Lynn M. Barber¹, Federico F. Krause¹, Shannon L. Greene¹, Jocelyn Keith-Asante²
(1) University of Calgary, Calgary, AB
(2) N.A.L. Resources, Calgary, AB

Abstract: Unconformity control on diagenetic overprinting of a fine-grained reservoir - Lower Triassic Montney Formation in the Sturgeon Lake South 'F' pool, west-central Alberta

An unconformity with a maximum time gap of 30 Ma separates the Lower Triassic Montney Formation from the Lower Jurassic Nordegg Member in west-central Alberta. This unconformity delimits the top of the Sturgeon Lake South ‘F’ pool reservoir and has provided the surface along which eogenetic fluids were introduced. Regional mapping is suggestive of additional unconformity sculpting events that are not readily apparent in Sturgeon Lake South ‘F’ pool. These events may be associated with the Triassic Worsley and Coplin unconformities.

Paleosolic processes have imprinted reservoir siltstones and very fine-grained sandstones. The original porous system has been modified by illuviation of clays and dolomite cementation during dolocrete development. Dolocrete deposits display at least 2-3 phases of dolomite cementation and karstification. The early dolomitization phases have altered original porosity and permeability.

Porosity and permeability in un-altered reservoir facies are approximately 13 to 18% and 5 to 20 md. Porosities drop to less than 10% and permeabilities less than 1 md immediately below the unconformity surface. Destruction of porosity and permeability is not restricted to the immediate vicinity of the sub-Jurassic unconformity surface, but is also observed as tight zones at other reservoir levels. These zones may be related to fluctuating water tables that may exist during unconformity sculpting events.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana