Constraints for plate reconstruction using gravity data—implications for source and reservoir distribution in Brazilian and West African margin basins

Marine and Petroleum Geology 20 (2003) 309–322 www.elsevier.com/locate/marpetgeo

Constraints for plate reconstruction using gravity data—implications for source and reservoir distribution in Brazilian and West African margin basins William G. Dicksona,*, Robert E. Fryklundb,1, Mark E. Odegardc,2, Chris M. Greend,3 b

a DIGs,4 Stafford, TX, USA Phillips Petroleum,4 Rio de Janiero, Brazil c GETECH, Stafford, TX, USA d GETECH, Leeds, UK

Abstract Evaluation of tertiary sequences of West Africa and Brazil typically requires large budgets and staffs to identify drilling targets. Since these tertiary sedimentary systems extend for hundreds of kilometers (Proc. Petrol. Geol. Deepwater Depositional Syst. (2001) 2; Marine Petrol. Geol. (1990) 94 – 122), well beyond the limits of individual 3D and many 2D seismic surveys, additional information is necessary to interpret the most favourable locations for detailed exploration. Potential field data are powerful but often underutilized assets in building an exploration framework for reducing costs and interpretation risks. Although drilling locations are normally based on 3D seismic interpretations, reservoir distribution is controlled by features mappable with other methods such as potential fields. Other factors crucial to petroleum maturation, migration and trap formation relate to deep-seated controls that are well imaged (and sometimes exclusively imaged) on potential field data. Our observations from potential field data help identify reservoir distribution; source pod locations, source maturity, possible migration pathways; and potential traps. This paper illustrates the power of combining both regional and basinal scale interpretations based primarily on gravity analyses with extensive knowledge of the underlying geology. We review data sets and methods employed and describe results of a regional overview based on special plate-tectonic reconstructions of the South Atlantic. Examples from basin-scale work offshore West Africa and Brazil are used to illustrate factors important to hydrocarbon exploration. We conclude with a look ahead to improvements in methodology and application. q 2003 Elsevier Ltd. All rights reserved. Keywords: South Atlantic; Plate reconstructions; Paleogeography; Continental margins; Geographic information systems; Gravity

1. Background This work is an offshoot of the Geophysical Exploration Technology (GETECH) and Dickson International Geosciences (DIGs) SAMBA (South Atlantic Margins Basin Analysis) project (Fryklund, Dickson, & Odegard, 2001; Odegard, Dickson, & Dombrowski, 2001). The work initiated with an independent E and P Company searching for exploration opportunities in the Gulf of Guinea, West * Corresponding author. Fax: þ 1-281-556-9213. E-mail addresses: [email protected] (W.G. Dickson), refrykl@ conocophillips.com (R.E. Fryklund), [email protected] (M.E. Odegard), [email protected] (C.M. Green). 1 Fax: þ55-21-588-8099. 2 Fax: þ1-281-240-6262. 3 Fax: þ44-113-242-9234. 4 Formerly Union Texas Petroleum, Houston, TX. 0264-8172/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0264-8172(03)00039-4

Africa, with minimal staff and budget. The first maps produced provided a sense of the source basins, controls on reservoir distribution and implications for fluid migration. That work became the start of a licensing round evaluation of offshore Gabon, West Africa for a group of E and P independents. The resulting block rankings proved robust even though staff size and budget remained small (Dickson, 1999a). The objective of SAMBA was to extend prior work to a better understanding of overall hydrocarbon prospectivity of the South Atlantic margin basins by reassessing sediment thickness, heat flow regimes, and structural and tectonic frameworks. SAMBA investigated the West African and Brazilian margins from continental to mini-basin scales, ranging across six epochs. A series of plate reconstructions was produced with regional gravity attributes displayed as

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images for each plate. Various regional structural elements were interpreted on the reconstructions; each element was compared or matched to a corresponding element on the opposite side of the Atlantic. After iterative revision, these features were used to select areas for more detailed interpretation. A final revision of the interpretations was based on the detail provided at basin scales. The results provided a powerful means for identifying and mapping the structural elements of the continental margin, the areas of recent sedimentation, defining the regional distribution of the offshore basins and determining the limits of the thinned continental crust. This became a framework to extrapolate interpretations from known to unknown areas, especially in the syn-rift basins, increasing the value of data already in hand. SAMBA also resulted in much reinterpretation, realignment and extrapolation of structural trends and anomalies, including some long-recognized features not previously imaged in their entirety. Many familiar anomalies and firstorder tectonic features were confirmed or refined in location or extent. A few surprises were encountered.

2. On the need for GIS A major requirement for SAMBA-type studies is streamlining data management, from organization to integration to display. Using off-the-shelf GIS software and some innovative data translation routines, we referenced and incorporated multiple sets of geophysical and geological data to evaluate the hydrocarbon potential of these conjugate margins. GIS allowed us to manage and display information at useful scales and to easily iterate back and forth between plate and sub-basin scales. In addition to easy cross-referencing and cross-correlation among all the relevant data, GIS methodology was used to perform quicklook evaluations and to build a framework for detailed, prospect-style work, while minimizing the drain on exploration resources. The GIS software provided excellent tools for data grooming, collating, and display (Weissel & Smith, 1995; M. Odegard pers. comm.) unlocked the value of under-utilized information in scattered archives to enable better exploration decisions (Dickson, 1999a,b) and allowed images and interpretation files to be accessed via a free multi-platform viewer providing simple access for collaborative sharing and review. The methodology was used both to perform quick-look evaluations and as a framework for detailed, prospect-style work. Referencing interpretive elements to published results allowed direct comparisons to well-accepted work.

3. On the need for gravity Historically, much excellent work has been achieved with offshore magnetic, gravity and deep seismic profiles (Asmus & Ponte, 1973; Lehner & deRuiter 1977; McMaster, De Boer,

& Ashraf, 1970; Mohriak, Hobbs, & Dewey, 1990; Sheridan, Houtz, Drake, & Ewing, 1969; Rona, 1970; Uchupi, Emery, Bowin, & Phillips, 1976 and many more recently). However, profile-based analyses are essentially two-dimensional. The selected gravity data provided extensive and complete coverage that could be analyzed and displayed in a quasithree dimensional sense in both onshore and offshore realms. Satellite altimetry can be used to derive gravity values over most of the world’s seas and oceans (Sandwell & Smith, 1997) and advanced processing can improve its resolution (Fairhead, Green, Maus, & Woollett, 1998) to scales relevant to basin and block ranking. This continuous coverage provided a unique means to rationalize and unify previous tectonic interpretations and link or extends features into areas not adequately covered by seismic data or drilling. Furthermore, advanced gravity processing provided anomaly attributes that can be correlated to known tectono-stratigraphic controls, allowing extrapolation into areas, where gravity is the only primary control. Map-based presentation of gravity field attributes also gave the nonspecialist a visually intuitive way to evaluate the geological meaning of the images, even if he or she was unfamiliar with the physics or mathematics behind their production.

4. Data preparation GETECH personnel have reprocessed public-domain satellite altimetry data to about 500 km seaward of the coastlines, spanning the continental shelves and adjacent sediment catchment areas of both margins (Fairhead et al., 1998; Fairhead, Green, & Odegard, 2001; Green, Fairhead, & Maus, 1998). The upgraded free air gravity data were then combined with improved continental margin bathymetry to generate Bouguer anomaly coverage. Onshore and marine Bouguer gravity data were selected from continental compilations for Africa and South America (each continent being covered by hundreds of land and marine surveys, all carefully merged to a common datum per Fairhead and Watts (1989) and Green and Fairhead, (1992)), from the 100 m isobath to roughly 500 km inland from the coastline. These primary data selections were merged into a regional grid with the public domain satellite-derived gravity covering the remainder of the offshore (Sandwell & Smith, 1997) and public domain data onshore (Green & Fairhead, 1996; Lemoine et al., 1998). The coverage could then support modelling of even the most distal effects of density variations from normal, unthinned continental crust far onshore to pure, fully-compensated oceanic crust far offshore. Fig. 1 shows the project area and gravity data coverage. Input data grids were 20 £ 20 (4 £ 4 km2) offshore and 50 £ 50 (11 £ 11 km2) onshore in the stippled area. Elsewhere, the onshore grid was about 50 £ 50 km2, whereas in the oceans the grid was still 4 £ 4 km2 but of lowerquality

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Fig. 1. South Atlantic study area with gravity data coverages after Fairhead and Watts (1989) (Africa), Green and Fairhead (1992) (South America), Sandwell and Smith (1997) (oceanic regions), Lemoine et al., 1998 (continental areas) and Green et al., 1998 (continental margins). Note principal basin names for Brazil (ES, Espı´rito Santo; C, Campos; S, Santos) and West Africa (G, Gabon; C, Congo; K, Kwanza).

data points. The output grid was uniformly sampled at 4 £ 4 km2. Multiple attribute displays were then generated for the merged gravity grid. The initial maps consisted of total horizontal derivative (THD) of Bouguer gravity, first vertical derivative (1VD), isostatic residual gravity (ISO) and especially Euler deconvolution of Bouguer anomaly (EDB). The care taken in the original compilation allowed full utilization of Euler deconvolution and derivative techniques (Fairhead, Bennett, Gordon, & Huang, 1994; Reid, Allsop, Granser, & Millett, 1990; Thompson, 1982) on local to regional/tectonic scales. Local depth-to-basement (D2B) maps were later constructed from the magnetic data using constrained spectral and Euler methods. The D2B work was confined to NW Africa and the Campos/Santos/ Espı´rito Santo basins of Brazil by considerations of cost and unfortunately is not illustrated in this paper because of confidentiality constraints. All these attributes were then displayed as geotiffs (geo-referenced images) in the GIS software and formed the backdrop to the subsequent interpretive work.

5. Attribute interpretation In the simplest examples, the patterns and lineations on the attribute images were compared to various published

geologic features. Where one-to-one correlations were clear, features were reinterpreted and extended in map view (Figs. 2 and 3). Usually, it was necessary to compare typical geologic correlation sections or seismic profiles to the attribute images to determine the depths from which the anomalies originated; interpretations were made for basement, syn-rift and post-rift/drift divisions of the total section.

6. Euler deconvolution results EDB results for the Aptian Salt Basin (Fig. 4a – d) were plotted in depth slices of 0 –4; 4 –8; 8– 10; 10– 15 and . 15 km. Each map showed the computed top of a density difference in a given depth range; the stacking of several colour-coded slices permitted the identification of the shape and nature of some of the features. For example, a normal fault will show several solutions (one at each notable density layer) with offsets corresponding to the dip of the bounding fault. Depth slices of EDB solutions behaved in intuitive ways with shallow sources reflecting known tectonic and basin structures while the deeper solutions helped to delineate limits of thinned continental crust. EDB solutions were displayed in different formats and depth ranges for the Brazilian margin basins of Pelotas, Santos,

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Fig. 2. Topography/bathymetry for South Atlantic region illustrating crustal features including mid-Atlantic ridge, transforms/fracture zones (FZ’s), leaky transforms (volcanics), hot spot tracks (HST’s).

Campos and Espı´rito Santo but were interpreted in the same fashion.

7. Depth to basement interpretation Because the raw D2B maps (not illustrated) were based mainly on magnetics, they required adjustments. D2B calculations often produce multiple depth values. The base of sediments may not be at the same depth as the changes in magnetization that drive the D2B solutions. Shallow or intra-sedimentary volcanics or crustal underplating also produces valid results different from the base of sediment depths. We reviewed the D2B values using comparisons to well depths, published profiles and the gravity attributes to select the final set of solutions. Errors bias towards greater depths (Reid, 1995; Reid et al., 1990; Thompson, 1982) so D2B values in this study typically represented the maximum possible sediment thicknesses.

8. First evaluation: Gabon margin, West Africa Fig. 3. Gulf of Guinea: Using isostatic residual anomaly (ISO) to discriminate recent deltas (red ellipses) from paleodeltas (blue) due to variations in crustal compensation.

Combinations of gravity attributes were used regionally to highlight areas for detailed study, while minimizing the exploration budget. In combination with a few published

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Fig. 4. (a)– (d) Euler Deconvolution of Bouguer gravity (EDB), West Africa. Four depth slices illustrate discrimination of continental versus oceanic crust; structural features Mayumba Spur; Congolide Fold Belt; Congo Canyon; COB. Note noise in solutions derived from public domain satellite altimetry (a).

seismic profiles and related studies and some very regional non-exclusive seismic lines, gravity maps allowed the determination of these key factors 8.1. Location of the continent – ocean boundary (COB) The COB matters because given the lower heat flows typical of oceanic regimes and starting with the cooler

temperatures of these bathyal to abyssal environments, source rocks may require up to 50% more overburden than the continental syn-rift source to reach maturity (Union Texas Petroleum internal work). Also, the tilted blocks associated with oceanic crust would not normally contain reservoirs or source rocks, in contrast to the syn-rift section above continental crust that has both. The outer envelope of the blue and red ovals (Fig. 3) along the margin served as an

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Fig. 5. EDB composite: five depth slices (Fig. 4 plus 8–10 km slice); tectonic features as Fig. 4. DT, dianongo trough, CFB, congolide fold belt.

approximate indicator of the COB. However, more rigorous interpretation was based in part on the EDB results of Figs. 4 and 5.

control erosional and depositional drainage. Hence, these oceanic features have consequences for hydrocarbon prospectivity, particularly near the continental margins.

8.2. Definition of oceanic transforms/fracture zones (FZs)/accommodation zones

8.3. Estimation of total sediment thicknesses

These features are well known in the literature (Karner, Driscoll, McGinnis, Brumbaugh, & Cameron, 1997; Reyre, 1984; Rosendahl, 1987; Rosendahl & Groschel-Becker, 1999, 2000), extending from the mid-ocean ridge and linking with onshore NE-SW trending faults. Simple transform/fracture zones (FZs) tend to segment the coastparallel trends and appear to act as open fractures. Well control offshore Gabon indicated local sand/shale ratio preferences along the transforms, implying control on sedimentation (J. M. Gibson, pers. comm.). These longlived zones of weakness may have acted as sediment conduits across the continental shelf and well into the abyssal deeps (Evans, 2001). Variations in hydrocarbon gravities (J. M. Gibson pers. comm.) associated with accommodation zones also suggested a continuing influence on fluid flow, as did the relative presence of hydrocarbon shows in the pre-versus post-salt intervals. Leaky transforms and hot spot tracks, especially where they intersect, produce volcanoes and intrusions that may act as sediment dams. Associated thermal doming and uplift stretches the crust and can induce fracturing, which may

The total sediment thickness above any postulated source interval must exceed the minimum for hydrocarbon generation. Traps can only be charged if they have access to mature source rocks. A quick-look determinant for sediment thicks was the ISO image (Fig. 3) with a rough indicator of a series of sediment thicks along the margin. Simple criteria included the masking of FZs as sediments thickened as for example the near disappearance of the Chain and Charcot fractures beneath the Niger Delta. More quantitative work derived from the Euler deconvolution (EDB) results. 8.4. Direction of structural trends Important structural trends may not be defined by widelyspaced seismic profiles because the latitudinal wavelength of continental structural change is 50 km or less (Mohriak, Barros, Rabelo, Matos, & 1993; Mohriak, Bassetto, & Vieira, 2000; Mohriak, Rabelo, Matos, & Barros, 1995; Rosendahl & Groschel-Becker, 1999, 2000). Tight seismic grids increase cost and require more interpretive effort. Gravity can clarify these trends, which

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often control reservoir distribution and hydrocarbon migration. A typical exploration problem in the Aptian Salt Basin was the difficulty in tying around many salt features. Our seismic line grid was so coarse (10 £ 20 km2) that trends aliased spatially. However, the gravity attributes (especially 1VD) showed the syn-rift trends clearly. A series of well known syn-rift tilted half-grabens (Belmonte, Hirtz, & Wenger, 1965; Brink, 1974; Herner, 1992; Karner et al., 1997; Robert & Yapaudjian, 1990; Teisserenc & Villemin, 1990) can be seen clearly on the 1VD as a set of coast-parallel features stepping coast-ward or basin-ward at the FZs. Other data sets used included multiple indicators of hydrocarbon sourcing, such as ODP and industry wells, seeps, and bottom simulating reflectors or BSRs (Cunningham & Lindholm, 2000). Coupled with observations from Sections 8.1 –8.4, we derived a robust block classification for hydrocarbon generative potential. Seismic signatures and water depth were factored into a preliminary block ranking. Based on this ranking, targeted seismic profiles were licenced on just a few high-graded blocks. This was a bold move because the area had been almost completely unmapped and there was concern that prospects would be overlooked without the review of vast amounts of seismic data. The staff was too small to evaluate all available lines anyway, so the interpreters concentrated on the lines actually licenced. They performed a detailed seismic – stratigraphic interpretation, drawing on the regional implications of the initial work. Line-to-line correlations were strongly influenced by the gravity products, which showed trends and general locations of highs and lows and aided the evaluation of the various ranked leads.

9. Licensing round ranking: results vs. costs The data expenditure was less than 1/4 the cost of all available seismic lines. The final block ranking matched the block popularity according to several industry scout reports. Management and partners were pleased with how limited budget and combination techniques were employed to yield a thorough, high quality appraisal. The study also formed the basis for the next step.

10. West African margin The ISO imagery (Fig. 3) may demonstrate the geometry and location of late Tertiary sedimentation in the Niger, Ogooue´ and Congo Fans. The significant positive anomalies are likely caused by sediments rapidly infilling bathymetry, faster than the crust can sink to isostatic equilibrium. The negative anomaly offshore southern Gabon represents the older Kouilou Fan. This area reached equilibrium after sediment supply was cut off by Oligo-Miocene stream

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capture by the Congo River (Kogbe & Me’hes, 1986; Norvick et al., 1998b). Importantly, EDB showed that the Congo (Zaire) River, while basement controlled onshore, where it trends enewsw, becomes offshore a function related to Tertiary lobeswitching (Vittori et al., 2000) and now trends east – west. Since hydrocarbon migration is affected by the basement trends, the distinction is important. Fig. 4a shows a plot of EDB solutions for the depth slice 0 –4 km. Near the coast, the few values lay mainly over the Congo Canyon. EDB solutions (Fig. 4a – e) were only on the shallow (0 –10 km, Fig. 4a – c) slices, disappearing from deeper slices although the crust is at least 20 km thick here. The explanation is that mis-located or under-sampled bathymetry caused an inadequate Bouguer correction for this steep, narrow channel, affecting all subsequent calculations. Most solutions on the 0– 4 km slice lie well outboard of the COB, indicating density contrasts at depths less than the sea floor, which is in the range of 4– 5 km. This discrepancy and the randomness of the solution vectors suggest they are related to noise in the data set. Per the striped areas on Fig. 1, the satellite data were reprocessed from the shore to the edge of this ‘noisy’ area beyond the COB, to improve the tie between onshore, marine and satellite data sets. We infer that reprocessing also eliminated a significant amount of short-wavelength noise (Fairhead et al., 1998, 2001) present in the public domain satellite gravity data. EDB solutions behaved intuitively and showed deepening to at least 20 km from the flanks into the center of an onshore anomaly associated with the Congolide Fold Belt. They also confirmed the THD and 1VD observations on most features, especially that the Mayumba Spur (Fig. 5) trend is east –west, probably with pre-rift origins (Teisserenc & Villemin, 1990) rather than taking the new syn-rift trend (Meyers, Rosendahl, & Austin, 1996a; Meyers, Rosendahl, Groschel-Becker, Austin, & Ronab, 1996b).

11. Crustal regimes The THD and 1VD maps (Fig. 6) showed two distinct regional domains: one of high amplitude lineaments immediately interpreted as continental and another, quiet zone, of oceanic crust. The continental domain was ‘hot’, with high amplitudes, dissected and full of linear features corresponding to the continental syn-rift fabric. The syn-rift pattern of half-graben development (Fig. 5) paralleled the coast although greatly masked under the main Congo Fan. Discontinuities in the syn-rift features correlated with known oceanic transforms such as the Fang and N’Komi FZs. Examples in the Gabon (Fig. 7), Camamu, Campos, and Congo basins delineated half-graben compartments that likely restricted circulation, promoting the development of lacustrine source rocks. Definition of these half-graben compartments is crucial in extrapolating charge areas. The accommodation zones linking half-grabens showed avenues

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Fig. 6. Gravity First Vertical Derivative (1vd) for West Africa showing FZ’s, Cretaceous magnetic quiet zone (CMQZ), proto-oceanic crust (POC), HST’s including the odd Sumbe Trend of southern Angola.

for paleo channels and fans, particularly those lineaments corresponding to known oceanic transforms. Refer to a companion paper, Dickson, Danforth and Odegard (2003) for details. Areas just outboard of the continental regime have been related to a third type, ‘proto-oceanic crust’ or POC (Abreu,

Fig. 7. Gravity ISO Dip-azimuth (DAzi) display in the Aptian Salt basin of West Africa. Note linear trends in the syn-rift and proto-oceanic regimes versus random trends in oceanic and basement (continental collision complex) regimes.

Vail, & Wilson, 1997). This material was emplaced at breakup near or even above sea level while the rift was not yet fully marine. Highly attenuated continental crust and/or earliest oceanic crust formed a series of rotated blocks that were then drowned as the margins cooled and tilted seaward. The region has a consistent signature on the gravity attributes and is interpreted on published deep seismic profiles of the PROBE study (Meyers, 1995, 1996a, b). Transforms dissected the POC into linear, stepped, coastparallel features. Wheras POC features are cored with non-reservoir material and ceased movement once subsided, they are long, typically 20– 60 km, and locally high relief, exceeding two km offshore Angola (A. R. Danforth, pers. comm.). Hence, POC blocks could have ponded sediments behind them in relatively restricted areas and could have promoted compactional drape traps over their crests. Recent exploration success in Rio Muni (Dailly, 2000) seems related to this crustal regime. The seaward domain of oceanic crust was relatively featureless, especially in the magnetically quiet area of Cretaceous crust (Goldflam, Hinz, Weigel, & Wissmann, 1980). Transform influences would also apply to sediments deposited on this true oceanic crust, where potential source rocks would all be marine and reservoirs would likely be unconfined fans at the mouths of channel systems guided perhaps by the FZ’s (Evans, 2001). Oceanic crust and POC both were punctuated by seamounts including the Cameroon line volcanoes. These showed spectacular artifacts on the THD maps related to inadequate bathymetric resolution, as in the Congo Canyon example and Fig. 8.

12. Large-scale reconstructions: using one side against the other Several factors benefit the risk assessment of hydrocarbon prospectivity, including improved understanding of the linkage of highlands and source provenances. Where the opening basins were young and hence narrow, early driftage delta systems could have extended across the full width of the rift. Basin separation would only have equaled the maximum syn-rift basin width of 250 km by about 80 Ma. Early drift-age source and reservoir provenances could easily have been from opposite sides of the rift than at present because modern analogues show deepwater transport of more than 1000 km in the Congo Fan (Mayall & Stewart 2001 Dickson & Macurda, 2000). From the interpretation on the West African side, it was natural to extend these observations to the Brazilian margin (Mohriak et al. 1989). Because segments of the two continents fit at break-up, a clear definition of one passive margin dictated the shape of the other. Many workers have looked at these plate margin processes and the fit of the opposing margins of the South Atlantic). Instead of using the conventional ‘wire-frame’ style of reconstructions (including Cande & Rabinowitz, 1978; Dias, 1993; Fairhead, 1988;

W.G. Dickson et al. / Marine and Petroleum Geology 20 (2003) 309–322 Fig. 8. (a)–(c) Plate reconstructions showing gravity total horizontal derivative (THD) at 55, 95, 120 Ma from left, respectively. Compare clear imaging of fracture zones (FZs), hot spot tracks (HST’s) and continental– oceanic crust boundaries (COB) with same features on Fig. 2.

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Lehner & de Ruiter, 1977; Meyers et al., 1996a,b; Norvick, Martin, & Hannfried Schaller, 1998a,b), this study restored full, present day gravity attribute grids for the South American and African Plate to their paleo-positions. Subsequent iterations also included points (such as ODP well locations) lines (i.e. COB) and polygons (i.e. areas of POC) from our present-day maps. Fig. 8a demonstrates the THD restoration to 55 Ma.

We used purpose-built plate tectonic reconstruction software (GET plate) to reproject the continents and associated data (Green, 1997) for a range of ages. Five time periods (Figs. 8 and 9) from 55 to 120 Ma were chosen for reconstruction. Data preparation only required removal from the gravity field of the third through tenth-order harmonics (normal gravity preparation has harmonics only to second-order removed) to eliminate image mismatches at

Fig. 9. (a)–(d) Reconstructions of the South Atlantic showing plate outlines and interpretations at four time steps: 55, 95, 105, 120 Ma. Note COB, Veeshaped HST’s, unclosed gap in the southernmost Atlantic at 120 Ma.

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the paleo-reconstructive margin, in this case the paleo-midAtlantic ridge. Data were simply rotated and translated back in time while values associated with intervening plate areas were consumed. Isochrons are based on magnetic reversals, which form matching crustal stripes (Mueller, Roest, Royer, Gahagan, & Sclater, 1997) on each side of the mid-Atlantic Ridge. Their age values plus known rotations control the simple plate model which will not close the syn-rift half grabens or restore continental stretch. Neither was any attempt made to excise seamounts or volcanic features younger than the crust on which they sat. A minor shortcoming of the method relates to representing

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the spreading center. Because the original plate model by Cambridge Paleomap Ltd. was global, plate outlines were somewhat coarse and the model under-sampled the actual mid-Atlantic Ridge with E – W errors up to 50 km. This mattered for the age of initial plate separation when basins of interest lay within the error zone; we compensated by working with the post-separation of 95 and 105 Ma steps. The maps clarify the configuration of pre-rift Brazil and West Africa (Fig. 8a – e). Sharply imaged transforms connect corresponding points on opposite sides of the Atlantic. The transforms fade to background in the oldest oceanic crust showing density uniformity or annealing

Fig. 10. (a) Amazon cone total horizontal derivative (THD). Note prominence of fracture zones (FZ’s) and the COB. The Amazon canyon has a clear signature across continental crust to the COB. (b) Amazon cone isostatic residual anomaly (ISO). Note prominence of fracture zones (FZ’s) and the COB; the Amazon canyon has a weak signature. Circled area is ISO high (compare to THD, above) showing thick, young sediments. Contrast the difference between images which underscores the need to use both to determine the areas of recent sedimentation and the limits of the continental crust.

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associated with the crust of the Cretaceous magnetic quiet period. Transforms east of the mid-Atlantic ridge fit the published NE – SW trends tracking across the prominent sub-parallel Cameroon Line of volcanoes. The Cameroon Line, like the Walvis Ridge/Rio Grande Rise does not follow a transform, in accord with a ‘hot spot’ origin. Some of the leaky transforms of the Brazilian margin (Chain/ Fernando de Noronja and Abrolhos FZs) exhibit strings of sea mounts possibly related to N – S separation besides pure transform motion. More troubling is the Sumbe volcanic (Duarte Morais, Melluso, Morais, Morra, & Sgrosso, 1998) trend offshore southern Angola which, when the continents close, links to NW-trending faults onshore Brazil. The implication of a trend, which is neither a transform nor a hot-spot track, is that a continental fracture trend or line of weakness can somehow propagate oceanward across newly forming crust. Since the Sumbe trend acts as a buttress or backstop for sediments in the Kwanza and Benguela basins, affecting the sedimentary and structural features, knowledge of how and why it formed may have implications for related features along both margins. Reconstruction emphasizes the zipper-like continental separation between the Chain FZ and the Walvis Ridge matching wide basins on one side with opposing narrow basins (Davison, 1997). The particularly well-defined West African plate margin helped locate the corresponding limit of the offshore Campos and Espı´rito Santo basins. Fitting the conjugate margins required adjustments to the published limits of most basins, especially for the Brazilian COB to move seaward off the syn-rift Campos and Espı´rito Santo Basins, enlarging the area of inferred lacustrine source potential (Fryklund et al., 2001). This limit was not clearly defined by the THD map on the Brazilian margin, partly due to masking by Tertiary to Recent sedimentation. The thickness of younger rocks diminished the THD response of the underlying syn-rift section, requiring combined THD and ISO attributes to image syn-rift and post-rift sections. This effect was most clearly observed over the Amazon Cone where the signature of the drift age sequence was on the ISO (Fig. 10a and b). A further masking resulted from the extensive volcanism along the Brazilian margin. Multiple attributes including ISO, EDB and various filters were again used to image the COB and syn-rift and post-rift sections. A major benefit of the reconstructions is the simple visual correlation of large amounts of data. The Cretaceous basins contain the main source rocks for the South Atlantic. Examining them in a present-day configuration meant large maps or small scales and left questions as to the nature of the outboard or distal limits of the basins. By turning back the clock, the interpreter can see the correct juxtaposition of the basins, visualize their limits and plot interpretations on a more specific canvas. Seeing the basins at their correct paleo-latitudes is also a plus for the sedimentologist and paleo-climatologist.

13. In the future This work has helped determine factors important to hydrocarbon generation (source rock limits, oceanic vs. continental crust and associated heat flows) and migration as well as reservoir rock distribution (transforms, FZs and intra-raft or intra-salt pathways and their spatial and temporal persistence). The costs were modest for the areas covered and in one example, allowed a small team of interpreters to create an excellent product for a fraction of a competitor group’s cost. However, several directions of improvement beckon. Future reconstructions require improved plate outlines with a greater level of detail to match the resolution of our data sets. Other efforts will use gravity inversion at the water –sediment interface to improve bathymetry, in turn upgrading all the offshore gravity data and reducing artifacts. This will enable the generation of more specific gravity attributes tuned for each sub-basin and sliced for specific depth ranges within these areas. Continued research to improve satellite altimetry precision is increasing resolution, perhaps to prospect levels. Expanded work with existing techniques also holds promise. Greater use of magnetics can often resolve ambiguous interpretations between volcanic or intrusive materials and sedimentary rocks. Magnetic data were used locally for depth to basement constraints and this could be extended over broader areas, improving the quality of hydrocarbon maturation estimates. Profile-based modelling allows the explicit use of many rock properties to minimize interpretation uncertainties down to typical prospect levels at the cost of increased data gathering and interpretation time. Work underway (SAMBA-SIP) is aimed at integrating extensive deep-imaging seismic profiles from the margins into the SAMBA package. Among other things, the work should result in trimodal (seismic, gravity and magnetics) calibration of each type of data set, and detailed density models of key seismic lines. The main thrust will continue to be derivation and presentation of results in cost-effective ways that allow ease of access and usage by the non-specialist.

Acknowledgements This paper is an expansion of two posters from AAPG Rio 0 98 augmented with material from SAMBA, a nonexclusive report by DIGs and GETECH. The posters included data from a detailed aerogravity survey onshore Gabon; block outlines and well locations for West Africa and some additional control points licensed to Union Texas/Arco. We have substituted GETECH gravity coverage onshore Gabon which is lower resolution but most adequate for our purposes here. Multiple sources have been combined to replace the earlier block and well location

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information. The authors thank Union Texas/Arco and GETECH for permission to publish the concepts and ideas that derive from our work with those firms. The authors thank Phillips Petroleum Co. for funding the colour figures. We especially thank Drs Webster Mohriak and Bruce Rosendahl for providing manuscript reviews.

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