Sedimentary processes and depositional environments of the gas-bearing Horn River shale in British Columbia, Canada

The Horn River Basin in the northeastern British Columbia, Canada, is one of the largest unconventional gas-bearing basins in North America. The main reservoir of this gas accumulation is the Devonian Horn River Formation that is stratigraphically divided into three members, the Evie, Otterpark and Muskwa in ascending order. This study focuses on sedimentary processes and depositional environments of the Horn River Formation based on sedimentary facies analysis by the aid of well-log mineralogy (ECS) and total organic carbon (TOC) data from the Kiwigana well. The shales of the formation consist dominantly of silicate minerals (quartz, feldspar and mica) and subordinate clay mineral and carbonate materials, with TOC ranging from 0.3 to 7.6%. Three sedimentary facies were identified on the basis of centimeter-scale description of sedimentary structures and texture in borehole cores: homogeneous mudstone (HM), indistinctly laminated mudstone (ILM), and planar laminated mudstone (PLM). Integration of sedimentary facies, lithology and TOC suggests that the Horn River shale was primarily deposited in overall distal marine setting deeper than shelf or marginal slope, possibly base-of-slope to deeper basin plain off the carbonate reef (or shelf). Facies HM is siliceous and organic-rich, and dominant in the Evie and the overlying lowermost Otterpark members. It is interpreted as a pelagic to hemipelagic deposit formed mainly by suspension fall-out in an anoxic setting below maximum storm wave base. Likewise, facies ILM shows relatively high proportion of silicate minerals and TOC. This facies is identified frequently in the Muskwa and rarely in the Otterpark; it reflects a deposition from hemipelagic settling with an influence of persistent and weak bottom currents or nepheloid flows. Facies PLM, dominant in the large part of the Otterpark, is relatively depleted in silicate minerals and TOC. This facies indicates more frequent inflow of episodic turbidity currents punctuating the hemipelagic settling of the background sedimentation process. During the deposition of the Horn River Formation, the depositional site has experienced an earlier relative sea-level fall changing from a deep basin (Evie) to shallower marginal slope (middle Otterpark), and subsequent relative sea-level rise turning back to a deeper marine environment (Muskwa).