbation is discriminating bioturbate texture from other types of soft-sediment deformation. Using petrographic observations from the mudstone-dominated Hare Indian and Canol formations in the Northwest Territories, Canada, this paper aims to further develop criteria for the identification and interpretation of bioturbation in the context of associated physical sedimentary structures. Additionally, comparisons between identified biogenic characteristics, geochemical proxies, and total organic carbon (TOC) contents are analyzed for potential merit in elucidating seafloor paleoredox conditions and estimating TOC trends. This paper identifies the physical and biogenic structures for which the highest degree of confidence is associated (i.e., examples where an interpretation of a sedimentary feature as a trace fossil is parsimonious). Using the ichnological dataset, we thereby identify small-scale fluctuations in both the physical and chemical conditions at and below the sediment-water interface during the deposition of organic-rich mudstone units in the Middle to Late Devonian Canol and Hare Indian Formations.

Some of the largest oil- and gas-producing zones in North America are fine-grained organic-rich mudstone deposits (e.g., the Eagle Ford Shale in Texas and the Niobrara Formation in Colorado). Both the Canol Formation and the Bluefish Member of the Hare Indian Formation are organic-rich siliceous mudstones that have the potential to be economically viable unconventional reservoirs (e.g., Fraser et al., 2011). Current oil-in-place estimates for the Canol Formation and Bluefish Member are 144.825 and 46.346 billion barrels, respectively (Northwest Territories Geological Survey and National Energy Board, 2015). With their economic significance, it is important to understand the nuances of these fine-grained hydrocarbon resources and how they form.