GEOTHERMAL RESOURCE CHARACTERIZATION OF THE SLAVE POINT FORMATION IN CLARKE LAKE FIELD, FORT NELSON, B.C., CANADA

Middle Devonian carbonates of the Slave Point Formation have been host to significant gas reserves (3.57 Tcf OGIP in the largest pool) since the discovery of Clarke Lake Field in 1957. The geothermal potential of the field was demonstrated by a 2005 to 2009 experiment in which Petro-Canada Oil and Gas attempted liberating trapped gas by pumping formation water out at rates of 2800m³/day to reduce reservoir pressure; instead pressure only dropped marginally (100 kPa after one year) due to a strong water drive. High geothermal gradients (> 50 °C/km) allowing formation water temperatures to be greater than 110 °C give the field the capability of producing 12 to 74 MW of geothermal energy for the Fort Nelson area (Walsh, 2013). High temperature water, a strong water drive and porous carbonate rock allow for a viable geothermal resource.

We describe and map depositional facies and relate these to porosity and permeability data to determine a flow model for the formation. In the late Givetian a relative sea level rise drowned the Keg River carbonate platform which allowed small, laterally discontinuous patch reefs of the Slave Point Formation to develop on the flanks of the Horn River Basin. Nine depositional facies have been described by Lonnee (2005) of which eight are coincident with Machel & Hunter’s (1994) reef facies classification. Pervasive dolomitization of the reef margin occurred by long-distance migration of halite-saturated brines while recrystallized matrix dolomite, replacive and cement saddle dolomites are products of hydrothermal alteration. More porous and permeable zones are related to dolomitization, while unaltered limestone facies are considered non-reservoir. Mapping and modeling the spatial variability of dolomitization is a key objective concerning optimization of geothermal well targets.

The feasibility of developing this geologically based geothermal reservoir model comes in part to the availability of a large-scale oil and gas well data set. The data include direct permeability/porosity core measurements, well-logs, DSTs and twenty core descriptions taken at the BC Oil & Gas Commission core research facility. Over 220 wells in the Clarke Lake area have been used in conjunction with the software Petrel to create stratigraphic cross-sections to interpret the 3D geometry of the reef.