Establishing a Sequence Stratigraphic Model for the Horn River Shale Succession, Horn River Basin, British Columbia
Kennedy, Michael; Dong, Tian; Harris, Nicholas B.
The Horn River Group of the Horn River Basin, northeastern British Columbia, is an Upper Devonian shale succession that has been targeted for its vast reserves of natural gas. This report is part of an ongoing study that aims to establish a sequence stratigraphic model for the Horn River Group, and to associate rock properties such as organic content, porosity, permeability and brittleness with sequences and systems tracts. This study advocates a thorough integration of sedimentological, geochemical and wireline log data from numerous long cores in the Horn River Basin and their shoreward equivalents, and cautions against an overreliance upon specific wireline logs.
The Horn River Group consists of the Evie, Otter Park and Muskwa Formations, overlying Keg River carbonates. Cores preserve two depositional phases that are lithologically and geochemically distinct. The first phase consists of gray calcareous mudstones and limestones, each containing numerous thin (1-2 cm) interbeds of bioclast-rich wackstones and packstones characteristic of carbonate shedding. These sediments comprise the Evie Formation, and were deposited under reducing ocean conditions based on elevated Mo/Al, V/Al, and U/Th ratios. The second depositional phase consists of black laminated silica-rich mudstones with abundant pyrite and no carbonate beds. These sediments comprise the Otter Park and the lower Muskwa Formations, and were deposited under less reducing ocean conditions based on a decrease of the same geochemical ratios mentioned above.
To the southeast, the shoreward equivalents of the Horn River units are two stacked reef complexes: the Presqu'ile Reef and the Slave Point Formation, both of which comprise the TSTs of two 3rd order sequences respectively within an overall 2nd order sea level rise. We correlate these two 3rd order sequences to the two depositional phases in the Horn River Basin. Therefore, the majority of carbonate shedding into the Horn River Basin originated from the Presqu'ile reefs and occurred during a 3rd order TST. Retrogradation of the Slave Point reefs due to the overall 2nd order sea level rise caused the shoreline to recede so that Slave Point shedding could not reach the basin during the second depositional phase, accounting for the absence of carbonate beds above the Evie Formation.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013