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Exploration potential of the Falher G shoreface conglomerate trend: evidence from outcrop

Geological Survey of Canada, 3303 - 33 Street NW, Calgary, AB T2L 2A7

Thomas F. Moslow¹

Midnight Oil and Gas Ltd., 1530, 140 - 4 Avenue SW, Calgary, AB T2P 3N3

The lower Cretaceous Falher Member shoreface conglomerate trends are the most prolific natural gas reservoirs of the Alberta and British Columbia Deep Basin. Individual pools can be in excess of 100BCF with discovery wells yielding AOFs of more than 100 MMCFD. Since the 1980s, five conglomerate shoreface trends (Falhers A, B, C, D and F) have been successfully drilled and mapped in the subsurface through observations of cores, cuttings, and well logs. Subsequent to their initial discovery and drilling in the subsurface each trend has been correlated to an outcrop equivalent in the Rocky Mountain Front Ranges of northeastern British Columbia. Until recently, no other Falher conglomerate shorelines have been recognized.

South (paleolandward) of the Falher A–F trends, an older shoreface conglomerate fairway, designated the Falher G, has been discovered through outcrop observations at Holtslander Ridge near Belcourt Creek, British Columbia, 110 km southeast of the southernmost described Falher shoreface conglomerate outcrop. This trend, which is oriented oblique to younger trends, can be extrapolated into the subsurface plains at the Narraway Field, Alberta, approximately 35 km south of the nearest (Falher F) shoreline conglomerate reservoir fairway.

Detailed analyses of measured sections, gamma log profiles, and photo mosaics show that the Falher G is a prograding shoreface facies association of clast-supported and sand matrix chert pebble conglomerate 8 to12 m thick. The outcrop is approximately 5.0 km in length, along a north–south-trending depositional dip exposure that grades basinward from clast B supported conglomerate to pebbly sandstone. The Falher G shoreface outcrop contains several internal ravinement surface that dip at 5–10° to the NNW (320°). Analysis of locally available coal exploration core limits the paleoshoreline width of the Falher G conglomeratic shoreface trend to less than 5.0 km, with a paleoshoreline orientation of 285°/105°. Extrapolation of this trend into the subsurface may yield a new and significant shoreface conglomerate reservoir.