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Modern and Ancient Alluvial Systems
Sedimentology and Hydrocarbon Distribution of the Lower Cretaceous Cadomin Formation, Northwest Alberta
The Cadomin Formation is the basal unit of the Lower Cretaceous Blairmore Group in northwestern Alberta, and is a wedge-shaped conglomeratic molasse deposit associated with the Columbian Orogeny. Laramide thrusting and subsequent erosion exposed the thickest sections of Cadomin in the Foothills of the Rocky Mountains. Beyond the eastern limit of overthrusting the deposit has been deeply buried in the Alberta Deep Basin. There it forms a thin, laterally extensive sheet-like unit which overlies a regional unconformity of low topographic relief.
The Cadomin was deposited in an alluvial fan – braidplain environment under humid climatic conditions. Braided river systems reworked the fan detritus laterally and distally to produce a thin, yet widespread distribution. A Scott to Donjek type braided-river deposit is inferred on the basis of systematic textural variations in facies assemblages. Proximal to distal trends include decreasing clast size and increasing sand content, with a resulting drop in abundance of bimodal textures.
In the subsurface, the unit forms a gently southwest dipping monocline in which a Deep Basin type hydrocarbon reservoir has developed. Porous and permeable water-bearing strata pass downdip into poor reservoir quality gas-saturated strata. Significant gas reserves have been established in the Elmworth Area but production is limited to higher permeability zones. Permeability and ultimate reservoir potential is a dual function of primary depositional texture and diagenetic modifications. Moderately to poorly-sorted conglomerate textures generally have higher porosity and permeability than bimodal textures. A variation in diagenetic modification exists in the Cadomin because of a geothermal gradient and an increasing depth of burial towards the southwest of the Deep Basin area. Permeabilities are reduced in zones of quartz overgrowth cementation and added authigenic clay matrix. Secondary porosity is developed and permeability enhanced in zones of dissolution and leaching by pore fluids. Successful exploitation of these conglomerates depends on knowledge of reservoir facies textures and diagenetic controls.
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