Abstract

White, zoned (Fe), rhombic and saddle shaped dolomite cements are present in association with fractures in the Frasnian stromatoporoid reef platform and buildup of the Swan Hills Formation in the Alberta Basin.

Twelve zones, as revealed by staining, microprobe and isotopic analyses, form the framework for the growth stratigraphy of the dolomite cements. Extensive mapping of the zones throughout the Swan Hills Field indicates that two distinct pulses of upward moving fluids occurred during cementation.

Computer produced cross-sections and structural maps based on the lateral and vertical distribution of 50 log markers (ranging from Devonian to Cretaceous) in 142 wells in township 67-10W5 show a correlation between linear structural offsets (5 to 13 metres (15 to 40 feet) in magnitude) and the distribution of dolomite zones.

The absence of pulse one dolomite cement zones above a distinct chronostratigraphic marker in the buildup suggests that pulse one was emplaced at the time the buildup was at (or near) sea level before sedimentation above the marker. Structural reconstructions using a Devonian datum show that the magnitude of the structural offsets is in excess of the effects of compaction, confirming an early timing. Sedimentological and morphological patterns in the upper part of the buildup, where only pulse two is present, show a correspondence with the structural lineaments. Hence, fluids, moving upward in an extensional structural framework are responsible for dolomite cementation, and at least pulse one dolomite cementation occurred penecontemporaneously with the development of the buildup.