A devonian hydrothermal chert reservoir: the 225 bcf Parkland field, British Columbia, Canada

The 225 bcf (original gas in place [OGIP]) Parkland A pool was discovered i n 1956 with the drilling of Imperial Pacific 6-29-81-15w6. This well initia lly production tested at 19 mmcf gas/day and has produced more than 95 bcf of sweet gas. The Parkland play type (as defined by the Geological Survey o f Canada) consists principally of fracture-associated hydrothermally karste d and dolomitized reservoirs hosted in the Upper Devonian (Famennian) Wabam un Group medial ramp carbonates resting on the axis of the Peace River arch and includes major fields such as Tangent, Teepee, and Gold Creek. Ironica lly Parkland itself owes little of its production to hydrothermally dolomit ized carbonates; instead, most of the reservoir pore volume can be attribut ed to microintercrystalline porosity within a pervasive replacement microqu artz (chert) that occurs at the dolomite-limestone interface. The thickness of this chert zone in the 6-29 well is 35 m and occurs near the top of the Wabamun. The origin of the chert is problematic. It postdates matrix dolomitization and crosscuts the early Tournaisian (Mississippian) Exshaw Ash but is in tu rn truncated by later saddle dolomite and calcite veins. The replacement ch ert is composed of a microporous (up to 30%) meshwork of microquartz crysta ls averaging 5-10 mum in size and is texturally distinct from early diagene tic chert nodules also found in the Wabamun. The two cherts are also distin guishable isotopically, with the replacement microquartz showing significan tly more depleted oxygen values (22 vs. 25 delta O-18 standard mean ocean w ater [SMOW]), consistent with precipitation from hot fluids having a temper ature range from about 140 to 200 degreesC. Silicification has had the grea test impact on slightly dolomitized limestone at the outer margin of the hy drothermal dolostone pod. Entrapped floating euhedral crystals of dolomite within the replacement chert show little evidence of microdissolution, sugg esting that the silicic acid-charged fluid, although undersaturated with re spect to the microspar lime matrix, was saturated with respect to dolomite. We postulate that hydrothermal fluids of common parentage, only marginally evolved in composition-if at all, were responsible for both dolomitization and silicification. Both replacement processes occurred in relatively rapi d succession at shallow burial depths and were related to an early Tournais ian period of hydrothermal activity associated with both wrench and minor e xtension structuring linked with the nascent development of the Fort St. Jo hn graben. The source of the silica is thought to be the immediately subjac ent Granite Wash or possibly the Precambrian basement. Stratigraphic proxim ity to the Granite Wash is likely a prerequisite condition for chert reserv oir development within the Wabamun and serves to explain, along with signif icantly different hydrothermal fluid temperatures (much hotter to the west) , why Parkland is such a distinct field compared to Tangent and Teepee. Hydrocarbon reservoirs that are dominantly chert-hosted are relatively unco mmon, but where recognized (e.g., Monterey Formation, California) they are related to the redistribution and transformation of biogenic opal-A silica derived from diatoms, spicules, and/or radiolarians in deeper water sedimen ts. To our knowledge, Parkland is the only hydrothermal chert reservoir tha t has been reported anywhere in the world.