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Fault Controlled Dolomitization of the Wabamun Group, Tangent Field, Peace River Arch, Alberta
Carbonates of the Upper Devonian Wabamun Group in the Peace River Arch region provide an example of fault-related dolomitization and reservoir creation. Sediments of the uppermost Wabamun generally comprise tight crinoidal lime wackestones to mudstones deposited in a deeper distal ramp setting. They show a number of depositional cycles, each becoming progressively deeper in aspect, culminating in the deeper-water Exshaw laminites. These sediments exhibit little or no depositional porosity or permeability. However, a number of fields in the Peace River Arch area produce from dolomitized uppermost Wabamun sediments. The Tangent field provides a model for understanding the evolution of this play type.
In the Tangent field area, production appears to be closely related to patterns of faulting. Adjacent wells, as close as 400 metres, show a change from tight limestone to productive dolomite. This dolomitization clearly cross-cuts the stratigraphic framework. Dolomitized wells often exhibit uncorrelatable high gamma ‘kicks’ at various levels within the section. These zones correspond to chaotic breccia units in core. Breccia units contain large angular clasts of various lithologic types in a silty dolomite or secondary anhydrite matrix. Some of the clasts are comprised of black laminite, similar to that from the overlying Exshaw Formation and occur sometimes 30 metres into the Wabamun section. Matrix material is comprised in part of glauconite and phosphatic pellets similar to the basal Banff siltstone lithologies.
The inclusion, within breccia zones, of clasts and matrix components apparently derived from the overlying Banff and Exshaw suggests early fault initiation prior to their lithification, and faulting appears to be no later than Upper Mississippian in age. These fault zones allowed the overlying Banff shales to dewater into the Wabamun, and the paragenetic sequence within breccia zones indicates initial solution and dolomitization followed by dolomite precipitation.
Subsequently, these faults became the conduits for hydrocarbon migration. Successful exploration for this play type relies on an evaluation of the relative timing of fault sets and determination of the prospective early faulting phase.
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