Abstract: Reservoir Characterization and Facies Analysis from Outcrop Exposures of the Falher D Member (Lower Cretaceous), Western Canada Sedimentary Basin

Thomas F Moslow, Allen Schink

Detailed sedimentologic descriptions and correlations from numerous, closely spaced measured sections of 3.0 km of laterally continuous outcrop

exposures of the Falher D Member (Gates Formation) have provided the geologic framework for characterizing the distribution, compartmentalization and behavior of analogous subsurface reservoirs in the Western Canada Sedimentary basin. Two parasequences are recognized in the Falher D from outcrop at Mt. Spieker, British Columbia. An underlying retrogradational parasequence ("Basal D") is overlain by a progradational parasequence that is internally characterized by a cyclic repetition of laterally discontinuous shoreface facies associations. Each facies association coarsens upward from fine-to medium-grained sandstone (lower shoreface) through plane bedded pebble-conglomerate (foreshore), and is interpreted as deposited in a graveliferous, wave-dominated strandplain depositional system. /P>

The foreshore facies in outcrop is up to 2.5 m thick and is a granule to pebble, unimodal clast-supported (open framework) conglomerate. As a function of wave swash processes of deposition, this facies has extremely high primary porosity and permeability. and is thus the principle reservoir facies. Along depositional strike, the foreshore facies has continuity for 10³ to 10⁴ m. In sharp contrast, foreshore conglomerates have minimal continuity (10¹ to 10² m) along depositional dip, pinching out rapidly up-dip (paleo-landward) and grading down-dip (basinward) to a trough-crossbedded, sand-matrix supported conglomerate and pebbly sandstone (upper shoreface facies). Therefore, each facies association in outcrop can be observed to grade laterall from the high reservoir quality foreshore facies up-dip, to non-reservoir facies (upper shoreface through offshore transition) down-dip. The shoreface facies association is repeated minimally five times over 2.8 km of depositional dip exposure with a lateral spacing of 250 m to 1000 m (maximum) between foreshore conglomerate facies trends. Each facies association is bounded by a ravinement surface that truncates underlying facies, and likely serves as a significant permeability barrier in subsurface reservoirs.

The Falher D is one of several (A--G) conglomerate Members in the Gates Formation, five of which are significant natural gas reservoirs in the Deep basin of Alberta and British Columbia. Characterization of these reservoirs from outcrop analogs provides powerful predictive capability for delineating reservoir trends and future development drilling.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994