ABSTRACT: Gas Prone, Incised Valley-Fill Sediments Within the Upper Albian Bow Island Formation, Southwest Alberta

COX, J., Mount Royal College, Calgary, Alberta, Canada, and B. P. J. WILLIAMS, University of Aberdeen, Aberdeen, Scotland

Regionally, the Bow Island Formation of southwest Alberta consists of three lithostratigraphic units. Unit 1, the lowest unit, is approximately 100 m thick and consists oft stacked, upward-coarsening clastic parasequences separated by marine flooding surfaces. Unit 2 is an easterly thinning wedge of nonmarine, coastal plain deposits, which conformably overlies Unit 1. Unit 3 is up to 20 m thick and consists of tidally influenced and marine shelf sediments.

The incised valley-fill sediments cut into Units 2 and 3 with a maximum identified thickness of 35 m. Diagnostic features of these sediments include: (1) a basal scour surface commonly overlain by a gravel lag; (2) variably interbedded sands and muds; (3) bioturbation restricted to a small variety of trace fossils; (4) numerous erosional surfaces, commonly overlain by graded sands and gravels exhibiting high-angle crossbedding; and (5) scattered gravel in bioturbated, muddy sandstone units. These features are consistent with an estuarine model of deposition, where the incised valleys filled with sediment during transgression of the Lower Colorado seas.

Bow Island estuarine reservoir rocks are typically heterogeneous with highly variable porosities and permeabilities, but are capable of hosting large volumes of gas in the Blood Field, which holds in excess of 30 BCF recoverable reserves.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)