Abstract

Very fine-grained sandstone beds in the lowermost Triassic (lower Induan = Griesbachian) lower Montney Formation are the primary reservoir lithology in the Pedigree, Ring-Border and Kahntah River gas fields of northwestern Alberta and northeastern British Columbia. Hydrocarbon production from Pedigree and Ring-Border fields occurs primarily from well-sorted, low-angle to hummocky and trough cross-stratified sandstone beds. Hydrocarbon production from Kahntah River field occurs primarily from highly bioturbated silty/muddy sandstone with hummocky and trough cross-stratified sandstone interbeds.

The three gas fields included in this study occur within shallow marine successions in strata that partially coincide stratigraphically, but are successively younger, from east to west. Thus the cored interval in Pedigree Field overlaps with, but is overall slightly older than, the cored interval in the Ring/Border Field, which overlaps with, but is overall slightly older than, the cored interval in the Kahntah River Field. Six lithofacies have been identified in the study interval: L-I (dark gray laminated shale); L-II (interbedded very fine-grained sandstone, siltstone and silty shale); L-III (low-angle to hummocky cross-stratified, very fine-grained sandstone); L-IV (convolute-bedded shale, siltstone and sandstone); L-V (bioturbated muddy/silty sandstone); and L-VI (trough cross-stratified, very fine-grained to fine-grained sandstone). These lithofacies comprise an overall shallowing upwards proximal offshore through shoreface succession.

Strata interpreted as proximal offshore contain few trace fossils, primarily isolated Planolites. Trace fossils in strata interpreted as offshore transition include Aulichnites, Conichnus, Cruziana, Diplichnites, Fustiglyphus, Halopoa, Helminthopis, Lockeia, Monomorphichnus, Palaeophycus, Planolites, Protovirgularia, Rhizocorallium, Rusophycus, Spongeliomorpha, Thalassinoides and Treptichnus. Although trace fossil diversity is very high, abundances are low and few of these taxa represent indigenous infauna. Most forms represent the dwelling and feeding traces of storm-transported, opportunistic colonizers. Strata deposited in the lower shoreface contain a diverse trace fossil assemblage consisting of Asterosoma, Cylindrichnus, Cruziana, Diplocraterion, Palaeophycus, Phycosiphon, Phycodes, Planolites, Rhizocorallium, Rosselia, Skolithos, Spongeliomorpha, Teichichnus and Thalassinoides. Upper shoreface sandstone beds are apparently barren.

Several lines of evidence, including low diversity of indigenous infauna, support the hypothesis of dysoxic depositional conditions in the lower Montney offshore transition during the Griesbachian. Decreased thickness and preservation of storm beds suggest that the intensity and frequency of storms in the study area appear to have decreased through the Griesbachian, resulting in longer interstorm intervals and deeper, more pervasive bioturbation. Periodic influx of turbid fresh/brackish water by local ephemeral deltas may have also had a dampening effect on local biodiversity.