The Nature and Significance of Erosion Surfaces in the Middle Triassic Doig and Halfway Formations, WCSB

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

Middle Triassic Doig and Halfway formations are important targets for petroleum exploration and have been studied using wire line logs and cores. This study has investigated the formations between townships 70 and 87, and W6th meridian to Alberta/BC border. The Doig-Halfway interval is particularly interesting in the sense that it can be considered as a complete petroleum system – the basal phosphatic zone of the Doig Formation serves as the source rock, coarser grained siliciclastic and bioclastic deposits of the Doig and Halfway formations provide the carrier and reservoir beds, and time equivalent carbonate-evaporites toward the paleobasin margin along with the overlying carbonate-evaporites of the Charlie Lake Formation serve as the seal.

Deposition was controlled by paleotopography initially, but was soon taken over by synsedimentary and penecontemporaneous tectonics. Reactivation along Late Paleozoic faults and karstification of Mid-Paleozoic carbonates may have played an important role. Other factors that influenced the deposition include shoreline morphology, eustasy, sediment supply and fluvial processes. Relative contribution of each of these factors may have varied temporarily and spatially during the deposition of the formations and were of utmost significance in generating the stratigraphically significant surfaces. Trace fossils combined with sedimentary structures suggest that depositional environments varied from offshore-shelf to (?fluvial) channels. Conodont biostratigraphy was used for age correlation within the Doig-Halfway interval. The conodonts also serve as thermal maturity indicators and aid in paleoecologic interpretation.

Five sequence boundaries (possibly 4th or higher order) have been identified from the interval using physical observations in core, well log character, and biostratigraphy, two of which are displayed in this core presentation of wells 07-15-073-08W6 and 11-01-078-11W6.

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