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AAPG Bulletin


AAPG Bulletin, V. 86, No. 9 (September 2002), P. 1609-1638.

Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

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

David W. Morrow,1 Mengwei Zhao,2 Lavern D. Stasiuk3

1Geological Survey of Canada-Calgary, 3303 33rd St. NW, Calgary, Alberta, T2L 2A7, Canada; email: [email protected]
2HC Geoconsulting Co., 128 Hampstead Rise NW, Calgary, Alberta T3A 6B4, Canada; email: [email protected]
3Geological Survey of Canada-Calgary, 3303 33rd St. NW, Calgary, Alberta, T2L 2A7, Canada; email: [email protected]


Dave Morrow received his B.Sc. degree (1967) from the University of British Columbia, Vancouver, Canada, and his M.A. degree (1970) and Ph.D. (1973) from the University of Texas, Austin. He worked for Mobil Oil of Canada in Calgary (1973-1974) and since 1974 has been a research scientist with the Regional Subdivision of Natural Resources Canada, Geological Survey of Canada in Calgary, Alberta, Canada, involved with lower Paleozoic stratigraphy.

Mengwei Zhao received his B.S. degree (1982) from Chengdu University of Technology, China, his M.S. degree (1984) from Northwest University in Xi'an, China, and his Ph.D. (1993) from the University of G–ttingen, Germany. Currently, he is a petroleum consultant providing oil companies and research institutions with geological services ranging from creating prospects to studying fluid inclusions.

Lavern Stasiuk received his B.Sc. (1984) and M.Sc. (1988) degrees and Ph.D. (1991) from the University of Regina, Saskatchewan, Canada. Since 1992 he has been a research scientist with Natural Resources Canada, Geological Survey of Canada, Calgary, Alberta, specializing in the organic petrology of Proterozoic and Phanerozoic kerogen, bitumen, pyrobitumen, and crude oil inclusions, as well as Holocene sedimentary organic matter.


We gratefully acknowledge the efforts of Keith Dewing at the Geological Survey of Canada-Calgary, who performed an early internal review of this article, and AAPG reviewers Ashton Embry and Jack Wendte, who provided many valuable suggestions that improved the article. We extend our appreciation to the editorial staff and to the AAPG editor for their help. We thank the Oil and Gas Initiatives Branch of the Government of British Columbia for their support and extend our appreciation to the staff at the British Columbia Government petroleum well core storage facility at Charlie Lake near Fort St. John in British Columbia. This article is Geological Survey of Canada (GSC) Contribution 2002010.


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.

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