Facies architecture and sequence stratigraphy of the Lower Triassic Montney Formation, NE British Columbia; fundamental controls on the distribution of 'sweet spots' in a world-class unconventional reservoir

In the Regional Heritage Field of northeastern British Columbia, Canada, the Lower Triassic Montney Formation consists of up to 320 m of bituminous, dolomite-cemented, arkosic, siltstone that forms a continuous, basin-centred, hydrocarbon accumulation. An integrated sedimentological and ichnological analysis of cores taken from across the field, suggests that the Montney Formation features three main facies: shelf, distal prodelta and proximal prodelta. The shelf facies records deposition below storm wave base in a suboxic environment. The distal prodelta facies reflects deposition above storm wave base, in a setting that was subject to continuous paleoenvironmental fluctuations. The proximal prodelta facies was deposited between fair-weather and storm wave-base, in a setting that was periodically influenced by storms. The stratigraphic architecture of the Montney Formation consists of a series of offlapping clinoforms that comprise at least three unconformity-bounded, depositional sequences, informally named Sequence 1, 2, 3, in ascending order. These sequences are interpreted to reflect the construction of a relatively shallow marine shelf that was fed, primarily, by ephemeral rivers in an arid to semi-arid setting. A reconstruction of the depositional history of Sequence 3 places the reservoirs of this extensively targeted interval into facies and sequence stratigraphic context. The reservoir and production characteristics associated with the lowstand systems tract of Sequence 3 is examined, in order to identify and establish relationships between reservoir quality distribution and horizontal gas-well productivity in the Regional Heritage Field. The analysis reveals that the observed regional variability in horizontal well productivity is controlled, primarily, by the thickness and quality of the reservoir, as well as by factors that influence the relative permeability to gas. These factors include the distribution of hydrocarbons within the reservoir, and the associated water saturation. This study demonstrates that a detailed and comprehensive understanding of the depositional processes and sequence stratigraphy associated with the Montney Formation is fundamental to the exercise of defining sweet spots in this otherwise continuous reservoir.