GEOCHEMISTRY OF ANHYDRITES AT THE IEA WEYBURN SITE: IMPLICATIONS FOR ASSESSING THE EFFECTIVENESS OF SEALING ROCKS

摘要

At the Weyburn Field, Canada, CO_2 is being injected into Mississippian carbonates of the Midale Beds, which are capped by sedimentary anhydrites of the Midale Evaporite and anhydritized carbonates within a diagenetically alterated zone that, in turn, are overlain by a regional aquitard, the Watrous Formation. Primary anhydrites in Mississippian units have □~(18)O values ranging 14 to 16 per thousand SMOW and □~(34)S compositions from 18 to 20 per thousand CDT which are within the range of isotopic compositions estimated for primary Mississippian anhydrite. Primary anhydrites from the Lower Watrous Formation have O and S isotopic compositions ranging from 12 to 14 per thousand SMOW and 15 to 18 per thousand CDT, respectively, corresponding to the isotopic values estimated for Triassic and Jurassic seawater. Sulphur isotope ratios of most late-stage anhydrite cements in Mississippian rocks are generally similar to those of Mississippian seawater, suggesting the source of sulphur for secondary anhydrite cements was dominantly Mississippian seawater. The distinct isotopic signatures of anhydrites in Mississippian units from those in the Lower Watrous suggests that fluid communication between Mississippian reservoirs and the Watrous sediments was limited, and that the Watrous Formation has been an effective long-term seal for the Mississippian reservoirs.