MORROW, DAVID W., Geological Survey of Canada, Calgary, AB; and ZHAO MENGWEI, HC Geoconsulting Co., Calgary, AB
Abstract: Dolomitization and Cementation History of Devonian Slave Point - Keg River Gas Fields of Northeast British Columbia, Canada
The Slave Point, Sulphur Point and Keg River formations of Northeast British Columbia contain numerous gas fields particularly along the preferentially dolomitized reefal shelf edge margin. The most typical reservoir facies is the very coarsely crystalline white Presqu'ile Dolomite follows the shelf edge within the Slave Point, but is regionally extensive within underlying strata.
The presence of abundant primary, single phase, aqueous fluid inclusions indicate that at least part of the white replacive and space-filling Presqu'ile dolospar in the Slave Point of the Peggo and Helmet gas fields was precipitated at temperatures <50°C. Almost all white dolospars from these fields also contain abundant primary, two-phase aqueous, high salinity (13 to 23 wt% NaCl eq) inclusions that have homogenization temperatures ranging from about 120° to 160°C. Oxygen isotope values for these same samples range from about -13.7 to -12.3 ‰ (PDB). Comparison of the homogenization temperatures with paleotemperatures calculated from the oxygen isotope geothermometer indicates that a Paleozoic age at shallow depth for the high temperature dolomitization event is more likely than a Cretaceous-Tertiary age at much deeper burial depths, partly because of the upwards temperature correction that must be applied to inclusion entrapment under high pressure. Oxygen isotope values for samples containing single phase inclusions range from about -13.4 to -12.3 ‰ indicating that the isotopic composition of the paleofluid may have been lower than -8.0 ‰ if these samples were precipitated at low temperature. Vug-filling calcite contains primary two-phase aqueous inclusions that are also highly saline (13 to 16 wt% NaCl eq).
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California