Gas Reservoir Potential of the Lower Ordovician Beekmantown … · 2010. 12. 10. · reservoir...

ABSTRACT

The Beekmantown Group in the Quebec Low-lands was deposited as part of an extensive EarlyOrdovician coastal and shallow marine complex onthe eastern margin of the North American craton.The Beekmantown is stratigraphically equivalent tothe Beekmantown, Knox, Arbuckle, and Ellen-burger rocks of the United States, and is subdividedinto two formations: the sandstone-rich TheresaFormation and the overlying dolomite-r ichBeauharnois. Dolomites of the Beekmantown pro-vide an important exploration target in both theautochthon and the overlying thrust sheets of theCanadian and U.S. Appalachians.

The reservoir potential of the autochthonousBeekmantown Group in the Quebec Lowlands canbe determined from seismic data, well logs, cut-tings, and petrographic analyses of depositional anddiagenetic textures. Deposition of the Beek-mantown occurred along the western passive mar-gin of the Iapetus Ocean. By the Late Ordovician,the passive margin had been transformed into aforeland basin. Faulting locally positioned UpperOrdovician Utica source rocks against the Beek-mantown and contributed to forming hydrocarbonreservoirs. The largest Beekmantown reservoirfound to date is the St. Flavien field, with 7.75 bcf oforiginal gas (methane) in place in fractured and pos-sibly karst-inf luenced allochthonous dolomiteswithin a thrust-fault anticline.

The Beekmantown below the thrust sheetsforms a northward-thinning wedge of peritidal and

subtidal deposits. Seven major depositional unitscan be distinguished in cuttings and correlatedwith wireline logs. Most of these units form north-ward-thinning sediment wedges and were deposit-ed on a gently dipping ramp. Quartz sandstonesdominate updip, whereas shallow, subtidal, pelletalto skeletal limestones dominate downdip. Awidespread blanket of shaly dolomite is the upper-most unit of the Beekmantown, but is of poorreservoir quality.

Dolomites in the Beekmantown contain vuggy,moldic, intercrystalline, and fracture porosity. Earlyporosity formed at the top of the major deposition-al units in peritidal dolomites; however, much ofthis porosity was later filled by late-stage calcitecement after hydrocarbon migration. Thus, a key tofinding gas reservoirs in the autochthonousBeekmantown is to define Ordovician paleostruc-tures in which early and continuous entrapment ofhydrocarbons prevented later cementation.

INTRODUCTION

Quebec is commonly considered to be relativelybarren of hydrocarbons, but significant oil seepswere noted in the province at least as early as themid-1800s, mainly at the end of the Gaspé Pen-insula as reported by Logan (1846). A particularlyintriguing aspect of Logan’s report is that manyyears before the importance of anticlines in trap-ping hydrocarbons became widely recognized, henoted that the seeps were most common along thecrest of an anticlinal fold in rocks now known to beSilurian in age. Following the discovery of oil bydrilling in Pennsylvania in 1859, the seeps nearGaspé became a focus for exploration, and twoshallow wells were drilled in 1860 (Hume, 1932).Unfortunately, neither yielded more than a trace ofoil, and the drilling of dozens of additional wells onthe peninsula in the years since has met with simi-larly disappointing results.

During the late 1800s and early 1900s, explo-ration efforts in Quebec moved westward along theSt. Lawrence River past Quebec City. There, farmersencountered significant amounts of gas in shallow

513AAPG Bulletin, V. 79, No. 4 (April 1995), P. 513–530.

©Copyright 1995. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received June 1, 1994; revised manuscript receivedNovember 15, 1994; final acceptance December 6, 1994.

2Talisman Energy Inc., 6611 Longmoor Way S.W., Calgary, Alberta T2P3R2, Canada.

3Consulting Geologist, 701 Harlan, #E-69, Lakewood, Colorado 80214.We thank Les Beddoes, Jeff Chisholm, and Henri Lizotte for useful

discussions and suggestions on early drafts of this manuscript. AAPGreviewers Rick Major, John Hobson, and Kenneth Stanley also providedmany useful comments. Thanks to Bow Valley Energy, Inc., and TalismanEnergy, Inc., and their partners for permission to publish this paper. BruceBailey prepared lithology logs for all study wells. The authors gratefullyacknowledge the financial and technical cooperation of the SociétéQuébécoise d’Initiatives Pétrolières (SOQUIP).

Gas Reservoir Potential of the Lower OrdovicianBeekmantown Group, Quebec Lowlands, Canada1

John C. F. Dykstra2 and Mark W. Longman3

wells drilled for water. Several enterprisinglandowners in the area were able to recover enoughgas for home heating and domestic use. The mostsignificant shallow gas discovery was made in 1955at Pointe du Lac field, located near Trois Rivières.Initially, this was another example of a local farmerseeking gas to heat his home, but his test wellencountered methane-filled unconsolidated glacialsands encased in clay at a depth of about 100 m(320 ft) and blew out. After six months, the wellwas finally controlled, and for the next 35 yr thefarmer used the gas to heat his home and those oftwo of his children.

The blow-out stimulated additional interest inthe shallow gas field, and Pointe du Lac field wasextensively developed between 1962 and 1972,producing about 2.5 bcf before being converted toa gas storage reservoir. Based on thermal matura-tion indices, Bertrand and Dykstra (1993) suggest-ed that the shallow gas was generated in theOrdovician Lorraine and Utica formations andseeped upward into the glacial sands within thepast 80 k.y.

The discovery of gas fields in the Alberta over-thrust belt, in combination with the gas found atPointe du Lac, spurred exploration of the Ordovi-cian section in the Appalachian overthrust belt ofthe Quebec Lowlands. That search resulted in thedrilling of about 40 deep (>1500 m or >5000 ft)wells, and the discovery of St. Flavien field in 1972.This field, which consists of two producing wellsand six dry holes, has now produced about 5.7 bcfof gas out of 7.75 bcf in place in an allochthonous(thrust-faulted) section of Beekmantown in a hang-ing-wall anticline. St. Flavien is currently in the pro-cess of being converted to a gas storage reservoir.Reservoir rocks are dolomites with secondaryporosity in which possible karst-related porosityhas been enhanced by faulting and fracturing (B.Bailey, 1992, personal communication). A numberof exploration companies joined the search for sim-ilar allochthonous Beekmantown reservoirs duringthe 1970s, but the dozens of wells drilled to testthe thrust sheets resulted in no other commercialdiscoveries.

A new approach to exploration for Beekman-town reservoirs in the Quebec Lowlands began in1989, following the discovery of OrdovicianArbuckle dolomite pay in Wilburton field inOklahoma in 1987 (Petzet, 1992). The Arbucklereservoir rocks at Wilburton field are stratigraph-ically equivalent to the Beekmantown Group inQuebec, and were originally estimated to contain asmuch as 600 bcf of gas (Petzet, 1990). The gas istrapped in a structurally high fault block beneathseveral thousand meters of thrust sheets. UsingWilburton field as an analog, it appeared that theautochthonous Beekmantown section in the

Quebec Lowlands, which has been similarly faulted,should also offer promising exploration targets.

This study describes the reservoir potential of theautochthonous Beekmantown Group in the QuebecLowlands. We emphasize understanding the deposi-tional units and controls on porosity that wouldinfluence reservoir development. The gas produc-tion from St. Flavien field, the presence good poros-ity locally within the autochthonous Beekmantownsection, and the common occurrence of bitumenand gas shows all suggest that the Quebec Lowlandscontains deeply buried gas reservoirs.

Specific objectives of the study were to (1) definethe major depositional facies in the Beekmantown,including grain-rich versus mud-rich facies, relativeshaliness, and the degree of early dolomitization; (2)interpret the diagenetic history of the Beekmantownwith particular emphasis on the origin and texturesof the dolomites; and (3) describe the origin and dis-tribution of porosity and pore types.

STUDY METHODS

The wells penetrating the Beekmantown section,and particularly the autochthonous Beekmantownbelow the thrust sheets, in the Quebec Lowlands(Figure 1) provided the basic data for this study.Wireline logs proved to be of limited use in differen-tiating lithologies, so detailed lithology logs wereprepared from the cuttings for each well. A fewcores from the Beekmantown were also availablefor study. Selected intervals were sampled and 125standard petrographic thin sections were prepared.These thin sections were used to identify the litho-logic variations in the Beekmantown, as well as todefine depositional and diagenetic fabrics. Specialemphasis was placed on sampling those intervalsreported to have yielded hydrocarbon shows andthose with visible bitumen stain. The samplesselected for study range in depth from 1356 to 4120m (4449 to 13,517 ft). The petrographic data werethen integrated with the wireline and lithology logsto subdivide the Beekmantown Group into sevendepositional units. Interpreting these units and theirassociated porosity forms the basis of this study.

The depositional and structural interpretationspresented in this paper are supported by the in-house evaluation of over 2000 km (1200 mi) ofrecently reprocessed seismic data and 917 km (570mi) of recently acquired seismic lines.

TECTONIC EVOLUTION OF THE QUEBECLOWLANDS

Understanding the depositional and tectonic his-tory of the Quebec Lowlands area is important to

514 Beekmantown Group Gas Potential

understanding the nature of the Beekmantown andits potential for reservoir development. St. Julienand Hubert (1975), Tremblay (1992), Dykstra(1993), and Shaw (1993) all provided regionalinformation on the rocks and structural history ofthe Quebec Lowlands area. Their work is com-bined with our own to provide the following sum-mary of the tectonic evolution of this interestingand complex region.

Most of what is preserved in the sedimentaryrock record was deposited during the Cambrianand Ordovician (Figure 2). Tectonically, the evolu-tion of the basin can be divided into six majorepisodes: (1) Middle to Late Cambrian, when nor-mal faulting of the passive margin occurred andsandstones of the Potsdam Group were beingdeposited; (2) Early to Middle Ordovician (Ibexianto Mohawkian), when gentle subsidence allowedwidespread deposition of the carbonate-r ich

Beekmantown, Chazy, and Black River rocks; (3)Middle to early Late Ordovician (late Mohawkianto early Cincinnatian), when initial continentalconvergence resulted in the onset of subductionfar to the east as the limestones of the Trentonwere being deposited in the Quebec Lowlands; (4)early Late Ordovician (early Cincinnatian), whencollision of continental plates and the onset of dis-tal thrust faulting provided a f lood of shale (theUtica Shale) across the Quebec Lowlands area; (5)Late Ordovician (late Cincinnatian), when proxi-mal thrust faulting of the lower Paleozoic sectionoccurred as the Lorraine f lysch and Citadelwildf lysch were being deposited; and (6) post-Ordovician, which has been characterized mainlyby relative tectonic stability in the QuebecLowlands region. Each of these events is illustratedin Figures 3 and 4, and described briefly in the fol-lowing paragraphs.

Dykstra and Longman 515

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Normal Faulting of the Passive Margin

During Potsdam deposition in the Middle to LateCambrian, the Iapetus Ocean was widening asLaurentia (paleo-North America) and Baltoscandiawere moving apart (Scotese and McKerrow, 1991;Huff et al., 1992). The area which later became theQuebec Lowlands was then along the westerncoastline of the Iapetus Ocean. New oceanic crustwas forming at a mid-ocean ridge far to the east(present-day coordinates), much as it is forming inthe Atlantic Ocean today (Figure 3A). The conti-nental margin was under tensional stress due to theopening of Iapetus and formed a passive marginwith a series of tilted fault blocks bounded bydown-to-the-basin and antithetic faults. Significantthickness variations in the Potsdam are seen onseismic lines, indicating that at least some faultswere active during Potsdam deposition.

Sandstones of the Potsdam Group were derivedmainly from the Laurentian highlands to the pres-ent-day northwest. Ephemeral sand dunes probablylined the shore, but most of the preserved Potsdamrocks were deposited in a shallow subtidal settingwhere waves and currents reworked the sedi-ments. The net result was deposition of a fairlymature, clean, quartz-rich to arkosic sand. By theend of Potsdam deposition, sand covered virtuallythe entire coastal margin complex, forming a nearlyflat plain that extended many kilometers basinwardfrom the shoreline.

Subsidence of the Ramp

Deposition of the carbonate-rich Beekmantown,Chazy, and Black River groups occurred during atime of relative tectonic stability. These sequencesincrease in thickness very gradually into the IapetusOcean, indicating that deposition occurred on anextremely broad, very gently dipping ramp. Minorfluctuations in thickness are attributed to subtletopographic highs and local sags. Carbonate depo-sition occurred immediately following majormarine transgressions, but siliciclastic sedimentswere transported onto the ramp during times of rel-ative sea level lowstand. The nature of this gentlydipping passive margin is shown in Figure 3B.

During the Early Ordovician when the Beek-mantown was deposited, the North American cra-ton was located along the paleoequator with theQuebec Lowlands area lying between 15 and 20°Slatitude (cf. Lindsay and Koskelin, 1993). This

warm, tropical setting favored the deposition ofcarbonates, many of which accumulated in veryshallow subtidal to peritidal settings. Aspects ofLower Ordovician deposition on what has come tobe known as the “Great American Bank” have beensummarized by Wilson (1993). Distinctive featuresinclude the common occurrence of upward-shoal-ing carbonate units ranging in thickness from 1–2m to tens of meters, extensive dolomitization of theupdip parts of these cyclic deposits, presence ofsandstone marker beds reflecting times of relativelylow sea level, and a major unconformity (the Saukunconformity) that terminated deposition at theend of the Early Ordovician.

Onset of Crustal Convergence

After the deposition of the Black River Group,the ancient continents of Laurentia and Balto-scandia started converging. This movement isrevealed by faulting visible on regional seismic sec-tions that show that the Trenton Limestone overly-ing the Black River varies significantly in thicknessacross some faults. Many of the passive margin nor-mal faults were reactivated during this time.Houseknecht (1986) reported similar patterns offaulting on the southern margin of the NorthAmerican craton at the onset of thrust faulting inthe Ouachita thrust belt during the Pennsylvanian.Figure 3C, based in part on the work of Jacobi(1981) and Stockmal et al. (1987), illustrates ocean-ic crust being subducted beneath Baltoscandia. Amajor marine transgression occurred during thistime, drowning the continental margin as far inlandas Ontario and burying the shallow-water sedi-ments of the Black River beneath deeper waterlimestones of the Trenton.

Crustal Collision and Distal Thrusting

During upper Trenton to lower Utica deposition,we interpret from the presence of a restrictedfacies that Laurentia and Baltoscandia began collid-ing. This collision marked the first phase of theTaconic orogeny, which mainly involved thrustfaulting of deep-water rocks along the edge of whatwas once the continental margin of Laurentia(Figure 4A). Distal flysch sediments from the thrustsheets were deposited, causing the TrentonLimestone to grade eastward and upward into theUtica Shale. Once the basinal thrust sheets had


Figure 2—Stratigraphic column for the Quebec Lowlands. The study area does not include the shaly Levis and Que-bec City formations, but they are shown here for the sake of completeness. Adapted from Dykstra (1993).


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480

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200

355

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moved far enough, a restricted basin formedbetween the thrust front and the shoreline. Therestriction of the basin, combined with the pres-ence of abundant organic material, contributed tothe Utica Shale becoming a rich petroleum sourcerock locally. Gradual loading of thrust sheets uponto the ramp reactivated many of the previouslyformed normal faults. In some places, this faultreactivation juxtaposed Utica source rocks againstBeekmantown reservoir rocks.

Thrusting of Carbonates

As the collision of the crustal plates continued,carbonate rocks of the shallow-water ramp wereincorporated into the thrust sheets (Figure 4B).The Utica Shale was conformably covered by thesilts and fine-grained sands of the Lorraine flysch,which prograded through time onto the craton.The Queenston Group is a coarser equivalent ofthe Lorraine, which was deposited closer to thethrust front. Shales of the Lorraine are not as rich inorganic matter as those of the Utica because rapiddeposition diluted the available organic matter. Asthe thrust front advanced, the thrust faults becameshallower and involved younger rocks. Eventuallythe Lorraine Group itself became imbricated alongthe foreland thrust belt.

Silurian to Present

Virtually no post-Ordovician rock record hasbeen preserved in the Quebec Lowlands, but thearea was once covered by younger strata. Silurianrocks are completely unknown, but Devonianrocks are known from inclusions in the CretaceousMonteregian intrusives, which occur on the islandof Sainte-Hélène in Montreal (Globensky, 1987).These intrusions produced contact metamorphismwhen they passed through the Ordovician carbon-ates that resulted in the release of carbon dioxideinto porous intervals in the Beekmantown. As indi-cated by carbon isotopes analysis (Bertrand andSavard, 1992), it was this type of carbon dioxidethat was tested in one of the deepest and southern-most wells in the Quebec Lowlands (St. Simon 1Awell) at a depth of 4110 m (13,500 ft). Similar con-tact metamorphism of carbonate strata reportedlyaccounts for the majority of the world’s naturallyoccurring carbon dioxide trapped in subsurfacereservoirs (Farmer, 1965).

Figure 4C is a present-day interpretation of theQuebec Lowlands area based on regional seismiclines and work by St. Julien et al. (1983). TheOrdovician rocks of the Quebec Lowlands areunconformably overlain by glacial sediments that

were deposited from approximately 80,000 to lessthan 9500 yr ago (Lamothe, 1989). Fine-grained flu-vio-deltaic sands were deposited in glacial LakeChamplain as the ice sheets retreated to the north-west. Several accumulations of methane, includingthe Pointe du Lac field, have been discovered insand lenses encased in the Champlain Clay.

STRATIGRAPHY

Beekmantown rocks were first described inQuebec and Ontario as a calciliferous sandstone bySir William E. Logan in 1864. Clarke and Schuchert(1899) were the first to assign the name“Beekmantown Group” to the type section in NewYork state. Based on extensive field work by Ells(1896) and Ami (1900), Raymond was the first touse the term “Beekmantown” in Canada in 1913.

The stratigraphic position of the LowerOrdovician (Ibexian–Whiterockian) BeekmantownGroup is shown in Figure 2. The Beekmantown istraditionally subdivided into two formations: alower interval rich in quartz sandstones, called theTheresa Formation, and an upper interval consist-ing of relatively clean dolomites, named theBeauharnois Formation. The Theresa has beeninterpreted as resting conformably on the UpperCambrian Potsdam, but the presence of a peritidaldolomite interval at the base of the Theresa in thedowndip St. Simon 1A well suggests that this con-tact is unconformable in updip areas to the north.The contact between the Theresa and Beauharnoisappears to be conformable. The contact of theBeauharnois with the overlying Chazy Group is awidespread unconformity commonly referred to asthe Sauk unconformity (also known as the St.George unconformity in Newfoundland; see Knightet al., 1991).

Dolomites are the primary exploration target inthe Beekmantown for two reasons. First, they con-tain good porosity locally. Second, they are strati-graphically equivalent to similar dolomites in theArbuckle and Ellenburger groups in the southernUnited States; these groups have yielded largeamounts of hydrocarbons (e.g., see Holtz andKerans, 1992; Bebout et al., 1993). Furthermore,dolomites of the Beekmantown form the reservoirfor the St. Flavien gas field.

The Lower Ordovician section in the QuebecLowlands represents cratonic deposition in periti-dal environments ranging from exposed tidal flatsto shallow-marine environments. Deeper waterwas to the south and east where the ramp carbon-ates grade into dominantly shaly facies (this facieschange occurs outside the immediate study area).Well-rounded and windblown quartz sand grainsderived from the Laurentian highlands, part of the



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Canadian shield, are abundant in the TheresaFormation and occur locally in the Beauharnois andChazy intervals.

Lower Ordovician dolomites in the United Stateshave been studied much more extensively thanthose of the Quebec Lowlands Beekmantown, inpart because excellent outcrops and large hydro-carbon reservoirs are present there. Thus, theseequivalent rocks provide a wealth of informationwith which to interpret the Beekmantown. Amongthe equivalent rocks are the Beekmantown typesection of New York state (Fisher, 1968), theArbuckle Group of Oklahoma, the EllenburgerGroup of Texas, the Knox Group of the Illinoisbasin, and the Prairie du Chien Group of theMichigan basin. Features shared by all of these rockunits include relatively great thickness (up to a fewthousand meters); locally interbedded sandstones,particularly in the lower parts of the section; depo-sition mainly in peritidal environments, whichcommonly contain distinct shallowing-upwarddepositional units up to a few meters thick; exten-sive dolomitization, much of which was penecon-temporaneous with deposition; and porosity that isdominantly secondary related to karst processesand/or fracturing.

Porosity development in these Lower Ordoviciandolomites has been the focus of numerous studies.Amthor and Friedman (1991) described dolomitetextures and porosity development in theEllenburger Group of west Texas, noting that up to12% porosity was present at a depth of 6477 m(21,250 ft) in the Delaware basin. They also dis-cussed the relative importance of karst processesand tectonically induced fracturing in these deeplyburied carbonate reservoirs. The importance ofkarst porosity in Lower Ordovician dolomites has

been emphasized by Loucks and Anderson (1985)and Kerans (1988), among others. Interestingly, nodistinct evidence of karst processes was observedin the Beekmantown samples examined for thisstudy.

A standard tool for defining depositional trendsis the isopach map. Unfortunately, the alloch-thonous Beekmantown sections in the Appalachianthrust sheets are incomplete and have been trans-ported too far to offer much insight into the origi-nal thickness of the Beekmantown, but theautochthonous Beekmantown is another story. Bydefinition, the autochthonous Beekmantown isessentially in situ, and seismic lines show that it isdipping very gently to the east and southeast. Wellspenetrating the autochthonous Beekmantown indi-cate that the section (and each of the two forma-tions comprising the Beekmantown) thickens veryregularly from north to south (Figure 5). This trendsuggests that the autochthonous Beekmantownwas deposited on a very gently southward-dippingramp. Minor irregularities in thickness probablyindicate subtle paleohighs and paleolows on theramp. The fact that the Theresa and Beauharnoisformations vary directly in thickness in the studyarea indicates that these paleodepositional featurespersisted throughout Beekmantown deposition.

From this data on thickness and assuming littleor no post-Beekmantown erosion (an assumptionsupported by the blanketlike nature of the shalydolomite capping the Beauharnois Formation), onecan calculate the angle of dip of the “ramp” onwhich the Beekmantown was deposited. The for-mation thickens consistently in a southward direc-tion (Figure 6) by 200 m (656 ft) over a lateral dis-tance of 140 km (86 mi). Converted to degrees, thismeans that the dip on the Beekmantown “ramp”


Figure 5—Cross-plot of the thick-ness of Theresa and Beauharnoisformations in the BeekmantownGroup relative to distance from aninferred paleoshoreline. Both for-mations steadily increase in thick-ness from north to south acrossthe study area.

was about 0.083°. Although the dip at any giventime during deposition may have been somewhatgreater than this extremely gentle slope, it is clearthat the Beekmantown units were deposited on anearly flat surface.

DEPOSITIONAL UNITS IN THEBEEKMANTOWN

Based on sparse outcrop data in the QuebecLowlands, the Beekmantown Group has traditional-ly been subdivided into the sandstone-rich TheresaFormation and the overlying, dominantly dolomiticBeauharnois Formation (as described by Globensky,1987). Based on subsurface information, includinglithology logs and cuttings, the Beekmantownsection can be subdivided into seven deposition-al units, with three in the Theresa Formation andfour in the Beauharnois Formation. These unitsare herein numbered in ascending stratigraphicsequence. The distribution of these units is sum-marized on the cross section in Figure 6, andeach unit is described brief ly in the followingparagraphs.

Theresa Formation (Unit 1)

Above the sandstones of the Potsdam Group, thefirst significant occurrence of dolomite marks thebase of the Theresa Formation. The lowest unit inthe Beekmantown is dominated by dolomite- andquartz-cemented quartz sandstones across most of

the study area. In general, these sandstones are tootight to offer any reservoir potential. In the south-ern part of the area, however, between the St.Simon 1A and St. Armand 1 wells, unit 1 thickensabruptly and the amount of dolomite present grad-ually increases upward over a few tens of meters toa point where it becomes the dominant lithology.Thick oolitic and peloidal dolomites in this intervalare nonporous in the St. Armand well, but couldoffer some reservoir potential if located withinstructural closure.


An interval of relatively pure dolomite called unit2 occurs in the southern part of the study area.This unit consists of very finely crystalline andunfossiliferous mudstones that were probablydeposited in peritidal to very shallow subtidal envi-ronments. More northerly wells contain little ornone of this dolomite, and sandstone dominatesdue to the more updip (proximal) position. Thedisappearance of the peritidal dolomites updip sug-gests that the Theresa/Potsdam contact is uncon-formable farther updip to the north (Figure 6).

Unit 2 is a very important interval because itforms a promising reservoir objective. Inter-crystalline and vuggy porosity occur in cores takenfrom this sequence in both the St. Armand 1 and St.Simon 1A wells. The porous dolomite in St.Armand 1 was not tested because no hydrocarbonshows were observed, but in the St. Simon 1A wellthe interval was heavily stained with bitumen and


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ST. ARMAND-OUEST1

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ST PT

ST PT

TD = 12,483 ft.

Potsdam

Unit 1

Unit 2

Unit 3

Unit 4

Unit 5

Unit 7

Unit 6

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Black RiverChazy

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Canadian Shield(Grenville)

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SAUK UNCONFORMITY

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Potsdam

Potsdam

Beekmantown Facies Vertical Scale

500 m

ST PT

Subtidal Peritidal

Figure 6—Regional cross section across the Quebec Lowlands area showing the geometry of the various units in theBeekmantown Group. Location of this section is shown in Figure 1.

did yield gas (mainly carbon dioxide, as describedin detail by Dykstra, 1993). This gas-rich interval,from 4107 to 4112 m (13,474 to 13,491 ft), con-tains up to 17% porosity.


The upper part of the Theresa Formation, herecalled unit 3, is composed primarily of sandstoneon the updip part of the ramp. This unit thickensgradually downdip and changes into interbeddedsiltstones and limestones. Cuttings samples suggesta fair degree of homogeneity in the lithologies in agiven well, indicating that conditions on the rampremained fairly constant during deposition. On theupdip part of the ramp where units 1 and 2 areabsent, unit 3 rests unconformably on the Potsdam.Updip erosion of the Potsdam probably providedsome of the sand in this interval. Almost every-where the rocks of unit 3 are quite tight, and eventhe cleanest sandstones are so extensively cement-ed with dolomite and quartz that they offer essen-tially no reservoir potential.

Lower Beauharnois (Unit 4)

A gradual decrease in the rate of clastic depositionat the end of unit 3 deposition, which was probablycoincident with a continued marine transgression,

resulted in the onset of carbonate deposition. Theserelatively clean carbonates mark the base of theBeauharnois Formation and the deposition of unit 4conformably over the Theresa. A regional limestonewith scattered skeletal fragments marks the base ofthe unit and is absent only in wells that were drilledon paleohighs (e.g., Ste. Francoise Romaine 1 well).

Unit 4 consists of subtidal limestones that gradeup into peritidal dolomites (Figure 7). In wells thatwere drilled into the more distal portion of theramp (e.g., St. Simon 1A and St. Armand 1), thisunit consists mainly of limestone and shaly lime-stone with little or no reservoir potential. In updipwells, however, unit 4 is a promising reservoirobjective. Not only does it contain the thickestdolomite interval in the Beauharnois, but it also haslocal porosity, and has yielded gas shows and bitu-men stain in the autochthon. This unit may alsoform the reservoir in the allochthonous thrustsheet that produces at St. Flavien field.

Unit 5

Unit 5 is a relatively thin wedge of rock that ispresent only in the deeper autochthonous wells.Downdip wells, such as St. Armand 1 and St. Simon1A, exhibit a dominant limestone and shaly lime-stone lithology, and the unit appears to lie con-formably on unit 4. Farther updip, the unit is domi-nated by finely crystalline (peritidal?) dolomite.


��

South

Below Fair Weather Wave Base

Deep RampShallow Basin

Shallow Subtidal toSupratidal Flats

Shallow Ramp

Increasing Faunal Content & Decreasing Clastics

Passive Margin-Basin Sag

St. Armand 1 St. Simon 1A

St. Ours 1St. Wenceslas 1

Gentilly 1

Du Chene 1Ste. Francoise Romaine 1

Ste. Croix 1Les Saules 1

Precambrian Grenville Basement

Wave AgitatedClean Dolomites Dolomites & Shaly Dolomites Dolomites & Sands

Low Energy Peritidal

North

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UNIT 3

UNIT 4

UNITS 1 & 2

POTSDAM

Figure 7—Schematic depositional profile of the carbonate ramp during unit 4 deposition.

The interval grades upward from limestone todolomite and represents a shallowing-upwarddepositional unit. More open-marine areasdowndip were subject to less dolomitization andless erosion. The reservoir potential of unit 5 is lim-ited by the fact that most of the updip shallow-water dolomites have been eroded. There may beareas where porous peritidal dolomites in this unitwere deposited and escaped erosion, but they havenot yet been found.

Unit 6

Unit 6 is very similar in lithology to the middlepart of the Beauharnois Formation. This unit repre-sents another shallowing-upward depositionalsequence and unconformably overlies unit 5 (far-ther updip where unit 5 is absent it overlies unit 4).Like the other units, the thickness of unit 6 increas-es in a basinward (southeasterly) direction.Lithologically, unit 6 is very similar to unit 4 becauseit was subjected to the same diagenetic processes.Limestones are more common in the lower part ofthe unit, particularly in the downdip wells, andgrade updip into dolomites. Even midway up theramp in the St. Ours 1 well, samples reveal that thedominant lithology is peritidal dolomite that oncecontained well-developed intercrystalline porosity.In that well, most of this porosity is now filled withlate-stage poikilotopic calcite, but these peritidaldolomites could form a good reservoir facies whereearly entrapment of hydrocarbons prevented thelate-stage cementation by calcite.

Unit 7

The uppermost part of the Beauharnois Forma-tion consists of a shaly dolomite mudstone interval

that is remarkably uniform in thickness andlithology across the study area (Figure 6). Thisinterval is absent only in the updip-most wellswhere erosion has removed the top of theBeekmantown section. This unit was depositedwhen a major marine transgression f looded theramp and just before the major regression thatcoincided with formation of the widespreadSauk unconformity. The blanketlike nature ofunit 7 is evidence that there was not a largeamount of differential erosion on the ramp fol-lowing Beekmantown deposition. We postulatethat unit 7 was relatively impermeable duringpost-Beekmantown exposure. This, in combina-tion with a moderately arid climate, limited largeamounts of meteoric water from penetrating theunderlying dolomitic units. Thus, the presenceof this shaly dolomite partly explains why evi-dence of karst processes is so rare in the underly-ing Beekmantown section.

PORE TYPES IN THE BEEKMANTOWN

Five types of porosity occur in the Beekman-town Group in the Quebec Lowlands. Listedapproximately in order of decreasing abundance,these are (1) vuggy (and minor moldic); (2) inter-crystalline; (3) fracture; (4) interparticle in carbon-ate grainstones; and (5) interparticle betweenquartz grains in sandstone. Except for the interpar-ticle pores in sandstones, examples of these poretypes are shown in Figure 8. Similar former poreswere commonly observed in other Beekmantownsamples to be filled with late cements, such as cal-cite, quartz, and dolomite.

Vuggy pores in the Beekmantown are irregular inshape and distribution. They formed by dissolution


Figure 8—Examples of pore types in the Beekmantown Group. (A) This thin-section photomicrograph from unit 2in the St. Simon 1A well shows a finely crystalline dolomite stained with pyrobitumen. Scattered vugs are impreg-nated with blue epoxy. Some vugs probably represent former fossil fragments and are now enlarged moldic pores.Other vugs are very irregular in shape and probably formed during dolomitization and dissolution of the matrix.Total porosity measured from core at this depth is about 17%. (B) The small vug visible just above the photo labelprobably formed by dissolution of a fossil fragment, such as an echinoderm columnal. Open intercrystalline porescontain a residue of black pyrobitumen. Sample from unit 2 in the lower part of the Theresa Formation. (C) Thiscutting from unit 6 shows a small vug (at upper right) that has been partly filled with quartz cement (which appearslighter beige than surrounding ferroan dolomite crystals). The crystals lining the vug are clearly zoned with iron-rich rims (stained blue). The fact that bitumen lines the vug beneath the quartz crystal indicates that the quartzpostdated hydrocarbon migration. (D) This cutting from unit 6 contains mainly intercrystalline porosity betweenthe dolomite rhombs. Some quartz sand grains (light beige to the far right and far left) with well-developed quartzovergrowths are present in the dolomite. The residue of black bitumen on the dolomite rhombs indicates that thispore network once contained hydrocarbons. (E) This photomicrograph shows partly cemented fractures. The hostrock is a sandy dolomite, and the fractures contain both quartz and calcite cements. (F) This dolomitized ooid grain-stone is from the Beekmantown in one of the thrust sheets associated with St. Flavien field. Interparticle pores arepresent locally, but are partly filled with dolomite cement. Such interparticle porosity was not observed in theautochthonous Beekmantown samples examined for this study, but its presence in the thrust sheets indicates itcould be important locally in forming reservoirs.

and some are simply enlarged moldic pores.Examples are shown in Figure 8A–C. Total vuggyporosity in the samples studied nowhere exceeded5%. Montañez and Stefani (1993) reported that simi-lar vuggy pores are locally present in the LowerOrdovician Knox Group of the U.S. Appalachians in

peritidal (cyclic) deposits, but that total vuggy poros-ity there typically averages only 3%.

Intercrystalline porosity occurs between crystalsof relatively similar size (e.g., Figure 8D) and iscommon in some peritidal dolomites. Some inter-crystalline porosity can also be seen in Figure 8A


and B near the vuggy pores. Much of the intercrys-talline porosity observed in the Beekmantown sam-ples is lined with a black hydrocarbon residue(bitumen or pyrobitumen). Other former intercrys-talline pores are filled with calcite cement. Thistype of porosity was originally quite common inthe peritidal dolomites at the top of the deposition-al units, but is now present only locally, particularlywhere bitumen staining is common (e.g., in unit 2of the St. Simon 1A well).

Fracture porosity is difficult to recognize in cut-tings samples because the cuttings break along thesurfaces of open fractures. However, as shown inFigure 8E, examples of fractures were observed insome core samples. Open fractures do not appearto be particularly important in the autochthon, butare probably important in the Beekmantown inthrust sheets. Bertrand and Savard (1992, personalcommunication) suggested that fracture porosityplays an important role in the reservoir at St.Flavien field. Most of the fractures observed in thisstudy were cemented with some combination ofcalcite, quartz, and dolomite cements.

Interparticle porosity in carbonate grainstoneswas not observed in samples from the autochthon,but does occur in dolomitized ooid grainstones inthrust sheets near St. Flavien field (Figure 8F).Considerable dolomite cementation between theooids filled much of the former interparticle porosi-ty, but as much as 12% interparticle porosityremains in a few beds. Such a network of interparti-cle pores can provide excellent reservoir quality, soits presence in the Beekmantown could be impor-tant. However, no porous grainstone samples wereobserved in the autochthon.

The least common type of porosity observed inthe Beekmantown is that occurring between quartzgrains in the sandstones. This was observed in onlya few samples of the Theresa Formation. In the fewsandstones with interparticle pores, the porositywas preserved by the presence of authigenic clays(mainly illite) which inhibited cementation byquartz and dolomite. These same clays serve toseverely limit the reservoir potential of this type ofporosity. The highest interparticle porosityobserved in Beekmantown sandstones is only about3%, and the pores are too isolated to be productive.Several factors contribute to the paucity of interpar-ticle porosity in the Beekmantown sandstones. Mostimportant among these are the early cementation ofthe sands with dolomite, and the extensive forma-tion of quartz overgrowths on sand grains in thosesandstones with little or no dolomite.

In summary, the best potential reservoir rockslikely to occur in the Beekmantown are dolomiteswith a combination of vuggy, intercrystalline, andfracture porosity. These porous dolomites are mostlikely to occur near the tops of the shallowing-upward

depositional units (units 2, 4, and 6) where peritidaldolomites accumulated. A factor that decreases thereservoir potential of the peritidal dolomites is thepresence of siliciclastic sediments (clays and sand)which seem to have aided porosity destruction. It isalso worth noting that early entrapment of hydrocar-bons probably helped preserve porosity locally inthese peritidal dolomites. This is indicated by thecommon occurrence of hydrocarbon residues inintercrystalline pores, and by the fact that off-struc-ture, formerly porous dolomites now tend to be tight-ly cemented, mainly with late-stage calcite cement.

DIAGENETIC SEQUENCE

Because of their age (more than 450 m.y.), previ-ous deep burial, and complex tectonic setting, therocks of the Beekmantown Group have undergoneextensive diagenesis. Important diagenetic eventsinclude dolomitization, neomorphism (of bothdolomite and limestone), precipitation of quartzovergrowths and ferroan dolomite, and precipita-tion of late-stage fracture-filling quartz and calcite.The relative timing of these events can be deter-mined from crosscutting relationships and othercharacteristics. This allows development of a para-genetic sequence (Figure 9) that provides insightinto the evolution of the pore system in the reser-voir rocks. The major diagenetic events affectingthe reservoir rocks are discussed briefly.

The earliest diagenesis in the Beekmantownoccurred penecontemporaneously with deposi-tion. Carbonate muds (micrites) deposited in periti-dal settings were probably dolomitized shortly afterdeposition. This early dolomitization produced theaphanocrystalline dolomites seen in the upperparts of the Beekmantown depositional units. Atthe same time, other processes, including down-ward seepage of hypersaline brines and/or mixingof fresh and marine waters, contributed to dolomi-tization of underlying calcareous sediments. In theupdip parts of the Beekmantown units, almost allof the carbonate sediments were dolomitized.Farther downdip, however, the peritidal environ-ment was more ephemeral (if it formed at all), anddolomitization was much less extensive. Thus, thedowndip parts of the Beekmantown units are com-monly less than 50% dolomite. A useful summarypaper on the nature and origin of dolomite inLower Ordovician peritidal cyclic deposits (in theKnox Group of the U.S. Appalachians) was pub-lished by Montañez and Read (1992). Many of theirfindings can probably be applied directly to dolomi-tization in the Beekmantown units.

Much of the dolomitization of the peritidal sedi-ments produced relatively low-porosity dolomitemudstones, but formation of euhedral crystals in


certain beds produced highly porous dolomites inother intervals. Thus, some of the porosity thatformed in the Beekmantown during early peritidaldolomitization has survived more or less intact duringthe hundreds of millions of years since the Early Ordo-vician. Similarly, most of those dolomites that weretightly cemented early remain tight dolomites today.Only fracturing has significantly improved reservoirquality in the low-porosity dolomites (and significantoccurrences of fractures are very localized).

During and after this early penecontemporane-ous dolomitization event, some skeletal fragmentsremained calcareous, in part because their largergrain size inhibited dolomitization. Some of theseskeletal fragments were subsequently dissolved toform moldic pores. This dissolution most likelyhappened while the sediments were still at fairlyshallow burial depths.

An interesting anomaly in the Beekmantown ofthe Quebec Lowlands is that there is almost noevidence of subaerial exposure and karst processes.

This absence is surprising considering that mostof the depositional units are capped by peritidaldeposits, and the fact that the Beauharnois is sepa-rated from the overlying Chazy and Black Rivergroups by a major subaerial unconformity. De-spite what must have been extensive subaerialexposure, karst features, such as solution cavernsor cave calcite, were not observed in the Beek-mantown [although B. Bailey (1992, personalcommunication) reported that some of the porosi-ty in the St. Flavien gas field may be karst related].Soil zones and pedogenic calcretes are alsoabsent, although this is partly due to the absenceof land plants during the Early Ordovician. Thepresence of regional truncation of some units(and the formation as a whole) updip to the north(Figure 7) proves that there was such subaerialexposure, but perhaps exposure occurred underrelatively arid conditions where limited amountsof fresh water limited the development of karstfeatures.


Figure 9—Paragenetic sequence for the autochthonous Beekmantown Group in the Quebec Lowlands. Fracturing,which was also an important porosity-enhancing process in the Beekmantown, is not shown, but would haveoccurred at a number of times across the time spectrum indicated.

After the Beekmantown was buried beneathyounger Ordovician sediments, a variety of neomor-phic events occurred in the limestones anddolomites. Small micron-size crystals commonlyrecrystallized to somewhat larger crystals, some pre-cipitation of calcite and dolomite occurred, and thesediments became completely lithified. Much of theporosity associated with deposition was probablydestroyed through compaction during this burial.

Still later, possibly after several thousand feet ofburial, quartz overgrowths began forming in thesandstones. The source of the silica for the forma-tion of these ubiquitous silica cements is unknown,but may be related to compaction in the sandstonesthemselves or to silica carried upward from moredeeply buried sandstones in the underlyingPotsdam Group. In any case, the net result is thatalmost all sandstones in the Beekmantown becametightly cemented with the quartz overgrowths,destroying reservoir quality prior to the generationof hydrocarbons.

Another event of some significance during thistime of burial was the formation of stylolites in thelimestones (and, less commonly, in somedolomites). High-amplitude stylolites indicate thatlarge amounts of limestone were dissolved in thelower parts of the major Beekmantown units. Someof the vast quantities of calcium carbonate releasedduring this dissolution was transported in the for-mation waters up and out of the formation, butsome of the remaining calcium carbonate probablyprecipitated as calcite or dolomite to plug much ofthe porosity remaining in the formation. The threemost common cements formed during this burialdiagenesis (but prior to hydrocarbon generationand entrapment) were equant calcite, ferroandolomite, and saddle dolomite.

Toward the end of the Ordovician, the Taconicorogeny resulted in burial of the Beekmantown andassociated formations to depths sufficient for theonset of hydrocarbon generation. Some of thehydrocarbons probably migrated along the sty-lolitic pathways and fractures in the Beekmantownto fill whatever pores remained open in the paleo-structures existing at the time. Small amounts of secondary porosity may have been created in CO2-enriched waters migrating just ahead of (orwith) the hydrocarbons, but this late-stage porosityis relatively unimportant. Once trapped, furthercooking of the hydrocarbons produced bitumenresidues that coated the walls of many of the poreand fracture surfaces in the rocks where the oil wastrapped. Gas was also generated during this ther-mal breakdown of the hydrocarbons. The bestpotential for Beekmantown reservoirs occurs inporous peritidal dolomites capping the deposition-al units on paleostructures where the gas wastrapped as soon as it formed.

Much Beekmantown porosity in the form of frac-tures, vugs, and intercrystalline pores containing aresidue of bitumen somehow survived deep burial,but was later cemented. Such late-stage cementscan be clearly identified because they postdate thebitumen residue (e.g., Figure 8C). In such cases,three stages of late cementation can be identified.The first stage consisted of precipitation of a fewdolomite crystals, the second was precipitation ofquartz in the form of euhedral crystals, and thethird was the precipitation of coarse equant calcitecement. It is these late-stage (post-hydrocarbon)cements that have now so completely destroyedthe reservoir-quality porosity of most of the former-ly porous dolomites in the Beekmantown. Onlywhere “permanent” gas entrapment predates theselate-stage cements is reservoir quality preserved inthe Beekmantown.

This paragenetic sequence makes it clear why itis so important that exploration for Beekmantownreservoirs focuses on finding Ordovician paleo-structures capable of trapping hydrocarbons. Onlyin these settings was the extensive late-stagecementation inhibited sufficiently for commercialamounts of hydrocarbons to be produced. Thehighly porous (and bitumen-r ich) peritidaldolomites at a depth of more than 4000 m (13,000ft) in unit 2 of the lower Beekmantown in the St.Simon 1A well clearly show that burial depth alonewas not a cause of porosity destruction. Instead, upto 17% porosity was preserved at depths that wereonce as great as perhaps 9000 m (30,000 ft) (basedon burial history reconstructions by Bertrand andDykstra, 1993), because the trapped hydrocarbonsand associated CO2 gas prevented precipitation oflate-stage cements.

CONCLUSIONS

(1) Based on sparse outcrop data, the Beek-mantown Group has traditionally been subdividedinto the sandstone-rich Theresa Formation and theoverlying dolomite-rich Beauharnois Formation.Based on subsurface information, including theanalysis of cuttings and wireline logs, theBeekmantown in the Quebec Lowlands can be sub-divided into seven depositional units, hereinlabeled 1 to 7 in ascending stratigraphic order.

(2) Units 2 and 4 (and to a lesser extent unit 6)contain the best potential reservoir rocks in theBeekmantown Group. In the autochthonousBeekmantown section, the most favorable rocksare found in areas along the depositional rampwhere peritidal dolomites with early porosityescaped later porosity-destructive processes suchas late-stage calcite cementation due to the earlyentrapment of hydrocarbons.


(3) Also offering some reservoir potential in theallochthonous Beekmantown section are fracturedand brecciated dolomites, such as those seen in thethrust-faulted anticlinal trap at the St. Flavien field.

(4) During development of the Sauk unconformi-ty, the slightly shaly unit at the top of theBeauharnois Formation (unit 7), in combinationwith a moderate to arid climate, limited largeamounts of meteoric water from penetrating intothe underlying dolomitic units. This partly explainswhy karst processes did not play a major role inporosity development in the Beekmantown.

(5) Although limestones within the Beek-mantown are present in almost all study wells, par-ticularly downdip on the depositional ramp, nonewere observed to have significant porosity.Stylolites are common in the limestones and appar-ently released large volumes of calcium carbonate,some of which reprecipitated as cements in porousdolomites after significant burial.

(6) As a general rule, most of the dolomites inthe Beekmantown are nonporous, but much of thelack of porosity is due to late-stage cementation bypoikilotopic calcite. This calcite clearly postdateshydrocarbon generation and migration because itcommonly fills vugs and intercrystalline pores linedwith bitumen.

(7) The common occurrence of bitumen andpyrobitumen in the study wells suggests that oilwas once widespread in the Beekmantowndolomites. Ordovician paleostructures, where earlyhydrocarbon entrapment limited later burialcementation, offer the best reservoir potential.These prospects can be defined on time structuremaps generated from seismic data (e.g., on thehorizon at the base Theresa).

(8) The findings from this study should be appli-cable beyond the area of the Quebec Lowlands toother deep Beekmantown prospects in theautochthon elsewhere in the Canadian and U.S.Appalachians.

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John C. F. Dykstra

John Dykstra graduated with aB.Sc. degree in geology from theUniversity of Calgary in 1982. Sincethat time, he has worked as anexploration geologist in frontierCanada and international areas formost of his career. His interestsinclude geochemistry, basin model-ing, and tectonic reconstruction.John is an active member of AAPGand a professional member of theAssociation of Professional Geologists, Geophysicistsand Engineers of Alberta (APEGGA).

Mark W. Longman

Mark W. Longman received hisB.A. degree from Albion College(Michigan) in 1972 and his Ph.D.from the University of Texas atAustin in 1976. He specializes inthe study of carbonate rocks andreservoirs and enjoys working withcores, thin sections, wireline logs,and seismic data to define andinterpret the nature and produc-tion performance of such reser-voirs. For the past eighteen years, first while workingwith Cities Service Company, Coastal Oil and Gas, andButtercup Energy, and later as a consultant, he has stud-ied a variety of carbonate reservoirs ranging in age fromOrdovician to Miocene in areas ranging from theWilliston basin to Texas, as well as southeast Asia,Mexico, the Middle East, and the Mediterranean region.

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