Exploring for dolomitized Slave Point carbonates in northeastern British Columbia

Abstract

In northeastern British Columbia, a number of significant gas fields have been discovered in dolomitized Slave Point carbonates located along the edge of the mid‐Devonian barrier‐reef system. Reflection seismic techniques have been used extensively in exploring for these fields. A common exploration practice has been to detect the reef edge on seismic data and then to drill on isochron thins and structural highs close to this edge. The remaining reserves, however, are proving more difficult to find, and this exploration technique does not always guarantee that the Slave Point formation is dolomitized at this location. As with many other complex play types, combining geologic concepts with geophysical exploration techniques can improve the explorationist’s success ratio. In this example, a geologic model (the Dorag dolomitization model) provides a possible explanation for the localization of the Slave Point dolomites. In this model dolomitizing solutions are created by mixing fresh, phreatic water with salt water. This condition could occur if porous reefal Slave Point carbonates were exposed to fresh‐water infiltrations adjacent to open‐marine conditions. The edges of the mid‐Devonian barrier reef and the edge of the Keg River embayments were areas that could have met these requirements. Thus the optimum condition for dolomitization should occur slightly basinward of the reef edge in the zone of possible mixing of fresh and salt waters. To increase the chance of encountering a potentially narrow band of dolomitized Slave Point reservoirs, exploration wells should be positioned on the edge of the carbonate reef slightly basinward of the isochron thin. An exploration example from a Keg River embayment area illustrates and supports this exploration concept.