Potential for lacustrine source rocks in Triassic synrift basins offshore Eastern North America

Abstract

Recent discoveries of super giant pre-salt oil fields in Brazil's offshore basins, and related discoveries in its African conjugates, have highlighted the great importance of syn-rift / pre-breakup fluvial-lacustrine successions to the success and efficiency of the petroleum systems. Improvements in seismic acquisition and processing technologies were keys in imaging the architecture of the underlying rift basins, and interpreting the basin fill and internal depositional facies later confirmed by drilling. This knowledge has highlighted the search for similar depositional settings and source successions offshore Nova Scotia. This poster display postulates that older Middle to Late Triassic lacustrine successions do exist, and previous interpretations of their source rock potential at the time of their deposition may have to be reconsidered.

Full text

Potential for lacustrine source rocks in Triassic
synrift basins offshore Eastern North America
David E. Brown
Canada-Nova Scotia Offshore Petroleum Board, 1800 TD Centre, 1791 Barrington Street, Halifax, Nova Scotia, B4A 3S5, Canada
dbrown@cnsopb.ns.ca
THESIS
Recent discoveries of super giant pre-salt oil fields in Brazil’s offshore basins, and related discoveries in its African conjugates, have highlighted the great
importance of syn-rift / pre-breakup fluvial-lacustrine successions to the success and efficiency of the petroleum systems. Improvements in seismic acquisition
and processing technologies were keys in imaging the architecture of the underlying rift basins, and interpreting the basin fill and internal depositional facies
later confirmed by drilling. This knowledge has highlighted the search for similar depositional settings and source successions offshore Nova Scotia. This poster
postulates that older Middle to Late Triassic lacustrine successions do exist, and previous interpretations of their source rock potential at the time of their
deposition may have to be reconsidered.
INTRODUCTION
Syn-rift basins of the Middle Triassic to Early Jurassic age Newark Supergroup (SG) of Eastern North
America are exposed onshore and extend into adjacent offshore areas, with equivalent basins in
Morocco / Northwest Africa and Iberia (Figure 1). These basins are dominantly extensional though
some reveal evidence of an Early to Middle Jurassic compressional event (Withjack et al. 2005). Well
documented lacustrine source rock successions occur in a number of the onshore U.S. basins.
Although no commercial petroleum discoveries have been made, hydrocarbon shows in outcrops and
a few wells are documented confirming that a working petroleum system existed at some point in
time. Comparison of the reflection profiles from the basins reveals strong similarities. Interpretations
of their filling successions are used to postulate the potential of Middle to Late Triassic age lacustrine
facies in both, and in turn the climatic conditions during their deposition. Together, this has significant
implications regarding the potential creation and preservation of organic material and subsequent
contributions to petroleum systems.
Figure 1: Paleo-reconstruction of the Central Atlantic and syn-rift basin distribution at approximately earliest
Jurassic time prior to breakup. The dashed green line indicates the approximate limits of the early Jurassic
tropical region (Olsen & Kent 1996; Whiteside et al. 2011). Modified after Olsen & Et Touhami (2008).
DISCUSSION
The basin-fill model for the first order
sedimentary successions of Newark SG
extensional basins reflects the coupling of
tectonically-driven accommodation and
paleolatitudinal changes over time. Four
tectonostratigraphic (TS) units have been
defined by Olsen (1997) and Olsen et al. (2000)
(Figure 2). TS I is an unconformity-bounded,
early synrift fluvial-lacustrine sequence of Late
Permian age. TS II is composed of dominantly
fluvial (and some lacustrine) strata believed
representative of an underfilled, hydrologically-
open basin (subsidence < sedimentation). This
is followed by either a closed basin or one in
hydrological equilibrium (subsidence ≥
sedimentation) dominated by lacustrine (TS III),
and later playa / lacustrine (CAMP volcanics)
successions (TS IV). These units tend to be
separated by subtle unconformities.
As a result of the northward drift of Pangea, the
climate reflecting the paleolatitudinal position of the
Newark SG basins had a direct influence on their
facies development and lithologies, particularly the
lacustrine successions (TS III) (Figure 3).
Figure 2: Tectonostratigraphic (TS) architecture of the
Newark Supergroup basins, Eastern North America. Olsen
(1997); Olsen & Et Touhami (2008). Figure 3: The influence of climate and latitudinal position
on cyclic lacustrine deposition (Olsen & Kent 2000; Olsen
& Et Touhami 2008).
In the Newark and Fundy-Chignecto basins, the TS units are well exposed (Figures 4 & 5) and clearly recognised in the subsurface of the latter (Withjack et al.
1995). Within the former, lacustrine successions of the Lockatong and Passaic formations have been studied in great detail through outcrops and a massive
coring program (Newark Basin Coring Program) that permitted the creation of an extraordinarily detailed and high resolution astronomically-calibrated
geomagnetic polarity time scale for over 5 km of strata of Late Triassic to earliest Jurassic in the Newark Basin (c.f. Olsen et al. 1996; Olsen & Kent 1996, 1999).
25 km
N-37
P-79
NEW BRUNSWICK
Line 82-29
Depth Structure Map North
Mountain Fm.
Middle - Late
Triassic Early
Jurassic
North Mtn.
Blomidon
Wolfville
Lepreau
Honeycomb Pt.
Late
Permian
SEISMIC LINE CONTOUR INTERVAL: 200 m
N
Figure 4: Fundy Basin geologic
map. The Chignecto Subbasin’s
areal extent is shown in grey,
with detailed geology in Figure 9.
Seismic mapping reveals at least
10,000 m of strata in the basin:
6000 m pre-CAMP sediments
(Wolfville and Blomidon Fms.,
1000 m basalts (North Mountain
Fm.), and at least 3000 m post-
CAMP sediments. The latter
McCoy Brook Fm. is estimated
to extend from the Hettangian to
perhaps the Aalenian (Middle
Jurassic, post-breakup). Modified
after Wade et al. (1996).
Figure 5: Geologic map of the
Newark Basin. The red
segment of the seismic line is
shown in Figure 6. Slightly
modified after Withjack et al.
(2012).
Line NB-1
Cabot KBI #1
.
.
Through time, the southern basins transited across
the paleoequator and remained in the humid tropics
whereas basins to the north moved from the humid
tropics into the drier subtropical region.
Newark
Fundy /
Chignecto
Note: Scales are identical in both figures.

Seismic profiles in the Fundy-Chignecto and Newark basins reveal expansion of the respective TS units toward the basin-bounding faults (Figure 6). In both
basins, high amplitude, laterally continuous reflections are clearly visible in the respective TS III units. Note that similar, if not better defined reflections, are
seen in the underlying TS II strata that are distal to the fluvial-dominated sediments observed in outcrop. In the Newark Basin, the Cabot KBI #1 well penetrated
lacustrine sediments (and some deep water facies) at the top of the Stockton Formation (Olsen et al. 1996; Olsen 2010).
Figure 6: Seismic profiles across the Chignecto Subbasin and Newark Basin, with their locations shown in Figures 4 and 5 (red segments of NB-1 not shown). Correlative TS
formations are coloured the same for each basin and in all related figures. The red horizon is the rift-onset unconformity. For simplification, the Boonton Formation includes other
related Early Jurassic formations and tholeiitic volcanic units. Vertical and horizontal scales are identical. Interpretations for the NB line are slightly modified after Withjack et al. 2012.
During the Carnian, climate-sensitive lacustrine facies (Olsen & Kent 1996) and faunal distributions (Whiteside et al. 2011) infer a narrow equatorial humid belt
about 6° wide centred on the paleoequator (Figure 7; green dashed lines). Geomagnetic modelling by Kent and Tauxe (2005) however suggest that this zone
was broader and more comparable to today’s humid belt. Increasingly arid climatic conditions dominated north of this region (5-20°) (Figures 7 & 8). During the
Late Triassic, lacustrine sediments of the Carnian-Norian Lockatong Formation (‘L’) were deposited in semi-tropical conditions. Similar successions are
proposed for the older (Anisian-Carnian) Wolfville Formation (‘W’) when the Fundy-Chignecto Basin occupied the same 6°-8°N paleolatitudinal position (i.e.
reversing the ~10° northward drift over the Late Triassic). The Newark Basin would shift southwards into the equatorial humid zone and Stockton lacustrine
successions presumably would be similar to Richmond-Newark and/or Richmond cycles defined by Olsen & Kent (2000) (see Figure 3).
Earliest
Carnian
Paleoequator
Figure 7: A reconstruction of the earliest Carnian paleoequator and
10° north and south latitude lines through reversal of the approximate
10° northward drift of Pangea from Middle Triassic to earliest Jurassic
(pre-breakup / ~Sinemurian). Compare with Figure 1. Modified after
Olsen & Et-Touhami (2008).
Figure 8: Nomogram of time and geography for the Newark
Supergroup, eastern North America illustrating the response of
sedimentation over climatic zones though time and northward plate
movement (Olsen et al. 2010). Interpreted lacustrine successions for
the fluvial-dominated Stockton and Wolfville formations are shown in
grey. Note that lacustrine strata are also recognized in the equivalent
TS-II succession of the proximal Argana Basin of Morocco. Formation
abbreviations and colours are keyed to Figures 4, 5 & 6. Modified after
Olsen et al. (2010).
*
Newark
Fundy /
Chignecto
M
B
P B
S
L ?
W
H
a
e
4°
6°
4°
6°
4° 6° *
CAMP
TS II
TS I
TS III
Post
Rift
TS IV
*
1800, 1791 Barrington Street, Halifax, Nova Scotia, B3J 3K9, Canada – www.cnsopb.ns.ca
TS II
TS III
TS IV
NW
600
650
700 550 500 450 400 350
1
2
3
4
0
TWTT –Sec.
SP
Cabot KBI #1 SE
Newark Basin –Line NB-1
B –Boonton Fm. (projected)
P –Passaic Fm.
L –Lockatong Fm.
S –Stockton Fm.
S? – ‘Buried’ Stockton Fm.
SL –Stockton Lacustrine facies?
P –Middle Proterozoic Basement
TWTT –Sec.
0
1
2
3
4
1100 SP
1200 1000 500
700800 600 300
900 400
NW SE
Chignecto Subbasin –Line 82-29
M –McCoy Brook Fm.
P –North Mountain Fm.
L –Blomidon Fm.
BL –Blomidon Lacustrine facies?
W –Wolfville Fm.
WL –Wolfville Lacustrine facies?
P –Late Paleozoic Basement
TS II
TS III
TS IV
TS II
TS III
TS IV
TS IV
TS III
TS II
TS IV
TS III
TS II
P – Paleozoic / Proterozoic Bsmt.
?
?

REFERENCES
Bohacs, K.M., Neal, J.E., Gabrowski, G.J. Jr., Reynolds, D.J. & Carroll, A.R. 2002. Controls on sequence architecture in
lacustrine basins – insights for sequence stratigraphy in general. 22nd Annual Gulf Coast Society–Society of Economic
Paleontologists & Mineralogists Bob F. Perkins Research Conference, Conference Proceedings CD-ROM, 403-423.
Formigli, J. 2007. Pre-Salt Reservoirs Offshore Brazil: Perspectives & Challenges. Petrobras Web Site,
www2.petrobras.com.br/ri/pdf/2007_Formigli_Miami_pre-sal.pdf, 21p.
Kent, D.V. & Tauxe, L. 2005. Corrected Late Triassic latitudes for continents adjacent to the North Atlantic. Science, 307,
240-244.
Leleu, S. & Hartley, A.J. 2010. Controls on the stratigraphic development of the Triassic Fundy Basin, Nova Scotia:
implications for the tectonostratigraphic evolution of Triassic Atlantic rift basins. Journal of the Geological Society, 167,
437-454.
Olsen, P.E. 1997. Stratigraphic record of the early Mesozoic breakup of Pangea in the Laurasia-Gondwana rift system.
Annual Reviews of Earth and Planetary Science 25, 337-401.
Olsen, P.E. 2010. Fossil great lakes of the Newark Supergroup – 30 years later. In: Benimoff, A.I. (ed) Field Trip
Guidebook, New York State Geological Association, 83nd Annual Meeting, College of Staten Island, 101–162.
Olsen, P.E. & Et-Touhami, M., 2008. Field Trip #1: Tropical to subtropical syntectonic sedimentation in the Permian to
Jurassic Fundy rift basin, Atlantic Canada, in relation to the Moroccan conjugate margin. Central Atlantic Conjugate
Margins Conference Halifax, Nova Scotia, Canada, 121p.
Olsen, P.E. & Kent, D.V. 1996. Milankovitch climate forcing in the tropics of Pangea during the Late Triassic.
Palaeogeography, Palaeoclimatology, & Palaeoecology, 122, 1-26.
Olsen, P.E. & Kent, D.V. 1999. Long-period Milankovitch cycles from the Late Triassic and Early Jurassic of eastern North
America and their implications for the calibration of the early Mesozoic time scale and the long-term behavior of the
planets. Philosophical Transactions of the Royal Society of London (series A), 357, 1761-1787.
Within the Chignecto Subbasin, the Wolfville succession rests unconformably on Late Carboniferous coal measures of the Cumberland and
Riversdale groups that are exposed along the Nova Scotia to the northeast (Figure 9). The estimated aerial extent of the interpreted lacustrine
succession is approximately 400 km2 with a maximum thickness of about 2700 m (estimate derived from the Chinampas N-37 well velocity
survey) (Figure 10). Note the proximity of the basin’s depocentre to a releasing bend of the adjacent Glooscap-Chedabucto strike-slip fault.
CONCLUSIONS
•Seismic profiles in the Newark and Fundy-Chignecto basins reveal identical structural geometries and tectonostratigraphic architectures reflecting
the influences of tectonism and climate.
•Late Triassic (Carnian-Norian) TS III lacustrine / playa successions were deposited when both basins were located adjacent to / north of the
equatorial humid tropic zone, with facies in the more northern Fundy-Chignecto reflecting increasing aridity as Pangea drifted northwards.
•Older Middle to Late (Anisian-Carnian) TS II fluvial successions reveal similar laterally equivalent, high amplitude reflections in the basin centres that
are interpreted as lacustrine sequences.
•This infers that the basins had significant tectonic extension from their inception, lakes formed immediately, and thus were hydrologically closed.
•Repositioning of the basins for the depositional period of TS II fluvial-lacustrine successions moves them into the tropic to near tropic climate zones.
•These humid-tropical climate conditions would be conducive to the creation and preservation of organic matter as demonstrated by other Newark
Supergroup basins.
•Similar subsalt, syn-rift half-grabens exist offshore Eastern North America and Northwest Africa / Iberia, and may contain source rock successions
that could be significant contributors to their petroleum systems.
Figure 10: Time structure map for the top of the Wolfville Formation
(base Blomidon). The grey area is the plane of the highest
detachment fault that over-rides lacustrine strata closest to the
assumed near vertical main border fault (Harvey-Hopewell fault; a
reactivated middle Paleozoic (Devonian Acadian orogeny)
transpressional feature).
Figure 9: Time structure map for the base of the Wolfville
Formation / top basal rift onset unconformity. The transverse fault
is the terrane-bounding Glooscap-Chedabucto fault with motion
that was dextral in the Devonian-Carboniferous (transpressional),
sinistral in the Middle Triassic to Early Jurassic (extensional), and
dextral (transpressional) sometime in the Middle Jurassic.
Figure 11: Well known seismic depth profile of the Tupi-Iracema
complex in the Santos Basin (Formigli 2007). Post-breakup salt
overlies the half grabens with possibly four tectonostratigraphic
units separated by unconformities (two in each “Syn-rifte” unit).
The lower unit 1 – Aratu Sequence - is composed of fluvial
sandstones, conglomerates, playa mudstones and volcanics,
and represents the initial syn-rift succession. The upper unit 2 –
Aratu / Buracica / Jiquia Sequence - is composed interbedded
lacustrine siliciclastic and limestone coquina reservoirs and
organic-rich source rock shales representing dominantly
lacustrine depsosition. The post-rift Sag – Alagoas Sequence –
directly underlies the salt and represents deposition of shales,
shaley limestones and microbialites (reservoir) under lacustrine
to hypersaline transitional marine conditions.
The Iracema and Tupi discoveries are believed to be a single
accumulation containing 12-30+ billion Boe initial hydrocarbons
in place (IHIP).
The pre-salt section shares geometric and stratigraphic
similarities with those of the Fundy-Chignecto and Newark
basins, however, these basins are inboard of the main rift axis
and salt basin. Similar half grabens exist under the thick salt of
the Hettangian Argo Formation in the offshore Scotian Basin,
though the salts are considered pre-breakup (sag phase).
Modified after Formigli (2007).
1800, 1791 Barrington Street, Halifax, Nova Scotia, B3J 3K9, Canada – www.cnsopb.ns.ca
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Bachmann, G. & Lerche, I. (eds) Epicontinental Triassic, Volume 3. Zentralblatt fur Geologie und Palaontologie, VIII,
1475-1496.
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basin (Early Mesozoic, Eastern North America). Geological Society of America, 108, 40-77.
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comparison of the stratigraphy and depositional environments of the Argana (Morocco) and Fundy (Nova Scotia,
Canada) Permian-Jurassic basins. In: Oujidi, M. & Et-Touhami, M., (eds) Le Permien et le Trias du Maroc, Actes de la
Premièr Réunion su Groupe Marocain du Permien et du Trias, Oujda, Hilal Impression, 165-183.
Olsen, P.E., Kent, D.V. & Whiteside, H. 2010. Implications of the Newark Supergroup-based astrochronology and
geomagnetic polarity time scale (Newark-APTS) for the tempo and mode of the early diversification of the Dinosauria.
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regional setting, stratigraphy and hydrocarbon potential. Atlantic Geology, 32(3), 189-231.
Whiteside, J.H., Grogan, D.S., Olsen, P.E. & Kent, D.V. 2011. Climatically driven biogeographic provinces of Late Triassic
tropical Pangea. PNAS, 108 (22), Proceedings of the National Academy of Sciences, 8972–8977.
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extension and shortening during passive-margin development. Tectonics, 14, 390-405.
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Triassic-Jurassic Newark basin of eastern North America. In: Mohriak, W.U., Danforth, A., Post, P.J., Brown, D.E., Tari,
G.C., Nemčok, M. & Sinha, S.T. (eds) Conjugate Divergent Margins. Geological Society, Special Publications, 369,
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