--> Detailed Lithofacies Analysis and High Resolution Sequence Stratigraphy of the Horn River Group, British Columbia, Canada

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Detailed Lithofacies Analysis and High Resolution Sequence Stratigraphy of the Horn River Group, British Columbia, Canada

Abstract

The Devonian Horn River Basin shales have been a target for gas exploration for the past decade in northeast British Columbia, Canada. The reservoir quality of these shales is highly variable due to the complex nature of the depositional setting and physical processes. Detailed sedimentological, ichnological, geochemical and stratigraphic analysis of nine Horn River cores indicated that depositional conditions range between anoxic conditions to oxygenated conditions. Integration of lithologic data, physical sedimentary structures, and geochemical signatures lets us better understand the fundamental architecture of the reservoir qualities. The Horn River Group consist of three formations: Evie, Otter Park, and Muskwa, which are relatively carbonate-rich, clay-rich and silica-rich, respectively. Ten lithofacies and three lithofacies associations were identified based on the sedimentological and ichnological observations. Massive and pyrite-rich mudstones show very rare current-generated structures and sparse bioturbation. These mudstones also show the highest total organic carbon (TOC) values and have been interpreted to represent anoxic deep-water (ADW) conditions. Heterolithic and laminated units show well preserved physical sedimentary structures, and they are typically moderately to intensely bioturbated. They show the lowest TOC values and have been interpreted to represent oxygenated deep-water (ODW) conditions. An additional facies association, representing transitional conditions, has been assigned to facies showing intense or sparse bioturbation (BI 0-1 or BI 4-6) depending on the core location, possibly indicating local changes in the energy conditions. Transitional facies generally show moderate TOC values. The Evie Member and Muskwa Formation are dominantly represented by ADW while Otter Park Member is represented by both ODW and transitional conditions. Changes in the lithofacies distributions and distinct geochemical signatures in space and time resulted in recognition of nine depositional sequences (3rd order) and eight major surfaces. The sequence boundaries are marked by noticeable changes in the sedimentological and ichnological signatures. The Evie Member and the Muskwa Formation are mainly represented by high-stand and transgressive system tracts while the Otter Park Member is mainly represented by low-stand and falling-stage system tracts.