ABSTRACT

The application of very high resolution stratigraphic controls to the study of the post-Montney/pre-Nordegg sequence in the Peace River area has made it possible to analyze its stratigraphy and facies at the reservoir scale. This high resolution control is based on differentiation of the sequence into 17 subunits, which were identified in more than 3000 well logs and complemented by core studies.

The post-Montney/pre-Nordegg sequence in the study area comprises four parasequences, separated by three unconformities of a regional magnitude, which are (base to top) the Halfway, Coplin and Baldonnel. These parasequences are the expression of four cycles of sedimentation. The lowermost cycle began with progradation of the phosphate beds and the Doig Formation to the west, which culminated in the Halfway unconformity. Subsequent movements along old faults in the area resulted in a temporary marine incursion or inundation associated with the deposition of the Halfway Formation. This cycle was followed by three others, each also consisting of: a) progradation, b) evolution of an unconformity, c) re-adjustment movements along old faults, and d) an inundation. The fourth cycle was interrupted near the end of Triassic time by a regional uplift that erased its upper levels with the Nordegg unconformity.

The four parasequences of the post-Montney/pre-Nordegg sequence represent different categories of tidal-flat sedimentation. The lowermost transitional parasequence is dominantly siliciclastic, whereas the other three are mixed silt-carbonate shoreline facies. The lateral continuity, facies changes and vertical stacking pattern of strata within these four parasequences are defined by the intermittent readjustment movements along old faults in the area. These faults are major, northwesterly trending step faults, downthrowing to the west. They have differentiated the area into blocks and played a decisive role in the distribution pattern of traps.

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