| The Weyburn Midale Pool in southeast Saskatchewan has been the focus of a large multidisciplinary research project on the feasibility of long-term geological storage of CO2. In order to determine the ability of CO2 to remain in the subsurface, a characterization of the sealing units of the Weyburn Midale pool was necessary.;There are three sealing units within the Weyburn Midale Pool, (1) The underlying Frobisher Evaporite, (2) the overlying Midale Evaporite, and (3) an updip zone of alteration at the sub-Mesozoic unconformity. In the case of the Midale and Frobisher Evaporites, they have been studied using traditional core and subsurface mapping procedures. The altered zone, in addition to core and mapping work, has also been studied geochemically using isotopes of carbon, oxygen, sulphur, and strontium.;The Frobisher Evaporite is an anhydrite unit up to 8m in thickness. It is composed of various facies deposited in a shallow shoreline sabkha environment. It is not present throughout the Weyburn Field, having been eroded to the north, and not deposited in the south. The southern margin is very irregular as a result of early erosion of the seaward edge of the sabkha.;The Midale Evaporite is an anhydrite unit up to 9.5 metres in thickness. This unit was deposited in a shore-parallel sauna environment as determined from facies analysis. Detailed study of the various types of fracturing in the Midale Evaporite indicates that there are no perceivable threats to CO 2 leakage through the unit.;The altered zone is a 2 to 10 metre thick zone of dolomitized and anhydritized Mississippian carbonates at the sub-Mesozoic unconformity updip from the Weyburn reservoir in the Midale Beds. Subsurface mapping, isotopic geochemical analysis, and petrography of the carbonates and anhydrites support a model of emplacement of the altered zone via a sabkha-like environment on the exposed Mississippian surface followed by a later anhydritization event coincident with oil migration. The various phases of dolomite and anhydrite present in this zone have served to make this an impermeable zone and updip seal to the migration of CO2. |
