Surface and subsurface signatures of gas seepage in the St. Lawrence Estuary (Canada): Significance to hydrocarbon exploration

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Marine and Petroleum Geology 25 (2008) 271–288 www.elsevier.com/locate/marpetgeo

Surface and subsurface signatures of gas seepage in the St. Lawrence Estuary (Canada): Significance to hydrocarbon exploration Nicolas Pineta,, Mathieu Duchesnea, Denis Lavoiea, Andre´e Bolduca, Bernard Longb a

Geological Survey of Canada, 490, rue de la Couronne, Que., Canada G1K 9A9 Institut National de la Recherche Scientifique, Eau-Terre-Environnement, 490, rue de la Couronne, Que., Canada G1K 9A9

b

Received 1 November 2006; received in revised form 7 June 2007; accepted 12 July 2007

Abstract The seabed of the St. Lawrence Estuary is characterized by many fluid releasing features. On multibeam bathymetric images these features correspond to crater-like depression (pockmarks) predominantly found on the northwestern shoulder of the 300 m deep Laurentian Channel, as well as on the channel floor. Aligned pockmarks, which define segments up to 12 km long, are frequent in the Laurentian Channel, whereas they are preferentially associated with submarine landslides on the northwestern shoulder of the channel. On high-resolution seismic profiles, pockmarks found in the Laurentian Channel are characterized by seismic chimneys that may be traced down to the autochthonous Paleozoic rocks (St. Lawrence platform), suggesting a thermogenic origin for the gas. On the northwest shoulder of the Laurentian Channel, the seismic signature of pockmarks does not extend downward to the reflector that corresponds to bedrock (St. Lawrence platform and/or Grenvillian province) suggesting, together with high sedimentation rate, a biogenic origin for the gas. These results are discussed in the light of the data collected in the onshore parts of the St. Lawrence platform and suggest the presence of a mature hydrocarbon source. The conclusions are supportive arguments for the hydrocarbon prospectivity of both the onshore and offshore segments of the Paleozoic autochthonous domain (St. Lawrence platform). Crown Copyright r 2007 Published by Elsevier Ltd. All rights reserved. Keywords: Pockmark; Multibeam bathymetry; High-resolution seismic; St. Lawrence platform; Hydrocarbon potential

1. Introduction Documentation of hydrocarbon seeps found at the earth surface or on the sea floor is not an ‘ultimate’ tool for petroleum exploration (Whelan et al., 2005) because the study of such surface features cannot characterize the whole hydrocarbon system. Some basic requirements for hydrocarbon accumulations such as the presence of a high porosity–permeability reservoir or of an efficient trap cannot be evaluated by the study of seeps. Moreover, the widespread occurrence of seep features around the world, some of them located in non-productive basins (e.g., Gulf of Maine, Kelley et al., 1994), shows that surface seepages are not indicative of the size of the hydrocarbon system. In poorly studied frontier sedimentary basins, surface seeps may, however, represent an open-window to the Corresponding author. Tel.: +1 418 654 3722; fax: +1 418 654 2615.

E-mail address: [email protected] (N. Pinet).

petroleum system and provide indirect evidence for the presence of mature source rocks within the geological system (Hunt, 1996). Therefore, the presence of seeps documents the first element of a hydrocarbon system and reduces exploration risks. Moreover, the relationship between hydrocarbon seeps and specific sedimentary beds or structural features may help to investigate the seal capacity at a regional scale (Loncke et al., 2004; O’Brien et al., 2005). From southern United States to Newfoundland (Canada), autochthonous Paleozoic sedimentary rocks border the north-American shield, and are in turn bounded on the south by the Appalachian orogen. In eastern Canada, the term ‘St. Lawrence platform’ is classically used to describe the autochthonous domain (Sanford, 1993). Onshore, few gas seeps have been documented in the St. Lawrence platform (Clark, 1964; St-Antoine and He´roux, 1993). They are located within the Quaternary succession and their relationship with Paleozoic rocks

0264-8172/$ - see front matter Crown Copyright r 2007 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2007.07.011

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70°W

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Offshore boundary of the Grenville province Offshore boundary of the Appalachians Normal fault Thrust fault Onland gas seep (PL, Pointe-du-Lac; VF,Vielles-Forges;Y,Yamachiche) Hydrocarbon producer (PP; Port-au-Port; SF; St-Flavien)

Fig. 1. Geological framework of the areas surrounding the St. Lawrence river, estuary and gulf. C, Charlevoix area; MI, Mingan islands, SLR, St. Lawrence River.

remains to be demonstrated. Isotopic analysis (St-Antoine and He´roux, 1993) attest to a biogenic origin for one of these seeps (Yamachiche, Fig. 1), and a mixed biothermogenic origin is suggested for two other seeps (Vielles-Forges and Pointe-du-Lac, Fig. 1). The aim of this paper is to document gas seepage in the marine realm of the St. Lawrence Estuary in eastern Canada and to show that: (1) many seeps are located over the autochthonous rocks of the St. Lawrence platform domain; (2) the seismic signature of the seeps found in the Laurentian Channel may be traced down to the Paleozoic bedrock suggesting a thermogenic origin for the gas; and (3) seeps are irregularly distributed in relation with specific stratigraphic and/or structural features. These results are discussed in the light of the geological data collected in the onshore parts of the St. Lawrence platform and indicate the presence of a mature hydrocarbon source. The conclusions are supportive arguments for the hydrocarbon prospectivity of both the onshore and offshore segments of the St. Lawrence platform. 2. Bedrock geological background and hydrocarbon potential The St. Lawrence Estuary roughly follows the Appalachian deformation front (Fig. 1). Its northwestern shore predominantly consists of Late Proterozoic Grenvillian metamorphic rocks with a few outliers of Cambrian– Ordovician autochthonous carbonate–siliciclastic rocks of

the St. Lawrence platform, whereas Early Paleozoic sedimentary rocks that belong to the Appalachian tectonic wedge form its southeastern shore. In detail, however, the submarine boundaries between the Grenvillian basement, the St. Lawrence platform and the Appalachians have not been precisely documented. In eastern Canada, Early Paleozoic slope and rise sediments that form the southeastern shore of the St. Lawrence Estuary (i.e., Humber zone, Fig. 1) have recorded a complex tectonic history and are involved in a number of structural slices in a thin- to thick-skinned tectonic scenario (St-Julien and Hubert, 1975; Lebel and Hubert, 1995; Lynch, 1998). Most of the deformation of the frontal thrust sheets of the Appalachians has been traditionally attributed to the Middle–Late Ordovician Taconian orogeny (St-Julien and Hubert, 1975; Tremblay and Pinet, 1994; Pinet and Tremblay, 1995). The imprint of younger tectonic events, recorded by both extensional and compressional structures, is, however, increasingly documented on the basis of field (Lynch, 1998; Rocher et al., 2003), seismic (Sanford and Grant, 1990), radiochronologic (Glasmacher et al., 2003; Sasseville, 2006) and maturation (He´roux and Bertrand, 1991) data. Within the St. Lawrence platform, sedimentologic studies document a long-lived extensional history. Major normal faults formed during the Late Proterozoic–Early Cambrian rifting period (Kumarapelli and Saull, 1966; Kumarapelli, 1985, Fig. 1) have been reactivated during the development and evolution of the distal Taconian foreland basin in Middle to Late Ordovician time (Lavoie, 1994)

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and during the Mesozoic opening of the Atlantic Ocean (Carignan et al., 1997; Glasmacher et al., 2002). 2.1. Stratigraphic setting of the St. Lawrence platform In the province of Que´bec, the Paleozoic St. Lawrence platform is divided in the central and eastern segments (Sanford, 1993). Such a division is based on a significant lack of platform outcrops between Que´bec City (central) and the Mingan/Anticosti Islands (eastern) (Fig. 1). The distinction is also expressed in some significant stratigraphic differences found at the base and at the top of the preserved successions (Fig. 2).

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2.1.1. Central St. Lawrence platform From the southwest (Montre´al area) towards the northeast (Charlevoix area), the width of the exposed St. Lawrence platform domain decreases significantly (Fig. 1). Moreover, basal beds that unconformably overlay the Grenvillian basement become progressively younger toward the north from the Late Cambrian (Lavoie, 1994; Salad Hersi et al., 2003) to the Middle (?) Ordovician (Lemieux et al., 2003) (Fig. 2). The rift and passive margin successions, which correspond in the Montre´al area to fluvial and shallow marine clastic deposits (Potsdam Group) capped by dolomitic sandstone, dolostone and limestone (Beekmantown Group), are notably absent north of Que´bec City (Charlevoix area, Fig. 2).

Fig. 2. Stratigraphic framework of the Paleozoic autochthonous domain (St. Lawrence platform) in eastern Canada. The along-strike changes of the sedimentary succession of the St. Lawrence platform are illustrated with three lithostratigraphic columns corresponding to the Montre´al, Charlevoix and Mingan-Anticosti islands areas (location in Fig. 1). Comments in the text.

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In the Charlevoix area, only the foreland basin succession associated with the Middle Ordovician building up of the Taconian orogenic wedge is preserved. This succession (Fig. 2) comprises basal siliciclastics which are overlain by a thick accumulation of shallow- to ultimately deep-marine carbonates deposited on a ramp affected by syn-sedimentary extensional faults (upper Chazy, Black River and Trenton groups and correlative units, Lavoie, 1994, 1995; Salad Hersi and Lavoie, 2001; Lemieux et al., 2003). The carbonate sedimentation was shut down with the progressive encroachment of deep marine shales (Late Ordovician Utica Shales), and overlying Late Ordovician Taconian flysch and final molasse derived from the Appalachian highlands (Sanford, 1993; Lavoie, 1994; Sharma et al., 2003).

exploration efforts indicate significant potential with some oil and gas discoveries (Cooper et al., 2001; Eaton, 2004a–c).

2.1.2. Eastern St. Lawrence platform This segment of the platform is also known as the Anticosti basin which covers the onshore and offshore area between Anticosti/Mingan islands and Newfoundland. In the Anticosti/Mingan islands, the base of the sedimentary succession corresponds to a passive margin, peritidal-dominated limestone and dolostone assemblage (Early Ordovician Romaine Formation, Desrochers and James, 1988; Lavoie et al., 2005, Fig. 2). The craton-wide Sauk-Tippecanoe unconformity has been recognized at the top of the Romaine Formation (Desrochers and James, 1988). This unconformity marks the inception of the Taconian foreland-basin succession in which basal siliciclastics were succeeded by predominantly open marine carbonates (mainly Middle Ordovician Mingan Formation, Desrochers and James, 1988; Lavoie et al., 2005). Sea level rise led to the drowning of the carbonate ramp at the end of the Middle Ordovician. The overlying Late Ordovician to Early Silurian units include: (1) a relatively thin (o 175 m) dark marine mudstone and shale (Macasty Formation); (2) a siltstone-dominated interval overlain by outer ramp shallowing-upward foreland carbonate (Vaure´al Formation); (3) subtidal carbonates with local biotherms (Ellis Bay Formation); (4) various carbonate facies with minor siliciclastics deposited on a stormdominated carbonate ramp (Anticosti Group, Sami and Desrochers, 1992). Seismic data collected offshore (Sanford, 1993; Pinet and Lavoie, 2007) show that a 2.5–3 km thick monoclinal succession overlies the sedimentary units recognized on Mingan and Anticosti islands, in close agreement with the assumed thickness of eroded strata on Anticosti Island based on thermal maturation data and basin modelling (Bertrand, 1987, 1990).

2.2.2. Maturation and generation Within the central St. Lawrence platform domain, the maturation conditions decrease significantly toward the northeast. The Utica Shales are in the dry gas zone in the Montre´al area, whereas the source rock sits in the upper part of the condensate zone in the Que´bec City area (He´roux and Bertrand, 1991). In wells, maturation positively correlates with depth, most likely suggesting a sedimentary burial control of maturation. On Anticosti Island, maturation increases southwesterly and positively correlates with depth (Bertrand, 1987). The Macasty Formation source rock is within the oil window in the northeastern half of the island and in the condensate zone in the southwestern segment of the island.

2.2. The St. Lawrence hydrocarbon system The Laurentian Cambrian–Ordovician autochthonous domain stretches from southern USA (Texas) to Western Newfoundland. Major oil and gas production occurs in most of the US segment (USGS, 2003). In Canada, recent

2.2.1. Source rocks In the St. Lawrence platform, detailed organic matter petrography and Rock Eval analysis indicate that the Late Ordovician Utica Shales (central St. Lawrence platform) and Macasty Formation (eastern St. Lawrence platform) have significant potential for oil and gas generation (Bertrand, 1987, 1990, 1991). Both units contain Type II and a subordinate Type I organic matter. The best total organic carbon (TOC) values range from 1.0 to 3.0 wt% with hydrogen index (HI) values up to 150 for the Utica Shales and from 1.5 to 4.9 wt% with HI up to 360 for the Macasty Formation.

2.2.3. Reservoir facies The first potential reservoir consists of porous units formed through early burial extensional (or transtensional) fault-controlled hydrothermal dolomitization of: (1) intertidal to shallow subtidal facies of the Early Ordovician Romaine Formation (eastern St. Lawrence platform, Chi and Lavoie, 2001; Lavoie et al., 2005) and Beekmantown Group (central St. Lawrence platform, Chi et al., 2000) and of (2) open marine facies of the Middle to Late Ordovician Mingan Formation (eastern St. Lawrence platform, Lavoie unpublished data) and Black River–Trenton groups (central St. Lawrence platform). A second potential reservoir consists of coarse-grained siliciclastic units represented by: (1) Middle Ordovician basal sandstones that overlay the Sauk-Tippecanoe unconformity and (2) the Ordovician flysch units that are characterized by number of gas shows in wells drilled in the central St. Lawrence platform. A last potential target corresponds to the very shallow subtidal to nearshore bioclastic limestone facies of the Early Silurian Chicotte Formation at the top of the preserved succession on Anticosti Island. The Chicotte Formation displays up to 30% of visible pore space (Desrochers, 2004). The reservoir potential of sedimentary units younger than the Chicotte Formation and identified on marine seismic data is unknown.

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2.2.4. Traps and seals In the central St. Lawrence platform, hydrocarbon exploration initially targeted structural traps associated with the Appalachian structural front. This lead to the discovery of the now exhausted St. Flavien gas field (Fig. 1) hosted in a slab of the Beekmantown platform dolomites beneath the Appalachians thrust sheets (Bertrand et al., 2003). Renewed exploration interest both in the central and eastern St. Lawrence platform focussed on diagenetic seals produced by the lateral transition from porous hydrothermal dolostones and breccia to tight carbonates (Lavoie et al., 2005). In this exploration model: (1) the SaukTippecanoe sequences boundary at the top of the Beekmantown Group and Romaine Formation could act as an upper seal, if not breached by faults and (2) the Utica Shale and Macasty Formation could have not only provided hydrocarbons but also likely sealed off the underlying rock units. In the St. Lawrence platform, structural closures associated with open folds such as those documented offshore the Gaspe´ Peninsula (Sanford, 1993; Pinet and Lavoie, 2007) and in the St. Lawrence Estuary (see below) have not been tested.

3. Quaternary geology A detailed review of the Quaternary geology of the St. Lawrence Estuary is beyond the scope of this paper and only a brief summary will be provided. The Laurentian trough is filled by a wedge-shaped Quaternary sedimentary succession that thickens from less than 50 m to the northeast to 4 400 m at the mouth of the Saguenay river. The regional seismo-stratigraphy of this Quaternary sedimentary succession has been described, among others, by Loring and Nota (1973), Syvitski and Praeg (1989), Josenhans and Lehman (1998), Masse´ (2001) and Duchesne et al. (in press). Several stratigraphic units that vary significantly in thickness and lateral extent have been described. These units have been interpreted as till and/or ice contact deposits, glaciomarine sediments, postglacial basinal muds and sand/gravel deposits, that together record, at least, the advance and retreat of the Late Wisconsinan ice sheet. The U-shape of the Laurentian Channel in the Charlevoix area, where it is bounded by major topographic scarps (see below) has been classically interpreted as the result of glacial erosion and over deepening of a pre-Quaternary drainage system (Josenhans and Lehman, 1998). Alternatively, Tremblay et al. (2003) proposed that the reactivation of normal faults controlled the development of the Quaternary basin. Most of the present-day sedimentary influx comes from the northeastern shore of the St. Lawrence Estuary and well developed submarine fans are associated with the Saguenay, Betsiamites, Outardes and Manicouagan rivers

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(Syvitski and Praeg, 1989; Masse´, 2001; Duchesne, 2005, Fig. 3). 4. Data sets The data presented in this paper originate from several recent and older multibeam bathymetry and high-resolution seismic (sparker and airgun) surveys located in the St. Lawrence Estuary between the mouth of the Saguenay River to the southwest and Pointe-des-Monts to the northeast (Figs. 3 and 4). The 2005 multibeam bathymetric data (Fig. 3) were collected with a Simrad EM 1002 (95 kHz) echosounder. The sounding pattern was stabilized for the ship’s roll movements and positioning corrections (including correction for tidal amplitude) were applied using two base stations. These data were used to generate a bathymetric map which has a sub-meter resolution. Older multibeam bathymetric data in the study area have been integrated but vary significantly in quality. A total of 3300 km of high-resolution single-channel sparker data were gathered in 2003 and 2004 along a NW–SE oriented grid with line spacing ranging from 2.5 to 10 km (Fig. 4). Data were collected with an EG&G 9-tip sparker array (2–8 kJ, 200 Hz) providing a vertical resolution of approximately 1.25 m (using an average velocity in water of 1500 m/s) and a maximum penetration under 300 m water depth of around 0.5 s two-way-time (TWT). Airgun single-channel seismic data were collected in the late 1980s by the Geological Survey of Canada (Fig. 4) with a 40 in3 Bolt source (200 Hz). Both the sparker and the airgun sources permitted full resolution of the Quaternary sedimentary column and allowed the imaging of the topmost part of the bedrock on 20–100 ms TWT, but the internal geometry of the bedrock was defined more clearly on the airgun profiles. 5. Multibeam bathymetric signature of the St. Lawrence Estuary The 7–35 km wide Laurentian Channel forms the dominant physiographic feature of the study area (Figs. 3 and 5). The channel, which has an average depth of 320 m, is characterized by a sub-horizontal sea floor that exhibits few morphologic features related to glacial processes such as glacially eroded grooves or iceberg scours. The NW edge of the Laurentian Channel comprises several major NE-trending topographic scarps parallel to its axis, bounded by NNE and ENE second-order transverse scarps (Fig. 5). In the southwestern part of the studied area, scarps are controlled by faults associated with the Late Proterozoic to Paleozoic St. Lawrence rift system (including the St. Lawrence fault described by Tremblay et al., 2003) and reach up to 260 m in height. The slope, height and along-strike continuity of these scarps decrease significantly toward the northeast (Fig. 5).

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Fig. 3. Multibeam bathymetric image of the studied area. The 2005 surveys are delimitated by black polygons. CS, DS, US, central, downstream and upstream segments of the SE edge of Laurentian Channel.

The SE edge of Laurentian Channel can be subdivided in three morphologically contrasting segments (Figs. 3 and 5): (1) an upstream area dominated by a steep NNE topographic scarp approximately 100 m high; (2) a central segment marked by several NNE trending discontinuous bathymetric features that form a relatively steep NW slope and mimic the onshore discontinuous relief associated with sandstone and quartzite units of the Appalachians domain; and (3) a downstream segment characterized by a slightly NW dipping slope with few Paleozoic ridges denuded by landslides. The analysis of multibeam bathymetric images reveals numerous seafloor instability features such as slope failures, submarine canyons and scars as well as debris lobes. On the NW edge of the Laurentian Channel, indicators of slope instabilities are preferentially located in zones of high river discharges and sedimentary influx and where the top

of the Proterozoic and Paleozoic basement (as imaged in seismic) dips toward the channel. 6. Seismic signature of the St. Lawrence Estuary 6.1. Quaternary deposits Within the Laurentian Channel trough, the thickness of the Quaternary succession varies between 0.55 s TWT to the southwest and less than 0.05 s TWT to the northeast (Fig. 6). On the shoulders of the Laurentian Channel, the Quaternary succession is thinner and generally does not exceed 0.1 ms TWT, except near river deltas. Seven major Quaternary seismic units can be distinguished in the surveyed area (Duchesne et al., in press). However, within the Laurentian Channel, most profiles exhibit only three clearly defined seismic units. The basal

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Fig. 4. Location map showing the high-resolution (sparker and airgun) seismic lines.

and top units are acoustically semitransparent and are separated by the central unit which is characterized by relatively high amplitude undulating reflectors (Fig. 7).

6.2. Bedrock In the NE part of the study area, slightly dipping reflectors (maximum dip of 201) are imaged on highresolution seismic profiles and define, within the Paleozoic bedrock, a broad synform structure that includes several second-order upright NE-trending open synclines and anticlines (Fig. 7). On profiles perpendicular to the channel axis, the amount of shortening tends to decrease from SE to NW. Because of the limited depth penetration of seismic data and of the relatively thick Quaternary succession, the structural style of the Paleozoic rocks is less firmly defined in the SW part of the study area.

The style of folding as seen in seismic data contrasts with the structural style described onshore within the Appalachian belt, where NE-verging folds are characterized by steep and locally overturned NW flanks. For this reason, we attribute the belt of open folds to the slightly deformed St. Lawrence platform domain. East of the studied area, Sanford (1993) reached a similar conclusion and pointed out that the age of folding must postdate the Ordovician Taconian orogeny as the youngest rock unit outcropping on Anticosti island (early Silurian Chicotte Formation) is clearly deformed. On seismic profiles, the open fold belt within the St. Lawrence Estuary is characterized by a smooth geometry of the top of the bedrock. This geometry may be correlated with the relatively flat topography of the onshore domain of the St. Lawrence platform and could have been accentuated by the differential glacial erosion of the autochthonous rocks compared with more

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discontinuous bathymetric highs NE-trending scarp

N

transverse scarp

scarp

Quaternary succession thickness (ms)

Fig. 5. Perspective view looking northeast of the multibeam bathymetry in the St. Lawrence Estuary. Illumination from the NW. Vertical exaggeration: 10. Note the decrease toward the north of the slope and height of the topographic escarpments that bound the Laurentian Channel.

SW

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600 Line 04C-21 Line 19

500 400 300 200 100 0 0

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40

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Along-strike distance (km) Fig. 6. Thickness of the Quaternary sedimentary succession along two profiles parallel to the Laurentian Channel (location in Fig. 4). The succession is the thickest in the south-western part of the study area where the Laurentian Channel is bounded by major topographic scarps.

erosion-resistant surrounding domains (Grenvillian basement and Appalachian tectonic wedge). In the NE part of the studied area, the boundary between the open fold belt and the Appalachian tectonic wedge is relatively well defined and is characterized by a marked change in the seismic amplitudes of the reflector that marks the interface between the Quaternary and Paleozoic successions (Figs. 7 and 8) and by the absence of reflectors within the Appalachian domain. South-eastward, this boundary is less well-expressed and its interpreted position on Fig. 9 must be considered as equivocal. On the northwestern shoulder of the Laurentian Channel, the boundary between the autochthonous domain

(St. Lawrence platform) and the Grenvillian basement is loosely defined. On Fig. 9, it has been drawn on the basis of a few seismic reflectors attributed to the St. Lawrence platform domain, on the presence of an interpreted cuesta morphology below the Quaternary succession that has been tentatively attributed to a homoclinal geometry, and on aeromagnetic data. On Fig. 9, the St. Lawrence platform domain varies significantly in width along the strike of the St. Lawrence Estuary from 40 km in the northeast to less than 10 km in the southwest. The autochthonous domain (St. Lawrence platform)/Appalachians contact exhibits a bend (also visible in the onshore structural grain) and lies farther from the south-eastern shore in the south. The location of the contact is in agreement with the fact that the islands located to the south of the study area (such as the ıˆ le-auxLie`vres island, Fig. 9) clearly belong to the Appalachian domain and with the distribution of the Cap-Chat me´lange (interpreted as a frontal unit; Cousineau, 1998) that is exposed only to the north of the bend (Fig. 9). 7. Location and types of fluid-releasing features Pockmarks are crater-like depressions at the sea floor which are commonly associated with the past and/or present release of fluids from the subsurface (Hovland and Judd, 1988). In the study area, pockmarks occur in water depths from 65 to 355 m. They range from less than 100 m to 300 m in diameter and are generally o10 m deep, with steep sides and relatively flat floors. The density of clearly identified pockmarks (number ¼ 612) varies significantly along the strike of the Laurentian Channel.

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TWT (s)

TWT (s)

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FA

c

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2 1

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TB 0.550

1 km Fig. 7. (A) Airgun seismic line showing the structural style of the folded belt. Location on Fig. 9. (B) Line drawing. (C) Enlargement of the southern part of the seismic line. The enlarged area in shown by a dashed box in A. AA, acquisition artefact; FA, fold axis; TB, top of the bedrock; 1, 2, 3, basal, central and top seismic units regionally defined in the Quaternary succession. Note the change of the seismic amplitude of the reflector that marks the top of the Paleozoic rocks at the Platform/Appalachians contact and the seismic blanking of this reflector below the pockmark.

SE

NW

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PLATFORM DOMAIN

2 km 0.500 Fig. 8. Sparker seismic line showing the change of the seismic amplitude of the reflector that marks the top of the Paleozoic rocks at the Platform/ Appalachians contact (black arrow). Location on Fig. 9. QS, Quaternary succession.

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Fig. 9. Location of the boundaries of the Paleozoic autochthonous domain (St. Lawrence platform) and of the pockmarks and seismic chimneys imaged by multibeam and seismic data sets. Numbers correspond to figure’s numbers. IL, ıˆ le-aux-Lie`vres island; SLP, St. Lawrence Platform domain.

This distribution is, in part, related to the poor quality of multibeam bathymetric data collected before 2005 (especially in the zone located between the areas surveyed in 2005, Fig. 3). Nevertheless, it also shows a clear decrease in the abundance of pockmarks toward the southwest (see below). Pockmarks are predominantly located within the Laurentian Channel and on its northwest shoulder (Fig. 9). Taking into account the proposed boundaries of lithotectonic domains, pockmarks appear to be preferentially located above the autochthonous rocks of the St. Lawrence platform (74%) and the Grenvillian basement (26%). Very few pockmarks (o1%), if any (considering the uncertainty of the lithotectonic boundaries), are located on the southeast side of the channel, within the Appalachians. Pockmarks are either associated with high backscatter values, which could be indicative of the presence of

indurated sea floor sediments, or have no distinctive backscatter signature. On the northwest shoulder of the Laurentian Channel, pockmarks are primarily concentrated in an elongated field near the submarine scarp that defines the northwestern boundary of the Laurentian Channel (Fig. 9) and at the head of major submarine landslides (Fig. 10). In the Laurentian Channel, pockmarks occur primarily as linearly distributed features and as subordinate discrete features. Aligned pockmarks follow a NE–SW trend and define segments up to 12 km long, with a maximum distance between individual features of 2 km (Fig. 11). In detail, these alignments comprise some coalescing structures forming pockmarks fields that reach 700 m in diameter (Fig. 12). The occurrence of groups of pockmarks in the same area (including some overlapping pockmarks) could indicate intermittent processes possibly controlled by

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N N

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Fig. 12. Coalescing pockmark within the Laurentian Channel. Location in Fig. 9.

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TB TB

4 km

Fig. 10. (A) Perspective view of the northwestern shoulder of the Laurentian Channel illustrating the location of pockmarks in a masswasting dominated area. The white line corresponds to the south-western part of the profile shown in Fig. 10B. The dotted and dashed lines mark, respectively, the head and toe of submarine landslides. Pockmarks are indicated by white arrows. Vertical exaggeration: 20. (B) Sparker seismic line across the northwestern shoulder of the Laurentian Channel. Location in Fig. 9. ER, enhanced reflector; QS, Quaternary succession; TB, top of the bedrock. Note the local enhancement of reflectors within the Quaternary sedimentary succession.

N

the local interaction between fluid escape, sealing phenomenon processes (by authigenetic mineral precipitation and/or collapse) and changes in the pressure regime of sediments. 8. Seismic expression of fluid escape structures Pockmarks seen on the multibeam bathymetric data set have a typical seismic expression that includes one or several of the following features: seismic chimney, local enhancement of reflectors and seismic blanking. The seismic expression of seep features differs with their location. For this reason, pockmarks on the northwestern shoulder of the Laurentian Channel and in the channel itself are distinguished. 8.1. Northwestern shoulder In the pockmark field located on the northwestern shoulder of the Laurentian Channel, high-resolution seismic profiles commonly exhibit zones where the seismic reflectors within the Quaternary succession are enhanced (Fig. 10B). Seismic chimneys, i.e., vertical zones of disturbance in the seismic data where amplitudes of reflectors are distorted, are common below individual pockmark. These chimneys extend from the seabed to various intervals in the Quaternary pile, but they do not extend downward to the bedrock seismic marker, which is characterized by a continuous high-amplitude reflector.

0

15 km

Fig. 11. Linearly distributed pockmarks within the Laurentian Channel. Location in Fig. 9.

8.2. Laurentian Channel In the Laurentian Channel, the most obvious seismic feature associated with pockmarks is seismic chimneys.

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A

S

N P

0.450

TWT (s)

0.500 A A

QS 0.550

0.600 1 km 0.650

B

SE

NW

TWT (s)

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0.500 1 km

C

SE

NW P

TWT (s)

0.400 SC

QS

0.450

0.500 A 1 km

0.550

TWT (s)

D

0.350

SE

NW P

0.400

QS

SC

Gas?

0.450 1 km Fig. 13. Seismic characteristics of pockmarks imaged in the Laurentian Channel. Location in Fig. 9. Black arrows show seismic reflectors within the Paleozoic rocks; A, artefact; P, pockmark; QS, Quaternary succession; SC, seismic chimney. Note the seismic blanking of the reflector that marks the top of Paleozoic rocks.

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In most cases, the seismic chimneys extend from the sea floor down to the reflector which marks the bedrock interface (Fig. 13). This reflector is commonly interrupted (i.e., seismic blanking) at the base of the seismic chimneys (Fig. 13). Bedding-concordant enhancements (bright spots) of reflectors within the Quaternary succession are also common near seismic chimneys. Such enhancements are imaged on seismic lines that are located more than 100 m away from the closest seafloor pockmark (Fig. 14). This indicates that if the vertical chimneys acted as the main migration corridor for the seeping gas, this gas likely encountered permeable sandy layers during their upward journey and laterally migrated into them. This situation has already been observed elsewhere (Zu¨hlsdorff and SpieX, 2004; Gay et al., 2005). Seismic chimneys associated with lateral seismic enhancement and/or blanking are also imaged in zones devoid of sea floor pockmarks (Fig. 15) and are interpreted as associated with lateral changes in the seismic velocity of sediments due to the presence of gas (seismic pull-down). In other cases, the concave geometry of reflectors within the Quaternary succession may possibly attest to the preservation of buried paleo-pockmarks.

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Because of the limited depth penetration of the sparker seismic data, the relationship between the distribution of pockmarks along linear trends and the structure of the Paleozoic bedrock has not been imaged for all cases. However, in the NE part of the studied area where the Quaternary column is thinner and seismic reflectors in the bedrock are best imaged, linear pockmark fields appear to be located on the flank of open folds (Figs. 7 and 13A). This suggests that unknown specific sedimentary units within the Paleozoic St. Lawrence platform act as pathway for the gas migration. The potential role of brittle faults in facilitating or enhancing gas migration cannot be discarded especially near the scarps that bound the Laurentian Channel (Fig. 13A). 9. Interpretation 9.1. Timing of pockmark formation As noted by recent studies (Ussler et al., 2003; Rollet et al., 2006, among others) seabed imaging and seismic studies do not demonstrate unequivocally the relationship

SW

NE

0.450 0.500

enhanced reflector

Mass-wasted deposit

QS

TWT(s)

0.550 0.600 0.650 0.700

2 km

0.750

Fig. 14. Reflector enhancement on a seismic line located at 120 m of the nearest pockmark (location in Fig. 9). P, Pockmark; QS, Quaternary succession. Black arrows show seismic reflectors within the Paleozoic rocks.

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NE

0.350 0.400 0.450

Seismic pul-down

0.500

QS

TWT(s)

0.550 0.600 0.650 0.700 0.750 0.800 0.850 0.900 0.950

Paleozoic bedrock

4 km

1.000 Fig. 15. Seismic chimney in a zone devoid of pockmarks (location in Fig. 9). QS, Quaternary succession.

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between seabed features and present-day active seepage. Direct observations (such as visual observation or echosounder imaging of gas plumes and high concentration of hydrocarbons in the water column) on the seep activity in the St. Lawrence Estuary are presently lacking. In this instance, two end-member hypotheses may be envisioned. First, pockmark formation may be interpreted as recording a short-lived event such as a major earthquake or a rapid change in physical parameters that prevail in the estuary. Second, pockmarks may be associated with past and present-day seepage. A relationship between gas venting and major earthquakes has been documented in several actively deforming regions (Field and Jennings, 1987; Soter, 1999; Hovland et al., 2002). The St. Lawrence Estuary is located in an intra-continental setting. However, this is one of the most seismically active regions of eastern North America (Mazotti et al., 2005) and large (Magnitude X7) earthquake(s) may have occurred in Holocene time. A comparison between the epicentre and pockmark location maps indicates that the relationship, if any, is tenuous as the south-western part of the study area, which is the closest to the more active deformation zone (the Charlevoix seismic zone, Mazotti et al., 2005), is characterized by the lowest density of pockmarks. In our understanding, earthquakes may have favoured regional-scale landsliding which in turn may have contributed to the breaching of impermeable Quaternary layers, through microfaulting and fracturing, and allowed the migration of gas previously trapped within the section. Alternatively, past changes in the confining temperature and pressure may have resulted in a short-lived period of extensive venting of gas hydrates possibly formed during periods of glaciation. Present-day physiographic conditions (water depth of the St. Lawrence channel: 320 m; temperature at the sea bottom: 4 1C) place the Quaternary succession of the St. Lawrence Estuary close to the gas hydrate stability zone (Sloan, 1990). For a system of zero salinity water and hydrocarbon gas of methane composition, gas hydrates may be stable approximately 150 m below the sea floor for the lower range (o14 1C/km) of geothermal gradient values (10–17 1C/km) considered by Lamontagne and Ranalli (1996) for the upper crust. Pockmarks found in the north-eastern part of the study area may record an almost instantaneous departure from the stability field of gas hydrates. This departure may have been not yet reach in the south-western part of the study area where the Quaternary succession is thicker. In this hypothesis, the lowest density of pockmarks to the southwest may be related to gas hydrates which are commonly considered to act as a buffer for upward migrating gas (Naudts et al., 2006). In our opinion, the short-lived hypothesis discuss above appears improbable for four reasons: (1) estimates of recent sedimentation rates in the study area range from 2 to 7 m/kyrs in the south-western part. Methods used to estimate sedimentation rates include: radiocarbon dating of

samples (core MD99-2220, St-Onge et al., 2003); 210Pb sediment accumulation rate (Smith and Schafer, 1999) and seismo-stratigraphy (thickness of unit 5 of Syvitski and Praeg, 1989, assumed to have been deposited over the last 8 kyrs). With such sedimentation rates, subtle (o5 m) morphological features should be buried in a few thousand years; (2) the observed range of morphological features, from well-defined pockmarks up to 10 m deep to less marked nearly circular features less than 2 m deep, suggests that they are related to the same process of seepage, but with different degrees of preservation, and thus different ages of formation; (3) some pockmarks are associated with high backscatter values suggesting an indurate seafloor (carbonate crust?) not buried by sediments; and (4) In the south-western part of the study area, seismic evidences for gas hydrates such as bottom simulating reflectors (BSRs) are presently lacking. Even if further field studies are necessary, the line of evidences listed above suggests that pockmarks are not related to a short-lived event and record both active and inactive gas venting. 9.2. Relationship between seeps and the Quaternary succession thickness Sedimentary overloading has been proposed as a parameter which controls fluid release (e.g., Loncke et al., 2004). In the St. Lawrence Estuary, the density of pockmarks strongly decreases in the south-western part of the study area where the Quaternary succession is the thickest. However, seismic chimneys and enhanced reflections interpreted as indicative of gas have been documented (Fig. 15). This suggests that part of the Quaternary succession acts as a seal for upward migrating gas. 9.3. Origin of seeping gas The density and location of pockmarks on the seabed are likely controlled by the fluid flux and by the nature of the seabed sediments and underlying bedrock geology. At least two origins for the seeping gas can be hypothesized: (1) thermogenic gas derived from either an underlying primary gas reservoir or source rock in the Paleozoic succession; and (2) biogenic gas that originate from the bacterial degradation of embedded organic matter in Quaternary sediments. Both types of gas may be present in a single setting as documented elsewhere (Schroot et al., 2005). Within the Laurentian Channel, seismically imaged pockmarks exhibit seismic chimneys that root into the Paleozoic bedrock. The chimneys likely result from migration of gas through the sediments. The chimneys can be seen as the result of several, non-exclusive mechanisms including hydraulic fracturing and associated disruption of the continuity of sedimentary beds, free gas rising and gas filling fractures (Riedel et al., 2006). Regardless of the mechanism(s), the vertical continuity of

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the seismic chimney from the sea floor to the bedrock is a characteristic of pockmarks located in the Laurentian Channel, above the Paleozoic autochthonous rocks of the St. Lawrence platform, which strongly suggests a thermogenic origin for gas. This interpretation agrees with the fact that most of the Quaternary succession would not be expected to contain enough organic material (except close to modern river mouths) to generate gas, as it was deposited in an ice proximal setting. Moreover, the occurrence of linearly distributed pockmarks in areas devoid of glacial features also indicates that fluid migration occurs via conduits corresponding to specific Paleozoic stratigraphic units and/or faults. Within the Quaternary succession of the St. Lawrence Channel, the presence of conduits enabling fluids to migrate to the seafloor such as neo-formed faults has not been directly imaged by multibeam and seismic data sets. This suggests that hydraulic fracturing during gas escape may have played an important role in the pockmark formation (Zu¨hlsdorff and SpieX, 2004). The presence of pockmarks on the northwest side of the Laurentian Channel, close to organic matter-rich Quaternary river sedimentary influx, suggests that these specific seeps are related to biogenic gas escape. This interpretation agrees with the observation that gas-related seismic anomalies do not root into bedrock. The occurrence of the pockmarks field close to the scarp that limits the Laurentian Channel appears to be mainly controlled by the topography of the bedrock. In this area, the morphology of the bedrock is characterized by several cuesta-like features forming ponded basins (Fig. 10B) that could have acted as a barrier during the transport of organic matter-rich river loads, enabling their sedimentation in several disconnected small depocentres. In the northeastern part of the study area, a close relationship between submarine landslides and pockmarks has also been documented. In such cases, mass wasting may have favoured the migration of gas through the formation of fresh scars and open fractures and/or by the destruction of the sealing capacity of surficial sediments (Naudts et al., 2006). 10. Discussion/conclusion In this paper, the pockmarks are interpreted as hydrocarbon seeps. Key observations and interpretation presented above indicate that pockmarks in the St. Lawrence Estuary can be divided in two groups: those located on the northwestern shoulder of the Laurentian Channel and those located in the channel itself. Pockmarks located on the northwestern shoulder of the Laurentian Channel are most probably related to the seafloor seeping of biogenic gas. Supporting evidence includes the lack of seismic expression of hydrocarbon escape at the top of the bedrock, a nearby source of Quaternary high influx of organic matter-rich sediments and their association with submarine landslides and

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second-order depocentres controlled by the bedrock morphology. Pockmarks found in the St. Lawrence Channel, are clearly located above the offshore extension of the Paleozoic autochthonous rocks of the St. Lawrence platform, far away from the locations of major deltaic fans of the Quaternary rivers which were mainly located on the Grenvillian basement. The seismic signature of these pockmarks includes vertical seismic chimneys that root into the top of the Paleozoic succession suggesting that the gas emanates either from a still active source rock or from breached reservoirs within the Ordovician–Silurian (?) platform itself. The occurrence of thermogenic gas seeps indicates that a mature and quality source rock exists within the geological system. Onshore data indicate that the Utica Shales (central St. Lawrence platform) and correlative Macasty Formation (eastern St. Lawrence platform) are the most probable source rocks. These source rocks that comprise Types I and II organic matter are in the upper part of the condensate zone in the northeastern part of the central St. Lawrence platform (Que´bec City area) and in the oil window/condensate zone in nearby eastern St. Lawrence platform (Anticosti Island). The quality of these source rocks is unequivocally documented in the fact that the Utica Shales have sourced the St. Flavien gas field near the NE limit of the onshore central St. Lawrence platform domain (Bertrand et al., 2003). Moreover, biomarkers (GC-MS) and carbon isotope ratios of hydrocarbon components (GC-IRMS) comparative studies of Paleozoic source rocks and oils collected in the adjacent Silurian–Devonian Gaspe´ Belt have linked the oils to an Ordovician source (Idiz et al., 1997; Rogers et al., 1998). The linear alignment of pockmarks above the flank of open folds indicates that some stratigraphic units acted as permeable carrier beds and could have potential for reservoir. The stratigraphic assignment of these units remains highly speculative because of the lack of platform outcrop and well data between the Charlevoix area and Anticosti Island and of the along-strike variability of the sedimentary succession. Of significant interest is the major gas production from the Middle–Upper Ordovician hydrothermal dolomites of the Black-River–Trenton groups in New York (Smith, 2006) with the Utica Shales as the source of the gas. Exploration for that type of Middle– Upper Ordovician play in southern Quebec has only been recently initiated. Finally, the open fold belt described in this paper and documented toward the east, offshore the Gaspe´ Peninsula (Sanford, 1993; Pinet and Lavoie, 2007), may form structural traps that remain unstudied at this time. Acknowledgements Gilles Bellefleur reviewed an early version of this paper. Special thanks go to Calvin Campbell who shared its experience of the marine geology of eastern Canada.

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Jacques Labrie (INRS-ETE) and Fred Learning (IKBTechnology) are thanked for their technical support offered during the seismic surveys. Gratitude is expressed to Roger Coˆte´, Louis Poliquin and Sonia Beaulieu from the Canadian Hydrographic Service for their involvement in the multibeam data acquisition. Multibeam data were gathered within the framework of the project ‘‘Geoscientific Mapping of the St. Lawrence Estuary’’ of the Geological Survey of Canada. Authors are in debt to officers and crew members of RV Coriolis II and CGCS Frederick G. Creed. Hydro-Que´bec Pe´trole et Gaz is acknowledged for its financial participation in the seismic surveys. This research paper has been realized within the framework of the Hydrocarbon Potential of the Gulf of St. Lawrence and the Surrounding Area Project of Geological Survey of Canada. The manuscript has been improved through reviews of Lies Loncke and David Piper This is contribution # 20060363 of the Geological Survey of Canada. References Bertrand, R., 1987. Maturation thermique et potentiel pe´trolige`ne des se´ries post-taconiennes du Nord-Est de la Gaspe´sie et de l’Iˆle d’Anticosti. Doctorate Thesis, University of Neuchaˆtel, Switzerland, 647pp. Bertrand, R., 1990. Maturation thermique et histoire de l’enfouissement et de la ge´ne´ration des hydrocarbures du basin de l’archipel de Mingan et de l’ıˆ le d’Anticosti. Canadian Journal of Earth Sciences 27, 731–741. Bertrand, R., 1991. Maturation thermique des roches me`res dans les bassins des basses-terres du Saint-Laurent et dans quelques buttes te´moins au sud-est du Bouclier canadien. International Journal of Coal Geology 19, 359–383. Bertrand, R., Chagnon, A., Duchaine, Y., Lavoie, D., Malo, M., Savard, M., 2003. Sedimentologic, diagenetic and tectonic evolution of the St. Flavien gas reservoir at the structural front of the Que´bec Appalachians. Bulletin of Canadian Petroleum Geology 51, 126–154. Carignan, J., Gariepy, C., Hillaire-Marcel, C., 1997. Hydrothermal fluids during Mesozoic reactivation of the St. Lawrence rift system, Canada: C, O, Sr, and Pb isotopic characterization. Chemical Geology 137, 1–21. Chi, G., Lavoie, D., 2001. A diagenetic study of dolostones of the Lower Ordovician Romaine Formation, Anticosti Island. Geological Survey of Canada, Current Research, 2001-D17, 16pp. Chi, G., Lavoie, D., Salad Hersi, O., 2000. Dolostone units of the Beekmantown Group in the Montre´al area, Que´bec: diagenesis and constraints on timing of hydrocarbon activities. Geological Survey of Canada, Current Research, 2000-D1, 8pp. Clark, T.H., 1964. Re´gion de Yamaska-Aston, comte´s de Nicolet, Yamaska, Berthier, Richelieu et Drummond. Ministe`re des Richesses Naturelles, Que´bec, RG-102, 148pp. Cooper, M., Weissenberger, J., Knight, I., Hostad, D., Gillespie, D., Williams, H., Burden, E., Porter-Chaudhry, J., Ray, D., Clark, E., 2001. Basin evolution in western Newfoundland: new insights from hydrocarbon exploration. American Association of Petroleum Geologists Bulletin 85, 393–418. Cousineau, P., 1998. Large-scale liquefaction and fluidization in the Cap Chat me´lange, Que´bec Appalachians. Canadian Journal of Earth Sciences 35, 1408–1422. Desrochers, A., 2004. Controls on ancient rocky shorelines development: examples from the Lower Silurian Chicotte Formation, Anticosti Island: Gulf of St. Lawrence. Geological Association of Canada— Mineral Association of Canada Joint Meeting Abstract vol., p. 112.

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