A depositional model for organic-rich Duvernay Formation mudstones

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• 作者：Levi J. Knapp ; ^{KnappLeviJ@gmail.com} ; Julia M. McMillan ; Nicholas B. Harris
• 关键词：Devonian ; Duvernay ; Sedimentology ; Mudstone ; Black shale ; Organic-rich
• 刊名：Sedimentary Geology
• 出版年：2017
• 出版时间：1 January 2017
• 年：2017
• 卷：347
• 期：Complete
• 页码：160-182
• 全文大小：7924 K
• 卷排序：347

文摘

The Upper Devonian Duvernay Formation of western Canada is an organic-rich shale formation now targeted as a hydrocarbon reservoir. We present a detailed sedimentological analysis of the Duvernay Formation in order to better understand organic-rich mudstone depositional processes and conditions and to characterize the vertical and lateral heterogeneity of mudstone lithofacies that affect petrophysical and geomechanical rock properties. Organic-rich mudstone facies of the Duvernay Formation were deposited in a dynamic depositional environment by a variety of sediment transport mechanisms, including suspension settling, turbidity currents, and bottom water currents in variably oxygenated bottom waters. Suspension settling dominated in distal relatively deep areas of the basin, but evidence for weak turbidity currents and bottom water currents was observed in the form of graded beds and thin grain-supported siltstone laminae. Organic enrichment primarily occurred in distal areas as a result of bottom water anoxia and low depositional rates of inorganic sediment. In deep water locations near platform margins, alternating silty-sandy contourite beds and organic-rich mudstone beds are present, the former interpreted to have been deposited and reworked by bottom water currents flowing parallel to slope. In shallower, more oxygenated settings, mudstone lithologies vary from calcareous to argillaceous. These sediments were deposited from suspension settling, turbidity currents, and bottom water currents, although primary sedimentary structures are often obscured by extensive bioturbation. Locally, organic enrichment in dysoxic rather than anoxic bottom waters was driven by a slightly increased sedimentation rate and possibly also by aggregation of sedimentary particles in the water column due to interaction between organic matter and clay minerals. Large variations observed in sediment composition, from siliceous to calcareous to argillaceous, reflect multiple biogenic, carbonate, and clastic sediment sources. Sediment composition is influenced by basin morphology, circulation patterns, sea level variation, and proximity to sediment sources.