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Aapg Memoir Habitat of Petroleum Along the South Atlantic Margins
Habitat of Petroleum Along the South Atlantic Margins
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Cilt:
73
Yıl:
2000
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english
Dergi:
AAPG Memoir
DOI:
10.0000/PMID14094
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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. REFERENCES CITED Bullard, E. C., J. E. Everett, and A. G. Smith, 1965, The fit of continents around the Atlantic, in A symposium on continental drift: Royal Society of London Philosophical Transactions, v. 258, p. 41–51. Cobbold, P. R., P. Szatmari, L. S. Demercian, D. Coelho, and E. A. Rossello, 1996, Seismic and experimental evidence for thin-skinned horizontal shortening by convergent radial gliding on evaporites, deep-water Santos Basin, Brazil, in M. P. A. Jackson, D. G. Roberts, and S. Snelson, Chapter 6—Habitat of Petroleum Along the South Atlantic Margins 50 OIL RICHNESS MM barrels/km CUMULATIVE PRODUCTION RESERVES MM barrels/km 40 Oil richness (Jan 1, 1996) Million barrrels per km Reserves and cumulative production (Jan. 1, 1996) MIillion barrels per km 50 40 30 20 10 0 N 75 30 20 10 Recôncavo Congo Espírito S. Campos Potiguar Se-Al Gabon N.Angola Cuanza Santos 0 S N Potiguar Se-Al Recôncavo Gabon Congo N.Angola Cuanza Santos Espírito S. Campos S Figure 7—Variation in original oil reserves (reserves plus cumulative production) per unit length from north to south along the South Atlantic Megatrend. 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