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1 EnCana Corporation, 150 9th Avenue SW, P.O. Box 2850, Calgary, Alberta, Canada T2P 2S5; richard.wierzbicki{at}encana.com
2 Dravis Geological Services, 4133 Tennyson, Houston, Texas 77005; jdravi{at}rice.edu
3 Department of Earth Sciences, University of Windsor, 401 Sunset, Windsor, Ontario, Canada N9B 3P4
4 EnCana Corporation, 150 9th Avenue SW, P.O. Box 2850, Calgary, Alberta, Canada T2P 2S5
Rick Wierzbicki earned his B.Sc. degree from the University of Alberta in 1980. He worked for Shell Canada Resources from 1980 to 1984. Since then, he has worked for PanCanadian Petroleum Limited, now EnCana, as an exploration and development geologist. He worked on the development plan for the Deep Panuke Project from 2000 to 2004. His interests are in carbonate sedimentology and reservoir characterization. He is a member of the Canadian Society of Petroleum Geologists, AAPG, and the Association of Professional Engineers, Geologists, and Geophysicists of Alberta.
Jeffrey J. Dravis is a consulting carbonate geologist located in Houston and an adjunct professor at Rice University. His company, Dravis Geological Services, conducts technical projects for oil and gas companies, specializing on carbonate diagenesis and porosity evolution and play development. Dravis Interests, Inc., presents applied in-house seminars and a field course to Caicos platform (>125 presented to date).
Ihsan Al-Aasm earned his B.Sc. and M.Sc. degrees from the University of Baghdad and his Ph.D. from the University of Ottawa. He worked at the Geological Survey of Iraq from 1978 to 1980 and as a postdoctoral fellow from 1984 to 1986, and then, he worked as assistant professor (1986–1988) at the University of Ottawa. He joined the University of Windsor in 1988, and he is now a full professor and head of the Department of Earth Sciences, University of Windsor. His principal area of research is on petrologic and chemical attributes of carbonate and siliciclastic diagenesis, dolomitization, and environmental geochemistry. He acted as an associate editor for the Journal of Sedimentary Research and holds memberships in AAPG, SEPM, the Geological Association of Canada, and the International Association of Sedimentologists.
Nancy Harland has been employed by PanCanadian/EnCana since 1985 after graduating from the University of Saskatchewan with a B.Sc. (honors) degree in geology. Exploration projects involved both clastics and carbonates in Saskatchewan, central and southern Alberta. Since 1999, Nancy Harland has been working as an exploration and development geologist on the Jurassic Abenaki trend, Scotian Shelf, with the East Coast Group. In 2005, Nancy became the team lead for the Atlantic Canada Exploration, Development and East Coast Drilling Group.
A large gas reservoir was discovered in the previously unproductive Jurassic-aged Abenaki carbonate margin in 1998. Most of the reservoir porosity is developed in dolostones. These dolostones replaced preexisting wackestones, packstones, and grainstones(?) associated with reefal and adjacent depositional environments. Many dolomites were subsequently recrystallized or dissolved, accounting for much of the preserved secondary porosity. Subsequent fracturing helped enhance reservoir permeabilities.
Enhanced petrographic techniques established that dissolution of previously dolomitized fabrics generated much of the secondary porosity in these dolostones. Diffused plane-polarized light revealed relict grains and textures invisible with standard microscopic observations. Petrographic and geochemical observations also confirmed that dissolution occurred under deep-burial conditions after incipient pressure solution.
Dissolution was not confined to the centers of dolomitized grains, as is commonly seen when remnant calcitic grains dissolve out during the advanced stages of replacement dolomitization.
Instead, dissolution was random within relict grains, as isolated dolomite crystals were also variably dissolved. The geochemistry of these dolomites and associated late-stage calcites implied precipitation from basinal hot fluids, as well as hydrothermal fluids. Later diagenetic fluids, either acidic or calcium rich, or perhaps both at different times (based on associated mineralization), seemingly promoted dolomite dissolution.
The presence of tectonic fractures and stylolites, helium gas, and faults observed in seismic data implied that dolomitization and subsequent dissolution along the Abenaki platform margin were controlled by reactivated wrench faults tied to basement. On a finer scale, diagenetic fluids moved through fractures and pressure-solution seams.
The data collected to date support our contention that the dolomitization and dissolution process, which has created most of the porosity in the Abenaki reservoir, was poststylotization and deeper burial in origin. Given the timing of tectonic activity in the area and its inferred connection to diagenesis, it is probable that at least a part of the diagenetic fluids were hydrothermal in nature.
This article has been cited by other articles:
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  K. Azmy, I. Knight, D. Lavoie, and G. Chi Origin of dolomites in the Boat Harbour Formation, St. George Group, in western Newfoundland, Canada: implications for porosity development Bulletin of Canadian Petroleum Geology, March 1, 2009; 57(1): 81 - 104. [Abstract] [Full Text] [PDF]
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 L. B. Smith Jr. and G. R. Davies Structurally controlled hydrothermal alteration of carbonate reservoirs: Introduction AAPG Bulletin, November 1, 2006; 90(11): 1635 - 1640. [Full Text] [PDF]
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G. R. Davies and L. B. Smith Jr. Structurally controlled hydrothermal dolomite reservoir facies: An overview AAPG Bulletin, November 1, 2006; 90(11): 1641 - 1690. [Abstract] [Full Text] [PDF]
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L. B. Smith Jr. Origin and reservoir characteristics of Upper Ordovician Trenton-Black River hydrothermal dolomite reservoirs in New York AAPG Bulletin, November 1, 2006; 90(11): 1691 - 1718. [Abstract] [Full Text] [PDF]
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