Ana Aapg Memoir Habitat of Petroleum Along the South Atlantic Margins

Habitat of Petroleum Along the South Atlantic Margins

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73
Yıl:
2000
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AAPG Memoir
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Szatmari, P., 2000, Habitat of petroleum along the South Atlantic margins, in
M. R. Mello and B. J. Katz, eds., Petroleum systems of South Atlantic margins:
AAPG Memoir 73, p. 69–75.

Chapter 6

Habitat of Petroleum Along the
South Atlantic Margins
Peter Szatmari
Petrobrás Research Center
Rio de Janeiro, Brazil

Abstract
On a predrift reconstruction of their facing margins, almost all of Brazil’s and southwest Africa’s petroleum reserves occur within a nearly continuous north-south-trending strip, the South Atlantic Megatrend,
whose formation was controlled by continental breakup. It took almost 40 m.y., from earliest Valanginian
to late Albian time, for Africa and South America to separate, as South America rotated clockwise relative
to Africa about a pole located in northeast Brazil. Rifting mechanism and continental deformation were
controlled by the distance from the shifting pole of differential rotation. The rift was cut into two approximately equal portions by the Ponta Grossa dike swarm and associated volcanics that formed over the
Paraná–Tristan da Cunha hot spot. South of the dike swarm continental, breakup was fast and characterized by intense volcanic activity (seaward dipping reflectors). In this area, a euxinic sea formed by Aptian
time over the oceanic crust. To the north of the dike swarm, continental breakup was slower, and a northward-tapering rift valley lying deep below sea level formed over extended continental crust. This depression was flooded catastrophically by sea water in late Aptian time, depositing a 2000-m-thick salt body.
The oil fields constituting the South Atlantic Megatrend occur either within the salt basin, both above
and below the salt, or near the pole of differential rotation in transtensional inland rifts that contain no salt.
The opening of the Atlantic divided this megatrend into segments that now alternate between the
Brazilian and African margins: an oil-producing segment on one margin is faced by a relatively sterile
segment on the other. This distribution may r; esult from alternating polarity of rifting by simple shear.
Oil richness, defined here as the volume of original reserves per unit length of a segment, increases
along the megatrend with distance from the pole of relative rotation. Far from the pole, as in the Lower
Congo and especially in the Campos Basin where the rift is wide, the rifted continental crust extends far
offshore, oil richness is high, and the largest petroleum accumulations occur offshore.

INTRODUCTION

CONTINENTAL BREAKUP BY ROTATION
ABOUT A POLE IN NORTHEASTERN
BRAZIL

On a prerift reconstruction of their facing margins,
almost all of Brazil’s and sub-Saharan western Africa’s
petroleum reserves occur within a narrow, straight, and
(relative to South America) north-south-trending strip
which is named here the South Atlantic Megatrend. This
trend extends for more than 2000 km from southern
Nigeria to northern Angola on the African side and from
the Potiguar Basin to the northern part of the Santos Basin
on the Brazilian side. Along this trend, oil is contained in
basins and structures that owe their existence, directly or
indirectly, to continental breakup. The mechanism of
breakup thus exerts a controlling influence on the distribution of hydrocarbon deposits.

Our model is based on the separation of Africa and
South America by differential rotation about a migrating
pole that was located in northeastern Brazil (Rabinowitz
and LaBrecque, 1979; Szatmari et al., 1985a) during most
of the continental breakup (Figure 1). As a result, along
Brazil’s eastern South Atlantic margin the rift propagated
to the north. Although such propagation locally resulted
in distinct rifting episodes, such as the one observed at
the end of the Early Cretaceous Rio da Serra stage, the
crucial point of the model is that rift propagation by
69

70

Szatmari

BRAZIL
Ponta
Grossa
Dike
Swarm

AFRICA

Figure 1—Schematic view of compression and extension
caused by separation of South America from Africa by
clockwise rotation about a pole located along Brazil’s
equatorial margin.

differential rotation of the separating continents was a
lengthy process. The angular momentum for such large
masses of thick continental lithosphere is very high, so that
it takes a long time both for initial acceleration to occur and
for differential rotation to grind to a halt. In the case of the
South Atlantic, it took almost the entire Early Cretaceous,
from earliest Valanginian to late Albian time, a period of
nearly 40 m.y., for the two continents to separate.
Differential rotation of the two continents about a pole
located in northeastern Brazil created the South Atlantic
rift, which tapers northward toward the pole of rotation.
The rift was divided into two structurally different
portions of about equal lengths by the Ponta Grossa dike
swarm (Figure 2) which formed over the Paraná–Tristan
da Cunha hot spot at about 130 Ma. To the south of the
dike swarm, in the southern Santos Basin, in the Pelotas
Basin, and between Argentina and southern Africa,
sublithospheric temperatures were high and continental
breakup was rapid. It was accompanied by intense,
mostly subaerial, volcanic activity that formed a volcanic
margin marked by seaward-dipping reflectors. To the
north of the dike swarm, from the northern Santos Basin
to Recife, temperatures were lower, producing a rift
floored by continental lithosphere that was wide near the
Ponta Grossa dike swarm but narrowed to the northeast
toward the pole of rotation (Figure 2).
Near the hot spot, in the Campos and Santos Basins,
where the rift was wide, the pre-Aptian sequence is
largely composed of volcanics (basalt flows and tuffs).
These are slightly younger than the continental flood
basalts of the Paraná Basin, reflecting the motion of the
hot spot to the southeast relative to the South American
plate. The thickness of pre-Aptian volcanics and the
width of the rift decrease to the north, toward the pole of
differential rotation, so that to the north of the Espírito
Santo Basin, instead of volcanics, a thick sedimentary
section occupies the pre-Aptian sequence. A similar
decrease in volcanics also accompanies decreasing extension in the East African rift, away from the hot spot
located in Ethiopia and the Afar triangle.

Rift slope
Deep pre-salt depression
Seaward dipping reflectors
Invading sea

Figure 2—Paleogeographic pattern of the South Atlantic
rift in middle Aptian time.

As a result of the differential rotation of the continents
about a pole located in northeastern Brazil, rifting near
the pole lasted longer and was completed later than in the
south. Rifting in northeastern Brazil took place in two
stages. During the first, pre-Aptian stage, faults propagated to the north, opening the Recôncavo–Tucano Basin,
now located inland, while en-echelon north–south faults
formed in the Sergipe–Alagoas Basin. During the second,
Aptian–Albian stage, northeast-trending transtensional
faults in the Sergipe–Alagoas Basin marked the transfer
of the differential rotation between the two continents to
the present continental margin (Szatmari et al., 1985a,
1987; Milani et al., 1987).
Both the inland Recôncavo–Tucano rift and the
Sergipe–Alagoas Basin along the continental margin
terminate to the north at the latitude of Recife, reflecting
the strong obstacle to rift propagation presented by the
transverse east–west trending Precambrian Pernambuco
and Patos fault zones (Szatmari et al., 1985a, 1987).
Therefore, north of Recife (between João Pessoa and
Natal), the clockwise rotation of South America relative to
Africa caused compression and left-lateral transpression
instead of extension, with no major rifting taking place
until Albian time (Françolin and Szatmari, 1987; Szatmari
et al., 1985b, 1987). Thus, the northeastern tip of South
America was turned counterclockwise relative to the rest
of the continent as it was pulled away from the facing
margin of Nigeria in the north while still being pressed
against Africa in the east (Figure 1). This created a stress
field that was inverted relative to most of Brazil’s eastern
margin, being characterized by east–west compression
and north–south extension. The northeast-trending rightlateral transtensional Pendência rift in the Potiguar Basin
formed in this stress regime (Szatmari et al., 1987;
Françolin and Szatmari, 1987; Françolin et al., 1994).
Along the rest of the equatorial margin, the differential
rotation of the two continents created north–south
compression (Figure 1) until Aptian time when the rift

Chapter 6—Habitat of Petroleum Along the South Atlantic Margins

71

ALLEVIATION OF PRESSURE BY
NORTHEAST-TRENDING FAULTS
ACROSS SOUTH AMERICA
Compression on the equatorial margin was alleviated by left-lateral strike slip movement along northeast-trending faults that cut through South America
and may have formed as shear faults simultaneously
with and parallel to Central and South Atlantic rifting
(Figure 3).

SD

P

PJ

1. The rift along the eastern (South Atlantic) margin
formed as South America moved away from the
asthenospheric high in the east, which increasingly
separated it from Africa, and onto the subduction
zone in the west, at the open margin of
Panthalassa (the Proto-Pacific).
2. This movement away from Africa was fast in the
south but slight in the north because of friction
along the equatorial margin. This difference
induced the clockwise rotation of South America.
3. All observed effects resulted from this rotation.
4. The eastern margin, characterized by nearly
east–west extension, tapered toward the pole
located in the northeast.
5. Instead of extension, the rotation resulted in
east–west to WNW-ESE compression at the northern end of the eastern margin, north of the
east–west trending Pernambuco and Patos ductile
shear zones.
6. East–west extension on the eastern margin took
place at the expense of north–south compression
on the equatorial margin as the continent rotated
clockwise.
7. This compression decreased toward the pole of
rotation in northeastern Brazil so that the easternmost segment of the equatorial margin was characterized by north–south extension and east–west
compression.
8. North–south compression along the equatorial
margin was in part alleviated by the breakaway
and lesser clockwise rotation of northwestern
Africa relative to the bulk of Africa.
9. Because the rotation of South America exceeded
the rotation of northwestern Africa, compression
along the equatorial margin increased westward.
10. Compression on the equatorial margin became
compensated by left-lateral movement of continental slices along southeast–northwest trending
faults toward the open Pacific subduction zone in
the southwest.

BO

propagated westward along the entire length of the equatorial margin (Szatmari et al., 1985b, 1987; Zanotto and
Szatmari, 1987).
The basic pattern of deformation can be interpreted in
terms of simple mechanical principles (see Figure 1):

Figure 3—Position of the Bocono (B), Pisco–Jurua (P-J),
and Sobral–Dom Pedro II (S-DP) faults on Bullard et al.’s
(1965) reconstruction of Pangea. During South America’s
clockwise rotation, left-lateral strike-slip movement along
these faults alleviated pressure on the continent’s northern margin. (Modified after Szatmari, 1983.)

The westernmost of these faults is the Bocono fault,
which trends northeastward in Venezuela. Present
motion along this fault is right lateral due to east–west
compression on the hot, hard-to-subduct Nazca and
Cocos plates. However, the earlier stress field was characterized by north–south compression and east–west
extension, resulting in left-lateral motion during Early
Cretaceous time.
Farther to the east is the northeast-trending Pisco–Jurua
fault system that cuts through South America from
Georgetown on the Atlantic to Pisco on the Pacific margin
(Szatmari, 1983). Left-lateral strike slip motion along this
fault system was accompanied by increasing compression to the southwest. Wrench tectonics is evident
throughout. In the northeast, the fault separates the
Guayana shield into two structurally distinct provinces.
In the exposed Precambrian basement, brecciation occurs
along the fault. The Tacutu graben, located along the
fault, is floored by Lower Jurassic basalt that is overlain
by Jurassic and possibly Lower Cretaceous rift sedimentary beds containing a thick evaporite horizon. The
east–northeast elongated graben presumably formed as
the Central Atlantic rift opened during Early Jurassic time

72

Szatmari

(Szatmari, 1983). It was segmented by Early Cretaceous
strike-slip faults created by the differential rotation of
South America and Africa during Early Cretaceous continental breakup.
The Pisco–Jurua fault cuts through the east–west
trending Upper Amazon or Solimões Basin. The
preserved sedimentary sequence of the Upper Amazon
Basin is mostly Paleozoic, ranging from Ordovician to
Permian; it was intruded by Early Jurassic diabase sills
and dikes simultaneously with Central Atlantic rifting.
The sills are widespread and have more than 500 m of
cumulative thickness. Both the sills and the Permian–
Carboniferous evaporite-bearing sequence which they
intruded were severely deformed in Early Cretaceous
time (Szatmari, 1983; Mosmann et al., 1984). The exact
time of deformation has not been determined; it is
younger than the deformed Early Jurassic sills but older
than the Aptian–Albian beds that discordantly overlie the
deformed structures. Deformation is characterized
mostly by reverse faulting and pop-up structures in the
lower Paleozoic and Lower Carboniferous beds and also
by folds in the Permian sequence. The axial plane of the
folds is often inclined and lies along the upward continuation of the reverse faults. The structures are mostly elongated in an east–west to northeast–southwest direction,
frequently forming arches concave to the northwest
(possibly over reactivated ductile shear zones that originally formed along a Precambrian wrench fault system
along the axis of the Amazon Basin). The heights of the
folds and the vertical offsets along the faults vary but
increase to the southwest; the structures hold significant
commercial gas and light oil reserves.
To the southwest of the Amazon–Solimões Basin, the
Pisco–Jurua fault passes through Precambrian basement
covered by Cretaceous–Tertiary sedimentary beds until
reaching the Eastern Cordillera of the Andes. At the
Abancay deflection, the Pisco–Jurua fault intersects, leftlaterally deflects, and offsets the Eastern Cordillera
(Szatmari, 1983), which is composed of lower Paleozoic
sedimentary rocks intruded by Permian granites.
Northeast-trending fault planes, some with distinct horizontal striations, can be observed in both the lower
Paleozoic sedimentary sequence and the Permian granites. They reach several hundred meters in height in the
Urubamba Valley near Machu Picchu. The fault system
reaches the Pacific coast near Pisco.
The next northeast-trending wrench fault system to
the east is the Sobral–Dom Pedro II fault zone. It reaches the
Atlantic coast near Fortaleza in Ceará State, Brazil. It is
well expressed offshore, where it disturbs Cretaceous
sedimentary beds at Icarai. On land, near Sobral, the fault
is well exposed as mylonite zones cutting through the
Precambrian basement, reactivating a fault formed
during late Proterozoic continental collision (Braziliano
cycle). A few kilometers to the southwest, at Santana de
Acarau, the fault borders a sliver of Lower Cretaceous
sedimentary beds that are nearly vertical along the fault
but dip gently only a few hundred meters away from it
(Destro et al., 1994). Partially sheared pebbles near the

fault provide evidence for high temperatures. Farther to
the southwest, the fault runs close to the southeastern
border of the Paleozoic Parnaiba or Maranhão Basin; silicified fault crests with horizontal striations rise high
above the surface along the fault.
Particularly interesting is the complex pattern of faults
in northeastern Brazil, in the vicinity of the pole of rotation, created by Early Cretaceous reactivation of late
Proterozoic ductile shear zones. The reactivated fault
zones are approximately radial, pointing toward the
center of the northeastern tip of South America. There is
a set of concentric strike-slip faults about this pole,
bordering the arcuate Brazilian coastline. Three major
areas can be distinguished: (1) the Potiguar Basin, where
the dominant movement was north–south extension
(away from the equatorial margin of Nigeria); (2) the
northeast coastline, where the motion was predominantly transcurrent; and (3) the southeast, where the
predominant extension was east–west to southeast–
northwest.

TECTONIC CONTROL OF THE SOUTH
ATLANTIC SALT BASIN
The northward-tapering shape of the South Atlantic
rift basin, created by the differential rotation of the two
continents, is reflected by the wedge-like shape of the
upper Aptian salt (Figure 2) that overlies the pre-Aptian
and lower Aptian rift sequence. Geophysical data along
the Brazilian margin indicate that the area of salt was not
significantly increased by postdepositional oceanward
salt flow during Late Cretaceous and Cenozoic time.
Instead, extension of the salt nearshore was compensated
by compression farther offshore (Demercian et al., 1993;
Cobbold et al., 1996). Thrusting and folding of strata
occurred over the salt near the limit of the oceanic crust
which divided the salt body that was originally continuous from Brazil to Africa. Thus the present distribution of
the salt permits reconstruction of the depositional limits
of the Aptian salt basin.
The South Atlantic salt basin is about 400 km wide at
its southern end in the Santos Basin, but it tapers toward
the pole in northeastern Brazil to less than 100 km in the
Sergipe–Alagoas Basin. The time of salt deposition is
bracketed by the Aptian ages of two marine faunas
located above and below the salt, respectively (SilvaTelles, 1996; E. A. M. Koutsoukos, personal communication, 1997). The subsalt fauna, in the Campos Basin,
marks a short marine incursion before salt deposition and
is of early Aptian age; the fauna above the salt, in the
Sergipe–Alagoas Basin, is late Aptian. The salt interval
itself is presumably short, on the order of a few hundred
thousand years, reflecting very high rates of salt deposition (2–4 cm per year).
Subsalt relief was shaped in a southward-widening rift
valley that was occupied, prior to its invasion by sea
water, by partially sediment-filled lakes separated by

Chapter 6—Habitat of Petroleum Along the South Atlantic Margins

AFRICA
AFRICA

SOUTH
AMERICA

BRAZIL

Potiguar
Sergipe-Alagoas
Tucano
Recôncavo

N

Espírito Santo
Campos
Santos

73

Figure 4—Distribution
of hydrocarbon-bearing sedimentary
basins on the facing
margins of the South
Atlantic.

NIGER DELTA
CAMEROON

GABON
CONGO
CABINDA
ZAIRE
CUANZA

ANGOLA

intralake sedimentary, volcanic, and basement highs.
Subsalt relief is more irregular near the pole in the
Sergipe–Alagoas Basin, where transtensional rifting at the
time of salt deposition was still active, than far from the
pole in the Campos–Santos Basin, where the depression
was wide and most of the faulting had ceased by the time
salt deposition started.
Mass balance calculations indicate original salt thicknesses of about 2000 m. We believe that most of this salt
was deposited in a preexisting rift depression dotted with
lakes that were about 2000 m below sea level (Figure 2).
This is because the rates of salt deposition were far too
high for tectonic subsidence to create space for syntectonic
salt deposition. Such a deep presalt depression is comparable to the present sub-sea-level depth of the surface of
the Dead Sea (–397 m) which lies in a much smaller and
less extended transtensional rift basin. Deposition in the
South Atlantic depression may have taken place in cycles
of recurrent catastrophic floods and evaporation, as the
sea located between southern South America and southern Africa overflowed the Ponta Grossa dike swarm
(Conceição et al., 1988) and associated volcanics of the
Paraná–Tristan da Cunha hot spot and filled the presalt
rift basin.
The flooding of the South Atlantic depression by the
sea in late Aptian time is reflected by a major, short-lasting drop of sea level at 112 Ma (Haq et al., 1987; SilvaTelles, 1996). Assuming that the wedge-shaped depres-

sion was 400 km wide at its mouth, 2500 km long to
Recife (where its width tapered to nearly zero), and 2000
m deep, its flooding required nearly 1 million cubic kilometers of sea water, causing about a 3-m drop in global
sea level.

INCREASE IN OIL RICHNESS AWAY
FROM THE POLE OF ROTATION
The oil-producing segments of the South Atlantic
Megatrend now alternate between the Brazilian and the
African margins; a productive segment on one margin is
faced by a relatively sterile segment on the other. This
distribution may result from the alternating polarity of
master faults during rifting. The major productive
segments are shown in the map of Figure 4.
The Gabon–Lower Congo Basin can be divided by
political boundaries into the Gabon, Congo, and northern
Angola segments, the latter including Cabinda. The
productive segment of the Gabon Basin is duplicated by
the parallel Recôncavo Basin in Brazil, which is separated
from the Gabon Basin by the Salvador–Jacuipe horst.
In the productive segments, both the oil richness and
the tectonic habitat of the oil are defined by the distance
from the pole of differential rotation, situated in northeastern Brazil near Brazil´s equatorial margin. Near the

74

Szatmari

4000

CUMULATIVE PRODUCTION
MM barrels

RESERVES
MM barrels
Reserves (Jan 1, 1996)
Million barrels

Cumulative production (Jan 1, 1996)
Million barrels

2000

1000

3000

2000

1000

0

0
N

Potiguar

Recôncavo
Congo
Espírito S.
Campos
Se-Al
Gabon
N.Angola
Cuanza
Santos

S

N

Potiguar
Se-Al

Recôncavo
Congo
Espírito S.
Campos
Gabon
N.Angola
Cuanza
Santos

S

Figure 5—Variation in cumulative oil production from
north to south along the South Atlantic Megatrend.

Figure 6—Variation in oil reserves from north to south
along the South Atlantic Megatrend.

pole, in the Potiguar, Recôncavo, and Sergipe–Alagoas
Basins, where the rift was relatively narrow and strikeslip motion was intense, petroleum occurs mostly on
land, along largely transtensional rift border and cross-rift
faults. Far from the pole, in the Lower Congo Basin and
especially the Campos Basin, where the rift was wide and
where rifted continental crust extends far offshore, oil
fields occur mostly offshore. In the Campos Basin, part of
the petroleum accumulated in lower Aptian subsalt sedimentary beds overlying thick synrift volcanics, but most
of the large reserves are in Upper Cretaceous to lower
Tertiary turbidites structured by salt flow.
The tectonic framework of the oil-producing basins
created during rifting was subsequently modified by two
interacting processes: salt flow and compressional reactivation of earlier faults during the Andean orogenic cycle
(Szatmari and Mohriak, 1995). Compression started in the
middle Cretaceous and culminated in early Tertiary time,
contemporaneously with major global plate reorganization. Strike-slip movement and related mountain building were accompanied by hot spot activity in southeastern Brazil from Late Cretaceous to early Tertiary time,
with several foci of igneous activity now marked by alkali
intrusions onshore and volcanic piles offshore. The
largest volcanic piles offshore formed in the area of the
present Abrolhos Bank and controlled the deposition of
extensive shallow-water carbonates. At the latitude of Rio
de Janeiro, a platelet composed mostly of oceanic lithosphere was dislocated to the south–southwest along a
reactivated late Precambrian ductile shear zone, creating
transtensional grabens and transpressional mountain
ranges (Serra do Mar and Serra da Mantiqueira). The
sediments derived from these mountains were deposited
in part as offshore turbidites; loading by these sediments
controlled salt tectonics, which in turn controlled the
distribution of turbidite channels.

The oil richness of a given segment along the South
Atlantic Megatrend is indicated by the cumulative
production (Figure 5) and the remaining reserves (Figure
6) of each basin or basin segment. These parameters,
however, are strongly dependent on the rather arbitrary
size of each segment, especially its length along the South
Atlantic Megatrend. To obtain a value that is independent
of basin size, we propose here to define oil richness as original reserves divided by the length of each basin or basin
segment. Oil richness so defined is shown for the
segments of the South Atlantic Megatrend along the
facing margins of Brazil and Africa (Figures 7 and 8); the
Niger Delta is omitted because of its different origin.
Figure 8 shows a sharp rise in oil richness along the megatrend from north to south as the rift widens with increasing distance from the pole of differential rotation.

Acknowledgments—The author wishes to thank reviewers
Juliano Macedo and Jan Golonka for helpful comments and
Petrobrás for support and permission to publish. This is a
contribution to IGCP Project No. 381.

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Chapter 6—Habitat of Petroleum Along the South Atlantic Margins

50

OIL RICHNESS
MM barrels/km

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RESERVES
MM barrels/km

40

Oil richness (Jan 1, 1996)
Million barrrels per km

Reserves and cumulative production
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MIillion barrels per km

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Se-Al

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Figure 7—Variation in original oil reserves (reserves plus
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Figure 8—Sharp rise in oil richness, defined as original oil
reserves per unit length, from north to south along the
South Atlantic Megatrend. Gaps in southward-increasing
trend may disappear with advancing exploration.

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