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INRS-Eau-TerreEnvironnement, 880 chemin Sainte-Foy, Québec, PQ G1S 2L2
Intragaz, 6565 Boul. Jean XXIII, Trois-Rivières-Ouest, PQ G9A 5C9
Ressources naturelles Canada, Commission géologique du Canada, 880 chemin Sainte-Foy, Québec, PQ G1S 2L2
The Beekmantown Group (Lower Ordovician) of the Saint-Flavien reservoir has produced 162x106 m3 (5.7 bcf) of natural gas between 1980 and 1994. The conversion of the field into gas storage was initiated in 1992 and the pool became operational in 1998. Integration of structural and sedimentologic features, carbonate and organic matter petrography and geochemistry for 13 drill holes is used to define a tectonic-sedimentologic-diagenetic model for porosity evolution in these reservoir dolostones.
The Beekmantown Group consists of numerous fifth-order shallowing-upward cycles 1.0 to 7.0 m thick (average of 2.4 m). Each cycle consists of a basal shale deposited during the initial flooding of the platform which was subsequently covered by a shoaling succession of subtidal and intertidal limestones to intertidal dolostones.
Early dolomitization has produced intercrystalline porosity and preserved some moldic pores in the intertidal facies. Near surface, post-dolomite karstification has created vugs that were subsequently filled by early marine calcite fibrous cement based on the 
18O and 13C ratios of calcite. Early burial elements consist of horizontal stylolites, pyrite and sphalerite.
Late migrated bitumen was thermally altered or vaporized as native coke under deep burial conditions exceeding 240°C, partly due to overthrusting of Appalachian nappes. Under these conditions, breccias and fractures were generated and subsequently filled with K-feldspar, quartz, illite, and xenomorphic and poikilotopic calcite. The 18O of the poikilotopic calcite and homogenization temperature of coeval fluid inclusions indicate formation under high temperatures (Th about 260°C). Horizontal shear zones and marcasite-rich vertical stylolites were produced during folding and thrusting. Dissolution has preferentially affected late fracture-filling calcite and generated most of the actual porosity during or soon after the Taconian Orogeny. The relationship between the occurrence of smectite and this type of porosity indicates the low temperature condition of this dissolution (T <100°C). Porosity in the Saint-Flavien reservoir has been mostly produced by fracture-controlled, late to post-Taconian dissolution of early to late calcite in the intertidal dolomitic slightly porous facies at the top of rhythmic cycles that compose the Beekmantown Group.
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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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R. Bertrand, R. Bertrand, D. Lavoie, and M. Fowler Cambrian-Ordovician shales in the Humber Zone: thermal maturation and source rock potential Bulletin of Canadian Petroleum Geology, September 1, 2003; 51(3): 213 - 233. [Abstract] [Full Text] [PDF]
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