Abstract

A 395 m continuous 7.5 cm (3”) full-diameter core of the entire Montney Formation and overlying “Anisian Wedge” (a.k.a. Sunset Prairie Fm) from the Progress Graham c-65-F /94-B-8, provides an outstanding opportunity to examine the complete sedimentologic and stratigraphic record of the entire Lower Triassic in its most basinward paleogeographic position in the subsurface of northeastern British Columbia. The well has been extensively sampled for rock properties, including routine core analysis, thin-section petrology, XRF, rock-eval geochemistry, and conodont biostratigraphy. Integration of a detailed sedimentologic description of the entire core with data from 30 samples analyzed for conodonts, supported by extensive previous sedimentologic and biostratigraphic studies of Montney core in offset wells, provides the basis for construction of a sequence biostratigraphic framework of the entire Montney Formation. Seven primary sedimentary lithofacies are identified: four siliciclastic facies with varying degrees of bituminous matrix, two bioclastic facies and a calcareous, calcispheric dolosiltstone facies. All facies were deposited on the distal through proximal portions of a clastic shoreface to inner shelf or mixed clastic/carbonate ramp. Three principal reservoir facies associations regionally targeted for horizontal drilling are observed in the core as follows:

1. Early Dienerian Claraia sp. biostrome. This facies association, which has informally been referred to for years as “the turbidite zone,” is an in situ, monospecific, life assemblage of Claraia sp. “flat clams,” interpreted as a biostrome.

2. Smithian mixed clastic/carbonate ramp facies association. Sharp-based, normally graded, 5–20 cm thick bioclastic event beds consisting of a low diversity mixture of bivalve fragments interpreted as tempestites and interbedded with variably cryptobioturbated suspension deposited siltstone.

3. Late Spathian siliciclastic shoreface-inner shelf facies association. Dominantly burrowed to bioturbated sandy coarse siltstone and silty very-fine grained sandstones were deposited in lower shoreface, offshore transition and offshore environments. This interval is bounded at the base by a maximum flooding surface and above by a coplanar sequence boundary/flooding surface demarcated by a Glossifungites trace fossil assemblage. The Spathian section is overlain by the “Anisian Wedge” (Sunset Prairie Fm.) stratigraphic unit.

Petrographic analysis in conjunction with core analysis data indicate that macro- and cryptobioturbation preserve intergranular porosity and permeability by inhibiting calcite cementation and developing dual pore throat size apertures, both of which play a key role in hydrocarbon deliverability. This has been demonstrated in MICP results from Late Spathian shoreface-inner shelf facies associations in nearby wells (Moslow and Haverslew, 2015).

The Montney is bounded by coplanar sequence boundary/flooding surfaces at the top of the underlying Paleozoic Belloy Formation and base of the overlying Middle Triassic Doig-Phosphate Formation. An informally named “Middle Montney” sequence boundary is regionally correlatable and coincident with the Smithian-Spathian contact. This surface is associated with significant erosion, brecciation and truncation of underlying strata. All three surfaces are likely the products of relative sea level fluctuations coincident with regional tectonism.

Conodont biostratigraphy of the c-65-F core provides the basis for subdividing the Montney into four sequences from oldest to youngest as follows: Sequence 1) Griesbachian-Dienerian (Lower Montney); Sequence 2) Smithian (Middle Montney); and Sequence 3) Spathian (Upper Montney). In addition, the overlying “Anisian Wedge” (Sunset Prairie Fm), sometimes included lithostratigraphically with the Montney Formation, is Sequence 4 and of Anisian (Middle Triassic) age.