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The breakup of the South Atlantic Ocean: formation of failed spreading axes andblocks of thinned continental crust in the Santos Basin, Brazil and its consequencesfor petroleum system development
I. C. SCOTCHMAN,1 G. GILCHRIST,2 N. J. KUSZNIR,3 A. M. ROBERTS4 and R. FLETCHER1,3
1Statoil (UK) Ltd, 1 Kingdom Street, London W2 6BD, UK (e-mail: [email protected])2Consultant, Statoil do Brasil, Rio de Janeiro, Brazil3Department of Earth and Ocean Sciences, University of Liverpool, Liverpool L69 3BX, UK4Badley Geoscience Ltd, North Beck House, North Beck Lane, Spilsby, Lincolnshire PE23 5NB, UK
Abstract: The occurrence of failed breakup basins and deepwater blocks of thinned continental crust is
commonplace in the rifting and breakup of continents, as part of passive margin development. This paper exam-
ines the rifting of PangaeaGondwanaland and subsequent breakup to form the South Atlantic Ocean, with devel-
opment of a failed breakup basin and seafloor spreading axis (the deepwater Santos Basin) and an adjacent
deepwaterblockof thinned continental crust (the SaoPauloPlateau) using a combination of2D flexural backstrip-
ping and gravity inversion modelling. The effects of the varying amounts of continental crustal thinning on the
contrasting depositional and petroleum systems in the Santos Basin and on the Sao Paulo Plateau are discussed,
the former having a predominant post-breakup petroleum system compared with a pre-breakup system in the
latter. An analogy is also made to a potentially similar failed breakup basin/thinned continental crustal blockpairing in the Faroes region in the NE Atlantic Ocean.
Keywords:Brazilian rifted margin, continental breakup, Santos Basin, Sao Paulo Plateau, Faroes, Atlantic
margin, subsidence, gravity inversion
Continental lithospheric thinning and rifted margin formation is
a poorly understood process, the kinematics of which can be
affected by many factors, including pre-existing lithospheric
heterogeneities, variations in plate kinematics and the presence of
mantle features such as plumes (e.g. Dunbar & Sawyer 1989;
Corti et al. 2003; Ziegler & Cloetingh 2004). Evidence of
complex kinematics of rifted margins is seen at many margins,
such as the South Atlantic Ocean off Brazil and the NE Atlantic
Ocean margin close to the Faroe Islands, both being discussed in
this paper.
The Santos Basin
The Santos Basin is the southernmost of the petroliferous chain of
basins along the western margin of the South Atlantic Ocean in
Brazil (Fig. 1). These basins, the Santos, Campos and Espirito
Santo Basins, resulted from rifting of the Gondwanaland super-
continent in the earliest Cretaceous with breakup and subsequent
seafloor spreading. The Santos Basin is a NESW-trending basin
that covers about 200 000 km2 of the Brazilian continental margin,
bounded by the Cabo Frio Arch to the north and by the Florianopo-
lis Platform to the south, both features being related to magmatic
activity associated with the westward prolongation of the ocean
fracture zones (Cainelli & Mohriak 1998). The western limit of
the basin is defined by the uplifted Precambrian rocks of the
Serra do Mar, a coastal range reaching up to 2000 m high, while
to the east the basin is flanked by the Sao Paulo Plateau (SPP).
The Florianopolis Fracture Zone (FFZ) is a major transform
feature which defines the southern limit of the Santos Basin and
marks a major break in South Atlantic Ocean breakup history.
To the south, seafloor spreading is constrained by the M4 and
M0 magnetic anomalies (Mueller et al. 1997), dated at 125.7 and
120.6 Ma, respectively. To the north of the FFZ, the age of breakup
is poorly constrained as it took place during the Cretaceous normal
polarity superchron between the M0 and C34 (83.5 Ma) magnetic
anomalies.
The Santos Basin and the Sao Paulo Plateau
Regional geology
Rifting and breakup of the Santos Basin and SPP areas appear to
have been very complex with several apparent attempts to extend
seafloor spreading north of the FFZ. This resulted in various
intrusive and volcanic features located in the southwestern SantosBasin and to the east of the SPP, where the Avedis volcanic
chain was interpreted by several authors as a failed spreading
centre (Cobbold et al. 2001; Meisling et al. 2001; Gomes et al.
2002). However, recent exploration and drilling indicate these fea-
tures to be probably of pre-rift origin and not the result of failed
Early Cretaceous-aged breakup. They form large regional struc-
tural highs (the Tupi and Sugar Loaf structures of Gomes et al.
2009) which drilling indicates to have trapped extremely large
volumes of hydrocarbons in overlying syn-rift and sag phase reser-
voirs. However, data presented in this paper suggest that an earlier
attempt at breakup took place in the centre of Santos Basin, extend-
ing northeastwards into the basin centre from the area of likely
oceanic crust emplaced in the southwestern Santos Basin (Meisling
et al.2001; Gomes et al.2002). The formation of the SPP and the
resultant development of the prolific pre-salt hydrocarbon province
recently discovered in the area appear intimately related to this
failed breakup event.
Rifting in the Santos Basin began around 140 Ma in the Neoco-
mian (Karner & Driscoll 1999), contemporaneous with eruption
of the Parana volcanics (Renne et al. 1996; Fig. 2). Syn-rift
deposition, overlying and interfingering with late stage basalts
VINING , B. A. & PICKERING, S. C. (eds) Petroleum Geology: From Mature Basinsto New Frontiers Proceedings of the 7th Petroleum Geology Conference,
855866. DOI: 10.1144/0070855# Petroleum Geology Conferences Ltd. Published by the Geological Society, London.
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(Chang et al. 1992), comprises fluvial lacustrine siliciclastics
which infilled much of the early rift topography. Subsequent depo-
sition, during the late rift sag phase, comprised basinal lacustrine,
organic-rich shales lapping onto coquinas of the Lagoa Feia
Formation, which were deposited on structural highs and the
basin flanks (Pereira & Feijo 1994). The sag phase is overlain, gen-
erally unconformably (Karner & Gamboa 2007), by evaporites
comprising intercalations of anhydrite and halite, reflecting devel-
opment of the post-rift South Atlantic Salt Basin (Fig. 3). Theseevaporites were deposited in the remnant sag basin system at or
at most several hundred metres below ambient sea-level (Karner
& Gamboa 2007) by marine incursions from the north (Davison
& Bate 2004). While Davison (2007) favours formation in a deep
rift basin, evaporite formation close to ambient sea-level is indi-
cated by several lines of evidence: the microbilitic limestones
immediately underlying the salt, forming the reservoir in hydro-
carbon discoveries such as Tupi, probably accumulated in less
than 3 m of water (J. Lukasik, pers. comm.), the layered evaporite
sequence overlying the basal halite section on the SPP was prob-
ably formed under shallow water sabkha-type conditions with
periodic emergence and, finally, the large rift block bounding
faults have a substantial post-salt displacement (e.g. Fig. 3,
Section 2 inset). Thick evaporitic sections up to 2 km thick devel-
oped in the adjacent Santos Basin where there was more accommo-
dation space, greatly influencing basinfill during the later evolution
of the basin with extensive halokinesis producing large salt diapirs
and walls within the deepwater basin.
In contrast, recent drilling on the SPP has found the structure to
have thicker, well developedsyn-rift andsag phase sections (Fig. 4;e.g. Gomeset al. 2009)whichare cappedby porous lacustrine lime-
stones of algal, stromatolitic or thrombolitic origin which form the
reservoir rocks in the recent large hydrocarbon discoveries in the
area, such as Tupi (Mello et al.2009). In contrast to the great thick-
nesses of evaporites seen in the Santos Basin to the west and
Campos Basin to the north (Davison 2007), on the SPP a generally
thinner layer of mobile salt overlies the syn-rift section; the rest of
the evaporitic section of up to 2 km in thickness appears from
seismic to be well bedded but is also tightly folded with diapiric
structures (Fig. 3). This well bedded section above the mobile
salt, previously identified as an extension of the Late Cretaceous
Fig. 1. (a) Bouguer gravity anomaly map (200 km high-pass filter) over the central area of Santos Basin showing the linear feature with strong negative gravity
anomaly adjacent to the SPP. (b) Bathymetry (metres) and (c) free air gravity (mgal) data for the Santos and Campos and adjacent segments of the Brazilian
rifted margin.
I. C. SCOTCHMAN ET AL.856
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turbidite play north of the Santos Fault, has been found by recent
drilling to consist of bedded evaporites, comprising halite and
anhydrite as well as complex evaporites such as carnallite, bischo-
fite, sylvite and tachyhydrite (Poiate et al. 2006). These complex
minerals are end members of the evaporitic system, indicating
development of extreme conditions.
Karner & Gamboa (2007) suggest a date for evaporite depositionin the Santos Basin as 110113 Ma, after which period major
marine flooding of the basin took place, probably linked to
breakup of the margin to the north adjacent to the Campos Basin.
This was marked in the Santos Basin by deposition of Albian-age
shallow marine carbonates, grey shales and sandstones which
were followed by coarse turbiditic sandstones and shales deposited
during progressive deepening of the basin, with the maximum
flooding conditions being marked by dark grey-black shales of
CenomanianTuronian age ItajaiAcu Formation. In the proximal
areas, a thick conglomerate package and shallow marine sand-
stones were deposited during the Santonian to Maastrichtian in
response to the first phase of Serra do Mar uplift (dated at 100
80 Ma by Lelarge 1993).
The age of the bedded evaporite deposition on the SPP is
equivocal as no biostratigraphic data from recent wells has been
published. Because of its extreme distal location, effectively iso-
lated between the African and Brazilian margins, evaporite depo-
sition under sabkha-type conditions may have continued up intothe Albian as no equivalent shallow marine carbonate platform
development is present, the evaporites being overlain by possibly
Cenomanian or younger Late Cretaceous turbidites and shales.
However, the age-equivalent section to the Albian carbonate plat-
form may instead be present as shales at the base of the marine
section overlying the evaporites.
By the Late Cretaceous and through the Cenozoic, deepwater
turbidite and shale deposition was predominant across both the
Santos Basin and the SPP, characterized by a basinward pro-
gradation of siliciclastics over platform/slope shales and marls.However, the SPP remained relatively sediment-poor with only
Fig. 2. Santos Basin stratigraphy and breakup history.
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around 2 km drift section compared with 45 km or more equival-
ent thickness inboard in the Santos Basin.
Regional structure
Regional evaluation of the Santos Basin andSPP, using an approxi-
mate4 4 km grid of 2D seismic data combined with both satellite
and ship-borne gravity and magnetic data, has shown that the struc-
ture of the basin is complex with a linear negative gravity anomaly
stretching from the outer southwestern part of the basin north-
northeastwards into the basin centre, with a large area to the ESE
comprising the SPP (Fig. 1). Regional seismic across the linear
gravity anomaly in the southwesterncentral Santos Basin
(Fig. 3) shows a linear anomaly with volcanic features of oceanic
Fig. 3. Seismic cross-sections across the deepwater Santos Basin and SPP.
I. C. SCOTCHMAN ET AL.858
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affinity extending from a crustal igneous feature in the southwes-
tern part of the basin northeastwards into the basin centre where
it terminates in a major fault, the Santos Fault. The feature forms
the northwestern margin of the SPP, giving rise to the observation
that these two features are connected. The contrasting structural
and stratigraphic characteristics of the SPP and the Santos Basin
to the north are illustrated by seismic line 3 of Figure 3, showing
the thick, well developed syn-rift section capped by over 2 km
of largely bedded evaporites in the former, while the latter
shows a thin syn-rift section capped by diapiric salt buried by
thick CretaceousCenozoic turbidites. In contrast, the equivalent
turbidite section on the SPP is only thinly developed, unlike
either the Santos Basin or the other basins to the north (Camposand Espirito Santo Basins).
In order to investigate the linear anomaly within Santos Basin
and its relationship to the SPP, along with the anomalous structural
and sedimentary development of the latter, crustal modelling was
undertaken. This used subsidence analysis from 2D backstripping,
based on a regional grid of eight seismic lines, in conjunction
with determination of crustal thickness from gravity inversion
based on the bathymetry and free air gravity anomaly (Fig. 1) as
detailed below.
Continental lithosphere thinning and crustal
thickness of the Santos Basin and Sao Paulo plateau
Crustal thickness, continental lithosphere thinning and Moho depth
for the Santos Basin and SPP areas of the Brazilian rifted margin
were studied by subsidence analysis using flexural backstripping
(Kusznir et al. 1995; Roberts et al. 1998) and gravity inversion
performed in the 3D spectral domain (Parker 1972), the latter
using a new method incorporating a lithosphere thermal gravity
anomaly correction (Greenhalgh & Kusznir 2007; Chappell &
Kusznir 2008).
Subsidence analysis
Subsidence analysis using flexural backstripping to produce
water-loaded subsidence, and gravity inversion using a new
method incorporating a lithosphere thermal gravity anomaly cor-
rection, were used to determine continental lithosphere thinning,
Moho depth and continental crustal thickness for the Santos Basin
and SPP areas of the Brazilian rifted margin. The results of the
subsidence analysis using flexural backstripping are described in
Scotchmanet al.(2006) and only a brief summary is given below.
Water-loaded subsidence was determined using the 2D flexural
backstripping (Roberts et al. 1998) of profiles across the Santos
Basin andSPP. Theassumption was made of thepalaeobathymetric
constraint of ambient sea-levelfor thebase Aptiansalt at thetime of
deposition, as discussed above. The water-loaded subsidence
was then processed using the extensional basin formation model
of McKenzie (1978) to determine continental lithosphere stretchingand thinning for (i) the assumption that base salt subsidence from
Aptian to present is due to both syn-rift and post-rift subsidence,
and (ii) alternatively that base salt subsidence from Aptian to
present is due to post-rift thermal subsidence only. An average
age of 120 Ma for rift development across the basin was used in
the modelling, an age older than the 110113 Ma estimates for
that of the Aptian salt (Karner & Gamboa 2007), to acknowledge
the existence of syn-rift deposition beneath the salt.
It is very likely that the base salt horizon within the Santos Basin
experienced syn-rift tectonic subsidence, followed by continued
thermal subsidence to the present day. In this likely scenario, treat-
ing the base-case salt water-loaded subsidence as only post-rift
leads to an overestimate of the bstretching factors required to
model the subsidence and the prediction of oceanic crust (with infi-
nite thinning) over much of the Santos Basin (Fig. 5a). This isclearly in conflict with the seismic and gravity data as any ocean
continent transition derived from this model is too far inboard.
Conversely, the salt does not represent the whole of the syn-rift
sequence and, as a consequence, treating the base salt water-loaded
subsidence as representing the whole of the syn-rift and post-rift
thermal subsidence underestimates continental lithosphere stretch-
ing and thinning (Fig. 5b). This assumption predicts finite (non-
infinite) thinning factors for the SPP, implying that this region is
underlain by thinned continental crust. A region of highly stretched
and thinned continental crust is predicted in the deepwater Santos
Basin to the NW, separating the SPP from the Brazilian margin.
Fig. 4. SPP seismic section illustrating the Tupi oil discovery.
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Crustal thickness determination
The input data used in the Santos Basin and SPP gravity inversion
study were satellite free air gravity (Sandwell & Smith 1997),
digital bathymetry (Gebco 2003), sediment thickness to base saltderived from 2D/3D seismic reflection mapping and ocean age iso-chrons to define unequivocal oceanic crust (Mueller et al. 1997).
Crustal thicknesses produced by the gravity inversion applied to
the Santos Basin and SPP are shown in Figure 6. Gravity inversion
results are shown both omitting sediment thickness information
(Fig. 6a) and including sediment thickness (Fig. 6b) to give a more
accurate depth to Moho where such data exists. Crustal cross-
sections with Moho depth determined using the gravity inversion
method are shown in Figure 7 and indicate that crustal basement
thickness in the Santos Basindecreases southwards. The low crustal
thicknesses in the south of the deepwater Santos Basin, shown on
cross-section 3 of Figure 7, and located to the north of the FFZ,
suggest that the SW Santos Basin is underlain by oceanic crust.
In contrast the SPP is underlain by crust between 12 and 16 km
thick which is interpreted as thinned continental crust (Figs 6 & 7).
Both the gravity inversion results and the flexural backstripping
subsidence analysis indicate that (i) the SPP is underlain by thinned
continental crust and (ii) that a tongue of oceanic crust extends
north of the FFZ into the deepwater Santos Basin.
Discussion and conclusions
The failed breakup model for the Santos Basin
The results of the subsidence modelling from flexural back-
stripping indicate a zone of high subsidence in the southwestern
Santos Basin, extending northeastwards into the central part of
the basin, albeit with decreasing subsidence. Estimates of crustal
thinning based on McKenzie (1978), backed by the regional geo-
logical reasoning, indicates that this represents both syn- and post-rift subsidence of thebase salt layer, suggesting theouterpart of the
southwesternSantos Basin is underlain by eitheroceanic crust or by
extremely thinned continental crust, which forms a tongue extend-
ing northeastwards into the basin centre. Seismic sections across
this area confirm this finding (Fig. 3), with associated igneous fea-
tures interpreted as seawards dipping reflectors observed within the
basin. The results also indicate thinned continental crust to the SE
of the feature beneath the SPP. The results of the gravity inversion
work (Fig. 6) lead to similar conclusions with thin potentially
oceanic crust in theouter part of theSantos Basin with decreasingly
thinned crust northeastwards along the feature and less thinned
crust beneath the SPP.
These results indicate that the feature is likely to be a failed sea-
floor spreading centre, representing an early attempt at breakup and
initiation of seafloor spreading through the centre of the SantosBasin north of the FFZ in the early Aptian. The results suggest
that extreme thinning occurred in the southern part of the feature,
which probably represents incipient oceanic crust. However,
the breakup and seafloor spreading event appears to have been
short lived, probably due to an adjustment of plate kinematics.
Rifting and breakup history of the Santos Basin/SaoPaulo Plateau
By 140 Ma in the Neocomian, rifting followed by breakup of the
southernmost South Atlantic Ocean took place south of the crustal
Fig. 5. Beta factormaps fortheSantos Basin andSPP from 2D backstripping from Scotchmanetal. (2006): (a) assuming subsidence ofbase salt is post-breakup
only; (b) assuming subsidence of base salt is both syn-rift and post-breakup.
I. C. SCOTCHMAN ET AL.860
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lineament subsequently re-activated as the Albian-aged FFZ. North
of this lineament, rifting began around 140 Ma (Fig. 8a), probably
contemporaneous with the intrusion of the Ponta Grossa dykeswarm and extrusion of the Parana volcanics, associated with the
Tristao da Cunha hotspot. Initiation of seafloor spreading occurred
south of the FFZ by 126 Ma, while to the north sag-sequence
deposition took place in the rift basin system with deposition of
the lacustrine source rocks and sandstones. During this time the
Tristao da Cunha hotspot appears to have moved into the rift area
north of the FFZ with intrusion of igneous bodies. Associated
with the hotspot, the spreading centre south of the FFZ may have
tipped-out into the rift basin on the northern side where it devel-
oped into incipient breakup and seafloor spreading on an ENE
SSW trend towards the Brazilian coastline around 120 Ma.
Crustal thinning appears to have affected the area with associated
thick development of sag-phase sedimentation, particularly over
what is now the SPP. Extreme crustal thinning took place alongthe line of the incipient breakup north of the FFZ, with likely
emplacement of oceanic crust in its southern part, north of the
FFZ, in what is now the southwestern part of the Santos Basin.
However, this phase of incipient breakup and seafloor spreading
appears to have failed in the early Aptian and resulted in the for-
mation of a failed breakup basin/seafloor spreading axis in whatis now the central part of the Santos Basin with an adjacent area
of thinned continental crust which became the present-day SPP.
During the late Aptian, deposition of thick halite took place in
the subsiding sag basins along the whole rifted margin, which had
been flooded intermittently by marine water, most likely from the
Fig. 6. (a) Crustal thickness map of the Santos Basin derived from gravity inversion assuming zero sediment thickness showing thick continental crust
underlying the SPP and thinned crust beneath the linear anomaly in the SW Santos Basin. ( b) Crustal basement thickness map derived from gravity inversion
incorporating sediment thickness data from seismic reflection grid showing thinned continental crust underlying the SPP and oceanic crust beneath the SW
Santos Basin.
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northviatheearlyriftsinthecentralpartoftheAtlanticOcean,asthe
RioGrandeHigh appears tohave preventedaccess bymarine watersto thebasin systemfrom thesouth (Scotchman et al. 2008). The SPP
area was at this time still on the African side of the rift system
and remained relatively high, forming the eastern flank of the
Santos failed breakup basin. Here the lack of large-scale regional
subsidence appears to haveresulted in thedeposition of veryshallow
water algal/thrombolitic/stromatolitic lacustrine/brackish waterlimestones capping the syn-rift shales, forming the reservoir in
recent discoveries such as Tupi. The area remained a positive
feature during the subsequent deposition of evaporites with
shallow, sabkha-like deposition of bedded evaporites. These
largely comprise halite and anhydrite, but occasionally with com-
plete evaporation leading to Mg- and K-salt precipitation.
Breakup finally appears to have taken place to the east of the SPP
in the late AptianAlbian, with evidence that this took place from
the north (Scotchman et al. 2008), with formation of the oceanicfracture zones such as the FFZ, which as noted above appears
to have re-activated an older crustal lineament. The Santos Basin
to the west was flooded by shallow seas with deposition of carbon-
ate platforms along the Brazilian flank and development of early
turbiditic sandstones and shales in the deepest parts of the basin.
The Albian carbonates appear to be not present or below seismic
resolution on the adjacent SPP, although a lateral facies change
to deep marine shales is also possible, making it difficult, without
biostratigraphic data, to resolve if deposition of the marginal
bedded evaporite sequence ended in the Aptian or continued into
the Albian. Regional evidence perhaps favours the latter hypothesis
as the Albian section in the deepwater Santos Basin flanking the
northern side of the SPP comprises anoxic organic-rich blackshales which form the main post-salt hydrocarbon source in the
basin (Katz & Mello 2000). Marine organic shale deposition gener-
ally requires a restricted basinal setting, for example, Demaison &
Moore (1980), and a narrow seaway between the coastal carbonate
platform and the SPP located over the thermally subsiding failed
spreading centre could easily fulfil such a role, providing additional
excellent hydrocarbon source potential.
When seafloor spreading finally occurred in the area, perhaps as
late as the end AlbianCenomanian, the split between Africa and
South America was completed and the Rio Grande High breached.
The whole area then underwent rapid subsidence with the spread of
deepwater turbidite deposition which had flooded both the Santos
Basin and the SPP. Figure 8 illustrates the simplified kinematic
model for the breakup of the BrazilianAfrican margin, showing
development of the failed breakup basin (the Santos Basin) andthe SPP.
Petroleum systems
The syn-rift Lagoa Feia lacustrine facies shales are the main hydro-
carbon source rock in the basin system along the Brazilian Atlantic
Ocean margin (Katz & Mello 2000), where they have charged post-
salt turbidite reservoirs of Late Cretaceous to Cenozoic age to form
a very prolific petroleum system with giant oil fields in basins such
as the Campos, Espirito Santo and Santos (Guardado et al. 2000).
Post-salt source rocks, particularly of Albian and Cenomanian
Fig. 7. Crustal cross-sections showing crustal basement thickness and Moho depth determined from gravity inversion across the Santos Basin and SPP.
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Fig. 8. Conceptual breakup models for (a c) the South Atlantic in the Santos Basin and SPP and (d f) the North Atlantic in the Faroes/FaroeShetlandBasin(modified from Fletcher 2009).
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Fig. 9. (a) Crustal thickness map for the NE Atlantic Ocean showing thinned continental crust beneath the FaroeShetland Basin while the Faroes continental
block comprises relatively un-thinned continental crust. (b) The iSIMM Deep Seismic line illustrating the deep structure of the Faroes/FaroeShetlandBasin (from Whiteet al. 2008).
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Turonian, also provide important charging to these drift-phase
deepwater reservoirs in the Santos and Espirito Santo Basins.
The syn-rift sourced petroleum system occurs in both the Santos
Basin and the SPP; in the former the main reservoirs are in the
post-salt turbidites while in the SPP the main reservoir is the
pre-salt carbonate section capping the thick syn-rift/sag phase sec-tion. Importantly, due to the greatly different heatflow and burial
depth between the Santos Basin and the SPP, the hydrocarbon
phase is different. Within the Santos Basin, the subsidence associ-
ated with the failed seafloor spreading ridge resulted in the deep
burial of the syn-rift source rock beneath a thick Late Creta-ceousCenozoic turbidite section of 45 km (e.g. Fig. 3 seismic
line 3), up to twice that of the equivalent section on the SPP,
which remained a relatively positive feature. It is therefore not sur-
prising that light hydrocarbons and gas, such as the giant Merluza
gas field, predominate in this part of the Santos Basin. On the
SPP, the large volumes of oil (28 328API) have been discovered,
reportedly in the syn-rift carbonate reservoir with estimated
resources of 2050 109 barrels, and a large gas discovery,
Jupiter, discovered on the flank of the SPP. As well as greatly
increased burial depths, the thermal regime in the Santos Basin
differs from that on the SPP: at a depth of 3 km below mud-line,
the typical temperature in the former is around 100 8C compared
with 60 8C on the SPP (Poiateet al.2006). The temperature differ-
ences appear to reflect both a lower heatflow on the SPP and theeffects of the thick evaporites blanketing the structure.
Therefore the development of the prolific pre-salt oil province
on the SPP appears related to its unique structural development
compared to the other basins along the Brazil Atlantic Margin:
the thinned crust allowed development of a thick, well developed
syn-rift/sag phase section with resultant excellent source andreservoir rocks, relatively low subsidence compared with the adja-
cent Santos Basin and relatively low temperatures due to lower
heatflows and the thick evaporites, allowing the source rocks to
remain in the oil window compared with the Santos Basin, where
the equivalent source rock kitchens are in the gas-window or are
burnt out.
Comparison of continental lithospheric thinning
and rifted margin formation in the Santos Basin/SaoPaulo Plateau with the Faroe Shetland Basin and
Faroes margin, NE Atlantic
The kinematics of continental lithosphere thinning and breakup at
the Brazilian margin can be compared with those at the NE Atlantic
Ocean in the FaroeShetland area (Fig. 9). The FaroeShetland
Basin (FSB) and Fugloy Ridge appear to be analogous features to
the Santos Basin and the SPP, respectively. The FSB, a major
Cretaceous Cenozoic depocentre, is located between the West
Shetland Platform north of Scotland and the Faroes (Dore et al.
1999) and contains a series of NE SW trending sub-basins
formed by a complex tectonic history involving multiple
phases of extension and volcanism (Carr & Scotchman 2003).
The FSB underwent several periods of rifting, accommodated by
extensional faulting from Devonian to Cretaceous times (e.g.Doreet al.1999; Robertset al.1999). The basin also experienced
lithospheric thinning in the Late Paleocene synchronously with
crustal rupture and the onset of Atlantic Ocean seafloor spreading
to the west of the Faroe Islands, at the Faroes margin (Fletcher
2009). This was accompanied by a massive outpouring of basalt
which covers part of the FSB. The crustal thickness map of the
Faroese region (Fig. 9a), derived using the method of Greenhalgh
& Kusznir (2007) and the iSIMM refraction line (Fig. 9b)
shows that the FSB is underlain by very thinned crust. Thin crust
beneath the FSB is coaxial with thin crust at the Mre margin,
and the FSB is postulated to be a failed breakup basin at the
palaeopropagating tip of the Atlantic. A schematic diagram of the
kinematics of breakup at the Faroes margin is shown in Figure 8b.
Both the Santos Basin and the FSB appear to be failed breakup
basins associated with attempted propagation of seafloor spreading
and consequent thinning of continental crust, while the adjacent
structural highs, the SPP and Fugloy Ridge respectively, represent
areas of relatively thick continental crust, albeit with greatly differ-
ing amounts of thinning. The SPP underwent considerable thinning
and subsidence which accommodated a thick syn-rift section con-
taining both source and reservoir rocks capped by a thick post-rift
evaporitic section, resulting in a prolific petroleum province.The Fugloy Ridge also experienced considerable crustal thinning
but comparison with the SPP is difficult as the crust has probably
been re-thickened by the addition of both igneous intrusions
and extrusive volcanics after it was thinned. Thick volcanics on
the Fugloy Ridge mean that the syn-rift section is hard to image,
although recent seismic data indicate the development of a sedi-
mentary sequence beneath the post-rift sequence (Roberts 2007),
indicating the potential for the development of a petroleum
province.
Conclusions
The modelling suggests that the linear gravity and magnetic feature
identified in the Santos Basin represent abandoned seafloor spread-ing propagation with the formation of oceanic or proto-oceanic
crust at its southwestern end. This represents an early attempt at
seafloor spreading initiation north of the FFZ during the early
Aptian. The breakup process also resulted in the formation of an
adjacent area of thinned continental crust, the SPP, with subsequent
deposition of a thick syn-rift/sag phase section containing bothhydrocarbon source and reservoir rocks capped by a thick post-rift
bedded evaporitic section. A prolific pre-salt petroleum province
developed on this feature due to preservation of the syn-rift/sagsource rocks and reservoirs by the relatively low heatflows and
subsidence compared with the adjacent Santos Basin. Breakup
and seafloor spreading to the east of the SPP, marking the final
breakup of the South Atlantic Ocean, appear to have been initiated
from the north (Scotchman et al. 2008) and occurred in the late
AlbianCenomanian.Similar analysis of the FSB area of the northeast Atlantic Ocean
suggests development of a similar failed breakup/seafloor spread-ing basin. Both the Brazilian and Faroes margins exhibit evidence
for complex breakup kinematics, where lithospheric thinning orig-
inallyoccurredat twoor more overlappingsegmentsbefore becom-
ing linked when the lithosphere ruptured. Sedimentary basins on
the regions of thinned crust on rifted continental margins have
potential to be hydrocarbon provinces.
The authors would like to thank Statoil for permission to publish this work
and John Kipps and Michael McCambridge for their draughting of the
figures and the many revisions.
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