The Upper Bow Island (Blackleaf) Formation of Southwestern Alberta: Geological Aspects and Exploration Approaches
PUTNAM, PETER E., and SHAWNA L. CHRISTENSEN, Petrel Robertson Ltd., Calgary, Alberta, Canada
The upper parts of the Bow Island Formation (Albian) of southwestern Alberta are significant gas reservoirs. The main westernmost reservoir zone is part of a complex package of interbedded lenticular sandstones, mudstones, and localized chert pebble conglomerates. The depositional setting for these sediments comprised a wave-dominated shoreline with conglomerates found proximal to drowned river mouths. The coarse nature of the upper Bow Island is related to tectonic
movements associated with Crowsnest (Vaughn) volcanism.
Conglomerates form the most impressive Bow Island reservoirs because of their thickness (up to 25 m) and petrophysical properties (17% porosity, 24 d permeability). Diagenesis dominantly comprises compaction features within grain-supported conglomerates. Increasing quartz content is related to decreasing grain size and is associated with porosity occlusion by quartz overgrowths.
Bow Island reservoirs in southwestern Alberta are cool (under 50 degrees C) and significantly underpressured (0.2 psi). The high permeabilities and low pressures at depths of 1000 to 1500 m suggest the potential for formation damage is high, and many wells in the region were targeted for deeper, high-pressure zones. In spite of the low pressures, however, many Bow Island wells are capable of excellent gas deliveries with individual well recoveries of up to 10 bcf. All significant Bow Island porosity in the deepest, undisturbed parts of southwestern Alberta is gas saturated with updip aquifers flanking the gas.
Seismic definition of the thickest Bow Island targets is feasible but has been hampered, in part, by difficult surface conditions and a prior emphasis on deeper targets.
AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)