Depositional framework and trace fossil assemblages of the Lower Triassic Montney Formation, northeastern British Columbia, Western Canada Sedimentary Basin

Abstract

Hydrocarbon production from the Lower Triassic Montney Formation in northeastern British Columbia, occurs primarily from unconventional reservoirs consisting predominantly of fine-to coarse-grained siltstone beds. In this area, the lithostratigraphy of the formation is difficult to characterize due to the complexities associated with subtle grain-size variation, diminutive biogenic structures, lateral facies variability, and distribution of local discontinuities. Detailed sedimentologic and stratigraphic analyses are essential for understanding and refining the depositional models and stratigraphic framework of the Montney Formation. Based on detailed core examination focused on sedimentological, paleontological and ichnological characteristics, eleven lithofacies and three recurring facies associations are identified within the Lower Triassic Montney Formation in northeastern British Columbia. The lithofacies identified are interpreted to have been deposited in offshore, offshore transition and lower shoreface settings, along a storm-dominated mixed siliciclastic-carbonate ramp. Facies associations include FA(A): siliciclastic distal offshore transition to distal offshore; FA(B): mixed siliciclastic carbonate, storm-influenced offshore to offshore transition; and FA(C): storm-dominated siliciclastic offshore transition to lower shoreface. Trace fossil assemblages correspond to a stressed Cruziana Ichnofacies, and are overall characterized by low diversity (1–6 ichnogenera), small size trace fossils (1–12 mm in diameter), and variable bioturbation intensity (0–6). The vertical variation in SDI values reflect changes in physico-chemical conditions during deposition of the Montney Formation, particularly across the Smithian-Spathian boundary. Understanding the lateral-facies variability and overall stratigraphic architecture of the Montney Formation in northeastern BC, constitute key elements in defining and correlating existing and new potential hydrocarbon reservoirs in the area.