sedimentologic, diagenetic and tectonic evolution of the

126

ABSTRACT

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 matterpetrography and geochemistry for 13 drill holes is used to define a tectonic-sedimentologic-diagenetic model for porosityevolution in these reservoir dolostones.

The Beekmantown Group consists of numerous fifth-order shallowing-upward cycles 1.0 to 7.0 m thick (average of2.4 m). Each cycle consists of a basal shale deposited during the initial flooding of the platform which was subsequentlycovered 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 fibrouscement based on the δ18O and δ13C 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 240ºC,partly due to overthrusting of Appalachian nappes. Under these conditions, breccias and fractures were generated and sub-sequently filled with K-feldspar, quartz, illite, and xenomorphic and poikilotopic calcite. The δ18O of the poikilotopic cal-cite and homogenization temperature of coeval fluid inclusions indicate formation under high temperatures (Th about260ºC). 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 soonafter the Taconian Orogeny. The relationship between the occurrence of smectite and this type of porosity indicates the lowtemperature condition of this dissolution (T <100ºC). Porosity in the Saint-Flavien reservoir has been mostly produced byfracture-controlled, late to post-Taconian dissolution of early to late calcite in the intertidal dolomitic slightly porous faciesat the top of rhythmic cycles that compose the Beekmantown Group.

RÉSUMÉ

Le Groupe de Beekmantown (Ordovicien inférieur) du réservoir de Saint-Flavien a produit 162x106 m3 (5.7bcf) de gaznaturel entre 1980 et 1994. La transformation du champ en une facilité d’entreposage souterrain fut initiée en 1992 etdevint opérationnel en 1998. Une intégration d’éléments structuraux et sédimentologiques, de la pétrographie et de lagéochimie des carbonates et de la matière organique provenant de 13 forages aide à définir le modèle tectonique-sédimentologique-diagénétique pour l’évolution de la porosité des roches dolomitiques du réservoir.

Le Groupe de Beekmantown est formé de nombreux cycles de cinquième ordre de diminution progressive de la tranched’eau, d’épaisseur variable comprise entre 1.0 et 7.0 m (moyenne de 2.4 m). Chaque cycle consiste en un shale de basedéposé lors de l’inondation initiale de la plate-forme lequel fut subséquemment surmonté par une succession régressiveconstitué de calcaires infratidaux à intertidaux culminant dans des dolomies intertidales.

BULLETIN OF CANADIAN PETROLEUM GEOLOGYVOL. 51, NO. 2 (JUNE, 2003), P. 126-154

Sedimentologic, diagenetic and tectonic evolution of the Saint-Flavien gas reservoir

at the structural front of the Quebec Appalachians

RUDOLF BERTRAND, ANDRÉ CHAGNON and MICHEL MALO

INRS-Eau-Terre-Environnement880 chemin Sainte-FoyQuébec, PQ G1S 2L2

DENIS LAVOIE and MARTINE M. SAVARD

Ressources naturelles Canada, Commission géologique du Canada880 chemin Sainte-FoyQuébec, PQ G1S 2L2

YVES DUCHAINE

Intragaz6565 Boul. Jean XXIII

Trois-Rivières-Ouest, PQ G9A 5C9

GEOLOGY OF THE SAINT-FLAVIEN GAS RESERVOIR, QUEBEC 127

INTRODUCTION

The Saint-Flavien structure is located about 50 km south-west of Quebec City. This structure hosts a gas field that wasproductive between 1980 and 1994. The Saint-Flavien field isthe only Paleozoic commercial gas pool found to date in south-ern Quebec. This pool was discovered in 1972 after a wellencountered gas shows. A third well was drilled in 1976 and itshigh productivity resulted in viable commercial production.The operation related to the conversion of the reservoir to gasstorage was initiated in 1992 and seven wells were later added.Since 1998, the dolostone subcrops of this structure are usedfor gas storage.

The origin of the Saint-Flavien reservoir gas was the subjectof an isotopic study, which showed that the hydrocarbons werethermogenic (Saint-Antoine and Héroux, 1993). However, theorigin of the reservoir was controversial. A hypothesis in themid-seventies suggested that porosity was essentially related to

fractures in the dolostones, whereas another one during themid-eighties suggested that porosity was of meteoric-karsticorigin and consequently randomly distributed (Dysktra andLongman, 1995). More recently, a hydrothermal-karstic originwas proposed (Béland and Morin, 2000).

This study was conducted by INRS-Géoressources andNatural Resources Canada (Geological Survey of Canada) inorder to define the nature of the porosity in the Saint-Flavienreservoir, understand its origin, and propose a model that couldexplain the irregular spatial distribution of the porosity beforethe reservoir conversion into a gas storage facility. These objec-tives require a structural, sedimentologic and diageneticapproach. This study attempts to clarify the debate about thenature of porosity, and provide the first integrated tectonic-sed-imentologic-diagenetic evolution model for thrusted platformrocks of the St. Lawrence Platform. The results will serve as avaluable baseline for ongoing hydrocarbon exploration effortsin this area.

GEOLOGICAL SETTING

The Saint-Flavien structure occurs at the Appalachian struc-tural front. The major regional tectonostratigraphic domains inthe area are, from northwest to southeast (Fig. 1): 1) thePrecambrian Grenville Province, 2) the Cambrian to UpperOrdovician rocks of the St. Lawrence Platform, the autochtho-nous domain, 3) the parautochthonous domain which consistsof fault-imbricated slices of platform rocks, and 4) theallochthonous domain of the Appalachians. This fourth domainis composed of Cambrian to Upper Ordovician slope and riserocks.

The Saint-Flavien wells are located on both sides of theFoulon Fault of the allochthonous domain (Fig. 1). At depth,however, Saint-Flavien wells cross the allochthonous rocks andthe southeast-dipping Logan’s Line to reach the reservoir indolostones of the parautochthonous domain (Fig. 2). Theparautochthonous domain is bounded to the southeast by the

Une dolomitization précoce a produit une porosité intercrystalline et a préservé quelques pores grossiers dans les faciès inter-tidaux. Une karstification précoce a suivi la dolomitization et a généré des pores lesquels furent subséquemment remplis de cal-cite fibreuse d’origine marine telle que suggéré par les valeurs de δ18O et de δ13C de la calcite. Les premiers élémentsdiagénétiques reliés à l’enfouissement précoce consistent en des stylolites horizontaux, de la pyrite et de la marcassite.

Des bitumes à migration tardive furent altérés thermiquement ou vaporisés comme coke natif lors de l’enfouissement profondà des températures supérieures à 240ºC, en partie reliée à la mise en place des chevauchements appalachiens. Sous ces condi-tions, des brèches et des fractures furent créées et par la suite remplies de feldspaths potassiques, quartz, illite et de la calcitexénomorphique et poikilotopique. Les valeurs en δ18O de la calcite poikilotopique et les températures d’homogénéisation desinclusions fluides associées indiquent une précipitation à haute température (Th environ 260°C). Des zones de cisaillements hor-izontaux et des stylolites verticaux riches en marcassite furent produites lors de l’épisode de plissement et de charriage. Une dis-solution a affecté de façon préférentielle la calcite tardive de colmatage de fracture et a généré la majeure partie de la porositéactuelle durant ou immédiatement après l’orogénie Taconique. La relation entre la présence de smectite et ce type de porositéindique des conditions de basse température lors de cette dissolution (T <100ºC). La porosité du réservoir de Saint-Flavien futprincipalement contrôlée par les patrons de fractures et produite par une dissolution tardi- ou post-taconienne de la calcite pré-coce et tardive présente dans les faciès intertidaux dolomitiques légèrement poreux au sommet des cycles rythmiques formant leGroupe de Beekmantown.

Fig. 1. Southern Quebec structural domains and location of Saint-Flavien studied wells.

128 R. BERTRAND, A. CHAGNON, Y. DUCHAINE, D. LAVOIE, M. MALO and M.M. SAVARD

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Logan’s Line and to the northwest by the Aston Fault (Figs. 1,2). Autochthonous platform rocks consist of a 2 to 2.5 km thicksuccession of Cambrian to Upper Ordovician rocks (Sandford,1993) which rest unconformably on Precambrian rocks of theGrenville Province (Fig. 2). Rocks of the St. LawrencePlatform are limited to the southeast by the Aston Fault and arein part bordered to the north by northeast-trending normalfaults (Fig. 1) which also affect the platform (Fig. 2). Southeastof the platform (Fig. 1), parautochthonous rocks are composedof Cambrian and Ordovician tectonic panels of the platformand foreland basin units (Fig. 2). Between Quebec City andMontreal, the parautochthonous domain forms a narrow zoneof 5 to 10 km wide zone with few good outcrops. However, theMERQ 2001 seismic line (St-Julien et al., 1983; Castonguay etal., 2001a) shows that this tectonostratigraphic domain is welldeveloped to the southeast under the allochthons. The seismicline exhibits large open, upright to northwest-verging folds,and southeast-dipping thrust faults (Fig. 2), a typical geometryof classical fold-thrust belt.

The Saint-Flavien reservoir within the fold-thrust belt is inthe Beauharnois Formation, the middle unit of LowerOrdovician Beekmantown Group (Bernstein, 1992; Salad Hersiet al., 2003), occurring at an average depth of 1500 m (Fig. 3).Deposition of the Beekmantown Group in southern Quebecoccurred during the Sauk III sub-sequence of Sloss (1963). Itrepresents the last passive margin unit deposited prior to incep-tion of Taconian subduction, development of a regional uncon-formity (Knight et al., 1991) and the transition to a forelandbasin type of continental margin (Lavoie, 1994).

The Beekmantown Group crops out only in the Montrealarea where it is composed of three formations: TheresaFormation (sandstone and dolostone), Beauharnois Formation(mainly dolostone) and Carillon Formation (dolostone-shale-limestone) (Bernstein, 1992; Salad Hersi et al., 2003) (Fig. 3).The classical tripartite succession of the Beekmantown Groupis dominated by nearshore intertidal–supratidal facies (SaladHersi et al., 2003). However, coeval subtidal-dominated car-bonate units are known in the Appalachian allochthonousExternal Domain of southern Quebec in the Philipsburg andActon Vale areas (Fig. 1). In Acton Vale, the Upton Group(Lavoie, 1992; Paradis and Lavoie, 1996) predominantly con-sists of strongly recrystallized and dedolomitized intertidal–subtidal platform facies which host high temperature barite andcopper mineralization. Thermal alteration of organic matterwas demonstrated to be a critical element in the diagenetic evo-lution of these carbonates (Paradis and Lavoie, 1996). In thePhilipsburg area, within the Philipsburg Group (Globensky,1981), the Beekmantown Group time correlative units are theWallace Creek–Morgan’s Corner–Naylor Ledge–HastingCreek formations (Salad Hersi et al., 2002a; 2003). These lastunits are dominated by open marine carbonate platform marginfacies (subtidal bioclastic and intraclastic limestones andthrombolites). Facies of the Beekmantown Group in the Saint-Flavien structure were likely located in the transition zonebetween the littoral facies of the classical Beekmantown sec-tions of the Montreal area and the platform margin facies in the

Philipsburg Slice (Salad Hersi et al., 2002a, 2003) and in theUpton Group (Lavoie, 1992; Paradis and Lavoie, 1996).

From top to bottom, the succession of the Saint-Flavien struc-ture is divided informally into zones (A1 to A6, B1 to B5 and C1to C4) based on diagraphic log analysis (Fig. 3). These zones canbe correlated approximately with two of the three units recog-nized by Bernstein (1991, 1992) and Salad Hersi et al. (2003).The basal Beauharnois Formation (Ogsdenburg Member) corre-lates with zone C, the upper Beauharnois Formation(Huntingdon Member) is equivalent to zone B, and the CarillonFormation to zone A. Dolomitic sandstones recognized belowthe C3 and C4 zones are correlated with the Theresa Formation(Fig. 3), but were rarely cored and not studied herein.

In the Saint-Flavien area, northeast thrust-displacement ofthe Beekmantown Group is nearly 25 km (St-Julien et al.,1983). On a cross-section (Fig. 2), the carbonates form an openanticline bounded to the northwest by a major thrust fault(Malo et al., 2001a).

TECHNIQUES AND METHODS

MINERAL PETROGRAPHY

A cumulative thickness of 3900 m of Beekmantown Groupstrata are intersected by the studied wells. Three hundred andseventy-nine (379) metres were cored (9.7% of the cumulativethickness). Core samples for petrographic analysis wereselected where diagenetic features would allow the reconstruc-tion of the paragenetic succession and the evolution of porosity(174 samples). All thin sections used for petrographic analysiswere polished and studied under transmitted light. Some werealso studied under cathodoluminescence (CL). Rock sampleswere impregnated with a blue resin prior to thin section prepa-ration for evaluation of the amount and type of porosity. Theoperating condition for cathodoluminescence (NuclideCorporations Model EEM2E luminoscope) was a 1 cm widebeam at 15 KV, 0.5 µA under a 50–70-millitorr vacuum. Themineralogical composition of nearly all samples studied in thinsection was verified through whole rock X-ray diffractionanalysis.

INSOLUBLE RESIDUE AND CLAY MINERAL ANALYSES

Insoluble residues are made up of pulverized powder of coresamples. Carbonates were dissolved in a heated hydrochloricacid bath. Dry powder samples were weighed prior to and afterdissolution in acid in order to calculate percentage of insolubleresidues. The proportion of calcite and dolomite were deter-mined by whole rock X-ray analysis.

Samples used for clay mineral and other silicate analyseswere crushed into 1 to 2 mm size fragments and the dust wasremoved. The fragments were agitated in demineralized waterand the resulting suspensions were decarbonated with diluted1N hydrochloric acid under constant agitation. The acid wasslowly added until the effervescence stopped, after which thesolution was neutralized. The clay fraction (<2µm) was sepa-rated from the total fraction by centrifugation. Both fractions



Fig. 3. Correlation between Beekmantown stratigraphy in the Montreal area and theinformal zones recognized in the Saint-Flavien gas field.

were oriented by sedimentation on aluminum slides prior to X-ray diffraction analyses.

Silicates, clays, and other minerals in the insoluble residueswere identified and quantitatively evaluated by X-ray diffrac-tion on two granulometric fractions. The total fraction was usedfor qualitative and quantitative evaluations of all silicates; the<2 µm fraction was used for clay identification and measure-ments of diagenetic parameters.

In order to evaluate mineral composition in total insolubleresidues, calibration curves were established using mixtures,in known proportions, of the same mineral types as thosefound in the analyzed samples. Quartz, K-feldspars, Na-Ca-feldspars and muscovite were used. Those artificial sampleswere prepared and scanned under the same conditions as theunknown samples. The relationships between X-ray diffrac-tion peak intensities and mineral relative abundance weredetermined and correction factors were calculated. These fac-tors were then applied to X-ray peak intensities measured ondiffractograms to calculate the proportions of the mineralspresent in the natural samples. In the standard samples, mus-covite was representing the clay matrix since other mineralssuch as chlorite, kaolinite and smectite could be present in thestudied samples. After having calculated the proportions ofquartz and all feldspars, the rest (100% – [quartz+feldspars])was attributed to clay minerals. Within the latter, the amountof different clay minerals, if present, was determined usingdiffraction profiles calculated with the NEWMOD program(Reynold, 1985). Diffraction profiles identical to those of themineral present in the samples are produced. These profilesare added in varying proportions until both calculated andexperimental diffractograms match.

Diffraction profiles from the <2 µm fraction were usedonly for clay mineral identification. Mineral assemblages, illite“crystallinity” (width of 001 peak of illite at half height) and the002/001 peak ratio of illite (H2/H1) are the major parametersused to define the origin and evolution (diagenetic andhydrothermal) of the clay fraction.

Isotope analysis

Sampling for isotope analysis of fine dolomites in dolostoneswas done by micro-drilling thin sections of polished parentrock slabs previously examined under cathodoluminescencefor the paragenetic study. Samples are from the dolomiticgroundmass of the rock which might contain intercrystal cal-cite, and from large cavities containing calcite only. Twenty-two analyses for δ13C and δ18O were performed at theDerry/Rust Laboratory at the University of Ottawa.

Dolomite and calcite were chemically isolated with phos-phoric acid prior to isotope analysis. For samples containing amixture of dolomite and calcite, the isotopic signals wereobtained by reaction with phosphoric acid until isotopic equi-librium was reached. (Al-Aasm et al., 1990). The equilibriumCO2 produced from calcite at 24ºC was isolated after four hoursand analyzed, and its residual CO2 pumped after 12 hours.Subsequently, equilibrium CO2 produced from dolomite at 50ºC

was isolated after 12 hours, then analyzed. The fractionationfactor used for dolomite at 50ºC is 1.0090. For carbonates, thedata are reported with the usual ‰ notation and in relation to thePeeDee Belemnite (PDB) standard. Precision of data is alwaysbetter than 0.1‰.

Fluid inclusion analysis

Fluid inclusions were studied from doubly polished thin sec-tions. The preparation method preserves low temperature fluidinclusions in carbonates. The samples were selected from thepetrographic analyses where fluid inclusions in quartz or cal-cite cements of veins and vugs were present. Fluid inclusionanalysis was performed on a Fluid Inc. adapted U.S.G.S Gas-Flow Heating/Freezing System. The homogenization tempera-ture (Th) and the final ice-melting temperature (Tm) weremeasured with a precision of 1ºC and 0.2ºC, respectively. Themethod of analysis is based on Roedder (1984).

Organic matter petrography and reflectance analyses

The petrography and reflectance of organic matter (OM)have been studied for most samples analyzed for diagenesisand insoluble residues (149 samples). Reflected lightmicroscopy was done on polished thin sections used for petro-graphic analyses, on extracted kerogen from core samples, andon extracted kerogen from composite samples. Composite drillcutting samples are composed of equal weights of five to tenconsecutive cutting sub-samples.

Kerogen extraction and polished petrographic preparationswere done following the method described in Bertrand et al.(1985) and Bertrand and Héroux (1987). Reflectance wasdetermined with a Zeiss photomicroscope reflectometerMPM 01K following standard procedures used in coal pet-rography. Some reflectance measurements were performed onmiscellaneous types of OM, but organoclasts (graptolites,chitinozoans, scolecodonts, migra- and pyrobitumen), whichare good thermal maturation indicators, were used for mostmeasurements. Based on the size of OM fragments, more thanone reflectance measurement could have been taken fromeach particle.

STRUCTURAL ANALYSIS

Observations were made on all available cores from Saint-Flavien No. 1 to No. 10 wells. Relative chronology of all strati-graphic and structural elements (stratification, stylolites, veins,joints, breccias, microfaults and shear zones) was determinedby crosscutting relationships. Fracture density was determinedfor each metre. Above a threshold of 25 fractures/metre, thefracture density was visually estimated. Fracture density wasreported on logs for all wells.

Strike and dip of structural elements come from reports byCore Laboratories for Saint-Flavien No. 8, No. 9 and No. 10wells. Cores were oriented with downhole logs such as FMSfrom Schlumberger (Formation MicroScanner) or CBIL fromBaker Atlas (Circumferential Borehole Imaging Log).


SEDIMENTOLOGY AND CYCLICITY

LITHOFACIES OBSERVATIONS

Detailed stratigraphy and sedimentology of cores, and pet-rographic examination of selected samples led to the recogni-tion of eight major recurrent lithofacies in the Beauharnois andCarillon formations. These facies are interpreted to representdistinctive depositional environments ranging from subtidal toupper intertidal (Fig. 4). This interpretation is based on sedi-mentological and paleoecological criteria and facies stackingpatterns in cores.

Lithofacies A consists of cryptomicrobial laminites and ismore than 95% dolomitized (Fig. 4). Millimetric laminationsrelate to alternating bands of various grain size dolomite.Intercalation of pellet laminites (spongiostromate), relic of cal-cimicrobes (Girvanella) and evidence of bioturbation and des-iccation are observed in this slightly porous (microporosity)facies. This facies can occasionally be very sandy and locallycontains quartz arenite laminae composed of well-rounded andwell-sorted grains. This facies is interpreted to have beendeposited in an intertidal setting.

Lithofacies B is composed of massive dolomicrite anddolosiltite beds. These beds are occasionally parallel-lami-nated, but more often they show a mottled structure followingintense bioturbation. This is the most abundant facies in thecored intervals of the Beekmantown Group. Dolostones of thisfacies are preferentially fractured or brecciated and commonlyshow intercrystal and vuggy porosity (Fig. 4), and locally haveghosts of cryptomicrobial laminae. This facies is interpreted asintertidal to shallow subtidal deposits.

Lithofacies C is volumetrically minor and is represented bydense micrite. The facies is commonly dolomitized and crudelybedded with fenestrae. It contains abundant relics of calcimi-crobes but also some marine bioclasts (mollusks) and calcare-ous green algae. The facies is interpreted as a product of lowerintertidal to very shallow and calm subtidal environments.

Lithofacies D consists of thrombolites (Fig. 4) and is aminor lithofacies of the Saint-Flavien structure. Thrombolite isa massive erected sedimentary structure formed by calcimi-crobes, including Renalcis and Epiphyton. Growth cavitiesbetween the erected frameworks are filled with thromboliteintraclastic rudstone and marine bioclasts and no preservedporosity is visible. This lithofacies is interpreted to represent ahigh-energy segment of the agitated subtidal domain based onsimilarities with modern examples in Australia and Bahamas(James and Bourque, 1992).

Lithofacies E (lime mudstone) and lithofacies F (wacke-stone) (Fig. 4) are discussed together because they are spatiallyclosely associated, petrographically similar, and differentiatedmostly by their allochem/lime mud ratio. Allochems are com-posed of bioclasts (trilobites, brachiopods and few mollusks),green algae, and pellets. The facies are intensively bioturbated,and burrows are more dolomitic than the host rock matrix.These facies are strongly stylolitized, but only slightly dolomi-tized and contain virtually no porosity. These facies weredeposited in a calm to slightly agitated subtidal marine envi-ronment.

Lithofacies G consists of packstone and grainstone bothcomposed of bioclasts (crinoids, trilobites and brachiopods),phytoclasts, pelsparite and intrasparitic intraclasts, pellets andrare oolites surrounded by lime mud or calcite cement (Fig. 4).Abrasion of bioclasts, presence of oolites and abundance ofrecycled intraclasts suggest a high-energy subtidal marine envi-ronment. Secondary dolomitization is sporadic in this facieswhich is nonporous but highly stylolitized.

Lithofacies H is composed of mudstones and is the mostcommon siliciclastic facies observed. It is generally finely lam-inated and contains various amounts of siliciclastic silt andeuhedral dolomite crystals. In general, this lithofacies sharplyoverlies a dolostone bed but is conformably overlain by lime-stone facies. These mudstones are the preferential locus forhorizontal shear zones; calcite cements and other diageneticminerals found in this facies are associated with deformed,bedding-parallel veins. Mudstone represents a deeper marine,calm, subtidal environment.

STACKING PATTERN

Cored intervals of Beekmantown Group in the Saint-Flavienstructure contain repetitive metre-thick cyclic subtidal to inter-tidal lithofacies. These cycles in the A5 and B2 zones of Saint-Flavien No. 3 and No. 9 wells are represented in Figures 5Aand B, respectively. The cryptomicrobial, laminated dolostone(at the left of the facies column) represents the shallowestdeposit in the intertidal environment. The siliciclastic mud-stone (at the right of the facies column) represents the deepestdeposit in the subtidal environment.


Fig. 4. Idealized 5th-order cycle in the Beekmantown Group.Theoretical arrangement of lithofacies in a cycle (H to A) and related relative marine level fluctuations (TR = transgression; RE = regression).Distribution of primary (Pφ) and secondary (Sφ) porosity and primary (Pd) and secondary (Sd) dolomite.

Figures 5A and B show that the thickness of these cyclesvaries between 1.0 and 7.0 m. They also indicate that the A5,A6, base of B1 and B2 zones are dominated by subtidal litho-facies but that the B1 zone, in both Saint-Flavien No. 3 (cores 2and 3) and No. 9 (mainly core 3) wells are dominated by inter-tidal lithofacies. The thickness of these cycles is greater nearthe top of the B1 zone compared to other parts of the coredintervals. In both wells, these cycles in the B1 zone are essen-tially dolomitic and correlate with the more porous interval ofthe core (Figs. 5A and B). The major pore types include: inter-crystalline, moldic and dissolution vugs, and fractures. The B1

zone is the main productive reservoir unit in the Saint-Flaviengas field.

INTERPRETATIONS

The uppermost lithology at the cycle top is lacking unequiv-ocal evidence for subaerial exposure which, if any, was likelyof restricted duration. The basal shale of each cycle was causedby flooding, which shut down the carbonate production, anddeposited undiluted siliciclastic muds on the marine shelf. Asthe carbonate factory re-established itself, carbonate sedimen-tation predominated, first in a subtidal environment (packstone,


Fig. 5. Vertical variations of sedimentologic, tectonic and diagenetic parameters in two wells of the Saint-Flavien structure showing reservoir quality: A) Saint-Flavien No. 3 (above) and B) Saint-Flavien No. 9 (next page). Lithofacies – usedFigure 4 as key. Physic = porosity by porosimeter; Int. = intercrystal; Diss. = dissolution; A6, B1, B2 = informal zones inBeekmantown Group used in Figure 3. Blank areas in Insoluble Residue Mineralogy and Organic Matter Petrography logsindicate no sample data.


Fig. 5. Continued. B) Saint-Flavien No. 9.

grainstone, lime mudstone, wackestone and thromboliteboundstone). The carbonate shelf became progressively shal-lower and very shallow subtidal (dense micrite) to intertidal(cryptomicrobial laminites) strata were deposited. The upperintertidal facies represent the shallowest facies in the succes-sion. These cycle tops are invariably overlain by deeper marinefacies (mudstone and/or subtidal limestones) marking the initi-ation of a new meter-scale shallowing-upward cycle. Our studyof the cyclicity within the Beekmantown strata in the Saint-Flavien structure is hampered by the limited thickness of avail-able cored rock succession. In the best case (Fig. 5B), 15complete cycles are developed. This is well below the 50 shoal-ing-upward cycles proposed as a minimum for detailed cyclic-ity study (Sadler et al., 1993).

This type of cyclicity is related to the combined effects ofseveral orders of relative sea level oscillations (Osleger andRead, 1991). Synsedimentary tectonism is ruled out as a poten-tial cause for relative Early Ordovician sea level fluctuations;recent studies in coeval strata in southern Quebec (Salad Hersiet al., 2002a, 2002b, 2003) and adjacent areas (Landing et al.,2003) clearly document the overriding control of eustacy.Limited and poorly expressed extensional tectonism is, how-ever, locally documented in uppermost Beekmantown strata(e.g. Carillon Formation or A zone in Saint-Flavien) in easternOntario (Dix and Molgat, 1998). Stacking patterns of metre-thick cycles (fifth-order) are used to identify larger-scale eventsrelated to lower order relative sea level variations (Osleger andRead, 1991). Attempts to recognize lower order eustatic sealevel fluctuation of sedimentary successions is done throughconstruction of Fischer plots (Fischer, 1964). As documented bySadler et al. (1993), less problematic constructions using thick-ness departure (positive or negative) from average cycle thick-ness are presented here (Fig. 6), although our average cyclethickness (2.4 m) is calculated from a limited data set (70cycles). Figure 6 plots fifth-order cycles recognized in coredintervals of Saint-Flavien No. 3 and No. 9 wells (Figs. 5A, B).The vertical axis of these plots is based on the cumulative depar-ture of individual cycle thickness (Sadler et al., 1993) from theaverage cycle thickness (2.4 m). The horizontal axis is theexpression of relative time represented by cumulative cycles. Inmost littoral and deep subtidal settings, missing beats are a com-mon problem; in our case, the mixed intertidal–subtidal settingis less problematic although this limitation has to be considered.A thicker than average sedimentary cycle indicates a relativeincrease in sea level whereas a thinner than average cycle cor-relates with a relative decrease in sea level. However, it isunderstood that these constructions are only rough estimates ofabsolute variations of sea level fluctuations. Their use hereinsimply demonstrates some patterns in the overall accumulationof sediments and the relationship with known porous intervals.

Close examination of the lower order curve (Fig. 6) showsthat the most significant porous zones in the B1 zone correlatedirectly with the interval having the thickest fifth-order cycles(cycles 10 and 11; Fig. 6). These thick cycles likely formed (ifno missing beats are present in this interval) during the timeinterval where most accommodation space was generated. This

defines the maximum low fourth-order sea level rise recordedin these cores. The rate of relative sea level rise was definitelylow because intertidal sediments (cryptomicrobial laminitesand dolomicrite; Fig. 5B) making up the total succession, wereable to keep pace with rising sea level. This resulted in thickaccumulation intertidal facies in fifth-order cycles. It will bedocumented that these thick intertidal dolostone successionsare most prone to later fracturing and development of economicsecondary porosity.

Although significantly less well cored, stacking patternresults from other B1 zone cores agree with the lower orderpattern shown for Saint-Flavien No. 9 well (Fig. 6). Locallythick fifth-order cycles can be developed by local conditionsfavouring sediment accumulation (e.g. the migrating tidal flatisland model of Pratt and James, 1986). The overall accumula-tion of the Beekmantown facies in the Saint-Flavien area waslikely responding to regional low order (fourth–third?) eustaticsea level fluctuations. However, the possibility of missing beatsin the predominantly intertidal-dominated reservoir intervalhas to be considered, and the thick fifth-order cycles docu-mented here could be expressed in a higher number of smallersubtidal–intertidal cycles in slightly more open marine settings.The only significant conclusion for this cyclicity study is thataccumulation of thick intervals of intertidal dolostone fromvarious potential mechanisms, results in the best case scenariofor development of secondary porosity. This observation is crit-ical for ongoing exploration efforts for hydrocarbon reservoirsin southern Quebec.

Stacking patterns of cycles near the top of the BeekmantownGroup clearly indicate a major shallowing of the platform. Thiscorresponds to a global event (Sauk–Tippecanoe sequencesboundary; Sloss, 1963) observed elsewhere in eastern NorthAmerica (Mussman and Read, 1986; Read, 1989; James et al.,1989; Bernstein, 1992; Salad Hersi et al., 2003). This generalshallowing was enhanced by the westward migration of aTaconian tectonic forebulge uplift (Knight et al., 1991). Thiscombined eustatic–tectonic regressive event generated thewidely occurring Knox–Beekmantown–St. George unconfor-mity that was later covered by clastic and limestone forelandbasin deposits (Lavoie, 1994).

INSOLUBLE RESIDUE MINERALOGY

OBSERVATIONS

Four mineral assemblages are recognized, based on mineralindicators such as smectite, or on the abnormal abundance ofcertain minerals that are usually minor elsewhere in the suc-cession. 1) The quartz assemblage contains more than 60%quartz. Peak ratios of other minerals compared to quartz arelow. Illite and K-feldspar are the other most common mineralsin the quartz assemblage. This assemblage is considered to bemainly detrital. Under cathodoluminescence, detrital quartz is atypical blue colour (Fig. 7), whereas, authigenous quartz isblack. 2) The feldspar assemblage can contain as much as 80%of feldspars, mostly potassic (Fig. 8), however the average is



Fig. 6. Plot of successive high order cycles recorded in Saint-Flavien No. 3 and No. 9 wells showing cumulativedeparture from average cycle thickness (2.4 m) as a function of successive cycles from B2, B1 and A6 zones. Thecurve joining successive cycles represents the assumed fourth-order relative sea level change. The plots are linkedto their respective stratigraphic column. The reservoir dolostone is located in cycles 10 and 11 characterized bythicker than usual intertidal dolostone interval.

about 40% feldspars. Locally, sodic feldspar is also abundant(Fig. 5A). Feldspars are identified from their peak position onX-ray diffractograms (see illite assemblage on Fig. 8). 3) Theillite assemblage (Fig. 8) contains between 40 and 50% of illitein the total fraction. The fine fraction contains this mineralonly. This assemblage also contains quartz and feldspars. In thefine fraction, illite is partly authigenic. The authigenic charac-ter is indicated by the high second to first peak ratios in the dif-fractograms. 4) The smectite assemblage (Fig. 8) containssmectite in variable amounts. Its presence in core samples isimportant because this mineral is entirely authigenic andindicative of low temperature conditions (Aja et al., 1991).

Vertical variation of insoluble residue mineralogy in Saint-Flavien No. 3 and No. 9 wells is illustrated in Figures 5A andB. The K-feldspar assemblage dominates the insoluble residueof cores 2 to 5 of Saint-Flavien No. 9 well. Maximum amountof K-feldspar is observed in the reservoir porous zone, at 1525m (Fig. 5B). Assemblages are more diversified in Saint-FlavienNo. 3 well and zoning is better developed (Fig. 5A). Samplesfrom the A5 and upper part of the A6 zones, above the reservoirzone, are typified by a quartz assemblage. The wackestoneinterval in the middle of the A6 zone is illite-rich with an illiteassemblage (Fig. 8). Downward, the next zones mostly containK- and Na-feldspars of the feldspar assemblage. Finally, thelowermost sample of core 3 contains insoluble residue ofquartz and relatively abundant smectite (Fig. 8).

INTERPRETATIONS

The abundance of potassic feldspars (up to 90%) in theinsoluble residue of some samples is incompatible with a detri-tal origin (Fig. 8). Very few igneous or metamorphic rocks con-tain such high percentages of feldspars, especially K-feldspars.These feldspars are very sensitive to meteoric alteration orhydrolysis and the probability of finding them in such a greatproportion in carbonate sediments from a stable platform isvery low. Moreover, they are locally observed as cement invugs and veins (Fig. 7). From these observations, it is thereforeconcluded that most feldspars are of diagenetic origin.

The coexistence of K-feldspar, illite and smectite in carbon-ates of the Beauharnois Formation (B zone) indicates a complexdiagenetic history. Activity diagrams for stability of authigenicclay minerals and K-feldspar (microcline) in the presence ofquartz and water, argue for such complexity (Fig. 8). These dia-grams illustrate the stability field of those minerals as a functionof temperature and the activity of potassium (K) and magne-sium (Mg) relative to the acidity of water (Aja et al., 1991).

The abundance of smectite in the reservoir zone suggests atemperature lower than 125ºC for smectite preservation, and atemperature presumably around 100ºC for its precipitation. Askaolinite was not observed in these rocks, both [Mg+2] and [K+]should have been relatively high compared to acidity [H+].Predominance of K-feldspar also indicates [K+] higher than7.0. However, the temperature has to be much higher, rangingbetween 125ºC and 200ºC. Conversely, illite has a wide rangeof possible precipitation temperatures and because muscoviteis not recognized in the Beauharnois Formation facies,

precipitation of the K-feldspar assemblage presumablyoccurred at Tº lower than 200ºC.

Based on the diagram of Aja et al. (1991), the co-precipita-tion of abundant K-feldspar and smectite is unlikely. The mix-ture of these two minerals observed in the smectite assemblageof Saint-Flavien No. 3 well is not chemically stable (Fig. 8). Thehigher temperature condition needed for K-feldspar precipita-tion would destroy any coexisting smectite. Consequently, thesmectite precipitation had to post-date that of K-feldspar in theporous zone of Saint-Flavien No. 3 well. The late diageneticfluid had to be cooler than the fluid responsible for K-feldsparprecipitation and acidic enough to dissolve it in order to main-tain high activity of K+.

Distribution of mineral assemblages in Figure 5A and Bindicates orderly precipitation of silicates surrounding theporous interval: smectite, K-feldspar, illite, and quartz. Thequartz zone characterizes the non-porous intervals of theBeekmantown Group. The occurrence of a feldspar zoneencasing the porous zone indicates that relatively hot, K- andNa-rich fluids circulated through pores, diffused in the hostrock and precipitated K- and Na-feldspars. Illite presumablyco-precipitated with feldspars where the diagenetic environ-ment was slightly cooler and depleted in potassium, mostlikely in a more distal setting. Later, a more acidic and coolerfluid altered K-feldspar, and precipitated smectite. This zono-graphy of insoluble residue minerals is recognized in all wellsin the Saint-Flavien structure where porous intervals areobserved. These silicate-rich zones around a porous carbonatehost show similarities with clay zones found around oredeposits (Héroux et al., 1996; Chagnon et al., 1998). This rela-tionship suggests that similar geochemical processes can oper-ate during generation of porosity in hydrocarbon gas pools.


Fig. 7. Compacted and tightly cemented dolosiltite composed ofxenomorphic cloudy crystals of dolomite (spotty red luminescence),cross-cut by vein cemented successively K-feldspar (according to X-raydiffraction of whole rock), quartz (black) and xenomorphic recrystallizedcalcite which shows alternating slightly to dull luminescent zones.Photomicrograph under cathodoluminescence. Saint-Flavien No. 8 well,1525.83–1526.09 m, Beauharnois Formation, B1 zone. Cd =Luminescent cloudy crystals of dolomite, Dq = Detrital quartz; Xc =Xenomorphic calcite cement; Aq-f = Authigenic quartz and feldspar.Scale bar is 400 microns.


Fig. 8. Some authigenic mineral assemblages and corresponding activity diagrams: system (K2O-MgO-Al2O3-H2O)in the presence of quartz (after Aja et al., 1991). Each diagram represents physico-chemical conditions that prevailed at the time of formation of the assemblage. Smectite assemblage from Saint-Flavien No. 3,1533.1–1534.1 m; Illite assemblage from Saint-Flavien No. 3, 1531.0–1532.2 m; K-feldspar assemblage fromSaint-Flavien No. 7, 1576 m.

CARBONATE DIAGENESIS

PETROGRAPHIC OBSERVATIONS

Two diagenetic patterns related to lithology are recognizedin these successions that pertain to the limestone and dolo-stone facies.

Limestone

Limestone porosity is commonly occluded by two differentcements: (1) isopachous rims of small bladed calcite crystalswhich surround allochems, and (2) xenomorphic crystals ofvarious sizes filling the remaining primary pores (Fig. 9A).Replacement dolomite is common in limestone facies of theBeekmantown Group in the Saint-Flavien wells. However,dolomitization is rarely pervasive and never creates secondaryporosity (Fig. 9A). Replacive euhedral dolomite is cross-cut bybedding parallel stylolites, which are very common in lime-stones (Fig. 10, Stage 4). Some post-stylolite fracturing occursin limestone, and fractures rarely show open vugs because theyare cemented by xenomorphic calcite.

Dolostone

Two types of dolomite-rich lithofacies are recognized: (1)dolostone composed of well preserved cryptomicrobiallaminites (Lithofacies A of Fig. 4) and (2) dolosiltite todoloarenite with poorly preserved sedimentary structures(Lithofacies B of Fig. 4). Excluding replacive, inclusion-richdolomite and open fractures and veins, lithofacies A containsfew observable diagenetic features. Our description conse-quently focuses on dolosiltite. Structures of dolosiltite are illus-trated relative to their position in diagenetic evolution (Figs. 9Bto H). The relative chronology of tectono-diagenetic processesthat produced these structures is shown in Figure 10.

Dolosiltite crystals are formed by three distinct phases(Fig. 9D). The crystal core is made up of cloudy to nearlyopaque, replacive, inclusion-rich dolomite. This early cloudydolomite shows a tightly interconnected anhedral crystalstructure (Fig. 9H). Following the generation of limited avail-able space during dolomitization, anhedral crystals areenveloped by more translucent early burial dolomite cement(Fig. 10, Stage 1). This overgrowth is best developed in drusydolomite, near vugs or in the matrix of the dolostone, whereintercrystal pores are abundant (Fig. 9D). Finally, the late bur-ial dolomite crystals in vugs (Fig. 9D) and veins or breccias(Fig. 9G) show the typical curved crystal faces of saddledolomite (Fig. 10, Stages 5A and B). The precipitation of sad-dle dolomite is generally interpreted to occur during mediumto deep burial at temperatures between 60ºC and 190ºC(Radke and Mathis, 1980; Lavoie and Chi, 2001). Beddingparallel stylolites are present but not very common indolosiltite (Fig. 10, Stage 4).

Other diagenetic elements

Following early replacive dolomitization, the first diage-netic events to occur in the dolostone lithofacies are limited

dissolution and karstification (Fig. 10, Stage 2). Early dissolu-tion of dolomite is suggested by corroded margins of cloudydolomite crystals (Fig. 9B) that are restricted to the cryptomi-crobial laminites and dolosiltite lithofacies at the top of somefifth-order cycles. These karst vugs show a maximum size ofa few centimetres. They were only recognized in Saint-FlavienNo. 8 well and are generally tightly cemented by fibrous cal-cite cements with local carbonate (calcite and dolomite) silts.Figure 9B shows the cement paragenesis of one of these karstswhere cements are composed of the following: 1) nodularpyrite, 2) fibrous calcite, and 3) xenomorphic calcite. Paralleland oblique oriented stylolites containing framboidal pyritecrosscut these calcite cements (Fig. 9B). Locally, sphalerite,euhedral crystal of dolomite and traces of organic matter areobserved in bedding-parallel stylolites. The oblique stylolitesare interpreted as late relative to calcite cements (Fig. 10,Stage 5B).

Veins are common features in the dolostone, and in additionto cross-cutting relationships, the occurrence of the varioustypes of cements or fillings indicates the presence of multiplegenerations of veins (Fig. 10). The most common cement in theveins is a cloudy, xenomorphic, inclusion-rich calcite com-monly associated with solid bitumen (see below; Fig. 9C, D).Under CL, this calcite cement is commonly zoned. It is inter-preted as being more or less coeval with hydrocarbon migration(Fig. 10). Under CL, this first cement has a pyramidal shapeand shows a medium light to medium dull luminescence withcommon nonluminescent terminations (Fig. 7). Poikilotopiccalcite is a common cement in the last generation of veins(Figs. 9F, 10). This cement is nonluminescent.

Veins with zoned calcite cement are sometimes outlined byxenomorphic quartz and other silicate cements (feldspar fromX-ray diffraction). In veins and breccias cemented by poikilo-topic calcite, the quartz cement is more common than in veinscemented by xenomorphic calcite (Fig. 10). The quartz iscomposed of euhedral crystals (Fig. 8F, G). In dolomitic brec-cia, where part of the cement is composed of large saddledolomite crystals, residual porosity is partially occluded byeuhedral quartz (Fig. 9G). Occurrence of anhedral and euhe-dral quartz cements in veins indicates that precipitation ofquartz predated that of calcite in veins but postdated that ofsaddle dolomite in breccia (Fig 10). The quartz is always non-luminescent (Fig. 7).

Dissolution features are abundant in the reservoir brecciamatrix of the Saint-Flavien N. 3 well (Fig. 8G) between drusyand saddle dolomite crystals. The most significant dissolutionfeature occurs in calcite cements in veins and breccia(Fig. 9H) where up to 18% of porosity is observed (Fig. 5A).Fragments of calcite cement and quartz are remobilized in thezone of dissolution (Fig. 9H). It is only in these zones thatillite-smectite cement is petrographically observed in theintercrystal pores of the breccia matrix (Fig. 9G). The rela-tionships between cements and dissolution features suggestthat this episode of corrosion of vein-calcite and precipitationof illite-smectite in the porous zones is late in the history ofporosity development (Fig. 10).


Interpretation

The complete cementation of limestone lithofacies, the goodpreservation of their sedimentary structures, and the rare occur-rence of dolomitization, suggest that limestones were cementedshortly after sedimentation and did not have good reservoirpotential.

The replacive dolomitization of cryptomicrobial laminites isinterpreted to be syn-sedimentary while dolosiltite was formedduring later diagenesis but before significant burial (pre-styloli-tization) (e.g. Morrow, 1990). The cloudy aspect of dolomitecores is likely a ghost of the original calcite or aragonite mudthat chemically formed in the intertidal environment. It is pos-tulated here that early dolomitization proceeded through mixedwater diagenesis (Fig. 10, Stage 1). This assumption is based onthe circulation of fresh water (local early karstification of somedolostones, see above). The abundant intercrystal porosityfound in this lithofacies was generated during this stage of earlydiagenesis. As dolomitization proceeded through burial, initialcrystals of dolomite were overgrown by clear and inclusion-freelayers of dolomite (Fig. 10, Stage 4). The presence of saddledolomite indicates that dolomitization was irregularly activeuntil deep burial was achieved (Fig. 10, Stages 5A, B). Somevolume of primary pores was preserved from dolomite cemen-tation as intercrystal porosity.

The presence of centimetre-size karsts indicates some sub-aerial dissolution affecting shallow intertidal dolomitic facies(Fig. 10, Stage 2). However, the rapid filling of these vugs byvarious cements indicates that secondary porosity created bykarsting had no significant impact on the Saint-Flavien reser-voir development (Fig. 10, Stage 3).

Fractures in dolostone formed during burial (Fig. 10, Stages4, 5A) and subsequent thrusting (Fig. 10, Stage 5B). The per-meable framework of intercrystal porosity in dolostone wasenhanced by the fracturing, without which it is unlikely thatany efficient permeable system would have developed.Hydrocarbon migration (see below) was intermittent between

episodes of calcite cementation (Fig. 10). Significant calciteprecipitation followed the main hydrocarbon migration event,as recorded by post-bitumen, pore-coating carbonate cementsin veins and breccias. Saddle dolomite, euhedral quartz, andpoikilotopic calcite cements are the last cements in veins andbreccia (Fig. 10, Stage 5B).

Dissolution of calcite in reactivated veins and breccias waslikely the main event for the formation of an efficient porosityand permeability system in the Saint-Flavien reservoir (Fig. 10,


Fig. 9. A) Recrystallized intra-pelsparite with complete filling of primary intergranular porosity by isopachous bladed calcite cement (Ic) and coarse equant calcite cement (Ec). Stylolite (S), partial dolomitization (Do) and vein filled with xenomorphic calcite (Xc). Transmitted light.Saint-Flavien No. 3 well, 1512.4 m, B1 zone of Beauharnois Formation. Scale bar is 100 microns. B) Contact between a karstified dolomite andinfilling karst cements: nodular pyrite (Np), bladed of fibrous calcite cement showing tectonically induced lamellar twinning (Fi). Approximate contour of karst outlines by the dashed line. Framboidal pyrite (Fp) in late stylolite (S). Transmitted natural light and incident, cross-polarized light.Saint-Flavien No. 8 well, 1520.82 m, B1 zone of Beauharnois Formation. Scale bar is 100 microns. C) Intercrystal pore in a calcite-filled (Cv) vein with solid bitumen (Py) thermally altered into an anisotropic pyrobitumen showing a fluid microstructure (Nk) (native coke). Transmitted lightand incident, cross-polarized light. Saint-Flavien No. 9 well, 1520.9 m, B1 zone of Beauharnois Formation. Scale bar is 50 microns. D) Vuggydoloarenite showing cores of xenomorphic to idiomorphic cloudy crystals of dolomite (Cd), overgrown by clear syntaxial dolomite cement (Ld). Inthe vug, dolomite crystals pass to saddle dolomite (Sd). The vug is successively filled by pyrobitumen (Py), and xenomorphic calcite (Xc). Someintercrystal porosity (Iφ) is preserved between dolomite crystals. Transmitted light. Saint-Flavien No. 13 well, 1500.4 m, B1 zone of BeauharnoisFormation. Scale bar is 200 microns. E) Contact between vein-filling saddle dolomite (Sd) and dolostone host (Dm). A film of coked pyrobitumen(Kp) lining the vein wall is displaced and squeezed by the saddle dolomite crystals. Incident light. Saint-Flavien No. 13 well, 1500.4 m, B1 zoneof Beauharnois Formation. Scale bar is 20 microns. F) Vein in a fine-grained sandstone cemented by authigenic quartz (Qz) and poikilotopic calcite (Pc). Some preserved porosity. Transmitted light. Saint-Flavien No. 3 well, 1702.6 m, Beauharnois Formation, C4 zone. Scale bar is 200microns. G) Secondary intercrystal matrix porosity (Sφ) in a partially cemented dolorudite breccia. The breccia is cemented by coarse saddledolomite (Sd), quartz (Qz) and an illite-smectite mixture (I-S) (confirmed by X-ray diffraction). Transmitted light. Saint-Flavien No. 10 well, depthof 1999.1 m, Theresa Formation. Scale bar is 200 microns. H) Secondary porosity (Dφ) in a dolomite breccia (Dm) where the xenomorphic calcite (Xc) and quartz (Qz) cements are partially dissolved. Dolostone fragments with minor dissolution. Transmitted, cross-polarized light. Saint-Flavien No. 3 well, 1533.1 m, Beauharnois Formation, B1 zone. Scale bar is 100 microns.

Fig. 10. Relative chronology of tectono-diagenetic processes basedon structural analysis, insoluble residue mineralogy and carbonate diagenesis. No absolute timing or quantitative importance of processesis implied. A = anhedral; E = euhedral; S = saddle dolomite; N = nodular; Fb = framboidal; X = xenomorphic; Z = zoned in cathodo-luminescence; I = inclusion-rich; Pk = poikilotopic; H = horizontal; T = translucent, V = vertical; Karst = subaerial Karst.

Stage 6). The high percentage of smectite associated with significant porosity values suggests that dissolution of calcitecements in the brecciated dolosiltite and laminated dolostone, is avery late diagenetic event that occurred at a much lower temper-ature than the maximum one reached during the deepest burial.

FLUID INCLUSIONS

Observations

The results of three of the six doubly polished thin sectionsthat contained workable fluid inclusions are presented in Table1. Analyzed inclusions are from three cement types: 1) poikilo-topic calcite in porous breccia (Saint-Flavien No. 3 well, B1zone), 2) vein-filling quartz crystal found in a brecciatedslightly porous dolomicrite (Saint-Flavien No. 3 well, C2zone), and 3) vein-filling xenomorphic calcite of a vein in afractured pelmicrite (Saint-Flavien No. 7 well, B1 zone).Homogenization temperatures (Th) in poikilotopic calcite flu-ids inclusions range from 223 to 282ºC (Table 1). This largevariation can be explained by the occasional fissuring alongcalcite cleavage when temperature reaches the critical point offluid inclusion homogenization. Ice-melting temperatures ofthese fluid inclusions indicate that salinity ranges from 2.9 to4.6% equivalent NaCl (Table 1).

Fluid inclusion analysis of secondary vein-filling quartzshows slightly lower homogenization temperatures (Table 1:172ºC < Th < 194ºC). The ice-melting temperature in quartz wasmore difficult to measure than in calcite, and consequently esti-mated salinities are more variable (Table 1: 0% <SalinityNaCl<3.7%). These results indicate that estimated salinity of fluidinclusions in poikilotopic calcite and quartz of dolomitic brecciasfrom Saint-Flavien No. 3 well, are similar.

In the Saint-Flavien No. 7 well, homogenization tempera-tures (Th) in vein-filling xenomorphic calcite show valuesranging from 81 to 84ºC (Table 1). Ice-melting temperature inthis calcite gave a high equivalent NaCl salinity (Table 1:21.0% < SalinityNaCl < 23.4%).

Interpretation

Based on fluid inclusion results, the late poikilotopic calciteis interpreted to have precipitated from a hot fluid (Th of260ºC) having lower than sea water salinity. This estimated This not corrected for pressure, and the percentage of methanedissolved in the fluid is unknown. Our results also suggest thatlow salinity fluids precipitated quartz cement in veins of theC2 zone. However, the Th of quartz cement is slightly lowerthan that of poikilotopic calcite (Th quartz about 180ºC). The


Table 1. Fluid inclusion microthermometric data.

high temperature and low salinity of this fluid and the rela-tively heavy δ18O composition of the poikilotopic calcite (seebelow) in the breccia suggest that this fluid could be ofigneous or metamorphic origin (Hoefs, 1987).

The Th differences between late fracture-filling quartz andcalcite cements could be related to the contrast of porositybetween C2 zone (porosity = 3.7%), where quartz fluid inclu-sions were analyzed, and porosity of the main reservoir, the B1zone (porosity up to 18%), where the calcite cement was ana-lyzed. Hydrothermal fluid likely circulates more rapidly in ahighly porous framework (B1 zone) than in a significantly lessporous one (C2 zone). Moreover, hydrothermal fluid shouldbetter preserve its original temperature when moving rapidlythrough porous intervals (B1 zone) than through a slow mov-ing fluid in a less permeable system (C2 zone).

In the Saint-Flavien No. 7 well, lower temperature (80ºC)and interpreted high salinity (22% NaCl equivalent) in fracture-filling xenomorphic calcite cement suggest a different fluid.The fluid inclusion data indicate a temperature of calcite pre-cipitation that is significantly lower than the maximum thermalcondition reached by the succession. This suggests that highlysaline brines precipitated calcite cement in the fracture either

before or after maximum temperature. Petrographic evidencessuggests that an early cementation is more likely than latecementation (Fig. 10).

C AND O STABLE ISOTOPES OF CARBONATE PHASES

Observations

Carbon and oxygen isotope ratios obtained from the diage-netic phases in the dolostones of Saint-Flavien No. 3, No. 7 andNo. 8 wells are shown in Table 2, and those of Saint-FlavienNo. 8 well are illustrated in Figure 11. The petrography hasshown that at least four distinct carbonate phases are present.The major phase (anhedral dolomite) is interpreted as early, andfollowed by fibrous calcite and vadose silt in karsts. Calcite inintercrystal dolostone porosity is of dubious origin. The vein-and breccia-filling xenomorphic and poikilotopic calcites arethe latest cements.

The early dolomite isotopic results range from –6.3 to–5.0‰ and from 0.2 to 1.0‰ for the δ18OPDB and δ13CPDB val-ues, respectively. The early dolomite values are higher thanexpected Ordovician values for marine calcite (Wadleigh andVeizer, 1992; –6.5<δ18OPDB<–6.0 and –1.8<δ13CPDB<–0.7;


Table 2. Carbon and oxygen stable isotope ratios in carbonates of the Beauharnois Formation, Saint-Flavien No. 8 well. % = percentage of the major carbonate phases. Calcite cement from

breccia sample in Saint-Flavien No. 3 (1534 m) and calcite cement in vein of Saint-Flavien No. 7 (1576.3 m) correspond to fluid inclusion analyses of Table 1.

Fig. 11). Fibrous calcite in cemented karsts shows δ18OPDBvalues (–6.3 to –5.5‰) similar to their associated dolomiteand dolomitic silt (–6.2 to –5.8‰) (Table 2). Moreover,δ13CPDB values of fibrous calcite (0.6 to 0.9‰) are similar tothose of dolomite and dolomitic silts associated in karsts (0.4 to 0.8‰) (Table 2). Calcite silts show lower δ18OPDB andδ13CPDB values than dolomite silts (Table 2). These values areintermediate between fibrous calcite and late calcite in veins.

The late poikilotopic calcite cement in veins contains thelowest δ18OPDB (–8.3 to –7.4‰) and δ13CPDB (–0.8 to –0.4‰)values (Table 2). The homogeneity of these isotopic valuesfrom top to bottom of the Saint-Flavien No. 8 well (near 200 mthick) indicates that the isotopic ratios are independent of thestratigraphic position of the samples.

Interpretation

The δ18OPDB composition of dolomite matrix in dolostone isslightly higher than values for Lower Ordovician normalmarine calcite (M on Fig. 11). Such high isotope values areexpected for dolomite compared to calcite theoretically co-pre-cipitated from the same parent fluids (Land, 1985). However,the δ18OPDB difference between the Lower Ordovician marinecalcite and the Beauharnois Formation early replacivedolomites is lower than that predicted for co-precipitates.

Dolomite should be enriched in δ18OPDB by 4 to 7‰ relative tocalcite (Hoefs, 1987). From Anderson and Arthur’s (1983)equation, crystallization of dolomite occurred at 45ºC, higherthan the estimated near surface temperature. The mixture ofearly replacing dolomite and syntaxial dolomitic cement in thedolomitic matrix of dolostone or isotopic re-equilibration dur-ing recrystallization could also explain the low δ18OPDB values.Karst-filling dolomite is slightly δ18OPDB depleted (Fig. 11); thispresumably also results from isotopic re-equilibration duringrecrystallization. The δ18OPDB values for calcite indicate thatthis cement precipitated from marine-like waters during shal-low burial. High δ13CPDB values of carbonates compared to con-temporaneous Ordovician marine calcite cement arepresumably due to the composition of the diagenetic water inwhich dissolved HCO3

–is preferentially removed during

methanogenesis by reduction of CO2 (Hoefs, 1987).A number of hypotheses can explain the low δ18OPDB values

in calcite veins relative to marine calcite (Fig. 11): 1) hot fluidof marine affinity, 2) precipitation from a non-marine δ18Odepleted fluid (e.g. the meteoric fluid case), or 3) a fluid withvariable salinity and temperature (e.g. the brine scenario). Noevidence for meteoric water involvement is recognized for thexenomorphic calcite cement. Therefore, if derived from a littlemodified marine fluid, the lighter δ18OPDB value for vein calcite(–8.3‰) relative to theoretically early marine calcite, indicatesa warmer parent fluid with a Tº slightly higher (+10ºC) thanseawater. A cross-cutting relationship with other diagenetic ele-ments (Fig. 10) suggests that xenomorphic calcite precipitatedin mid to deep burial, hardly reconciliable with such a low tem-perature scenario. Higher salinity of the parent fluid as sug-gested by some fluid inclusions (>21% NaCl equivalent) from80ºC xenomorphic calcite cements can explain the smallamount of δ18OPDB depletion compared to that of normal marinecalcite (Fig. 11).

Fluid inclusion results indicate that the precipitation tem-perature of the late fracture-filling poikilotopic calcite inbreccia of the Saint-Flavien No. 3 well was about 260ºC, a value irreconcilable with the δ18OPDB values (around –8‰)if the parental fluid was of marine origin. Similarly, the meteoric water scenario has to be ruled out. Knowing the pre-cipitation temperature of the poikilotopic calcite cement(Table 1: mean value = 257ºC) and its isotopic composition(Table 2: δ18OPDB = –7.8‰), the isotopic composition of theparent fluid can be estimated with O’Neil et al.’s (1969) equa-tion (d18Owater (SMOW) = δ18Ocalcite (PDB) – (2.78 X 106) ÷ (TºK)2 +33.35). The calculated isotopic value (δ18O(SMOW) = 15.6‰)indicates, as expected, that this fluid is highly different fromseawater (δ18O(SMOW) = 0.0‰). The inferred isotopic compo-sition and the lower than marine salinities of the parent fluidfound for that poikilotopic calcite (Table 1), suggest that theparent fluid of the breccia-hosted calcite cement in the Saint-Flavien well No. 3 was of metamorphic origin. Indeed, meta-morphic waters produced by dehydration (Hoefs, 1987;Sheppard, 1986) in deeper strata of the sequence, and whichmigrated to the site of breccia cementation, can explain thedata set discussed here.


Fig. 11. Isotopic composition of Beauharnois Formation dolomiteand calcite cements. M = composition of Lower Ordovician marine calcite (Wadleigh and Veizer, 1992). Arrows indicate isotopic theoreticalevolution trend in carbonates that precipitate from water of increasingsalinity or temperature.

Poikilotopic calcite in veins

calcareous silt in karst

dolomitic silt in karst

fibrous calcite in karst

dolomite karstified

intercrystal xenomorphic calcite

dolomite

ORGANIC MATTER PETROGRAPHY AND REFLECTANCE

OBSERVATIONS

Results of organic matter (OM) petrography in Saint-Flavien No. 3 and No. 9 wells indicate that kerogen composi-tion is related to lithofacies (Fig. 5A, B). Macerals areidentified according to ICCP (International Committee for CoalPetrology) nomenclature (Taylor et al., 1998) or, when theICCP classification is not applicable (for zooclasts), they areclassified following Alpern (1980). Three groups of maceralsare present: amorphinite, zooclasts and migrabitumen.Zooclasts are composed of four types of organisms: chitino-zoans, graptolites, hydroids and scolecodonts. Reflectance val-ues of chitinozoans, graptolites, hydroids and scolecodonts inmudstones and limestones of Saint-Flavien No 3. and 9. wells(Fig. 5A and B) are listed in Table 3. Four types of migrabitu-men are identified: vaguely zoned migrabitumen formingdroplets; nearly isotropic migrabitumen; migrabitumen show-ing a waving anisotropy; and migrabitumen with mosaic, pal-isades, spherulites, ribbons, and flow structures (natural cokeof Bertrand, 1993 and Taylor et al., 1998). Natural coke isformed when a rapid increase in temperature affects OM. Forvitrinite, coke formation occurs at temperatures between 300ºCand 500ºC (Taylor et al., 1998). One of the various forms ofnatural coke is shown in Figure 9C. Migrabitumen is the mostabundant component of kerogen, zooclasts are the next mostabundant macerals, and amorphinite is the least abundant.

Maceral distribution is zoned within and around dolostoneporous interval. Migrabitumen most commonly occurs in inter-crystal and vuggy pores in the dolomitic matrix of dolostones,but it also occurs in residual pores of veins partially filled bycalcite where it is commonly transformed into pyrobitumenwith a mosaic or fluidic structure (Fig. 9C, D). Migrabitumenis also locally dispersed in the siliciclastic mudstone and

limestone lithofacies (Fig. 5A, B). It is not present in poikilo-topic, calcite-filled fractures. In the main reservoir zone, thekerogen is commonly composed of natural coke (Fig. 5A, B).Below and above this zone, natural coke is nearly absent.

Two types of natural coke occurrence are observed. In thefirst type, natural coke fills most of the intercrystal pores andvugs in the veins (Fig. 9C). In the second type, it consists of athin film on the walls of intercrystal pores (Fig. 9E). In Saint-Flavien No. 13 well, this lining of pore walls is commonly bro-ken and displaced into residual porosity filled with saddledolomite (Fig. 9E). In Saint-Flavien No. 12 well, precipitationof poikilotopic calcite has produced the same feature.

Our data indicate that anisotropy of migrabitumen and natu-ral coke relates to reservoir zone proximity (Fig. 12). The rela-tionship between minimum and maximum reflectance andbireflectance allows quantification of the anisotropy of migra-bitumen and natural coke (Kilby, 1988; 1991). Kilby’s methodis applied to some samples containing migrabitumen and natu-ral coke in cores 2 to 5 of Saint-Flavien No. 9 well (Fig. 12).With bireflectance lower than 1% and intermediate reflectanceabout 2%, migrabitumen in the A6 and B2 zones shows low tomedium anisotropy (Fig. 12, 1511.9 m and 1554.52 m, respec-tively). Moreover, for a given bireflectance, maximum andminimum reflectance largely overlap. In the B1 zone, the dif-ference between maximum and minimum reflectance increaseswith proximity of the porous zone (Fig. 12, 1531.5 m).Bireflectance shows values above 4% and intermediatereflectance ranging around 2%. A similar relationship isobserved for all Saint-Flavien wells containing porous dolo-stones. Away from the main reservoir, particularly in lime-stones, the OM is mostly isotropic to slightly anisotropic(Fig. 5A, B). However, near the reservoir, the OM anisotropyincreases in the dolostone. Adjacent to and within the reservoir,the OM is entirely composed of natural coke (Fig. 5A, B).


Table 3. Zooclasts (chitinozoans, graptolites, hydroids and scolecodonts) reflectance in core samples of Saint-Flavien No. 3 and No. 9 wells. Lms = Limestone; Muds = mudstone/shale; Mix. = mixing of lithologies; Mean = mean values; N = number of measurements;

SD = standard deviation; Vitr. equiv. = equivalent vitrinite reflectance derived from zooclast values.

INTERPRETATION

The reflectance of natural coke cannot be used as a thermalmaturation indicator. The relationship between the chemicalcomposition of natural coke and reflectance varies widely(Taylor et al., 1998). Equivalent vitrinite reflectance is esti-mated from that of zooclasts (Bertrand, 1990; Bertrand and

Malo, 2001). The equivalent reflectance of vitrinite variesbetween 2.3 and 2.5% in Saint-Flavien No. 3 well and between1.9 and 2.3% in Saint-Flavien No. 9 well (Table 3).

The maximum paleotemperature recorded by the limestonesand shales on both sides of the reservoir zone in Saint-FlavienNo. 3 and No. 9 wells can be estimated using the Barker andPawlewicz’s (1996) equation: ln (Ro) = 0.0078*Tmax –1.2.


Fig. 12. Differentiation between the maximum and the minimum reflectance (Ro) of solid bitumen, or pyrobitumen (increasinganisotropy), with proximity of the reservoir in the B1 zone of Saint-Flavien No. 9 well (e.g. 1531.5 m). Based on Kilby’s (1988,1991) diagrams. Bireflectance = maximum Ro – minimum Ro.

This equation considers the reflectance as strictly related totemperature and neglects the effect of time. This relationship isvalid here because the estimated temperature is higher than130ºC (Taylor et al., 1998). Maximum paleotemperaturesreached in Saint-Flavien No. 3 and No. 9 wells are between 260and 270ºC, and 240 and 260ºC, respectively. These tempera-tures are calculated from estimated vitrinite reflectance(TºCvitr) derived from zooclast reflectance (Table 3).

Temperature values estimated from reflectance data agreewell with homogenization temperatures obtained from fluidinclusions in poikilotopic calcite cement in the reservoir brec-cia (Th = 260ºC). This similarity suggests that the maximumtemperature recorded in the reservoir breccia is similar to theone in the surrounding rocks. This also indicates that poikilo-topic calcite precipitation in the reservoir breccias is mostlikely coeval with maximum temperature.

The natural coke that fills intercrystal pores and vugs in theveins was formed through coking of in situ bitumen. Thepyrolitic carbon lining the walls of intercrystal pores resultsfrom local condensation of high-temperature hydrocarbons.The remobilization of the pore-coating coke by saddle dolomiteor poikilotopic calcite and its displacement into residual porespace suggests that the migration of hydrocarbons in residualpores and the alteration to pyrobitumen with a coke texture pre-dated the last pulse of saddle dolomite growth (Fig. 10).

A model of organic matter zoning around the reservoir isbuilt from the above constraints dealing with temperaturereached during the thermal maturation and the zoning of pyro-bitumen. Because of a slower rate of temperature increase, theOM is mostly isotropic to slightly anisotropic away from themain reservoir zone. Near the reservoir, the rate of temperatureincrease is faster due to higher rock permeability and easierhydrothermal circulation. This results in an increase in OManisotropy and eventually in the local presence of natural cokein the close vicinity of the reservoir itself.

STRUCTURE

OBSERVATIONS

In addition to stratification, five structural elements wereidentified in cores: stylolites, fractures, breccias, small-scalenormal faults, and sub-horizontal shear zones. The stratifica-tion is defined by thin shaly or silty laminations in homogenousdolostone or limestone, beds of silty sandstone, and abruptlithologic changes (shale–limestone, sandstone–limestone).The bedding is generally horizontal. Stylolites are common inlimestone but rare in dolostone. Most commonly, stylolites arethin, regular and parallel to stratification, and are cross-cut byall other structural elements (Fig. 9A). Less regular and thickerstylolites, showing higher amplitude, are also observed. Somethick stylolites are associated with sub-horizontal shear zones,whereas others are inclined to sub-vertical and locally parallelto brecciated zones. The formation of stylolites parallel to bed-ding is related to burial, and their formation occurred earlierthan other stylolites that are presumably of tectonic origin.

Some of the inclined stylolites in limestones are probably pres-sure solution cleavage associated with folding. Fractures areclassified into three types: 1) joint — a fracture without offsetand filling, 2) veinlet — a thin fracture (<1 mm) filled with cal-cite, and 3) vein — a fracture (>1 mm) filled with calcite andquartz. All fracture types could be partially opened. Joints andveinlets sometimes form conjugate systems, and veins are com-monly arranged en echelon.

The intensity of fracturing is directly correlative with rocktype. Dolostones are more fractured than limestones; and lime-stones are more fractured than argillaceous limestones andshales (Fig. 5A and B). Highly fractured dolomites form brec-ciated rocks (Fig. 13) at the top of the Beauharnois Formation(B1 zone in Figs. 5A, B) and within the Beauharnois C zone. Inboth Saint-Flavien No. 3 and No. 9 wells, the correlationbetween the intensity of fracturing and the thick interval ofdolomicrite with or without cryptomicrobial dololaminite, isclear. Thin dolomite intervals, less than 2 m thick, alternatingwith limestone, are not significantly more fractured than thicklimestone intervals (Fig. 5A, above 1524 m; Fig. 5B, below1535 m). Breccias are commonly associated with conjugatefracture systems, brecciated rock being located in the acuteangle of the fracture intersection.

Small-scale faults showing downthrow of millimetre to anunknown larger displacement (because of the limited diameterof the core) are associated with veins and breccia. When thedisplacement is large, the fault plane is stylolitized. Small-scaleshear zones are common and are preferentially located withinthin shaly beds, often associated with thick stylolites. They arealso expressed as laminated bedding-parallel veins in limestoneor dolostone. In both cases, sub-horizontal shear surfaces arestriated, suggesting horizontal displacement related to thrustingor to interbed slip associated with flexural-slip folding. In ori-ented cores, shear zones indicate northwest-directed thrusting.


Fig. 13. Hand specimen of a dolosiltite (Ds) tectonic breccia in the reservoir zone of Saint-Flavien No. 3 well. Calcite cement (Cc) ispartially dissolved as shown by the blue colouration. Abundant mm-wideand cm-long open pores are of dissolution origin. Variously oriented stylolites (S) are limited to breccia fragments. 1533.1 m, B1 zone ofBeauharnois Formation.

Lithologies are generally similar on both sides of the shearzones, and bedding-parallel veins suggest that displacementsare small.

Fractures are mainly sub-vertical, but a few joints and vein-lets are sub-horizontal (Fig. 14). Horizontal fractures are com-monly bedding-parallel veins. Two major fracture sets arefound in all wells. Fractures in the first set strike north–southand dip steeply east and west, whereas those in the second setstrike ENE–WSW and dip steeply SSE (e.g. well No. 9,Fig. 14). Other less developed fracture sets are also observed;for example, the sub-horizontal fractures (Fig. 14, Set 3). Theyare well developed in some wells (Saint-Flavien No. 10 well)and less abundant in others (Saint-Flavien No 8. well). The attitude of fractures varies in relation to their location in thestructure. However, the north–south striking vertical fracturesare dominant in all wells.

INTERPRETATION

In the Saint-Flavien structure, the north–south striking frac-ture set is nearly perpendicular to the fold axis whereas theENE–WSW striking fracture set is sub-parallel. These majorfracture sets correspond to type 1 and 2 of Stearns (1968), asso-ciated with folding in many fold-thrust belts (Price, 1967;Mitra, 1988; Cooper, 1992). North–south striking fractures rep-resent an extension structure parallel to the regional shorteningaxis. ENE–WSW striking fractures are perpendicular to thisshortening axis but can be related to extension fissures parallelto the fold hinge in the outer fold arc of the anticline (Ramsayand Huber, 1987). The sub-horizontal fractures correspond tobedding-parallel veins associated with flexural-slip folding and

to sub-horizontal small-scale shear zones related to thrustfaults. The fact that those horizontal veins and previouslydescribed vertical veins are mutually intersecting indicates thatthe majority of veins are contemporaneous and most can beassociated with compressive deformation during regionalshortening (Fig. 15, Stage 5B). Some vertical fractures, how-ever, can be associated to some early (Fig. 15, Stage 5A) and/orlate (Fig. 15, Stage 6) normal faults recognized in the Saint-Flavien structure (Malo et al., 2001a; 2001b). Early normalfaults might have occurred in the platform during the develop-ment of the foreland basin before folding and imbrication of theBeekmantown Group, whereas a late NNE-trending sinistralstrike-slip fault with a normal component of movement isobserved on the 3D seismic survey of the Saint-Flavien struc-ture (Malo et al., 2001a).

A MODEL OF RESERVOIR FORMATION

Development of the dolostone reservoir in metre-thick shal-lowing-upward cyclic successions, such as those observed inthe Carillon and Beauharnois formations in the Saint-Flavienstructure, are well known in Upper Cambrian and LowerOrdovician successions of the St. Lawrence Platform andAppalachians (James et al., 1989; Osleger and Read, 1991;Bernstein, 1992; Hardie, 1993; Chi et al., 2000; Chi andLavoie, 2001; Cooper et al., 2001). The observed cyclic litho-facies can be related either to episodic progradation of inter-tidal shoals (autocyclic) or to eustatic sea level fluctuations(allocyclic). The factors controlling such short-term fluctua-tions in sea level are known as Milankovitch climatic rhythms(Fischer, 1964). Osleger and Read (1991) determined that thedominant control on the simultaneous development of inter-tidal and subtidal cycles along the Lower Paleozoic passivemargin in Eastern North America is eustacy. The model pro-posed by these authors for fifth-order cycles can explain thegeneral pattern of lithofacies cyclicity in the BeekmantownGroup. The most significant conclusion for cyclicity recordedby sediments of the Beauharnois Formation is that all signifi-cant matrix porosity developed in the dolostones (reservoirfacies) is found in thick dolostone-dominated fifth-order cyclesthat were possibly deposited during the late stage of a signifi-cant but low rate (third- or fourth-order) sea level rise.

These thick dolostone intervals were also the site of prefer-ential fracturing, the intensity of which was critical for the gen-eration of an efficient porosity and permeability system.Otherwise, the preserved interdolomite minute pore spaces inthe dolostone would not have resulted in a viable plumbing sys-tem for hydrocarbon migration. Our detailed structural analysisshowed that fracture generation events were multiple and ofeither early or late origin. The most significant event for frac-ture development relates to the Taconian Orogeny where hori-zontal and vertical fractures were formed. Locally, in the thickdolostone intervals, the density of fractures resulted in the formation of tectonic breccia. These breccias form the reservoirzones at Saint-Flavien. This highly permeable framework acted


Fig. 14. Orientation of fractures and veins in the B1 zone of Saint-Flavien No. 9 well from analysis of oriented core. Fold axis deducedfrom 3D seismic and S0.


Fig. 15. Temporal evolution of the porosity in Beauharnois Formation successions in the Saint-Flavien reservoirstructure. See text on evolution of reservoir porosity for details on stages.

as the main plumbing system used by diagenetic fluids andhydrocarbons. The succession of various cements (mineral andbitumen) observed in the fractures and related vugs records thethermal history of the host succession.

The precipitation of the late poikilotopic calcite in the reser-voir breccias occurred at high temperature (Th = 260ºC). Basedon temperatures estimated from reflectance values, precipita-tion of late poikilotopic calcite occurred at the maximum tem-perature reached by the reservoir.

This maximum temperature could either be related to burialof the succession or to circulation of high temperature fluids inthe dolostone reservoir. The latter is more likely, based on theδ18O signature and high precipitation temperature (Th =260ºC) of the poikilotopic calcite. Isotopic and fluid inclusionanalyses suggest that the parental fluid of poikilotopic calcitewas δ18O-enriched and slightly saline (<3% NaCl equivalent).Circulation of this high temperature fluid explains the rapidcoking of migrabitumen near the reservoir and the precipitationof a coked film of hydrocarbons in open intercrystal pores.Outside the high temperature fluid migration pathways, theOM matured over a longer period of time and consequentlyshows decreasing anisotropy values. This model explains thezoning of OM types and anisotropy around the reservoir dolo-stones of Saint-Flavien No. 3 and No. 9 wells (Figs. 5A, B).

The K-feldspar mineralogical assemblage indicates a tem-perature of precipitation less than 200ºC but greater than 125ºC(Fig. 8). This temperature range is significantly lower than thatprovided by OM reflectance (240 to 270ºC) and the Th of fluidinclusions in poikilotopic calcite (260ºC). Two explanations arepossible: 1) the K-feldspar assemblage formed either before orafter maximum temperature (and/or burial), or 2) rocks withauthigenic K-feldspars did not reach the high temperaturesrecorded by the main reservoir breccia. This second explana-tion is more likely because the K-feldspar assemblage onlyoccurs near the reservoir zone and is not developed in thereservoir breccia itself. With maturation occurring over a longperiod of time, temperatures between 150 and 200ºC can pro-duce reflectance values between 2.0 and 2.5% (Héroux et al.,1979). The first hypothesis cannot be ruled out, however,because the K-feldspar assemblage and the natural coke arelocally associated.

EVOLUTION OF THE RESERVOIR POROSITY

Figure 15 summarizes the evolution of porosity in theSaint-Flavien structure and the zonation of diagenetic indica-tors around the dolostone breccia. Based on petrographic fea-tures and the slightly δ18OPDB-enriched composition ofreplacement dolomite (about –5.0‰) compared to theoreticalcoeval marine calcite (about –6.0‰), we suggest that the earlydolomitization occurred in the presence of seawater (Fig. 15,Stage 1). Coeval high-frequency relative sea level falls fromeither allocyclic, autocyclic or even from these two combinedmechanisms produced metre thick shoaling-upward cyclescapped by short episodes of subaerial exposures. Limitedmigration of meteoric water lenses resulted in small dissolu-tion karsts (Fig. 15, Stage 2) which were rapidly occluded

(Fig. 15, Stage 3). Horizontal stylolites, with embedded fram-boidal pyrite and sphalerite were the next diagenetic phases tobe recorded (Fig. 15, Stage 4). The formation of stylolites pre-sumably occurred at medium burial depth because they arecross-cut by veins cemented by xenomorphic calcite withδ18OPDB values up to –6.6‰ (normal Early Ordovician marinecalcite cement) and fluid inclusions with low Th values (80ºC).The marine-like δ18OPDB signature of the fracture-filling cal-cite is explained by the high salinity of its parental formationwater (Salinity NaCl > 21%).

In Middle to Late Ordovician time, rocks of the Saint-Flavien structure reached their maximum burial during thedevelopment of the foreland basin in front of the Taconian oro-genic wedge. The maximum thickness of the foreland basin(Sainte-Rosalie and Lorraine groups) was likely located at itssoutheastern margin (e.g. closer to the orogenic wedge) whererocks of the Saint-Flavien structure were located prior to theirthrusting at their present-day position. Concurrently, theTaconian penetrative deformation and metamorphism werealready ongoing in the more southeasterly internal zone(Castonguay et al., 2001a).

Early vertical fractures might have been generated duringthis flexural extension of the lithosphere and the inception of theforeland basin (Fig. 15, Early Stage 5A). The precipitation tem-peratures of the xenomorphic calcite cement in veins are about80ºC. Imbrication of the platform was coeval with folding andwas followed by the development of fold- and fault-related frac-tures and migration of hydrocarbons (Fig. 15, Late Stage 5A).The newly-formed fractures and breccias were filled succes-sively by calcite, bitumen, quartz and feldspars as temperaturesincreased. Porous zones are typified by a large number of openfractures or veins also partially filled by high-temperature sad-dle dolomite, quartz (160ºC) and late poikilotopic calcite(260ºC). Sub-horizontal shear zones and associated horizontalveins intersect some vertical veins and consequently, postdatesome of these vertical veins. Other vertical fractures associatedwith folding were developed at the same time, as indicated bytheir cross-cutting relationship (Fig. 15, Stage 5B). These struc-tures resulted from horizontal stress.

In the deep burial realm, migrabitumens were altered intonatural coke, and this coke precipitated in porous zones andopen fractures in the dolostones (Fig. 15, Stage 5B). This alter-ation likely resulted from circulation of the hot metamorphicfluid that precipitated the late vein- and breccia-filling poik-ilotopic calcite (Fig. 15, Stage 5B). The reflectance data sug-gest that the studied successions reached a maximumtemperature of 270ºC, indicating either very significant burialdepths or major heat transfer from the hot metamorphic fluidstowards the host succession.

Finally, an undated reactivation episode of deformationaffected previously cemented breccia and veins (Fig. 15,Stage 6). Late NNE-trending vertical faults might be responsiblefor this fracturing. Intense dissolution of calcite, dolomite, andpresumably of quartz and K-feldspar, generated secondaryporosity. Thereafter, in the new highly porous breccia, significantprecipitation of illite and smectite occurred at lower temperatures


(<125ºC). Smectite is not preserved in a dry and hot diageneticenvironment, so the reservoir had to be cold enough when itbecame dry (<80ºC). This late precipitation probably occurredafter erosion of Taconian highlands.

ZONING OF INDICATORS AROUND THE RESERVOIR

Figure 16 summarizes the zoning of sedimentologic, diage-netic and tectonic indicators of porosity in the Saint-Flavienstructure and around the dolostone breccia. The relationshipbetween the lithofacies and the porosity generated during dia-genesis is shown in Figure 16A. The most significant porousinterval in the B1 zone correlates directly with the interval having the thickest original dolostone lithofacies. The matrixporosity in these dolostones is mainly intercrystalline moldicbut is significantly enhanced by dissolution in the reservoir zone(Fig. 16A). The reservoir zone is characterized by an intensedegree of fracturing and the presence of breccia, indicating thatfracturing and brecciation were critical factors in improvingreservoir quality. This relationship is illustrated in Figure 16B.

The dolostone hosting the reservoir is also characterized by aK-feldspar assemblage. K-feldspar is not encountered in thereservoir breccia itself, which is typified by the presence ofsmectite (Fig. 16C). Away from the porous zone, quartz domi-nates the silicate assemblage. Due to different rates of maturationduring hydrothermal fluids circulation (which was related to theporosity and the permeability of dolostones), the OM is mostlyisotropic to slightly anisotropic away from the main reservoirzone but typically transformed into natural coke near the reser-voir itself. This relationship is illustrated in the Figure 16D.

CONCLUSIONS

The Saint-Flavien reservoir results from a complex sedi-mentary, diagenetic and tectonic history. The BeekmantownGroup consists of numerous meter-scale fifth-order shallow-ing-upward cycles. Thickness of cycles is not randomly dis-tributed in the successions; variations in cycle thickness areused to recognize lower order cyclicity.

Reservoir intervals are found in rock successions depositedduring a low order (third or fourth) sea level rise, resulting inthe thickest fifth-order shallowing-upward cycles for theBeauharnois Formation. These thick cycles are almost entirelycomposed of intertidal dolostones, and early dolomitizationproduced some intercrystal and vuggy porosity. The fifth-ordercycles developed through local conditions favouring sedimentaccumulation, but the overall accumulation of theBeekmantown facies in the Saint-Flavien structure clearlyresponds to regional low order eustatic sea level fluctuations.

Porosity generally occurs in dolostone beds; limestones arenonporous. Local minor meteoric karstification of intertidaldolostone created vugs that were subsequently filled by vadosecarbonate silts, pyrite, and early marine calcite cements basedon the δ18OPDB and δ13CPDB ratios of marine calcites. Thisepisode of dissolution did not contribute significantly to theoverall porosity of the Saint-Flavien reservoir.

A number of burial dissolution-cementation episodes fol-lowed. An early episode of stylolitization was more pervasivein limestone than dolostone, and occurred at an assumedmedium burial depth. The first generation of fractures postdat-ing stylolites are cemented at medium burial depth byxenomorphic calcite cement (δ18OPDB = –6.6‰) precipitated by


Fig. 16. Spatial distribution of key elements helping to define theproximity of the reservoir in the Beauharnois Formation in the Saint-Flavien structure. A) type of porosity; B) fracture density; C) composi-tion of insoluble residue; D) organic matter facies. See text for details.

relatively low temperature (80ºC) but very saline (SalinityNaCl>21%) formation water.

Deep sedimentary burial was followed by episodes of fold-ing and thrusting during the Taconian Orogeny, leading to thegeneration of a dense network of veins and fractures that con-nected the dispersed zones of minute matrix porosity in dolo-stones. The dominant system of veins was vertical, associatedwith shear zones and vertical stylolites that resulted from thehorizontal stress that produced the folds and thrust-faults.During those episodes, intercrystalline and vuggy porosity,fractures and breccias were successively cemented by repeti-tive episodes of xenomorphic calcite cements, saddle dolomite,migrabitumen, illite, K-feldspar and quartz (Th about 160ºC).The parent water of the last episode of calcite cementation hada likely metamorphic origin. It was hot (Th about 260ºC) andslightly saline (SalinityNaCl <3%). At this stage of the burial history, migrated bitumens were thermally altered or vaporizedas native coke. Zooclasts in shale and limestone adjacent todolostone intervals underwent significant maturation resultingin reflectance values ranging from 2.0 to 2.5%.

A significant late fracturing event restricted to dolostonepartly reopened and led to dissolution of previously cementedveins and breccias. The absolute timing of this last event isunknown. However, instead of being cemented again, the lowtemperature late fluids (T<100ºC) that circulated in these frac-tures dissolved a significant part of the older calcite cementsand precipitated a small amount of smectite.

Aureoles of structural and diagenetic indicators characterizereservoir-hosting intertidal dolostones of the Saint-Flavienstructure. The main tectonic indicators are the fracture densityand the presence of breccias. Reservoir diagenetic indicatorsare: calcite cement dissolution, presence of feldspars and smec-tite, and evidence of abnormal thermal alteration of the organicmatter (natural coke). The diagenetic zones around the reser-voir show strong similarities to those found around mineraldeposits.

The generation of an efficient porosity and permeability sys-tem in the Saint-Flavien reservoir is related to fracture-con-trolled, low temperature, post-Taconian dissolution of hightemperature poikilotopic calcite, in the intertidal dolomiticslightly porous facies at the top of rhythmic cycles that com-pose the upper part of the Beauharnois Formation.

Over the past two decades few geological models based ongas fields discovered in North America, especially in theAppalachian thrust belt, have been applied to the QuebecHumber zone. In spite of relative success in finding reservoirunits, the few wells drilled to date have failed to intersect eco-nomic reservoirs. The geological model established for theSaint-Flavien structure provides a unique example that will beuseful as a case history.

ACKNOWLEDGMENTS

We acknowledge critical review of the manuscript byDr. Yvon Héroux and by Bulletin reviewers, Drs Mike Shieldsand Ian Muir, which resulted in a much improved product; their

comments and suggestions are greatly appreciated. We thankM.M. André Hébert and Jean-Claude Bérubé of InstitutNational de la Recherche Scientifique (INRS) for their highquality slide of organic matter concentrate and petrographicfluid inclusions and their rock thin sections. This work was sup-ported by Intragaz Inc., the INRS-Géoressources and NaturalResources Canada (Geological Survey of Canada). We wish tothank the management of Intragaz Inc. for permission to publishthis work. M. Malo acknowledges NSERC for a continuousgrant. This is a Geological Survey of Canada Contribution No.2002173. This is also a contribution of the GroupeInteruniversitaire de Recherche en Géodynamique et Analyse deBassin (GIRGAB); all authors are part of this group.

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Manuscript received: July 5, 2002

Revised manuscript accepted: January 15, 2003


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