Regional subdivisions, sequences, correlations and facies relationships of the Lower Triassic Montney Formation, west-central Alberta to northeastern British Columbia, Canada — with emphasis on role of paleostructure

Abstract Stratigraphic subdivisions and sequences established for the Lower Triassic Montney Formation based on extensive core control in west-central Alberta have been extended to the northern sector of northeastern British Columbia (NE BC). The paper draws extensively on unpublished data from multiple industry sources. The correlations are based on six (6) cross-sections incorporating 63 wells with a cumulative section length of over 1000 kms. The cross-sections are presented in two formats: proportionally-spaced LAS log sections, included as text figures; equally-spaced raster log sections with extensive labels (including selected core summaries) in an appendix. The Montney in this paper is divided into three unconformity-bounded, third-order sequences. The divisions and their internal subdivisions (from base up) are: the Griesbachian-Dienerian Lower Montney, built up by transgressive and highstand systems tracts; the Smithian Middle Montney, with a basal lower Smithian lowstand wedge, a ‘mid’ Smithian transgressive systems tract, and a thick mid to upper Smithian highstand systems tract; the Spathian Upper Montney, with a basal lower Spathian lowstand wedge, and an overlying mid to upper Spathian siltstone unit (the former Lower Doig Siltstone of earlier GDGC reports) built up by a basal transgressive systems tract and an upper highstand systems tract. The bounding unconformities for these Montney divisions (from the base up) are: the global first-order Permian-Griesbachian unconformity and sequence boundary (SB), a complex, multi-event, paleokarst boundary correlative with Permo-Triassic mass extinction event/s (base Lower Montney); the global third-order Dienerian-Smithian unconformity and SB (Lower to Middle Montney boundary); the global third-order Smithian-Spathian unconformity and SB (Middle to Upper Montney boundary); and the global second-order Spathian-Anisian unconformity and SB (Upper Montney to Doig boundary). Paleostructure on the Paleozoic unconformity below the Montney is a key control on the thickness of the total Montney, and of thicknesses of internal subdivisions, of facies, and of depositional trends. Upper Devonian Leduc reef and platform margins are the principal controls on Montney paleostructure from the southern Peace River arch area southeastward into south-central Alberta, while reactivated basement faults, including the Dawson Creek Graben Complex (DCGC) and the Hay River Fault Zone (HRFZ), are the major paleostructural controls northward into NE BC. Paleostructure is expressed best by third-order residual structure mapping of the top-Paleozoic unconformity, with three residual structure map-figures incorporating cross-section overlays included in this paper. A paleostructural location summary is included below each well log on the raster cross-sections. In the mid 1990s, preliminary biostratigraphic dating of the Montney in west-central Alberta was based on palynology. Within the last five years or so, new conodont biostratigraphic dating of the Montney in NE BC, from one published source but mainly from data released for this paper by Progress Energy Canada, has provided strong support for the proposed ages of subdivisions in the mid to upper Middle Montney (mid to late Smithian) and throughout the Upper Montney (early, mid, late Spathian). The only apparent discrepancy between conodont age dates and log-based lithostratigraphic picks and correlations in this paper centre around the placement of the Dienerian-Smithian (Lower to Middle Montney) boundary. As all conodont-dated wells lie within or peripheral to the HRFZ or other basement faults, structural movement may be a factor in this apparent discrepancy, but it is more likely due to uncertainties concerning the age range of key conodont species around or across the Dienerian-Smithian boundary, and of some stratigraphic picks. Some of the most intense bioturbation observed by the first author in any Montney cores occurs in the updip, shallower part of the lower Spathian lowstand wedge in westernmost Alberta and NE BC (where it is productive). A similar bioturbated interval also is recognized now in the updip, shallower part of the lower Smithian lowstand wedge in west-central Alberta. Ichnological analysis included in this paper demonstrates that the ichnogenera in both wedges essentially are the same, albeit more robust in the Spathian wedge. Extensive cryptobioturbation in HCS-dominant event beds in the upper Middle Montney enhances reservoir properties. The subdivisions, correlations and nomenclature for the Montney defined in this paper may stir some disagreement, but should serve as a base for an eventual industry-wide nomenclature and subdivision framework for the Montney.

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