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Hibernia Area Tectonics
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Regional Tectonic History
The Jeanne d'Arc Basin is one of several Mesozoic extensional-sag, cratonic
margin basins that underlie the Grand Banks of Newfoundland. The basin
covers an area greater than 10,000 km2 and comprises a
Mesozoic-Cenozoic sedimentary succession as much as 20 km thick. The basin
is fault bounded and plunges northeastward. A large basement platform,
called the Bonavista Platform, borders the basin to the west and a series of
basement ridges, referred to as the Central Ridge Complex, defines the
eastern boundary. The Avalon Uplift borders the basin to the south.
Multiple Mesozoic rifting episodes on the Grand Banks were initiated in the
Late Triassic, preceding break-up of the supercontinent Pangea and the
ancestral opening of the North Atlantic Ocean. These rifting episodes
dominated the tectonic and sedimentation style of the Jeanne d'Arc Basin.
Three distinct episodes of rifting affected the Jeanne d'Arc Basin during
the Late Triassic through Albian. Each rifting event was followed by a
period of passive subsidence.
The first Mesozoic episode of rifting occurred in the Late Triassic through
Early Jurassic. This rifting event defined the limits of the immense
northeast-trending half-graben that underlies the central and southern parts
of the basin. Continued lithospheric extension during this early Mesozoic
tectonic phase led to Middle Jurassic separation of Nova Scotia and Africa.
Within the Jeanne d'Arc Basin, the initial rifting event was followed by a
late Early to Late Jurassic subsidence phase and the deposition of the most
prolific source rocks.
A second episode of rifting occurred from the Late Jurassic to the earliest
Cretaceous, and resulted in the growth of a series of mainly north-south-
trending faults and coeval uplift of a broad area at the southern end of the
basin, known as the Avalon Uplift. This phase of extension may have resulted
in limited separation of the southern Grand Banks and southern Iberia.
Deposition of the hydrocarbon-bearing reservoir rocks of the Jeanne d'Arc
Formation occurred during this interval.
A third episode of rifting took place in the mid-Aptian to late Albian. This
period of extension resulted in the growth of the major
northwest-southeast-trending ("trans-basin") normal faults in the basin.
Separation of the central Grand Banks from the northern Iberian margin and
the British Isles occurred at that same time.
There is considerable variation in the published interpretations of the
timing, type and range of the third tectonic event in the basin. The
majority of these tectonic interpretations are summarized in a 1993
publication by Sinclair.
Re-activation of major basin faults Murre, Egret, and Spoonbill in the Late
Cretaceous and Early Tertiary are interpreted as related to salt tectonics.
It is possible, however, that these faults are related to an additional
phase of subdued extension which may have preceded the opening of the
Norwegian-Greenland Sea in the Middle Eocene. Since the Late Cretaceous,
the Grand Banks area, including the Jeanne d'Arc Basin, has formed part of a
passive margin.
Regional Stratigraphy and Depositional Environments
Since the Triassic, the Jeanne d'Arc Basin has been dominated by terrigenous
clastic deposition. Lithologically defined, seismically delineated,
unconformity-bounded megasequences in the basin can be related to
distinctive regional tectonic pulses.
The earliest record of deposition in the Jeanne d'Arc Basin consists of
Upper Triassic to Lower Jurassic (Carnian-Pliensbachian) continental redbeds,
restricted-marine evaporites, and carbonates the Eurydice, Argo, and
Iroquois formations, respectively. This succession is resolvable on seismic
and has been encountered, in a few wells, in the extreme southern part of
the basin. This initial syn-rift succession was overstepped by a thick,
post-rift succession of Lower to Middle Jurassic (Pliensbachian - Callovian)
marine mudstones and carbonates of the Downing Formation. The overlying
conformable Voyager Formation is diachronous and consists of a Middle
Jurassic (Bathonian - Callovian) extensive lower unit of dominantly shallow-
to marginal-marine, interbedded sandstone, shale and limestone, and an upper
unit of shale deposited during a widespread regression in the basin. The
overlying Rankin Formation is conformable and marks the termination of this
post-rift sequence. This Upper Callovian to Kimmeridgian, dominantly marine
interval consists of a heterogeneous mix of massive limestone, fine clastics
and thinly interbedded limestone, marl and shale in the southern part of the
basin, and an interval of interbedded sandstone, siltstone, shale and
occasional limestone in the northern part of the basin. The prolific source
rocks of the Egret Member are found in the upper part of the Rankin
Formation. The source rocks are regionally extensive and consist of thinly
interbedded and laminated marls, calcareous shales, and claystones deposited
in a low-energy, restricted-marine environment. The deposition of the Egret
Member is interpreted as representing the "onset warp" phase of the second
phase of extension in the basin. This "onset warp" phase is interpreted as
representing a phase of differential subsidence in the basin without
significant faulting.
The Kimmeridgian to Tithonian Jeanne d'Arc Formation, a coarse grained to
conglomeratic fluvial braidplain deposit with associated restricted-marine
shales, unconformably overlies the carbonates of the Rankin Formation. Most
researchers believe the Jeanne d'Arc Formation represents the beginning of a
second rifting episode in the basin in the Late Jurassic. The
Tithonian Fortune Bay marine shales and siltstones, and the prograding
Berriasian to Valanginian deltaic succession of sandstones and shales of the
Hibernia Formation continued to fill the basin during this syn-rift episode.
Widespread deposition of the B marker limestone above the
mid-Valanginian unconformity and deposits of the nearshore-marine,
fine-grained clastics and oolitic limestones of the Catalina Formation, in
addition to the distal equivalent shales of the Whiterose Formation,
constituted a return to passive subsidence in the basin.
The A marker has been included by one researcher as part of the Barremian to
lower Aptian Avalon Formation, and the succession interpreted as
representing the "onset warp" phase of the third tectonic episode to affect
the basin. The Avalon Formation consists of a stacked succession of marine
to marginal-marine calcareous sandstone, bioclastic limestone and minor
shale of varying thickness across the basin. The overlying syn-rift
mid-Aptian to upper Albian Ben Nevis Formation consists of a succession of
transgressive shoreface sandstone and back-barrier facies, which were in
turn flooded by the laterally extensive offshore shales of the Nautilus
Formation.
An alternative interpretation has been provided for the timing of the third
tectonic episode affecting the Jeanne d'Arc Basin. The interpretation is
based on dating of the sequences using an extensive palynology database and
an analysis of the rotation and divergence of seismic reflectors (stratal
geometry). The stratal geometry patterns observed attest to differential
subsidence, which documents an episode of extension and block rotation in
the basin within the Aptian Avalon Formation. The lithostratigraphic scheme
has been used for the Cretaceous interval in the Terra Nova well correlations,
which will be discussed later in the report. All of the Upper Cretaceous
post-rift succession, ranging from Cenomanian to Maastrichtian, is assigned
to the Dawson Canyon Formation, which consists mainly of marine shales, but
also includes the deltaic members of the Otter Bay and Fox Harbour, the
Turonian chalky Petrel Member and the Coniacian to Maastrichtian chalky
Wyandot Member. The Tertiary passive margin sequence is represented by the
marine shales and minor chalks, siliceous mudstones and rare sand-silt beds
of the Banquereau Formation.
Regional Structure
Structural analysis of the Jeanne d'Arc Basin is based on seismic
interpretation and mapping at various stratigraphic levels, integrated with
well data. Timing of structural deformation has been constrained by
stratigraphic geometries and detailed biostratigraphic analysis.
The Late Triassic to Early Jurassic structural episode was responsible for
the creation of the large half-graben that dominates the architecture of the
southern and central portion of the basin. The principal extensional
basin-forming faults, including the Murre-Mercury faults, and the associated
listric synthetic and antithetic faults, trend northeast. These
northeast-trending faults either offset or terminate against transfer faults
that impart a regular orthogonal pattern to the basin margin. An alternative
interpretation suggests that there was no evidence to support the presence
of contemporaneous, orthogonal transfer faults offsetting the en echelon
normal faults of this syn-rift phase. The northeast-striking faults were
separated by tilted relay ramps that accommodated the diminishing amount of
extension on overlapping normal faults. Locally, the presence of growth
faults in syn-rift strata and the presence of growth faults related to
sediment loading, salt mobilization and diapirism (halokinesis) within the
Middle Jurassic post-rift sequence in the extreme southern part of the basin
are demonstrated. However, most researchers agree that the Late Triassic to
Early Jurassic rifting episode is difficult to unravel because later
episodes of extension largely obscure the Late Triassic to Early Jurassic
structural trends.
Structures related to the Late Jurassic to Early Cretaceous phase of
extension include uplift of a broad region referred to as the Avalon Uplift
in the southern part of the basin and the development of north-south-oriented
faulting, which characterizes the central Jeanne d'Arc Basin and Central Ridge.
A third structural episode involved the growth of a series of
northwest-southeast-oriented faults. These trans-basin faults are
interpreted as normal faults, initiated in the mid-Aptian and terminating in
the late Albian. Alternatively, the trans-basin faults are interpreted as
representing "relay transfer structures," accommodating
northwest-southeast-oriented strike-slip motion between fault blocks during
the Late Callovian to Barremian before fault detachment and dip-slip
movement beginning in the Barremian. Transtensional and transpressional
structures were associated with these transfer faults. Other researchers
interpreted most of the trans-basin faulting as having occurred in the
Albian, related to salt tectonics. Regardless of their origin, the
northwest-southeast faults detached at various levels within the sequence.
Detachment zones have been interpreted within basement, between basement and
sedimentary infill, within salt between carbonates and clastics, and within
thick, ductile shale. These faults form local grabens, horsts, tilted blocks,
reverse drag folds and local rollovers, and constitute excellent hydrocarbon
traps in the basin.
Transpressional and transtensional structures have also been interpreted as
being associated with mid-Cretaceous reactivation of northeast-southwest,
en echelon, Late Triassic to Early Jurassic normal faults. Linkage of these
en echelon faults created a series of restraining and releasing bends that
formed the locations of distinctive structures. Transpressional elements
related to oblique-slip motion along the restraining bends include "pop-up"
structures, reverse faults, forced folds and wrench-related folds, including
the large-scale convergent anticline that formed the Terra Nova Arch.
Reactivation of some of the major faults in the Upper Cretaceous and Lower
Tertiary (e.g., Voyager, Trinity, Egret, Spoonbill and Murre faults; along
with the occurrence of local salt diapirism, mark the end of significant
tectonic activity within the basin.
Regional Geochemistry
Commercial quantities of hydrocarbons in the Jeanne d'Arc Basin (Hibernia
and Terra Nova) demonstrate the coexistence of a mature, organic-rich,
oil-prone source rock, good reservoirs and traps, proper structural timing
and good migration pathways.
Source rocks in the basin were first reported in 1980. An immature Upper
Jurassic potential source-rock interval was recognized in the Egret K-36
well. This lower Kimmeridgian interval is defined as the Egret Member of the
Rankin Formation.
Geochemical studies conducted by the Geological Survey of Canada and industry
have shown the Egret Member to be the major source for the oils in the Jeanne
d'Arc Basin. This source rock is generally 200 to 300 m thick in the southern
and western parts of the basin and thickens in the northeast to more than 700
m. This thick sequence of dominantly calcareous, organic-rich shale was
deposited under euxinic conditions and consists primarily of amorphous
organic matter (Type II-I kerogen).
Potential source rocks exist within the Voyager Formation and the Fortune Bay
Formation. These source rocks are described as having more terrestrial,
gas-prone organic material (Type III kerogen). In addition, the Fortune Bay
source rock appears to have a limited regional extent and is confined to the
northeastern part of the basin.
Biomarker data on recovered oils suggest that the contribution from the
Voyager and Fortune Bay Formation source rocks may be relatively minor.
However, it has been concluded from detailed geochemical analysis of the
oils, condensates and extracts that some of the hydrocarbons in the Ben
Nevis area were derived from a source rock distinctly different from the
Egret Member.
The youngest immature marine source rock present in Lower Tertiary strata is
interpreted as the source of the oil located in the Adolphus 2K-41 well, in
the centre of the basin. Heat conduction, associated with salt diapirism,
creating the Adolphus structure, rendered the Tertiary source rock fully
mature in the Adolphus area. Regional source-rock maturity and distribution
of oils in the basin suggests a primarily vertical migration pathway for the
hydrocarbons sourced from fully-mature or late-mature source beds. The
numerous listric normal faults and fractures dissecting the Mesozoic and
Cenozoic sections provide excellent conduits for vertical migration during
episodes of extension. Direct feeding of reservoir sands has been observed
where reservoirs are in direct contact with the source beds (e.g., Terra
Nova). Although the Jeanne d'Arc Basin oils are similar, they have been
generated over a wide range of maturation levels. In addition, vertical
variations in maturity of the oils is evidence of more than one episode of
oil migration in some areas of the basin. Significant lateral migration on
the South Tempest and Trave structures on the east side of the basin has been
postulated because highly-mature oil and condensate are trapped above
marginally-mature Jurassic source rocks. However, vertical migration along
a major north-south fault adjacent to the structures may have sourced these
reservoirs from late-mature and overmature Jurassic source rocks.
Timing of hydrocarbon generation and migration has been estimated by
determining when the source rocks reached thermal maturity. Using vitrinite
reflectance (Ro) data for Type II kerogen, oil generation is expected to
begin at a 0.5 (Ro) level, peak at 0.8 (Ro), and end at about 1.35 (Ro).
Present maturation levels for the Egret Member source rocks, as well as
time-temperature modelling of hydrocarbon generation near drilled structures,
suggest that oil generation began about 100 million years ago and that peak
generation was not reached until about 50 million years ago, during the
Early Tertiary. Pre-Tertiary hydrocarbon generation and expulsion were
possible only in the deepest part of the basin, where the Jurassic source
rocks are buried to an estimated depth of 10,000 m. Generation and migration
of hydrocarbons could have begun there as early as the Early Cretaceous.
Although the structural framework of the Jeanne d'Arc Basin was nearly
complete by late Albian time, faulting and subsidence in the Late Cretaceous
and Early Tertiary (mid-Eocene) probably contributed significantly to the
generation, migration and distribution of hydrocarbons in the basin. In
addition, areas affected by Early Tertiary extension and sag, such as Hebron,
Ben Nevis, Mara, and North Trinity, show heavy oil of moderate to extensive
biodegradation in shallow reservoirs.
Download entire Terra Nova Geoscience (MS Word) document dp2.zip
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