Primary sealing units in the Weyburn field:

Geological nature and implications for long-term CO₂ storage

Erik H. Nickel

The Weyburn field in southeastern Saskatchewan is situated in the northern portion of the Williston Basin, and is the focus of a detailed investigation into geological storage of CO₂ as part of the IEA Weyburn CO₂ Storage and Monitoring Project. A significant portion of this project is to determine the nature and integrity of the primary sealing units for the reservoir, which is in Mississippian carbonates of the Midale Beds. Three units form the primary seals to the Weyburn Midale reservoir: 1) the overlying Midale Evaporite, 2) the underlying Frobisher Evaporite, and 3) the up-dip diagenetic cap-rock subjacent to a regional erosional surface, the sub-Mesozoic unconformity. Together, these three units are the principal factors in containing both the oil in the reservoir, and the injected CO₂ for long-term geological storage.

The Midale Evaporite is a 5 to 11m thick anhydrite-dolostone bed and is present at a depth of about 1400m below surface. It occurs as an east-west arc across southeast Saskatchewan and is a succession of laminated/bedded to massive anhydrites at the base, and nodular to chicken-wire anhydrites at the top, with interspersed local dolostone interbeds. Various types of fractures have been identified within the Midale evaporite. Most fractures are synsedimentary in origin and although relatively few late-stage fractures are observed, none of them currently appear to have the capacity to transmit fluids.

The Frobisher Evaporite is 0 to 7m thick and underlies the Weyburn Midale reservoir in the northern portion of the Weyburn field but is absent in the southern half of the field. The Frobisher Evaporite consists of nodular to chicken-wire anhydrite intercalated with reddish dolomudstone. Underlying the Frobisher Evaporite are the Frobisher Beds, a series of oolitic shoals with wackestones deposited in intershoal lows that appear to have controlled the deposition and thickness of the Frobisher Evaporite.

Located updip and north of the Weyburn Midale reservoir is a 2 to 10m thick zone of alteration associated with the sub-Mesozoic unconformity surface. Within this alteration zone, porosity in the Midale has been occluded by anhydritization and micritization, creating a highly effective updip lateral seal. Micritization of the Midale carbonates was one of the earliest diagenetic events, followed by later emplacement of diagenetic anhydrite in the form of satin-spar veins. The final stage of diagenesis was the formation of brown metasomatic anhydrite in disseminated megacrystic blebs. The combination of these diagenetic processes has destroyed porosity in the Midale Beds subjacent to the sub-Mesozoic unconformity surface, and is considered to provide an effective up-dip seal to constrain CO₂ migration.