The gas-bearing Devonian Presqu'ile Dolomite of the Cordova embayment region of British Columbia, Canada: Dolomitization and the stratigraphic template

Abstract

White dolospar of the Presqu'ile Dolomite forms the reservoir for many gas fields of northeastern British Columbia within the Keg River, Sulphur Point, and Slave Point formations along the shelf margin of the Cordova embayment. The embayment fill succession of the organic-rich Klua Shale, the Otter Park shale, and marlstones equivalent to the Sulphur Point and Slave Point formations are aquicludes that blocked the upward and lateral flow of dolomitizing brines. Presqu'ile dolomitization along the Slave Point shelf edge is interrupted in places where the Klua Shale extends shelfward underneath the Slave Point Formation. The Watt Mountain shale was also a regional aquiclude that prevented upward circulation of dolomitizing fluids in areas behind the Slave Point shelf edge. Flow of dolomitizing NaCl-CaCl2-MgCl2-H2O brine solutions began in the shallow subsurface and precipitated dolomite cements (type 1) at temperatures less than 50 degreesC, possibly even during Slave Point deposition, as northwestward-directed shallow subsurface brine reflux from the Elk Point basin began. This was followed by upward-directed subsurface convective flow of NaCl-CaCl2-MgCl2-H2O and NaCl-MgCl2-H2O brines and precipitation of dolomite cements at temperatures of about 130 degreesC during the Late Devonian-Carboniferous. Lower salinity brines that precipitated calcite cements and some dolomite (type 2) may have interacted with connate fluids of marine origin or, less likely, with meteoric fluids. Modeling of organic maturation indicates that the Muskwa shales above the Slave Point Formation passed through the oil generation window in the Permian and the onset of dry gas generation in the middle Mesozoic. Postdolomitization calcite cements precipitated during these times contain abundant liquid hydrocarbon-filled and methane-filled inclusions. High temperatures of close to 200 degreesC experienced by Slave Point strata in the Mesozoic caused stretching, but no leakage or refilling, of fluid inclusions in all dolomite cements.