Scotchman et al 2011 -The Breakup of Gondwana in Santos Basin.pdf

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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.

THE BREAKUP OF THE SOUTH ATLANTIC OCEAN 857


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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.



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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.



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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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