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The absence of large-scale tectonic disturbances along the eastern flank of the Alberta syncline has made the search for structurally entrapped oil and gas deposits rather minor in comparison with the stratigraphically entrapped deposits. Closed structures occur, but the controlling factors for the closed anomalies lie in draping. Draping is over reef buildups, residual topography on surfaces of unconformity, and structural noses along ancient tectonic trends.
Oil and gas fields along the Edmonton reef chain provide examples of many complex stratigraphic relations which in turn control the entrapment of hydrocarbons. The principal trap along the trend is the carbonate buildup of the Devonian reef entirely enclosed by shale. The geometry of the oil and gas accumulations in the reefs simulates the normal fluid segregations found in simple anticlinal traps. Differential compaction of overlying sediments above the reef massifs causes draping in the overlying sediments and supratenuous anticlines are formed somewhat symmetrically with the reef body. Such features enable seismic exploration to delineate the reef trends, even though seismic records give poor reflections from the reef massifs themselves. Subreef structural relations are unknown at resent because of lack of deeper drilling.
The structural closures in the sediments above the reefs account for another type of trap for oil and gas. The Devonian Nisku and Wabamun carbonates, the Cretaceous Basal Quartz and Viking sandstones, and in several places the Cretaceous Cardium sandstone contain oil and gas accumulations with geometry of fluid segregations similar to that found in anticlinal closures. It must be pointed out, however, that in many places, porosity-permeability conditions in these reservoirs modify the geometry of the fluid accumulations to a great degree.
A third type of trap results from erratic porosity development related to the effect of buried highs on subsequent sedimentation. In several places improved porosity conditions in the Devonian Nisku formation account for the extension of oil accumulation some distance beyond the structural closures above the reef massifs. The development of thicker, more porous sands can be found in the Cretaceous Basal Quartz above the inter-reef channels. Some old topographic depressions in the unconformity surface between the Paleozoics and Cretaceous are filled with porous sand lenses in the Basal Quartz sequence.
A fourth type of trap is found in the truncated edge of several Mississippian units. Accumulations of oil and gas are found in porous Rundle carbonates as their regional updip termination crosses the Edmonton reef chain.
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