Abstract

Starting in 2011, a multiyear modern 2D long offset broadband seismic survey was acquired offshore Labrador, Canada, by TGS and PGS in partnership with Nalcor Energy. This regional survey covers the slope and deepwater portions of the margin. Three Mesozoic and Cenozoic-aged basins were informally defined from these data, the Chidley, Henley, and Holton; also, the poorly constrained Hawke Basin was remapped. The 2D data set imaged for the first time a very large Cenozoic-aged delta adjacent to the mouth of Lake Melville. We have mapped this delta on a 5×5  km 2D seismic grid. The delta is 5–8 km thick, and its aerial extent is 17,500  km2. The age of this delta has been interpreted to be Eocene to Miocene. Adjacent to this Cenozoic delta on the Labrador shelf, there is a working petroleum system within the proximal Hopedale and Saglek Basins where there are five gas discoveries and one oil discovery. The modern long-offset 2D data set appears to indicate a working petroleum system within the newly mapped Cenozoic delta, and two phases of hydrocarbons may be present.

Introduction

From the early 1970s to the present day, offshore Labrador has been an area of interest for hydrocarbon exploration. To date, only the shelfal region has been explored with the first exploration well, Leif E-38, drilled in 1971, followed by Leif M-48 in 1975 and then the Bjarni H-81 gas discovery well also drilled in 1975. Through a 12-year time period (1971–1983) 29 wells in total were drilled on the shelf offshore Labrador. Of these, 5 wells were gas discoveries with an estimated volume of 4.2 trillion cubic feet, Tcf (C-NLOPB, 2000), and one oil discovery also made at North Lief I-05, which tested and recovered 22 barrels of 32° API high-wax oil to surface (C-NLOPB, 2000).

After 1983, a long hiatus in hydrocarbon exploration began in offshore Labrador with limited seismic acquisition in the area and no wells were drilled. There were two known sedimentary basins explored on the Labrador shelf, the Hopedale and the Saglek. A third, more distal, basin, the Hawke Basin, was known but was fairly unconstrained. In 2013, three additional sedimentary basins were defined from new 2D seismic data acquired in the slope and deepwater offshore Labrador. They were named the Chidley, Holton, and Henley Basins; as well as the geometry of the previously known Hawke Basin was refined (Carter et al., 2013).

This paper examines through seismic interpretation a Cenozoic delta within the Chidley and Hawke Basins based on the new 2D seismic data acquired over the region from 2011 to 2017, as well as the evidence of active petroleum systems.

Previous work

Scientific work began in offshore Labrador in the 1960s with aeromagnetic and seismic surveys over the margin. In 1966 and 1967, the first aeromagnetic survey was conducted by Tenneco, whereas the first seismic survey was acquired from 1965 to 1969 by the Geological Survey of Canada. This initial 2D seismic survey offshore Labrador identified a 7 km seaward thickening wedge of sediments on the Labrador shelf. These data were then used by Grant (1972) to first describe the morphology of the Labrador margin.

The first industry seismic data were acquired in 1968 following the first issuance of exploration permits in 1966. A total of 80,000 line km of multichannel reflection seismic data and a small amount of refraction data were acquired, and the first exploration well, Leif E-38, was drilled in 1971. Postdrilling of E-38, 28 additional wells were drilled on the Labrador shelf in the Hopedale and Saglek Basins between 1971 and 1983. Out of these 29 wells, five were gas discoveries (Bjarni H-81, Gudrid H-55, Hopedale E-33, North Bjarni F-06, and Snorri J-90) and one oil discovery (North Leif I-05).

After 1982, there were no seismic data acquired offshore Labrador until Geophysical Service Incorporated acquired 2D reflection seismic data between 2003 and 2006. Then in 2010, Nalcor Energy with Infoterra (now Airbus Defence and Space) conducted a regional satellite oil seep mapping study of the entire Newfoundland and Labrador offshore. This initial seep study was then used in 2011 to plan a multiyear 2D long-offset broadband regional seismic program by TGS, PGS, and Nalcor Energy (Figure 1). These modern seismic data have provided high-quality imaging over the entire Labrador margin, including the deepwater areas, and have led to the reconfiguring of the sedimentary basins offshore Labrador.

New geologic interpretations and insights

From 2011 to 2017, a regional 2D broadband seismic survey was acquired by TGS/PGS in partnership with Nalcor Energy over the Labrador slope and deepwater. The improvement in image quality from this data set al.owed for new regional mapping that showed that the Labrador margin was more complex than originally perceived. The modern seismic data showed that six sedimentary basins could be defined within the Labrador Sea. These basins include the previously known Saglek, Hopedale, and Hawke Basins with the addition of the Chidley, Henley, and Holton Basins. The newly defined basin configuration has been outlined by Carter et al. (2013). The Chidley Basin (Figure 2) has been defined by Carter et al. (2013) as a series of connected rotated fault blocks with preserved Mesozoic section within the slope to deepwater region of the Labrador Sea. Bound to the south by the Cartwright Fracture Zone (CFZ) where it intersects with the Hawke Basin and to the north where it connects with the Saglek, the Chidley Basin is a large half-graben structure that has 3–9 km of Mesozoic and Cenozoic section in the west and gradually pinches out to the east.

The Hawke Basin (Figure 2) was inferred in geologic interpretations pre-2013 (Government of Newfoundland and Labrador, 2000), but became more constrained with the new 2011–2012 2D data acquisition over the slope and deepwater. Mapping by Carter et al. (2013) define the basin as a northwest–southeast striking half-graben feature formed during mid-Mesozoic rifting. To the south, it connects with the northern limits of the Orphan Basin, whereas in the north, the boundary is tentatively placed near the CFZ (Figure 3). The eastern extent, as currently mapped, consists of a series of basement highs that form a northwest–southeast set of lineaments that mark the transition into the western edge of the Holton Basin.

The geology of the sedimentary basins within the Labrador Sea can be segmented into three phases: rift, drift, and postdrift. This tripartite megasequence division of Balkwill et al. (1990) has been retained herein and provides a logical separation between the rift, drift, and postdrift sediments.

Rifting between Greenland and Labrador began at 140 Ma (Tappe et al., 2007) when continental stretching led to the development of rift basins along the Greenland and Labrador margins. The rifting phase of the Labrador Sea created a landscape of northwest–southeast trending grabens and half grabens where synrift sediments were deposited (Dickie et al., 2011), followed by postrift subsidence and the development of sag basins in the Late Cretaceous (Dickie et al., 2011).

The timing of the rift-drift transition is debated by different authors. One proposition for the initiation of seafloor spreading is the Late Cretaceous; this timing has been proposed based on the oldest magnetic anomaly identified in the Labrador Sea (Chron 33), which is Late Cretaceous, Campanian (Roest and Srivastava, 1989) (Figure 3). The other time proposal for the onset of seafloor spreading is in the Early Cenozoic; Paleocene, during Chron 27 (Selandian) (Chalmers, 1991; Chalmers et al., 1995). Chalmers (1997), also states that there is no continent ocean boundary at the base of the bathymetric continental slope as proposed by Roest and Srivastava (1989); instead, continental crust extends more distally and is separated from the oceanic crust by a transition zone of variable width.

While seafloor spreading was ongoing in the Labrador Sea, a triple junction formed south of Greenland at 55 million years and seafloor spreading began on the eastern side of Greenland (Kristoffersen and Talwani, 1977; Roest and Srivastava, 1989). This continued until 33 million years, when seafloor spreading became more focused in eastern Greenland and spreading in the Labrador Sea slowed and then stopped between Chron 21 (mid-Eocene) and Chron 13 (early Oligocene) (Welford et al., 2020). With the end of seafloor spreading at 33 million years, the Labrador margin transitioned to a passive margin, very similar to its modern setting.

More than 170,000 line km of modern 2D seismic data have been acquired offshore Newfoundland and Labrador since 2011; of this, 47,000 line km covers the Labrador margin. Recent mapping of this 2D seismic grid over the slope and deepwater portions of the Labrador south study area was completed on a 5×5  km grid. This grid was infilled over several seasons of seismic acquisition in the area. As mentioned within this paper, the tripartite megasequence division of rift, drift, and postdrift sediments put forward by Balkwill et al. (1990) was retained to separate the tectonic phases of sediment deposition within the Labrador margin. However, the semiformalized formation names proposed by Balkwill et al. (1990) have not been used within the seismic mapping. This is due to the large-scale region of the 2D seismic grid that has been interpreted over the entire margin of Newfoundland and Labrador and tied together regionally; therefore, the authors have instead opted to move to the convention applied by Dickie et al. (2009) using approximate age of the seismic megasequence within mapping studies. Once these regional seismic megasequences were mapped, interpretation then focused on infilling the sequences between these unconformities using regional seismic markers that were tied to wells on the shelf.

Initial mapping of the geology in the slope and deepwater of Labrador south was completed on a sparser regional seismic grid and focused on seismic megasequence packages and regional unconformities that provide a logical separation between the rift, drift, and postdrift phases of deposition (Carter et al., 2013). These regional unconformities include the base Mesozoic (K_140 marker), the Cenomanian unconformity (K_100), the base Cenozoic unconformity (C_65), the mid-Eocene unconformity (C_45), and the top Oligocene unconformity (C_24). In the regional mapping of Labrador south, the synrift sediments were defined as those deposited in Cretaceous time, lying beneath the C_65 unconformity. The synrift sedimentary package has been divided into two sequences: the early synrift (K_100) to base Mesozoic (K_140) and the late synrift (C_65) to (K_100) (Figure 4).

The base Mesozoic (K_140) was picked at the base of the sedimentary package observed on seismic. The structure map for the base Mesozoic (K_140) shows the deep low created from the early Cretaceous rifting and subsequent thermal subsidence. This low is oriented in a northwest –southeast direction adjacent to the Labrador shelf and is infilled with Cretaceous sediments. Defined within this basement map is the CFZ adjacent to the Lake Melville/Hamilton Inlet (Figure 5). The CFZ had an influence on the morphology of the Chidley and Hawke Basins because it delineates the southern boundary of the Chidley Basin and the northern boundary of the Hawke Basin.

North of the CFZ in the Chidley Basin, a series of half grabens with a north-northwest–south-southeast orientation is noted almost perpendicular to the large sag basin feature that overlies the CFZ. The base of the synrift Cretaceous-aged sediments within the Chidley and Hawke Basins is defined at the base Mesozoic unconformity (K_140). The top of the Cretaceous-age sediments is defined by the base Cenozoic unconformity (C_65). The Cretaceous section is divided into Early Cretaceous and Late Cretaceous by the Cenomanian unconformity (K_100). Sediments of Cretaceous age are spatially extensive over the Chidley and Hawke Basins (Figure 6) and have a maximum thickness of 6 km in the deepest areas of the basins.

The base of the Cenozoic package in Labrador is defined by the C_65 regional seismic marker. The Cenozoic sedimentary package ranges in thickness from 2 to 8 km within the study area (Figure 7). Mapped within the Cenozoic sedimentary package are five regional surfaces or sequences. The C_54 top Paleocene, C_45, mid-Eocene, C_34 top Eocene, C_24 top Oligocene, and C_10 mid-Miocene.

The thickest Cenozoic sediments were deposited in a previously unmapped delta in the deepwater area of the Chidley and Hawke Basins. This delta may be a prime candidate for liquid hydrocarbon accumulations.

The petroleum system

As discussed previously, the Labrador shelf has an active petroleum system that has proved to host dual phases of hydrocarbons — oil and gas proven in discoveries in the Hopedale and Saglek Basins. The slope and deepwater offshore Labrador are unexplored with no wells drilled to date; however, seismic, geochemical analysis of seabed piston core and seep data suggests that an active petroleum system exists here and that there is potential for oil and gas phases to be present.

The discoveries made within the Hopedale Basin in the 1970s and 1980s were sourced from the Cretaceous-aged Bjarni and Markland Formations (McWhae et al., 1980; Balkwill et al., 1990; Fowler et al., 2005; Enachescu, 2006). The structures that were targeted during that phase of exploration focused on the Early Cretaceous reservoir (the Bjarni Formation) preserved on the crests of basement highs (i.e., North Bjarni F-06) or pinched out against the flanks of tilted fault blocks (i.e., North Leif I-05) (Figure 8). As well, Paleozoic-aged carbonates underlying the Mesozoic section proved to be successful reservoir at Gudrid H-55, with the discovery of gas.

With the acquisition of the new 2D seismic data set, satellite seep, and shallow seabed coring projects over the 2011–2017 period, our knowledge of the Labrador slope and deepwater areas has increased significantly. The satellite seep project, undertaken in 2010 in partnership with Infoterra (now Airbus Defence and Space), used data from Synthetic Aperture Radar satellites to identify oil slicks on the surface of the ocean that could be linked to natural seepage. Based on the results of this survey, the 2D seismic survey was initiated in 2011 and designed to target areas where satellite slicks were observed, but sedimentary basins were not defined. This seismic program was also tied to the wells on the shelf to extend the mapping of the regional sequences seen on the shelf into the slope and deepwater areas where the sequences are expanded. Structural and stratigraphic plays have been identified within the newly acquired data and additional infill seismic has been acquired in areas of interest to further delineate. Additionally, a shallow seabed coring project has been undertaken to focus on key areas of the Labrador Sea where geochemical analysis can be used to deduce thermogenic versus biogenic hydrocarbon signatures over prospective areas.

Potential source — Shelf and slope Labrador Sea

On the Labrador shelf, the source rock for the hydrocarbons encountered was widely thought to be the Late Cretaceous-aged Markland Formation (McWhae et al., 1980; Balkwill et al., 1990); however, in recent years, it has been proposed that the interbedded lacustrine shales contained within the Early Cretaceous Bjarni Formation could also act as a potential source (Fowler et al., 2005; Enachescu, 2006). Based on bottomhole temperatures recorded in legacy exploration wells, the geothermal gradient of the Labrador shelf is calculated to be 2.8°C to 3.2°C per 100 m (present day). In this thermal regime, the Cretaceous-aged sediments buried in the strata of the Labrador shelf are currently sitting in the late oil window to gas window. New work completed through data collection and basin modeling on the slope and in deepwater areas is showing a major shift and reduction of geothermal gradients in some regions (Beicip-Franlab, personal communication, 2017). This is a critical insight for the petroleum potential of the slope and deepwater as multiple source rock intervals may have been in the maturity window at the critical moment, and in some cases, still reside there today.

Seismic megasequences tied to the shelf wells in the Hopedale Basin can be mapped further east into slope and deepwater of the Chidley and Hawke Basins. As the geology is mapped into the more distal regions of the Labrador Sea, fundamental changes in the crust underpinning these Mesozoic-aged basins are imaged. The proximal Hopedale Basin, which resides on the shelf, is underlain by continental crust. As mapping continues into the more distal Chidley and Hawke Basins, located into the slope and deepwater, the underlying crust changes from continental to transitional and then to oceanic (Welford et al., 2020). Modeling of the gravity data collected in conjunction with the 2D seismic survey in 2011 and 2012 has indicated that a zone of hyperextension must exist within the transitional crust to reproduce the observed gravity values. Distal to the interpreted transitional crust, the crust begins to thicken to 8 km, consistent with normal oceanic crust (Welford et al., 2020). Whereas crustal hyperextension would suggest a possible increase in geothermal gradient for the slope and deepwater of central to southern Labrador, basin modeling indicates that it does not. The main driver for the thermal gradients within the study area is in fact the geologically young, thick sedimentary wedge of Cenozoic-aged sediments that has a cooling effect on the sedimentary package within the study area and lowers the geothermal gradient in the slope and deepwater from the values seen on the shelf (Beicip-Franlab, personal communication, 2017).

One of the key areas of interest in the Chidley and Hawke Basins, related to petroleum occurrence, is the area that resides outboard to the mouth of Lake Melville/Hamilton Inlet, where the Churchill River system deposits eroded sediment from the continental interior. Seismic mapping in the area adjacent to the discharge point of the Churchill River system shows that ancient delta system deposits are present to the east of the modern river system in the slope and deepwater portion of the sedimentary section (Mitchell et al., 2017). Detailed seismic mapping of this package displays a 6–8 km thickness range of Cenozoic-aged sediments deposited in a deltaic system from the Paleocene to the Miocene. This sediment wedge, deposited over a 40 million year timeframe, can be mapped seismically over an area of approximately 17,500  km2 and will be known herein as the Churchill Delta (Figure 9).

The Churchill Delta, with its immense sedimentary thickness, is postulated to have a major effect on the geothermal gradient in this area of offshore Labrador. Geothermal gradients over the Churchill Delta area were calculated in the basin modeling work conducted for Nalcor Energy by Beicip-Franlab. In this work, Beicip-Franlab used three main inputs to determine the geothermal gradient in the slope and deepwater: heatflow data, seismic mapping of bottom simulating reflectors (BSRs), and temperature profiles from lithospheric models. Beicip-Franlab’s findings through basin modeling work were that the geothermal gradient for the slope and deepwater ranged from of 2.4°C to 2.7°C per 100 m, a significant reduction in gradient from the shelf values of 2.8°C to 3.2°C per 100 m (Beicip-Franlab, personal communication, 2017).

Within the Chidley and Hawke Basins of the Labrador Sea, it is hypothesized that based on the rift history, shelf well data, depositional environments, seismic, heatflow data, geothermal gradients, and basin modeling information on the slope and deepwater, there is potential for multiple source rock intervals.

The Early Cretaceous shales — Bjarni Formation age equivalent

Within the Chidley and Hawke Basins, there is a package of Early Cretaceous-aged strata (interpreted to be Valanginian to Albian) that was deposited within the grabens of the early rift stage of the Labrador Sea opening; these oldest sediments were likely deposited in a lacustrine environment (Figure 10). As rifting continued in the mid-Cretaceous, the depositional environment shifted to marginal marine and subsidence led to flooding of the topographic lows from the proto-Atlantic to the south. These newly formed marine basins may have had a fairly restricted setting, with potentially high inputs of fresh water from fluvial sources on the basin margins creating favorable conditions for a positive water balance basin, leading to a lack of mixing in the water column and anoxic bottom waters. Globally, Cretaceous sediments deposited at the same time as the Bjarni Formation have numerous documented occurrences of ocean anoxic events (OAEs) recorded within them. In particular, the globally significant are early Aptian Selli event, OAE1a (Coccioni et al., 1987), and the early Albian OAE1b Paquier Event (Breheret, 1985)

The Late Cretaceous shales — Markland Formation equivalent

Within the Chidley and Hawke Basins, the Late Cretaceous package from the K_100 (Cenomanian) to the C_65 (base Cenozoic) is geographically extensive and is thought to contain sediments of Cenomanian/Turonian age as well as Maastrichtian age. During deposition of the Late Cretaceous, the early rift sediments of the Bjarni Formation were overlain by the marine Markland Formation. At this time, the Labrador rift was well-developed and the proto-Atlantic began to invade the subsiding rift valley and marine shale deposition was prevalent. During the mid to Late Cretaceous, there are two main OAEs documented worldwide. The Cenomanian-Turonian Bonarelli event and the Coniacian-Santonian event (OAE 3). This is of interest because the Itilli-type source rock occurs on Labrador’s conjugate margin in western Greenland and consists of marine shales of Cenomanian/Turonian age that are highly oil prone (Bojesen-Koefoed et al., 2004). Geographically to the north of Labrador, the Kanguk Formation of the Sverdrup Basin in the Canadian Arctic also hosts dark marine shale of Turonian age that was deposited in a disoxic to anoxic low-energy marine environment (Nunez-Betelu et al., 1994). As well, Cenomanian/Turonian was encountered at the ODP 1276 site located 1300 km south of the Flemish Cap. One of the horizons in this core was interpreted as Cenomanian to lowermost Turonian in age and correlated to the OAE 2 or Bonarelli event. It had total organic content (TOC) values up to 7 wt% and a hydrogen index (HI) up to 452 mg HC/g TOC, which is indicative of type II marine-derived kerogen (Shipboard Scientific Party, 2004).

The existence of Cenomanian/Turonian marine shales covers a regionally significant area from the Canadian Arctic to western Greenland south to ODP 1276 and has been proven as a type II oil-prone source rock in western Greenland. It has the potential to be a regionally extensive significant source rock for the Labrador and Newfoundland margins.

Paleocene shale — Cartwright Formation equivalent

Within the Chidley and Hawke Basins offshore Labrador, there is a thin, regionally extensive section of Paleocene-aged sediments. In the study area, the Paleocene has been mapped as the condensed shales interpreted as the décollement surface for a listric fault system (Figure 11). The Cartwright Formation is interpreted to be a shallow to marginal marine setting, similar to the Paleocene-aged source rocks deposited simultaneously in west Greenland. These source rocks have been hypothesized as the source rock of the Marraat oil, within Paleocene lavas on Nuussuaq peninsula, Greenland (Christiansen et al., 1996). The source rock for the Marraat oil has a dominance of terrestrial organic matter deposited in a marine setting (Christiansen et al., 1996).

The Paleocene strata mapped in Labrador’s slope and deepwater are coeval to the Cartwright Formation on the shelf and presently reside in the upper oil window. This interpretation is based on geothermal gradient modeling, which adopts the hypothesis that the large volume of Cenozoic sediment contained within a major delta system has a cooling effect and reduces the geothermal gradient in the slope and deepwater area, versus the higher geothermal gradient encountered on the adjacent shelf in the Hopedale Basin.

The discovery of Marraat oils within the volcanics of the Nuussaq peninsula (Christiansen et al., 1996) further proved that Paleocene-aged source rock on the margin could liberate a liquid-phase hydrocarbon. In the study area, there are multiple examples within the 2D broadband seismic data that may indicate the potential for two phases of hydrocarbons in fault block traps within the Cenozoic section offshore Labrador.

Seismic evidence for a dual-phase hydrocarbon system

Seismic amplitude signatures, potentially associated with hydrocarbon indications, are observed on some of the first long-offset seismic lines acquired in the slope and deepwater area of offshore Labrador (Cameron et al., 2013; Wright et al., 2013). The 2D seismic data in this area were reprocessed with prestack depth migration (PSDM) in 2015, and imaged in the data set were many structurally related amplitude variation with offset (AVO) anomalies that could be consistent with hydrocarbon indicators (Montevecchi et al., 2019). One area of such anomalies is in the Miocene in the Chidley Basin shown in Figure 12.

This area shows two broad groups of amplitude anomalies encircled in white and yellow. The white circled area, at approximately 900 m below the mudline, contains AVO class III anomalies truncated against an angular unconformity. The yellow circled area, further down the slope at a slightly deeper depth below the mudline (1100 m), contains class II and III AVO anomalies truncated structurally by listric faults with the far angle amplitude signatures observed in the up-dip portion of the blocks. Differing classes of AVO anomalies have been identified to be consistent with models of different pore fluid phases in the deepwater West Orphan Basin to the south of this area where similar depositional compaction trends are expected (Montevecchi et al., 2016; Wright et al. 2016). To examine the relationship of the various amplitude anomalies further, Figure 13 shows an enlarged view of one of the fault blocks shown in Figure 12.

In this example, there is an anticlinal feature denoted by the top horizon that has a class III amplitude response, characterized by negative amplitude in the near-angle stack and stronger negative amplitude in the far-angle stack at the crest of the feature. Down the flanks of the anticline, below the black dashed line flat in depth, the amplitude class changes to a class II amplitude denoted by the two green circles (Figure 13). This amplitude signature has been imaged in several fault blocks in the area (class III above and class II below) and is modeled to have signatures consistent with potential gas-saturated reservoir structurally conformed over an oil-saturated reservoir over a brine-filled reservoir. Additionally, in the near-angle stack, there is an amplitude peak (an increase in impedance) that cuts flat across the structure in the near-angle stack at the deeper dashed line at the termination of the class II amplitude on the limbs and could be indicative of an amplitude flat spot along an inferred hydrocarbon-water contact. These amplitude responses provide direct seismic evidence of potential multiple fluid phases (gas and oil) imaged in situ in the section. In an area thought to be gas-prone, these observations provide insight into potential oil presence in the Chidley Basin.>

Reservoir — Chidley and Hawke Basins

There are multiple potential reservoirs within the Mesozoic and Cenozoic sections of the Chidley and Hawke Basins. In the Cretaceous section, the Bjarni Formation, equivalent to what was deposited in the early rift stage, is a primary reservoir candidate. During early rifting, rift shoulders were eroded and deposited thick sandstone intervals into the newly formed rift valleys, as is seen within the Hopedale Basin where the Bjarni Formation has been cored. Because the rift system has now been imaged into the shelf and deepwater, it is highly probable that those same early synrift Bjarni sandstones were deposited further in the outer rift basins.

The Cenozoic section within the study area is mainly composed of a large shelf edge delta deposit, the Churchill Delta. The age on this delta is Eocene to Miocene, and it has a thickness of more than 8 km in its central portion. Although the Eocene was a time of globally high sea level, the Churchill Delta was able to develop because the sediment supply from the ancient Bell River system was large enough to prograde and reach the shelf edge. The Bell River system drained the eastern Cordillera to the Labrador Sea during the Cenozoic and Quaternary times. This river system transported great volumes of sediment from the eastern slopes of the Cordillera, the Interior Plains, and the Canadian Shield (McMillan, 1997). Shelf edge deltas, like the Churchill Delta, with a high sediment supply are able to transit moderately wide shelves during sea-level rise. Sediment supply-rich deltas have the potential to deposit at the shelf margin, and they also have the ability to deliver large volumes of sand to the deepwater slope and basin floor (Uroza and Steel, 2008).

Within the Churchill Delta complex there are multiple potential reservoir sequences throughout the Cenozoic section. They range in age from 54 to 6.5 million years and are coeval in age to the Kenamu, Mokami, and Saglek Formations encountered on the Labrador shelf. The North Leif I-05 and Cartier D-70 wells, which are located between the mouth of Lake Melville/Hamilton Inlet and the Churchill Delta, have sandstones within these stratigraphic intervals. Shelf edge deltas commonly develop significant growth faults; these growth faults control the morphology of the sand bodies, aligning the sand deposits parallel to strike (Bhattacharya, 2010). This characteristic is seen within the Churchill Delta, with major growth in the sedimentary section occurring within the Eocene and Oligocene sections with some faults within the listric set of faults displaying 1–1.5 km of growth (Figure 11).

Whereas deltaic deposits dominate the Cenozoic section of the study area, there are some indications of turbidites within the system at the early Eocene time at the base of the Churchill Delta complex. These turbidites are likely the result of instability of the prograding delta during its initial stages in the early Eocene on the edge of the shelf. As the Bell River system deposited sediment at the shelf edge at the beginning of the Eocene, the large sediment accumulation failed at the shelf break (where the listric fault system begins) and initiated turbidity currents into the basin (Figure 14). Postfailure and turbidite deposition, the Bell River system continued to transport large amounts of sediment to the outer shelf where the Churchill Delta was deposited.

Traps and seals

There are multiple trap types within the Churchill Delta system (Figure 15). Within the delta succession, there are rotated fault blocks, roll-over anticlines, toe thrusts, stratigraphic pinch outs, and pure stratigraphic traps. Within the Cretaceous section, the onlap of siliciclastics onto rift basin margins is the primary trap.

The Churchill Delta system has multiple seals. The primary seal for the system is the interbedded shales within the upper Miocene to Pliocene that has an average thickness of 1 km near the shelf slope break and 2–3 km in the distal region of the study area. Internally the delta traps may be sealed with interbedded shales and aided by fault placement to adjacent fault blocks acting as lateral seals.

Timing and critical moment

Based on the seismic mapping completed over the study area and structural reconstruction work done on the listric fault system by Nalcor Energy geoscience staff, the critical timing for hydrocarbon migration into the Churchill Delta system is less than 10 million years ago. It was at this time that reservoir, traps, and seals were in place, and source rocks for the system were buried deep enough to generate hydrocarbons.

Conclusion

The acquisition of modern 2D seismic in the Labrador Sea has provided increased geoscience knowledge over the area. Imaged within the new 2D seismic was the presence of a large-scale shelf edge delta complex, herein named the Churchill Delta.

The elements of a working petroleum system for the Churchill Delta have been discussed. Of all the petroleum system elements, the source rock presence for this area is the major unknown. Several intervals within the Cretaceous and Paleocene have been considered as candidates for potential source rock for the Churchill Delta. Occurrences of Albian/Aptian, Cenomanian/Turonian, and Paleocene source rocks have been encountered along the Newfoundland and Labrador margin and the conjugate margin in Greenland. As well, proven source rocks of Cretaceous age are found in the Hopedale Basin that is proximal to the Churchill Delta. Basin modeling work carried out in the area hypothesizes lower geothermal gradients in the slope and deepwater due to the volume of Cenozoic sediment in the area, thus putting the Paleocene source rocks within the oil window and the Cretaceous source rocks in the lower oil to wet gas window. Geophysical AVO observations within the 2D seismic data may indicate the presence of dual-phase hydrocarbon system within the Churchill Delta’s structural, stratigraphic, and combination traps and open a new play fairway within the study area.

Acknowledgments

The authors would like to acknowledge TGS/PGS for permission to show the seismic images. The authors thank A. Spencer for her work and contribution to the maps and figures within the paper; they also appreciate the efforts of the entire geoscience team at Nalcor Energy Oil and Gas who reviewed the manuscript.

Data and materials availability

Data associated with this research are confidential and cannot be released.

Erin Gillis received a B.S. and an M.S. in geology from the Memorial University of Newfoundland. She joined Nalcor Energy-Oil and Gas in February 2010 and presently holds the position of manager, leads and prospects team within the Exploration Group. In this role, she and her team are focused on the interpretation and mapping of play trends, leads, and prospects as well as hydrocarbon resource potential within Newfoundland and Labrador’s frontier basins. Before joining Nalcor, she worked as a geologist on the thermal team for Canadian Natural Resources Limited. She has 14 years of experience in the oil and gas industry, and she is a registered member of the Professional Engineers and Geoscientists of Newfoundland and Labrador (PEGNL).

Richard Wright received a B.S. (Hons) and a Ph.D. in geophysics from Memorial University of Newfoundland with a thesis evaluating 4D seismic at the Hibernia oilfield. He is the exploration manager of Nalcor Energy-Oil and Gas. He joined Nalcor Energy-Oil and Gas in 2009 and presently holds the position of exploration manager. His team is responsible for Nalcor’s exploration strategy to delineate Newfoundland and Labrador’s frontier basins to assess the province’s oil and gas potential. Before joining Nalcor, he was part of the frontier exploration team for Chevron. Prior to Chevron, he was employed with a geophysical consulting firm in Orange County, California, where he worked on several exploration and development projects in basins located in California, South America, North Sea, Middle East, West Africa, and offshore Australia. He is a member of multiple professional associations, including SEG, PEGNL, and EAGE. In 2018, he was recognized by the PEGNL with the Early Accomplishment Award, and he was the 2016 recipient of the Newfoundland and Labrador Oil and Gas Industries Association Rising Star Award. Nalcor’s Exploration Strategy was awarded the 2015 Natural Resources Magazine Industry Excellence Award for Resource Development and, in 2017, Nalcor Oil and Gas was a finalist for the Petroleum Economist (London) Award for Exploration Company of the Year.

Victoria Mitchell received a B.S. and an M.S. in geophysics from Memorial University of Newfoundland and Labrador. She is a geophysicist with Nalcor Energy whose primary role is to identify and assess areas of oil and gas prospectivity in the Newfoundland and Labrador offshore. Prior to joining Nalcor in 2015, she worked with Husky Energy in St. John’s primarily as an exploration geophysicist working in the Jeanne d’Arc Basin. During this time, she also worked as a joint venture geoscientist and development geophysicist within the North Amethyst field. Before Husky, she worked with Chevron Canada Resources in Calgary with the frontier group where she worked on exploration opportunities in the Beaufort Sea, NWT.

Nicholas Montevecchi received a degree from Memorial University of Newfoundland and is a professional geoscientist with 13 years of exploration experience in Canada, South America, and offshore West Africa. Currently, he is working as a QI geophysicist within Nalcor Oil and Gas Exploration Team focusing on rock physics, prospectivity evaluation, and petroleum systems analysis in frontier exploration environments, offshore.

Freely available online through the SEG open-access option.