Jeffrey J. Packard¹, Ihsan S. Al-Aasm², Iain Samson², Zeev Berger³, Jim Davies³

(1) Burlington Resources Canada Energy Limited, Calgary, AB
(2) Department of Earth Sciences, University of Windsor, Windsor, ON
(3) Image Interpretation Technologies, Calgary, AB

ABSTRACT: A Devonian "Hydrothermal" Chert Reservoir: The 225 Bcf Parkland Field, British Columbia, Canada

The 225 Bcf (OGIP) Parkland "A" Pool was discovered in 1956. The discovery well production tested at 19 Mmcf/day and has produced over 95 Bcf of sweet gas. The "Parkland" play type (as defined by the Geological Survey of Canada), consists principally of fracture-associated hydrothermally-dolomitized reservoirs hosted in the Upper Devonian (Famennian) Wabamun Group medial ramp carbonates resting on the axis of the Peace River Arch. Parkland itself owes little of its production to dolomite, instead most of the reservoir pore volume can be attributed to micro-intercrystalline porosity (12-30%) within a pervasive replacement microquartz zone (up to 35 m thick), that occurs at the dolomite-limestone interface surrounding irregular bodies of tight hydrothermal dolostone. The replacement silicification post-dates matrix dolomitization and cross-cuts the Early Tournaisian (Mississippian) Exshaw Ash. It is texturally and isotopically distinct from early diagenetic chert nodules. The replacement microquartz shows more depleted oxygen values (22 vs. 25 d18O SMOW), consistent with precipitation from hot fluids (140 to 200ºC). Entrapped floating euhedral crystals of dolomite within the replacement chert show little evidence of microdissolution suggesting that the silicic-acid-charged fluid, although undersaturated with respect to the microspar lime matrix, was saturated with respect to dolomite. It is postulated that hydrothermal fluids of common parentage were responsible for both dolomitization and silicification. The two replacement processes occurred in relatively rapid succession at shallow burial depths. They were related to an early Tournaisian period of hydrothermal activity associated with both wrench and minor extensional structuring, linked with the nascent development of the Fort St. John Graben. The source of the silica is thought to be the immediately subjacent Granite Wash. To the authors' knowledge, Parkland is the only hydrothermal chert reservoir that has been reported anywhere in the world.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado