Exploration Significance of Anomalously Thick Sandstone and Mudstone Bodies of the Middle Triassic Doig and Halfway Formations in the Western Canada Sedimentary Basin

Bazlur Rahman and Charles M. Henderson
University of Calgary, Calgary, AB

Doig and Halfway formations in the WCSB are underexplored due to stratigraphic correlation problems, and warrant an improved paleodepositional facies model with age constraints. Detailed well-log and core examination highlights some of the complexities regarding deposition and preservation of the anomalously thick sandstone and mudstone bodies (ATSB and ATMB), as well as documenting previously unrecognized units/sequences within the formations, between townships 70 and 87, west of 6th meridian to Alberta/BC border, Canada. Synsedimentary and penecontemporaneous tectonics exerted major control on the creation of accommodation space and deposition and preservation of stratigraphic sequences, and fluvial processes played a secondary role.

Thirteen sedimentary facies are grouped into five lithofacies associations: offshore/shelf, offshore-transition, barrier island-shoreface, estuarine and carbonate ramp. Five potential higher order sequence boundaries have been identified within the Doig-Halfway interval, two of which lie within the Doig phosphatic zone. Conodont biostratigraphy has not only been used to discriminate age correlation, but also to serve as a thermal maturity indicator and aid for paleoecologic interpretation.

ATSB within the Doig-Halfway interval are located parallel to subparallel to paleodepositional shoreline, were deposited in a transgressive barrier island setting on an overall regressive background and have excellent reservoir potential. ATMB occurring inboard of ATSB were possibly deposited in a tectonically induced estuary. Remnants of bedforms in the ATSB show a paleolandward migration pattern in the dip-logs as opposed to the paleobasinward migration of their thin lateral equivalents. Pale yellow to amber coloured conodonts in the phosphatic zone suggest that it still has petroleum generation potential.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005