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Petroleum Geology of Gabon Basin
Abstract
Sediments of the Gabon basin, 16,000-18,000 m thick, range in age from Early Cretaceous, or perhaps latest Jurassic, to recent. A salt layer of late Aptian age separates the almost completely continental facies of the presalt or Cocobeach sequence from the largely marine postsalt sediment. Since its inception, the basin has taken the form of a half graben whose eastern edge consisted of a series of three, or perhaps more, hinge zones which migrated successively farther west and which controlled the distribution of depositional environments and facies changes. The recognition of the hinge zones is of great importance in predicting reservoir trends. In its central part, the basin was bordered on the west by the Anguille basement high, and as the position of this high remained more or less fixed at the (present) continental margin, the basin became narrower with time. It was not until Miocene time that the Anguille high subsided strongly and ceased to influence deposition. Mainly during late Cocobeach deposition the Lambarene-Ikassa Kongo-Gamba horst zone was formed. After the peneplanation of horsts and grabens alike, the sea invaded the Gabon basin for the first time. The relatively thin, transgressive, coastal-marine sequence between the unconformity and the overlying salt is called the "Gamba formation," whose sandstones are important oil producers. The main productive trend is related to the structural configuration of the underlying horsts and grabens. The third hinge zone (Atlantic hinge belt) was active during deposition of much of the postsalt sequence. This hinge zone probably extends over the whole length of the basin and controlled the separation of deeper marine (source) environments on the west from shelf (reservoir) environments on the east. The lack of shale members to cap potential reservoir rocks along at least parts of the hinge belt may explain why no important oil accumulations have been found so far. The principal oil fields producing from the postsalt sediments are 60-100 km west of the Atlantic hinge belt, partly on the eastern slope of the Anguille basement high. The environments of deposition of the producing sediments vary from brackish-water estuarine to marine-distal deltaic; the largest oil fields are on the west not as a result of optimum reservoir conditions but because of the timely development of nonpiercing salt-induced domal structures of large areal extent. Steep salt piercements are present farther east in deeper parts of the depositional basin, and oil accumulations related to these piercements tend to be smaller.
... The first part of this section outlines the geology of the Congo-Gabon region, mainly based on VINCOTTE (1938), HIRTZ (1964), GIRESSE & KOUVOUMONTZAKIS (1971, HOURCQ (1966), REYRE (1966aREYRE ( , 1966b, FRANKS & NAIRN (1973), BRINK (1974), GIRESSE & TCHIKAYA (1975), GIRESSE & CORNEN (1976), CORNEN et al. (1977, and on data from petroleum exploration boreholes (Elf-Congo, Petrango, personal communication). The Atlantic coastal plain is predominated by the relief of the Precambrium Maycombe Chain which shows an intensive Pan-African folding, granitisation, metamorphism, overthrusting, etc. Near Mayumba, outcrops stretch as far as the beach and form the boundary between the rather deep Gabon Basin in the north and the Congo Basin in the south which extends over the enclave Cabinda, Zaire, and north Angola. ...
... When the incumbent load was large enough, an intensive diapirism arose. The salt domes are believed to have reached their present form during the Oligocene erosional period (BRINK, 1974). Diapirs occur in the subsurface of the Gabon mainland, while in Congo their presence is only demonstrated on the shelf and continental slope. ...
... -During the Cenomanian, continental conditions prevailed at the present inner shelf area of the Gabon Basin. The deposits consist mainly of reddish brown sandstones and marly and shaly intercala-tions with anhydrite and gypsum (BRINK, 1974). Towards the present outer shelf they merge laterally into fully marine siltstones and shales. ...
Seismic and acoustic reflection profiles demonstrate the presence of 4 seismic and 2 acoustic units. From the inner shelf outwards they are successively: Unit 1, interpreted as Upper Cretaceous starting from the Cenomanian, and a Palaeocene and Eocene Unit 2. Both units are folded, the latter more gently. The Oligocene regression induced the discordant boundary with two monoclinal units 3 and 4 representing the Miocene, and separated by a Middle to Late Miocene regressional unconformity. Late Cretaceous tensional tectonics, accompanying the early opening of the South Atlantic, was followed by at least 2 phases of compressional tectonics during Palaeocene to Eocene times. The orientation of the axes, 90° to 120°, is different from the known directions in western Africa. Fractures belonging to a Miocene tensional regime are proposed to have caused offsetting of the mapped strata. The thickness distributions of 2 acoustic units were mapped. The Lower Unit, less than 5 m thick, consists of Middle Weichselian marine clays. Their relicts are concentrated at the outer shelf, due to the fall of the sea level (100 to 115 m) around 18 000 yBP. The Upper Unit forms an at maximum 15 m thick Holocene lobe of clays originating from the Congo River. Both units show strips oriented ∼110°, controlled by the direction of the Cretaceous and Tertiary subcrops.
... The Kwanza Basin formed during the Early Cretaceous opening of the south Atlantic Ocean, when the Congo–Sao Francisco craton was rifted along the grain of the Proterozoic west Congolian–Braziliano orogen. In the Gabon – Angola region, synrift strata were deposited in two margin-parallel troughs separated by a chain of basement horsts (e.g., Brink, 1974; Karner et al., 1997 Karner et al., , 2003 Karner and Driscoll, 1999; Bate et al., 2001). The nomenclature for this chain of horsts has become confused. ...
... Some authors refer to it as the Atlantic hinge zone (e.g., Karner et al., 1997Karner et al., , 2003 Karner and Driscoll, 1999; Marton et al., 2000). However, this is inconsistent with the original usage of Brink (1974), who defined the Atlantic hinge belt (also referred to as ''hinge zone 3'') as a prominent flexure in the base of salt. In the type area of Gabon, this flexure is located several tens of kilometers west of the basement high. ...
... Brink's usage has been followed by several subsequent workers, including Peel et al. (1998), Cramez and Jackson (2000), Hudec and Jackson (2002a), and Tari et al. (2003). Our paper uses Atlantic hinge zone in the sense that Brink (1974) originally defined it, both because of historical precedence and because the term ''hinge zone'' seems better applied to a flexure than to a horst block. We refer to the chain of basement horsts as the Gabon –Angola horst system (Figure 1). ...
Restoration of a 375-km (230-mi)-long section across the Kwanza Basin, Angola, shows three stages of deformation detaching on Aptian salt, each caused by basement tectonics. First, tilting related to postrift thermal subsidence initiated early Albian deformation, shortly after salt deposition ended. Deformation waned in the late Albian, probably because of thinning of salt lubricant beneath the extensional province. The second phase of deformation was triggered by hitherto unrecognized crustal uplift beneath the continental rise around 75 Ma (Campanian). Uplift led to salt extrusion and seaward advance of the Angola salt nappe over the abyssal plain. Exposure of the nappe toe removed the buttress provided by abyssal-plain cover, which rejuvenated seaward translation. Third, Miocene basement uplift below the shelf steepened the bathymetric slope and greatly accelerated downslope translation. This deformation is now slowing because accelerated sedimentation on the abyssal plain reduced the relief of the system and blocked salt-nappe advance. Minor changes in basin configuration led to profound changes in detached deformation. Miocene uplift was only a few hundred meters on the shelf, but this was sufficient to destabilize the system and increase the translation rate from 300 to 3200 m/m.y. (980 to 10,500 ft/m.y.) Deposition of 600 m (2000 ft) of sediment on the abyssal plain in the upper Miocene shifted contractional deformation 150 km (95 mi) landward. We conclude that driving and resisting forces have been precariously balanced for much of the Kwanza Basin's history.
... Furthermore, the interstitial waters of sediment cores from two of these diapirs have yielded salinity values (40°/oo to 50%o) which are higher than those for normal sea water (approximately 35°/oo) (Leyden and others, 1978). Evaporites have been drilled in many coastal basins on both sides of the Atlantic (Ponte and Asmus, 1976;Brink, 1974). Hence, it is reasonable to assume that most of these diapirs in the outer part of the continental margin are indeed composed of salt. ...
... The drilling terminated at a depth of 741 m in sediments of late Albian age (approximately 100 m.y.; Perch-Nielsen, Supko, and others, 1975). In all the coastal basins of Brazil and Africa that contain evaporites, the end of the evaporite phase is always of late Aptian-early Albian age (approximately 106 m.y.; Ponte and Asmus, 1976;Brink, 1974). The sediments that presumably lie directly over the evaporitic layer in the deep margin have been drilled at site 364 in the Angolan margin and are also late Aptian to early Albian age (Bolli, Ryan, and others, 1975). ...
... The magnetic-anomaly pattern (Ladd, 1974) and the location of proposed poles of opening (Le Pichon and Hayes, 1971;Francheteau and Le Pichon, 1972) require the rate of spreading to be higher in the southern than in the northern South Atlantic. The early opening in the northern South Atlantic (areas to the north of Rio Grande Rise and Walvis Ridge) was marked by deposition of thick terrigenous, shallow-marine, fluvial, and lacustrine sediments in the coastal basins of the Brazilian as well as of the West African margin (Brink, 1974;Asmus and Ponte, 1973;Asmus, 1975;Campos and others, 1974). This stage in the opening of the northern South Atlantic has been referred to as the "rift-valley stage" (Asmus, 1975;Ponte and Asmus, 1976). ...
The São Paulo Plateau is a prominent marginal plateau in the southeastern Brazilian margin. Its surface displays a rough morphology either because of diapiric structures that have pierced and/or domed the overlying sediments or because of outcrops of igneous basement rocks. The plateau is bounded to the north and south by fracture zones and appears to be situated on oceanic crust created between Neocomian and Aptian time. At the end of Aptian, the segment of the initial spreading center that was bounded by the fracture zones that now form the northern and southern boundaries of the plateau shifted to the east. The newly created segment of the spreading center formed the eastern boundary of the plateau. The stratigraphy of the plateau has been inferred through correlation of seismic-reflection profiles with the drilling data from Deep Sea Drilling Project site 356, located on the southeastern corner of the plateau. We have attempted a comparison of the sedimentary facies of the plateau with those of the Santos Basin, a shelf basin located adjacent to the plateau. A continuous evaporitic layer of Aptian age extends from the basin to the plateau. In Albian time, shallow-platform limestone was deposited in the basin, while open-marine limestone was deposited on the plateau. Coarse conglomerates were deposited in the basin during Turonian-Coniacian time, whereas distal turbidites were contemporaneously deposited on the plateau. A major transgression in Maestrichtian time trapped the terrigenous sediments within the Santos Basin and halted terrigenous sedimentation on the plateau. Cenozoic sediments in both areas are open marine. The sedimentary history of the Santos Basin and the São Paulo Plateau thus suggests that the two regions have been structurally continuous at least since Late Cretaceous time. The region occupied by the plateau has acted as a depocenter and has evolved as a marginal plateau since the termination of evaporitic deposition in the South Atlantic.
... As on most continental passive margins throughout the world, conservative thinning models were prevalently invoked to explain the formation of the Brazilian-Angolan Margins. The pure stretching model was first applied to the Santos Basin (Chang and Kowsmann, 1984), then to the Sergipe-Alagoas Basin (Chang and Kowsmann, 1986;Pontes et al., 1991;Mohriak et al., 1995Mohriak et al., , 1998Mohriak et al., , 2000, the Campos Basin (Mohriak and Dewey, 1987;Mohriak et al., 1990), the Pelotas Basin (Fontana, 1987), the Espirito-Santo Basin (Costa, 1988;Chang et al., 1992), and the Gabon Basin (Brink, 1974;Reyre, 1984;Teisserenc and Villemin, 1990;Meyers et al., 1996;Rosendhal and Groschel-Becker, 1999). In order to illustrate the problem associated with these kinds of models, we investigated the interpretation of one of these margins, the Sergipe-Alagoas Basin, just north of our study area. ...
... These reconstructions present a shift of more than 150 km for the Ascension FZ (see Moulin et al., 2007) and cannot be used to define the conjugate margin. Maximum sediment thickness areas come from Brink (1974), Cordani et al. (1984) and Lana (1985), in Castro (1987). in our area, whereas constant crustal volume models require at least 235 km (Fig. 6). (Rosendahl et al., 1991;Meyers et al., 1996;Dupré, 2003) and 239-RL-343 profile (Mohriak et al., 1995(Mohriak et al., , 1998, the Espirito Santo-Angola System, based on 38-RL-250 profile (Mohriak et al., 1990), Zaïango Profiles 7-11 (Moulin, 2003;Contrucci et al., 2004;Moulin et al., 2005;Boavida et al., 2000) and industrial profiles (Moulin et al., 2006), the Santos-Namibe System, based on industrial profiles Moulin et al. (2006). ...
... Core chemistry and mineralogy. The core was sampled at 1153-1154 m depth in a lower Cretaceous sandstone, underlying the transgressive Gamba Formation and thick salt deposits 9 . Reservoir pressure and temperature were respectively 12 Mpa and 43 °C. ...
... However, there are few reports on the predominance of aerobic microorganisms in oil reservoirs that are generally inhabited by strict anaerobes including sulfate-reducers and methanogens 6,7 . The reservoir rock studied here belongs to a cretaceous formation dominated by evaporitic conditions 9 . Protective presence of salt, adsorption on clays and low water activity could have preserved macromolecules such as DNA 56,57,59 and may therefore explain the repeated molecular detection of aerobic hyperhalophilic Archaea. ...
Microbiology of a hypersaline oil reservoir located in Central Africa was investigated with molecular and culture methods applied to preserved core samples. Here we show that the community structure was partially acquired during sedimentation, as many prokaryotic 16S rRNA gene sequences retrieved from the extracted DNA are phylogenetically related to actual Archaea inhabiting surface evaporitic environments, similar to the Cretaceous sediment paleoenvironment. Results are discussed in term of microorganisms and/or DNA preservation in such hypersaline and Mg-rich solutions. High salt concentrations together with anaerobic conditions could have preserved microbial/molecular diversity originating from the ancient sediment basin wherein organic matter was deposited.
... A compressive tectonic episode from Late Cretaceous through the Cenozoic has been described in the West African margin Fairhead and Binks, 1991). In the neighbouring basin of South Gabon, such structures have been attributed to salt tectonics dating to Late Cretaceous (Leyden et al., 1972;Brink, 1974;Reyre, 1984;Teisserenc and Villemin, 1989). In the Kribi-Campo sub-basin, it was difficult to link the folds to diapiric structures (Aloïsi et al. 1995;Benkhelil et al., 2002). ...
... Erosional surface "J" could correspond to the peneplanation of the basin margins as a result of active subsidence towards the final phase of the rifting at the Upper Aptian. In the Gabon and Angola basins, this surface of peneplanation is observed to ca 112 (Brink, 1974, Brice et al., 1982Reyre, 1984;Teisserenc and Villemin, 1989) where it would mark an angular unconformity (Braun and Beaumont, 1989). In Douala/Kribi-Campo basin the surface "J" could correspond to the discontinuity reported and dated 112 Ma by Wornardt et al. (1999). ...
The present study is based on an approach including offshore seismic data and outcrops to better understand the tectonic evolution of the Kribi-Campo sub-basin. Tectonic markers recognized (normal and reverse faults, listric faults, fractures, drag folds, anticlines, grabens, slump and divergent structure) confirm the existence of syn-rift extensional and compressive phases. Two structural regions were recognized on the seismic lines: the western and eastern depocentres. Within both depocentres, the synrift succession is bounded above by the "J" erosive unconformity, which marks the end of the rifting in the Kribi-Campo sub-basin. In the Campo outcrops, the major active faults in the syn-rift deposit are those having a N000-020°E, N160-180°E, N80-110°E trend. Structures such as normal faults are connected to extensional syn-rift. Reverse faults and folds are indicative of a compressive episode, which is placed in the Lower Cretaceous (105-95 Ma). These faults and tectonic activity indicate that the Campo outcrops belong to the transitional or ending rifting phase.
... These, moving oceanwards, are the Inland Hinge, the Atlantic Hinge and the Continent-Ocean Boundary (COB). In West Africa these hinges were first identified from Gabon by Anton Brink (1974): Anton was subsequently appointed Vanco's Chief Geologist and his name has been given to one of the company's pair of deepwater blocks in southern Gabon. The track of the hinges has now been determined from seismic and gravity for the entire West African margin, though much of this work remains to be published. ...
... In Gabon, the primary population of offshore fields (Anguille, Grondin etc.) is associated with deepwater sands related to late Cretaceous (Senonian) drainage changes in the Ogooué River basin triggered by regional tectonism in Africa. Salt tectonism associated with younger Tertiary loading created the traps (Brink, 1974;and Teisserene and Villemin, 1990). Though these discoveries (1500 MMbo in total) were drilled from modern shallow water settings and can, therefore, be considered as "classic" plays, they represent the first fields in West Africa related to depositionally deepwater settings. ...
The West African deepwater is, as is evident from the just subsided bidding "frenzy" for Angola Blocks 31-33 and the growing excitement generated by the Agbami giant in Nigeria and now the La Ceiba find in Rio Muni, of global importance to the oil industry. The objectives of this contribution are to examine, in terms of the regional geological framework, why the West African deepwater (Angola to Ivory Coast) is of such interest and to suggest what the future could hold for this already exceptionally rewarding region (Knight and Westwood, 1999). The traditional onset of the deepwater is the 200-metre (660-feet) shelf edge isobath. Increasingly, the deepwater commences at the 500 metres (1640 feet) isobath with the ultra-deepwater commencing at 1500 metres (4920 feet). Now, as the exploration and technology horizon advances oceanwards, terms such as ultra-ultradeepwater are emerging for the 2500 metres (8200 feet) plus frontier. These distinctions are not utilised in this review and all water depths greater than 200 metres are referred to as deepwater. To provide adequate space to present the great variety of geological settings, only passing reference is made to existing, developing and planned deepwater production facilities. Other contributors to this conference will be addressing some of these developments. Emphasis is placed on Angola since this where the main discoveries have been. However, as will be argued, Nigeria seems to be destined to become a second focus.
... They are located over the major western bounding fault system of the Kribi/Campo high. They overlie the Mundeck Formation from which they are separated by a probably breakup unconformity [29][30][31]. This suggests that salt was deposited in the final stage of continental rifting. ...
... Unrestricted ocean circulation is thought to have commenced in the Turonian, when the North and South atlantic gateways were joined. According to [29], the post-rift phase comprises three stages of drift: ...
The aim of this research work was to report a facies analysis of the N'Kapa formation, identified the clay minerals pre-sent in those facies and evaluate their oil potential. For that to be done, Lithostratigraphic descriptions were performed on three natural outcrops chosen in three different localities of the northern border of Douala sedimentary basin. Ten shaly samples were then collected on those outcrops and submitted to X ray diffraction and Rock-Eval pyrolysis. Lithologically, the N'Kapa formation is made up of dark to grey shales and fine to coarse sandtones. The mineralogic content of the shales is made up of Kaolinite, dickite, low quartz and vaterite. Those shaly facies present high amount of immature organic matter (average TOC content around 2%). The petroleum potential is fair to poor (average S2 for all the samples around 3.33 mg HC/g of rock) though some samples (M1 and M2) presenting a good petroleum potential up to 6.62 kg HC/t of rock and 6.44 kg HC/t of rock respectively. They have undergone a low degree of diagenesis (early to burying diagenesis). This is evidenced by the predominance of kaolinite and dickite, low quartz and vaterite which are minerals stable at low temperature.
... They are located over the major western bounding fault system of the Kribi/Campo high. They overlie the Mundeck Formation from which they are separated by a probably breakup unconformity [29][30][31]. This suggests that salt was deposited in the final stage of continental rifting. ...
... Unrestricted ocean circulation is thought to have commenced in the Turonian, when the North and South atlantic gateways were joined. According to [29], the post-rift phase comprises three stages of drift: ...
The aim of this research work was to report a facies analysis of the N’Kappa formation, identified the clay minerals present in those facies and evaluate their oil potential. For that to be done, Lithostratigraphic descriptions were performed on three natural outcrops chosen in three different localities of the northern border of Douala sedimentary basin. Ten shaly samples were then collected on those outcrops and submit to X ray diffraction and Rock-Eval pyrolysis. Lithologically, the N’Kappa formation is made up of dark to grey shales and fine to coarse sandtones. The mineralogic content of the shales is made up of Kaolinite, dickite, low quartz and vaterite. Those shaly facies present high amount of immature organic matter (average TOC content around 2%). The petroleum potential is fair to poor (average S2 for all the samples around 3.33 mg HC/g of rock) though some samples (M1 and M2) presenting a good petroleum potential up to 6.62 kg HC/t of rock and 6.44 kg HC/t of rock respectively. They have undergone a low degree of diagenesis (early to burying diagenesis). This is evidenced by the predominance of kaolinite and dickite, low quartz and vaterite which are minerals stable at low temperature.
... When they are poorly affected by dissolution, the Loémé evaporites are composed of 11 correlative depositional cycles, named I through XI, interpreted as solar-driven evaporation cycles, from 15 m to 130 m thick (Belmonte et al., 1965;Lambert, 1967;Brink, 1974;De Ruiter, 1979) (Figs. 2, 3). Each cycle is typified by a mineralogical succession consisting of basal dolomitic and bituminous shale, followed by halite, then a series of carnallite and halite beds (Belmonte et al., 1965;De Ruiter, 1979). ...
The evaporites of the Aptian Loémé Formation occur along the stable structural domain of onshore Congo as horizontally well-stratified depositional halite interbedded with Mg-poor potash minerals including carnallite, bischofite, tachyhydrite and sylvite. Eleven correlative depositional recharge-to-evaporation cycles are composed of suites of organic-prone shale followed by primary halite and carnallite beds, sometimes punctuated by a bischofite-tachyhydrite end-member.
A sedimentological and petrographical analysis performed on cores and thin sections allowed an interpretation of nine facies associations, named as FA1 to FA9, relying upon the occurrence of bands of fluid inclusions, grain-size, dissolution features, detrital sediments, minute inclusions and sedimentary structures. A depositional model reconciles the spatial distribution of primary textures, hydrology, brine saturation and palaeo-depth during a standard regressive evaporation cycle. Shallow burial eo-diagenesis led to compaction-driven dissolution and recrystallization into fine-grained halite and carnallite laminites but poorly affected the overall preservation of primary deposits.
Measurements and profile analysis of bromine trace element concentrations reveal a palaeo-salinity increase of the parent brine for cycle II to base IX caused by a relative brine level fall and a gradual confinement of the salt basin, likely associated with an excess of saline elements inflow from hydrothermal vents. Similar depositional textures and bromine concentration variations are observed in the lower cycles of the Aptian evaporites from the counter-part Sergipe Basin in the northern part of the Brazilian margin, testifying of a progressive brine salinity rise due to a palaeogeographic confinement along the northern segment of the South Atlantic Ocean.
In Congo, the sylvite displays horizontal “pseudo-stratifications” that unconformably overlay the crests of anticlines and propagate along flanks as a sharp-based caprock. The flushing and transformation of depositional carnallite – halite into secondary sylvite – halite is interpreted as having been caused by down-stepping incremental dissolutions sourced by younger pervasive connate to phreatic waters. In Sergipe, the sylvite is interpreted as secondary but further results from syndepositional dissolution of carnallite and tachyhydrite due to a regional North Atlantic Ocean ingression.
... There is evidence to support this. First, the oldest supra-salt strata vary from proximal platform carbonates to pelagic facies going oceanwards (Brink, 1974;Teisserenc and Villemin, 1990), implying varying bathymetry of the top salt immediately after its deposition. Second, the relative levels of salt and oceanic crust provide critical information by using the concept of "regional" (Hossack, 1995), i.e., the level of a horizon where it has not subsequently been deformed other than by regional uplift or subsidence. ...
In this study, we present an interpretation of a 3D depth-migrated dataset imaging the Ocean Continent-Transition (OCT) of the southern Gabon margin. Located in an area where salt is not strongly overflowing the OCT, it allows the geometrical relationships of salt deposition and its relative timing to breakup to be determined. The OCT is formed by extensional propagators floored by exhumed mantle limited by either fault-bounded upper-crustal or pre-salt sediments and/or magmatic additions. Based on our interpretations, we conclude that the pre-salt sequence was deposited over a wide area spanning crustal thinning to the onset of hyperextension and exhumation. The relative timing of salt deposition is constrained by the observations that detachment faults break away in the pre-salt sequence and that some of these faults also strongly offset the base salt and are linked to the creation of accommodation space in the salt. This is direct proof that salt was deposited during mantle exhumation. Salt deposition is interpreted to have occurred at variable depths. Evaporites formed approximately 1 km below sea level in the north, above a 4 to 6 km thick pre-salt sequence, and as deep as 2 km in the south. In this area, salt is interpreted to directly overlie exhumed mantle, but whether it was deposited on mantle or juxtaposed during exhumation remains more debatable. Space created in the faulted outer trough was filled partly by autochthonous salt and possibly also by downslope flow. Rifting continued after the cessation of salt deposition, followed by magmatic additions in the outer high and ultimately the onset of seafloor spreading and the formation of Penrose oceanic crust.
... A thick layer (locally up to 4 km; Hudec & Jackson, 2004) of Aptian salt is deposited shortly after rifting, draping the residual syn-rift relief. The salt is deposited in two sub-basins separated by a margin-parallel chain of platforms, known as the Atlantic hinge zone, where the salt is thin or absent ( Figure 3b) (Brink, 1974;Hudec & Jackson, 2004). The sub-basins are termed the Inner (eastern and landward) and Outer (western and seaward) Kwanza Basins. ...
We use a 3D seismic dataset from Offshore Angola to document the salt-influenced translation and evolution of structures on the margin.
... A thick layer (locally up to 4 km; Hudec & Jackson, 2004) of Aptian salt is deposited shortly after rifting, draping the residual syn-rift relief. The salt is deposited in two sub-basins separated by a margin-parallel chain of platforms, known as the Atlantic hinge zone, where the salt is thin or absent ( Figure 3b) (Brink, 1974;Hudec & Jackson, 2004). The sub-basins are termed the Inner (eastern and landward) and Outer (western and seaward) Kwanza Basins. ...
Salt‐influenced passive margins are widespread and commonly hydrocarbon‐rich. However, they can be structurally complex, with their kinematic development being poorly understood. Classic models of salt tectonics divide such margins into updip extensional, mid‐slope translational, and downdip contractional kinematic domains. Furthermore the faults, folds, and salt walls associated with each kinematic domain are typically assumed to form perpendicular to the maximum principal stress, which in gravitationally driven systems means broadly perpendicular to base salt dip. We use high‐resolution 3D seismic reflection data from the Outer Kwanza Basin, offshore Angola to show that these models cannot explain the diversity of salt structures developing on passive margins, especially those defined by considerable relief on the base‐of‐salt surface. Overburden seismic‐stratigraphic patterns record the basinward translation and rotation, allowing us to reconstruct the origin and evolution of the salt structures. We show structures in the transitional domain of the Outer Kwanza Basin display three dominant trends, each characterised by different structural styles: i) salt walls perpendicular to the overall base salt dip, ii) salt walls parallel to the base salt dip, and iii) salt walls oblique to the base salt dip. We show that each set of walls has a unique history, with synchronous phases of extension and compression occurring in adjacent structures despite their close spatial relationship. Our analysis suggests that, in the Outer Kwanza Basin, the structural evolution of the salt and overburden is predominantly controlled by translation over relief on the base‐salt surface formed above fault scarps associated with a preceding phase of rifting. Changes in the downdip volumetric flux and velocity of the salt over topographic features can cause local extension or contraction of the salt and its overburden, associated with local acceleration or deceleration of the salt, respectively. This interaction with base‐salt relief creates locally variable stress fields that deform the salt and its overburden, overprinting the broader, margin‐scale salt tectonics typically associated with gravity gliding and spreading. This article is protected by copyright. All rights reserved.
... Hinge zones were important features along the African margin during the Early Cretaceous development of the South Atlantic. They controlled the location of the larger half-graben structures, which migrated westwards as the rifting progressed and which influenced the distribution of depositional environments and facies changes within the African basins (Brink, 1974). ...
By using a deformable plate tectonic framework to map known tectonic elements and how they reconstruct through time, one can better understand geological risks for exploration. This framework was recently applied to the South Atlantic region.
... In ancient rifts, petroleum is being generated today from Early Cretaceous lakebeds in rift basins along the South Atlantic margin in Angola, Brazil, and Gabon (Ghignone and Andrade, 1970;Brink, 1974;Brice and Pardo, 1981) (Fig. 1). (The location of rift basins discussed in this paper is shown in Fig. 1.) Lead, zinc. ...
Robbins, E.I., 1983. Accumulation of fossil fuels and metallic minerals in active and ancient rift lakes. In: P. Morgan and B.H. Baker (Editors), Processes of Continental Rifting. Tectonophysics, 94: 633–658. A study of active and ancient rift systems around the world suggests that accumulations of fossil fuels and metallic minerals are related to the interactions of processes that form rift valleys with those that take place in and around rift lakes. The deposition of the precursors of petroleum, gas, oil shale, coal, phosphate, barite, Cu-Pb-Zn sulfides, and uranium begins with erosion of uplifted areas, and the consequent input of abundant nutrients and solute loads into swamps and tectonic lakes. Hot springs and volcanism add other nutrients and solutes. The resulting high biological productivity creates oxidized/reduced interfaces, and anoxic and H2S-rich bottom waters which preserves metal-bearing organic tissues and horizons. In the depositional phases, the fine-grained lake deposits are in contact with coarse-grained beach, delta, river, talus, and alluvial fan deposits. Earthquake-induced turbidites also are common coarse-grained deposits of rift lakes. Postdepositional processes in rifts include high heat flow and a resulting concentration of the organic and metallic components that were dispersed throughout the lakebeds. Postdepositional faulting brings organic- and metal-rich sourcebeds in contact with coarse-grained host and reservoir rocks. A suite of potentially economic deposits is therefore a characteristic of rift valleys.
... The early salt movements actually occurred under a relatively thin overburden. This was probably due to the high density of the superimposed deposits (dolomites of the Coimbra Formation), a phenomenon also know from West Africa (Brink, 1974). The main phase of pillow development was, however, during the Late Jurassic-Early Cretaceous as exemplified at the locations of well Mo-1 offshore, and well MRW-9 (Monte Real structure) onshore (2-3 in Fig. 13A and B). ...
A regional seismic study of the Lusitanian Basin, located along the western Iberian margin, has been carried out as a contribution to the EC-sponsored MILUPOBAS R & D project. This is an ongoing multidisciplinary investigation with objective to enhance knowledge of the Lusitanian and Porto basins of Portugal and their hydrocarbon potential.
... 2002). These salt deposits are likely to have been formed at the same time with those reported from Abakaliki and Ogoja area [52,53], Douala basin [54], and Gabon basin [55]. ...
... The origin of the South Atlantic (Angola Basin) salt deposit (Belmonte et al., 1965;Brink, 1974;Brognon and Verner, 1966a, b;Vidal et al., 1975;Pautot et al., 1973;Delteil et al., 1975;Leyden et al., 1972Leyden et al., , 1976von Herzen et al., 1972;Roberts, 1975;Burke, 1975) remained an unsolved problem particularly in light of whether the salts were laid down in ancient coastal lagoons or sabkhas beneath the modern continental slope (Kinsman, 1975) and within the confines of former rift valleys of a subsiding continental crust, or whether the salt was deposited in a hypersaline sea whose axial area was floored by oceanic crust extruded since the initial separation of Africa from South America. Paleobathymetric considerations were of paramount interest (Sclater and McKenzie, 1973) and resolution of the ancient depths depended critically on the correct interpretation of the origin of Walvis Ridge. ...
... From ca. 100 Ma, rivers fl owing from the interior toward the continental margin breached the eroding shoulder uplifts, and wedges of siliciclastic sediment propagated onto the carbonate platform at the continental margin. Examples on the South Atlantic margins of Africa include the Cretaceous Orange River (Jungslager, 1999, especially Figs. 2 and 14 therein), the Cretaceous Congo River (Karner and Driscoll, 1999, p. 290), the Cretaceous rivers of the Kwanza Basin (Brognon and Verrier, 1966), and the Cretaceous rivers of Gabon (Brink, 1974;Teisserenc and Villemin, 1989). The timing of the change from carbonate to siliciclastic rocks for basins at the margins of Afro-Arabia is set down in Figure 5. ...
This outline of the topographic evolution of Africa tied to the history of the African Surface illustrates how a unique geomorphic history over the past 180 million years refl ects the continent's distinctive tectonics. The African Surface is a composite surface of continental extent that developed as a result of erosion following two episodes of the initiation of ocean fl oor accretion around Afro-Arabia ca. 180 Ma and 125 Ma, respectively. The distinctive tectonic history of the African continent since 180 Ma has been dominated by (1) roughly concentric accretion of ocean fl oor following those two episodes; (2) slow movement of the continent during the past 200 m.y. over one of Earth's two major large low shear wave velocity provinces (LLSVPs) immediately above the core-mantle boundary; (3) the eruption during the past 200 m.y. of deep mantle plumes that have generated large igneous provinces (LIPs) from the coremantle boundary only at the edge of the African LLSVP; and (4) two episodes during which basin-and-swell topography developed and abundant intracontinental rifts and much intra-plate volcanism occurred. Those episodes can be attributed to shallow convection resulting from plate pinning, i.e., arrested continental motion, induced by the successive eruption of the Karroo and Afar plumes. Shallow convection during the second plate-pinning episode generated the basins and swells that dominate Africa's present relief. By the early Oligocene, Afro-Arabia was a low-elevation, low-relief land surface largely mantled by deeply weathered rock. When the Afar plume erupted ca. 31 Ma, this Oligocene land surface, defi ned here as the African Surface, started to be fl exed upward on newly forming swells and to be buried in sedimentary basins both in the continental interior and at the continental margins. Today the African Surface has been stripped of its weathered cover and partly or completely eroded from some swells, but it also survives extensively in many areas where a lateritic or bauxitic cover has accordingly been preserved. Great Escarpments, which are best developed in the southern part of the continent, have formed on some swell fl anks since the swells began to rise during the past 30 m.y. They separate the high ground on the new swells from low lying areas, and because they face the ocean at some distance from the African coastline, they mimic rift fl ank escarpments at younger passive margins. The youthful Great Escarpments have developed in places where the original rift fl ank uplifts formed at the time of continental breakup. Their appearance is therefore deceptive. The African Surface and its overlying bauxites and laterites embody a record of tectonic and environmental change, including episodes of partial fl ooding by the sea, during a 150-million-year long interval between 180 Ma and 30 Ma. Parts of African Surface history are well known for some areas and for some intervals. Analysis here attempts to integrate local histories and to work out how the surface of Afro-Arabia has evolved on the continental scale over the past ∼180 m.y. We hope that because major landscape development theories have been spawned in Africa, a review that embodies modern tectonic ideas may prove useful in re-evaluation of theory both for Africa itself and for other continents. We recognize that in a continental-scale synthesis such as this, smoothing of local disparities is inevitable. Our expectation is that the ambitious model constructed on the basis of our review will serve as a lightning rod for elaborating alternative views and stimulating future research.
... Pourtant, les bassins· du Cuanza et de Moçamedes n'ont livré pour l'instant que des sédiments de base supposés néocomiens. La stra~igraphie de ces bassins côtiers est bien connue grâce aux forages pétro~iers(BROGNON et VERRIER, 1965BRINK, 1974;REYRE et al., 1966) et, dès 1966, REYRE soulignait le synchronisme des principaux événements sédimentaires du Cameroun au bassin Moçamedes. Nous avons synthétisé dans la ~igure 25 les principaux renseignements concernant la strati-gr~phie des bassins Cuanza et Moçamedes. ...
This work tries to bring explanations to genesis and evolution of the reliefs bordering the South central Atlantic, in Angola and Brazil. According to this purpose, it finds back the major stages of the morphologie evolution for the three great unities and draws up a synoptic list of them, which allow: the following conclusions l -The Relief modifications on land and at sea have been absolutely synchronous during the Cretaceous, partially during the Tertiary, when both "swells" are only affected by the same strong beats. 2 -Marginal swells have been rhythmically heaved in Neocomian-Aptian, Turono-Senonian, Oligocene, Pleistocene-Holocene. The upheavals were unequal leading to under and over-raised parts. 3 -The terracing of erosion surfaces, present everywhere although unequally, arises from this rythmic upheaval as well as from the evolution of forms under a climate whose basic trend was -to the semi-arid, shaping pediplain. The morphologic evolutions are thus liable to be interpreted as balance-sheets 4 -Swells have been strongly standing out as major reliefs since the lower Cretaceous with certainty, that is to say, since the unquestionable opening of the South Central Atlantic. Topographic~ permanence, synchronism of evolution require explanations whose value is global. Under and over-raised parts of swells, morphologic dive:. fication demand elucidations of local, regional scope. -The topographic permanence arises from : thermal heritages, juxtaposition of oceanic and continental lithosphere. - The thermal heritage is twofold, resulting from the tectono-thermal event (late Precambrian) and from the splitting of the gondwanian continent (lower Cretaceous). The continental litho-sphere building the swells was thermically outbalanced, heated such is not the case with the one which is to bi found inland, denser. - The swells locate the contact between oceanic and continental lithosphere. The first one cools and subsides, whereas one speaks of thermal inertia for the second one. A distorsion arises from this difference, well marked by a strong flexuration. -The synchronism of evolution calls out compensating mechanisms in relation with the differences between lithostatic pressures, and thermal heritages. The differences are brough about on the very swell by the juxtaposition of both lithospheric types, whence compensation by' creep' of partially molten material from ocean to continent under a mini-mal depth of about 40 km. The extensional faults result of pressure differences at lesser depth, make easier the rising of material and explain the wealth of dykes, heating and lightening the continental crust, provoking isostatic upheaval. The mid-atlantic ridge pushes, resulting from the relief differences between a ridge and oceanic basins, which varies according to the rhythms, of expansion, increase the creep of material under the swell, adding to the balance of constraints an horizontal component, unsteady. Indeed, periods of dyke intrusions and periods during which the rhythms of expansion vary, are firmly synchronous. In the Tertiary, however, the synchronous upheaval could be explained by the delay required between heating of the lower part of the crust during, the cretaceous events, and conveyance of thermal energy by conduction. -Under and over-raised .parts of swells are primarily owed ta the passing of lithospheric plates over hot spots. Distribution of marks let back explains a very clear alternation of parts in Africa, whereas the brazilian situation appears quite unlike. The morphologie diversification of swells arises from a varying lithologic composition of the precambrian basement, too. The cratons, not concerned by the last precambrian event, contribute with their strong lithologie heterogeneity to the shaping of disordered forms . . Thus, the marginal swells stand out first as thermically inherited reliefs. This notion of heritage is easily to be deduced from the two basic elements which explain them : lithospheric justaposition and effects of hot spots.
... The salt formations and their overlying strata have become deformed by the buoyancy of salt, differences in load, gravity, thermal convection, compression and extension to form complex salt tectonic structures. Salt diapirism and its related fractures provide channels and a driving force for hydrocarbon migration, and abundant traps and caps for hydrocarbon accumulation (Liu Y L et al., 2008;Brink, 1974). The irregular shape and density inhomogeneity of Aptian evaporites combined with the plastic rheology of halite lead to poor quality seismic imaging of subsalt formations due to shielding and interference effects (Hudec and Jackson, 2007;Tang et al., 2005;Fort et al., 2004;Jia et al., 2003;Jackson et al., 2000;Jackson and Roberts, 1993;Nalpas and Brun, 1993;Jackson and Talbot, 1991), which restricts accurate interpretations of sedimentary sequences and structural features, and thus full understanding of basin evolution. ...
The Gabon Coastal Basin is a typical saliferous basin located in the middle portion of the West African passive continental margin. Complex salt tectonics make sedimentary sequences and structural frameworks difficult to interpret and can lead to difficulties in construction of balanced cross-sections and reconstruction of basin evolutionary processes. Sedimentary facies and salt structural patterns displaying zonation are based on seismic reflection profiles and drilling data. Two near-vertical fault systems, NW-SE and NE-SW, caused basin to be subdivided E-W zoning and N-S partitioning. Scarp slopes and extension faults formed in the Hinge belt III zone where salt diapir piercement occurred and numbers of salt pillars, salt stocks and salt rollers developed under transtension of coupled near-orthogonal fault systems. The zone east of Hinge belt III is characterized by small-scale salt domes and salt pillows. To the west are large-scale salt walls and salt bulge anticlines caused by diapirism promoted by tension and torsion that also resulted in formation of numerous salt pillars, salt stocks and salt rollers. Our modeling of salt tectonic structures indicates that they were produced by plastic rheological deformation of salt under regional stress fields that varied during three distinct phases of extension, compression and re-activation. Hinge belt III was active from Coniacian to Early Eocene, which was a critical period of formation of salt structures when many extension-related salt structures formed and salt diapirism controlled the distribution of turbidite fans. Rootless extrusion-related salt stocks developed throughout the Late Eocene to Early Oligocene as a result of local ephemeral low-intensity tectonic inversion. Post Oligocene salt diapirism was weak and salt tectonics had a weak influence on sedimentation. Balanced cross-sections of two saliferous horizons crossing different tectonic units from east to west reveal that the basin tectonic evolution and sediment filling processes can be divided into three stages containing seven episodes of rifting, transition and drifting.
... 2002). These salt deposits are likely to have been formed at the same time with those reported from Abakaliki and Ogoja area [52,53], Douala basin [54], and Gabon basin [55]. ...
A 42 m thick outcropping portion of the Mamfe Formation is subdivided into Manyu (31 m thick) and Kesham (11m thick) Members on the basis of textural, mineralogical and structural differences. The Manyu Member (Albian) consists of folded and indurated, medium to coarse grained arkosic sandstones and thickly laminated organic-rich shales deposited in a lacustrine environment. The Kesham Member (Cenomanian) consists of subarkoses intercalated with massive green shale and mudstone deposited in a fluvial environment. The change in depositional environment was tectonically controlled. The mid Cretaceous paleogeography of the embayment was governed by the NE-SW trending "Ikom ridge" which prevented marine incursion from adjacent the Benue Sea. Evaporites found within the basin were precipitated from ocean water that was periodically spilled by strong tides and storms across the ridge into the embayment. The filling-up of the embayment to base level in the Cenomanian resulted in a shift in the depositional center downstream to adjoining lower Benue Trough. Similarity in heavy mineral composition and maturity of the Cenomanian sandstones with recent clastics in the embayment indicates their derivation from the same source terrain and relatively stable tectonic and climatic conditions at the source area since the Cenomanian time. Keywords: embayment • tectonics • depositional environment • Cretaceous © Versita sp. z o.o.
... Il comprend trois sous-bassins, séparés par des limites d'origine structurale : le bassin côtier Sud-Gabon ; le bassin côtier Nord-Gabon, séparé du bassin côtier sud par l'accident de N'Komi, et le bassin intérieur, de direction N140, ouvert vers le nord-ouest et limité par rapport au précédent par le horst de Lambaréné ( Fig. 1). L'analyse du remplissage sédimentaire du bassin intérieur permet de distinguer trois ensembles [7] Alors que les bassins côtiers ont été largement étudiés par les géologues des compagnies pétrolières [2,6,[12][13][14][15][16], le bassin intérieur reste mal connu, car ses potentialités pétrolières sont considérées comme faibles. C'est pourtant le seul bassin où la série présalifère affleure et sert de référence pour le bassin côtier Nord-Gabon. ...
The Interior Basin of Gabon, created during the break-up between South America and Africa, displays thick Neoproterozoic to Aptian p.p. fluvio-lacustrine deposits overlain by Aptian to Albian marine facies. Rock–Eval analyses from outcrop and drillhole samples show high content in organic matter (up to 25%) related to types I and II. These intervals are encountered within Permian, Neocomian–Barremian as well as Aptian siliciclastic succession. They constitute fairly good to excellent potential petroleum source rocks, which are most probably at the origin of oil indices recognized both in drillholes and in surface.
... However, recent studies of canyon formation on the slope area off South-Gabon (Rasmussen, 1994(Rasmussen, , 1997 concluded that there was a marked sea level lowstand at the Aquitanian±Burdigalian boundary of Prentice and Matthews (1988), corresponding to an important tectonic event in West Africa (Walgenwitz, Pagel, Meyer, Maluski, & Monie, 1990). Consequently, the Mid±Late Miocene boundary will be considered a more or less continuous progradational interval which is referred to the Alewana Formation (Brink, 1974). On the basis of our palynological and lithological analysis, it is inferred that the dark dolomitic sandstones correspond to the N2 facies, which is Early Miocene in age. ...
A geological and structural sketch of the southern Cameroon shelf was drawn on the basis of a high-resolution seismic survey, dredgings and Kullenberg core sampling of the outcropping rocks at sea bed. Palynological and mineralogical data (X-ray diffractometry), supported by occasional foraminiferal evidence, revealed four main lithostratigraphic sets resting on a Precambrian basement: Upper Cretaceous (Albian to Senonian), Paleocene–Lower Eocene, Miocene–Pliocene and Pleistocene. Three successive deformational types characterise the structure of this margin: (1) N–S trending fractures parallel to the onshore Precambrian structural directions formed during the rifting period and crosscut by N60°E trending cross-faults induced by the transform movement of the South Atlantic; (2) compressive structures governed by local stresses during the Eocene transcurrent reactivation along the N60°E transform faults excluding a salt-controlled deformation; (3) a set of approximately N–S faults affecting the unfolded sequence of the Miocene–Pliocene or the Quaternary and indicative of distensive conditions. The geodynamic evolution is placed in the general framework of the Central and South-Atlantic rifting and passive margin history. Interestingly, the age of the compressive deformation varies from Late Albian to Mid-Eocene depending on the regional plate motion events which are generated along the narrow fracture zones extending as far as or even into the continental domain.
... Another member ofthe Getty team, Dr. Antoni Brink, previously worked for Shell International in Gabon and was excited about the presence of the carbonateevaporite sequences found in the Bahamas test wells that were drilled by other parties. His original work on the Gabon Basin preceded major discoveries there (Brink 1974). The Gabon Basin is a half graben with a series of hinge zones that controlled the distribution of the depositional environments and ultimately the hydrocarbon reservoir trends (Fig. 2). ...
In the early 1980’s, the Getty Oil Company developed petroleum play concepts for several lease areas that it controlled in the Bahamas. Interest in these areas was based upon the facts that both Cuba and Florida had producing oil fields, oil shows were reported in test wells that had been drilled in the Bahamas and the well logs from these test wells indicated that the amount of evaporites in the Bahamas Platform carbonate-evaporite complex increased with depth. Evaporites indicate arid and often anoxic and reducing depositional environments which is favorable to the preservation of organic material.
The Getty frontier exploration team addressed the formation and occurrence of source rocks, the thermal maturation histories of the potential source rocks, the presence of reservoir quality rock and possible migration pathways. The southern Bahamas was identified as the area that probably had the greatest amount of mature source rock. Thermal maturation (Lopatin) profiles for the Cay Sal well indicated that the Jurassic and Lower Cretaceous were within the oil generation window and the Upper Cretaceous was immature. Thermal maturation (Lopatin) profiles for the Long Island well indicated that the Lower Jurassic was in the gas generation window, the Upper Jurassic and Lower Cretaceous were within the oil generation window and the Upper Cretaceous was immature. Thermal maturation (Lopatin) profiles for the Great Isaac well indicated that the Jurassic and Lower Cretaceous were within the early oil generation window and the Upper Cretaceous was immature. Dolostone, with estimated porosities of between 10% and 18% were thought to be likely reservoirs. Secondary reservoir objectives were skeletal, oolitic and reefal limestones. Evaporites, mainly in the form of anhydrites, and nonporous carbonates were identified as probable seals.
Successful deep wells in the Gulf of Mexico are targeting subsalt structures at depths greater than 20,000 feet (6,100 m) (McMoran Exploration Co., 2009). The presence of significant hydrocarbons and reservoir quality rock at these depths, coupled with advances in seismic imaging, has opened up the possibility of the existence of other successful deep plays in frontier areas, such as the Bahamas.
... Hence, we can assume that the northern part of South Atlantic (areas to the north of Rio Grande Rise and Walvis Ridge) was opening more slowly than the southern South Atlantic. The Neocomian early opening in the northern South Atlantic was marked by the deposition of thick terrigenous, shallow marine, fluvial, and lacustrine sediments in the coastal basins of the Brazilian as well as of the West-African margin (Brink, 1974;Asmus and Ponte, 1973;Asmus, 1975;Campos et al., 1974;Mascle and Renard, 1976). This stage in the opening of the northern South Atlantic has been referred as the "rift-valley stage" (Asmus, 1975;Ponte and Asmus, 1976). ...
The Sào Paulo Plateau is a prominent marginal plateau in the southern Brazilian margin. It is underlain by oceanic crust and is bounded to the north and south by fracture zones. Seismic profiles and drilling data from Site 356 (located on its southeastern corner) indicate that the region occupied by the plateau has acted as a depocenter and has evolved as a marginal plateau since the Aptian time. The Oceanographic conditions and depositional environments prevailing over the plateau during its early evolution were almost identical to those suggested in the Angolan margin by the drilling data at Site 364 (DSDP). Reconstruction of the evolution of Sào Paulo Plateau and com- parison between the drilled geologic sequences on the opposing margins suggest the following history for the northern South Atlan- tic. Coarse terrigenous sediments were deposited in coastal basins at both the margins following the opening of the South Atlantic in the Neocomian. By Aptian time, a linear narrow basin, with barriers to the south and north, was created. The Walvis and Sào Paulo ridges formed the southern barrier whereas the equatorial fracture zones appear to have formed the northern barrier. An evaporite layer, which continuously extended from Brazil to Africa, was deposited in the basin during middle to late Aptian. At the end of Aptian, the evaporite deposition ceased and the evaporite layer was split along the present-day seaward boundaries of the diapir zones located in the Brazilian and West African margins. As evidenced by limestones with sapropels of late Aptian to early Albian age at Site 364, stag- nant conditions prevailed in the basin at the end of evaporite deposi- tion. Open-marine limestones, drilled at Site 356 as well as at Site 364, suggest that the basin became well oxygenated by the Albian time. Another basin-wide period of stagnation is suggested by black shales and mudstones of Turonian-Coniacian age at Site 356 and of Turonian-Santonian age at Site 364. The terrigenous nature of these sediments suggests supply from the adjacent shelves in the two margins. The waters of the South Atlantic have remained well cir- culated and oxygenated since then.
A sedimentary basin is an area of the Earth’s crust that is underlain by a thick sequence of sedimentary rocks. Hydrocarbons commonly occur in sedimentary basins. Most basins cover tens of thousands of square kilometers and may contain more than 5 km of sedimentary fill. Basins may be divided into true basins (subcircular in plan) and troughs (elongate). Embayments are basins that open into larger basins.
Syndepositional basins form where subsidence and deposition occur simultaneously. Postdepositional basins form where paleocurrent directions and facies are discordant with and clearly predate the present basin structure.
Basins formation occurs in four ways. The first group, rift basins, forms as a result of crustal tension during sea floor spreading. The second group forms as a result of crustal compression at convergent plate boundaries. The third group forms in response to vertical crustal movements. The fourth group forms following simple crustal loading due to sedimentation.
Many schemes have been proposed for classifying sedimentary basins. Using a descriptive and genetic approach, basins can be classified as (1) cratonic, (2) troughs, (3) rift–drift, and (4) strike-slip. Cratonic basins can be subdivided into intracratonic basins (wholly on continental crust) and epicratonic basins (partly on continental crust and partly on oceanic crust). Troughs are linear basins that are complex in structure and facies and largely result from three types of subduction zones (continent–continent, continent–oceanic, and ocean–oceanic). Basins associated with subduction zones are termed back-arc troughs and forearc troughs. Rift basins are bounded by major fault systems. Symmetrical rifts are bounded by two sets of faults, and asymmetrical rifts are bounded by one set of faults. Rift–drift basins occur at divergent plate boundaries and are the major petroleum provinces. These basins are typically filled with the following sequence of facies: (1) continental clastics associated with volcanics, (2) evaporites overlain by organic-rich muds (restricted); and (3) carbonate shelves and prograding clastic wedges. Strike-slip basins are bounded by major wrench faults. The basins can be very deep and petroliferous.
The sedimentary rocks overlying
the crystalline basement rocks of the
area north lat. 24° 15' to lat. 24° 30'
on the Red Sea coast comprise 7
mappable formations, subdivided into
11 members. A classification based on
the l i t h o l o g i c and p a l a e o n t o l o g i c
characteristics is proposed. It includes
13 vertical f a d e s association.
A generalized stratigraphic column
s h o w s a close similarity to facies sequence of ocean margin b a s i n s ,
involving two alternating stages of
s e d i m e n t a t i o n , and imply M i d d l e -
Miocene-Early Pliocene transgression
and Late Pliocene-Holocene regression.
In recent years, West African passive continental margin basins become the rapid growth area of global oil and gas resources. To reveal the hydrocarbon accumulation regularity of Gabon coastal basin, the basin's structural characteristics, salt sequence, play and hydrocarbon accumulation model were researched. The results show that: two near-vertical fault systems, north west-south east fault system and north east-south west fault system, which was formed in the period of rift, transitional and passive continental margin stages, make the basin have the tectonic framework of east-west zoning and north-south blocking. Salt sequence is the key factor to hydrocarbon enrichment. Salt structural traps formed by its flow deformation are favorable to hydrocarbon accumulation of the post-salt sequence, and salt rock remains a contiguous sequence because of no sliding window of salt, which provide a good seal to pre-salt sequence. Salt tectonic evolution is different during different periods. Coniacian-early Eocene is the main formation period of salt structures, which corresponds to the active period of hinge belt III. Lots of extensional genetic salt structures were formed, and salt diapirism rate approximately equal deposition rate, which controlled facies distribution and favorable traps. During Late Eocene-Early Oligocene, rootless salt plug of extrusion genesis was developed by tectonic inversion in some areas. In post Oligocene, salt diapirism rate was slow, which influenced sedimentation weakly, and favorable traps were few. Comprehensive analysis shows that favorable play of pre-salt sequence developed in fault-block and faulted anticline of hinge belt I, and favorable play of post-salt sequence distributed in the superimposed area of turbidite fan and salt structural traps on the west side of hinge belt III.
Sedimentary basins can be classified according to their structural genesis and evolutionary history and comprise tectonic and sedimentary cycles and stages, to which common elements of petroleum systems and plays may be linked. We describe a new method that allows easier and more efficient comparisons between petroliferous sedimentary basins with similar geologic characteristics. Using this method, we can make predictions of potential petroleum systems and plays occurring in underexplored basins or regions. In this article, the methodology is applied to the well-explored Gabon coastal basin (west African margin) and the conjugate Almada-Camamu Basin (Brazilian margin), which is a frontier basin. The two basins experienced a similar tectonostratigraphic evolution and share many similarities. We propose that petroleum systems and plays identified in the Gabon coastal basin might potentially also be present in the Almada-Camamu Basin. These could include a second lacustrine synrift petroleum system, as well as a fluviomarine transitional petroleum system. Potential, but not yet proven, reservoirs in the Almada-Camamu Basin include coarse sandstones surrounding basement highs within the synrift, synrift lacustrine carbonates, fluviomarine sandstones at the base of the transitional cycle, and early postrift ramp carbonates. The methodology and newly developed figures are particularly useful to obtain a first impression of the tectonostratigraphy, likely petroleum system and play development, and exploration history of potentially analogous basins. Furthermore, the approach allows for the recognition of important differences, raising questions that can be answered by more direct techniques.
Significant lateral velocity heterogeneity occurring near the surface complicates processing of Gabon onshore seismic data. While implicitly PSDM imaging from the recording surface can deal with this type of heterogeneity, this paper discusses how in PSTM imaging a velocity-adaptive pseudo datum in primary static corrections may help coping with the near-surface heterogeneity. Tests performed on synthetic seismic data and the review of processing flows of real seismic data suggest, that the utilization of velocity-adaptive pseudo datum results in PSTM images superior to those based on primary static corrections using a flat pseudo datum.
We describe a regional grid of West African PROBE Study (WAPS) deep penetrating multi-channel seismic reflection (MCS) and potential field data crossing the transition from rifted continental crust (RCC) to normal oceanic crust (OC) in the offshore Douala and North Gabon Basins. In profiles oriented sub-parallel to oceanic fracture zones RCC is shown to terminate seaward into a very different form of crust interpreted to be composed of highly attenuated blocks of RCC, wedges of seaward-dipping reflectors representing intermixed mafic volcanics and sediments, mafic lower crust, and possibly exhumed mantle rocks, all of which form a complex faulted terrain. This heterogenous type of crust is referred to here as ‘proto-oceanic crust’ (POC). POC grades seaward into normal to thick OC.
The Lower Congo Basin and Congo Fan of Angola contain giant oil discoveries in Oligocene and Miocene deep-water sands; the Late Cretaceous and Tertiary fans in the Campos Basin of Brazil contain some 90% of Brazil's reserves. Petroleum exploration in these vast, deep-water regions is resource-hungry, increasing the value of techniques that facilitate quick and inexpensive extrapolation from discoveries into undrilled areas. Our work demonstrates a strong correlation in these basins between reservoir fairways and various attributes of the gravity signature. We provide comparisons between published interpretations based on well and seismic data and our interpretations of gravity attribute images. We demonstrate the use of geographic information systems (GIS) techniques to facilitate the comparisons, discuss the possible basis for these correlations and speculate on their application to other basins, We conclude with notes on current research to improve the resolution of the gravity data and future refinements to our processing and interpretation methods.
Analysis of structural, stratigraphic, and gravimetric data from the
Reconcavo, Tucano, and Sergipe-Alagoas basins in northeastern Brazil and
the Gabon basin in Africa shows that these basins originated in a double
rifting system associated with multiple crustal detachment surfaces and
that the direction of dip of the detachment surfaces was reversed at the
Vaza-Barris fault system. This geometry is in agreement with models and
data from modern rifts, but it requires the existence of a predominantly
extensional stress regime in the northern part of the South Atlantic
during the early stages of oceanic evolution. Evidence for the existence
of such a stress regime is given by application of kinematic constraints
to Early Cretaceous reconstructions of the South Atlantic Ocean.
Interpretation of the regional geologic setting suggests that the double
rifting system and the point of final continental rupture were
controlled by the preexisting structural orientation of the tectonic
provinces in the Precambrian basement.
Seismic images along Australia's southern continental margin reveal the internal geometry and depositional history of the Hammerhead Delta, a Late Cretaceous shelf-margin delta complex in the Ceduna Subbasin of the Bight Basin. The Hammerhead Delta comprises the late Santonian to Maastrichtian Hammerhead supersequence, which is divisible into three, third-order sequence sets and their component sequences. Sequence set 1 (late Santonian to early Campanian) comprises three progradational sequences deposited after a major fall in sea level under a high sediment supply regime. Sequence set 2 (early Campanian to early Maastrichtian) comprises two, sandy progradational sequences. Basinward shifts in facies caused by forced regression are apparent between each progradational sequence. Sequence set 3 (early to late Maastrichtian) is a thick, aggradational succession deposited when the rates of creation of accommodation space and sediment supply were balanced. A transgressive episode at the top of sequence set 3 marks a rapid decrease in sediment supply and the end of deltaic sedimentation. A long-lived sand-rich sediment supply, most likely derived from erosion of the eastern Australian highlands to the northeast, was the major influence on delta formation. Rapid progradation and the formation of thick shelf-margin clinoforms resulted in slope instability, growth-faulting, and load-induced collapse of the shelf margin during the Campanian. The Hammerhead Delta is characterized by sandy, progradational clinoforms; and lacks the thick coeval prodeltaic shales and shale tectonism that are common to many other large deltas. The results of this study, which included seismic facies mapping, well correlations, and comparisons to other large shale-poor deltas, suggest that the Hammerhead Delta has excellent reservoir potential.
This paper is part of the special publication No.153, The oil and gas habitats of the South Atlantic. (eds: N.R. Cameron, R.H. Bate and V.S. Clure). The opening of the South Atlantic created a series of passive margin basins on both sides of the new ocean. This paper reviews the distribution of petroleum reserves in these basins in terms of their tectono-stratigraphic position within the framework of the rift-drift succession. Seven megasequences are recognized. Three are in the drift succession, three are in the rift succession and one comprises the pre-rift succession. Ninety- three per cent of the presently discovered recoverable hydrocarbons are reservoired in the drift succession, 6% are located in the rift succession and 1% is associated with the pre-rift units. The basins group geographically into seven sectors within which both margins share some common features. Sixty-five per cent of the reserves are contained in Sector V which is dominated by the Niger Delta, and 28% are positioned in Sector III, which includes the Campos and Lower Congo Basins. The new deepwater giant discoveries of Angola, which are located in Sector III, are the Africa counterparts of the earlier deepwater Campos Basin discoveries. In general sectors III-IV, located between the Walvis Ridge and the Equatorial Atlantic transforms, are most favourable as they are characterized by the most prolific source rocks at all horizons.
The Erris Ridge is a narrow structural high that separates the Erris and Rockall Troughs. It has a composite internal character and its orientation is thought to be controlled by a pre-Caledonian structural fabric which was frequently reactivated during Caledonian to Cenozoic time. In the north it is fault-bounded and probably contains rocks of Palaeozoic and older age. II formed a positive topographic feature which was progressively onlapped during Permo-Triassic to Cretaceous times. Fault reactivation occurred along its western boundary in Tertiary times and it was submerged by approximately latest Miocene times. Further south the Ridge is more complex. The Palaeozoic and older core is flanked to the SE by interpreted Permo-Triassic to Lower Jurassic strata which constitute a fault-controlled, uplifted, remnant of a half-graben basin. This composite structure was probably inverted during Late Jurassic limes and remained as a topographic feature until Cretaceous times. It was progressively onlapped during Cretaceous and Tertiary times, with a minor phase of fault reactivation during the Early Tertiary. The Ridge acted as a rigid block which facilitated the focusing of several phases of crustal extension in the Erris Trough and in the Rockall Trough.
The economic importance of the Cretaceous sequence of the Benue Valley is highlighted. The linear sedimentary basin or trough, as sometimes described, is underlain essentially by marine Cretaceous rocks, starting from the Middle Albian, and ranging to the Maastrichtian. There is, so far, no evidence of the occurrence of younger rocks in the Upper and Middle Benue area, though in the Lower Benue and the Niger Delta regions, Palaeocene to Recent formations overlie the older formations.The depositional processes and the tectonics associated with this period caused the emplacement of an accumulation of a number of important minerals of sedimentary and igneous origins, including lead-zinc, barytes, limestone and coal. There are indications of the possible occurrence of evaporites as evidenced by brine-springs associated with collapse features and fracture systems related to diapiric dome structures. The use of the minerals in the Nigerian economy is discussed. There are also favourable prospects for uranium and hydrocarbons. The immediate future will usher in a period of intensive exploration work aimed at evaluating these resources.
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.
The present investigation of the Tertiary succession offshore South Gabon is based on seismic surveys of 1985–1989 vintage and wells covering the southern part of Gabonese waters.In general, the Tertiary basin off South Gabon evolved during two structural stages. During the Paleocene and Early-Middle Eocene, a trough-like basin subsided slowly towards the west. During this period there was a cyclic development of the succession which consists of up to four sequences mostly bounded by Type 2 sequence boundaries. The sequential changes were induced by eustatic sea-level changes. This pattern changed during the Neogene when the evolution was characterized by continental crustal flexuring in Early-Middle Miocene times. This resulted in strong subsidence of the western part of the basin and uplift of the eastern part. Consequently, the eastern part was exposed to strong erosion and a thick Middle Miocene succession was deposited west of the shelf (approximately the present-day shelf), especially on the slope. The clinoformal and lens-shaped Middle-Late Miocene succession which consists of four sequences bounded by Type 1 sequence boundaries has been controlled by eustatic changes in sea level.In addition to the overall subsidence pattern, salt tectonics has modified the external and internal architecture of the sequences on a local scale. Canyon formation in connection with slumping results in the establishment of well developed fan systems on the lower part of the slope and on the basin floor area.
The study provides a regional seismic interpretation and mapping of the Mesozoic and Cenozoic succession of the Lusitanian Basin and the shelf and slope area off Portugal. The seismic study is compared with previous studies of the Lusitanian Basin. From the Late Triassic to the Cretaceous the study area experienced four rift phases and intermittent periods of tectonic quiescence. The Triassic rifting was concentrated in the central part of the Lusitanian Basin and in the southernmost part of the study area, both as symmetrical grabens and half-grabens. The evolution of half-grabens was particularly prominent in the south. The Triassic fault-controlled subsidence ceased during the latest Late Triassic and was succeeded by regional subsidence during the early Early Jurassic (Hettangian) when deposition of evaporites took place. A second rift phase was initiated in the Early Jurassic, most likely during the Sinemurian–Pliensbachian. This resulted in minor salt movements along the most prominent faults. The second phase was concentrated to the area south of the Nazare Fault Zone and resulted here in the accumulation of a thick Sinemurian–Callovian succession. Following a major hiatus, probably as a result of the opening of the Central Atlantic, resumed deposition occurred during the Late Jurassic. Evidence for Late Jurassic fault-controlled subsidence is widespread over the whole basin. The pattern of Late Jurassic subsidence appears to change across the Nazare Fault Zone. North of the Nazare Fault, fault-controlled subsidence occurred mainly along NNW–SSE-trending faults and to the south of this fault zone a NNE–SSW fault pattern seems to dominate. The Oxfordian rift phase is testified in onlapping of the Oxfordian succession on salt pillows which formed in association with fault activity. The fourth and final rift phase was in the latest Late Jurassic or earliest Early Cretaceous. The Jurassic extensional tectonism resulted in triggering of salt movement and the development of salt structures along fault zones. However, only salt pillow development can be demonstrated. The extensional tectonics ceased during the Early Cretaceous. During most of the Cretaceous, regional subsidence occurred, resulting in the deposition of a uniform Lower and Upper Cretaceous succession. Marked inversion of former normal faults, particularly along NE–SW-trending faults, and development of salt diapirs occurred during the Middle Miocene, probably followed by tectonic pulses during the Late Miocene to present. The inversion was most prominent in the central and southern parts of the study area. In between these two areas affected by structural inversion, fault-controlled subsidence resulted in the formation of the Cenozoic Lower Tagus Basin. Northwest of the Nazare Fault Zone the effect of the compressional tectonic regime quickly dies out and extensional tectonic environment seems to have prevailed. The Miocene compressional stress was mainly oriented NW–SE shifting to more N–S in the southern part.
A study of active and ancient rift systems around the world suggests that accumulations of fossil fuels and metallic minerals are related to the interactions of processes that form rift valleys with those that take place in and around rift lakes. The deposition of the precursors of petroleum, gas, oil shale, coal, phosphate, barite, Cu-Pb-Zn sulfides, and uranium begins with erosion of uplifted areas, and the consequent input of abundant nutrients and solute loads into swamps and tectonic lakes. Hot springs and volcanism add other nutrients and solutes. The resulting high biological productivity creates oxidized/reduced interfaces, and anoxic and H2S-rich bottom waters which preserves metal-bearing organic tissues and horizons. In the depositional phases, the fine-grained lake deposits are in contact with coarse-grained beach, delta, river, talus, and alluvial fan deposits. Earthquake-induced turbidites also are common coarse-grained deposits of rift lakes.Postdepositional processes in rifts include high heat flow and a resulting concentration of the organic and metallic components that were dispersed throughout the lakebeds. Postdepositional faulting brings organic- and metal-rich sourcebeds in contact with coarse-grained host and reservoir rocks. A suite of potentially economic deposits is therefore a characteristic of rift valleys.
This chapter summarizes the sedimentary characteristics of non-marine environments. Non-marine evaporite deposits are common features of modern arid closed basins, but relatively few have been recognized in the geologic record. These evaporites are important because their depositional setting reflects both climatic and tectonic conditions and they may contain economically important mineral resources. Most modern non-marine evaporites and almost all ancient non-marine evaporates occur in saline lake deposits or their associated facies. The term saline lake includes perennial lake, ephemeral lake, playa-lake, playa, salina, salar, chott, salt pan, inland sabkha, and any other terms used to refer to areas that, at least intermittently, hold standing bodies of water. The general distribution of modern non-marine evaporites is shown in a diagram and references are presented in a table in the chapter. The distinction between marine and non-marine evaporites is not trivial and many ancient evaporites that are literally assumed to be marine may be, at least in part, non-marine in origin Furthermore, some of the deposits interpreted as non-marine may actually be marine (for instance, the evaporites of the Precambrian McArthur Basin, Australia).
This paper is part of the special publication No.153, The oil and gas habitats of the South Atlantic. (eds: N.R. Cameron, R.H. Bate and V.S. Clure). Findings drawn from the Tertiary pre-rift, rift and post-rift sequences of Africa and the Red Sea are applied to the South Atlantic margins with a view of aiding hydrocarbon exploration. In relation to gravity and magnetic prediction of top basement, template rifts tend to follow pre- existing lines of weakness and the upper crust underlying these rifts may have geophysical characteristics which are different from those of the rift flanks. Template rifts may or may not be associated with volcanicity. In sparsely volcanic rifts, syn-rift volcanism is located on rift axis structural highs. Extensive volcanism is accompanied by regional doming and erosional loss of the pre-rift section is likely in rift sectors preceded by volcanism. Consequently, it is doubtful whether pre-rift sediments are preserved beneath the major volcanics offshore from the Parana-Etendeka areas in the South Atlantic. The Kalahari Group of southern Africa is identified as a good analogue for pre-rift reservoir objectives in the South Atlantic, and facies and thickness criteria are discussed that help discriminate pre-rift sequences from rift sequences. Where sedimentation has kept pace with rift subsidence, so that fault scarp facies have not developed, then the Okavango Delta provides a useful analogue. Parallels are drawn between major sand dykes interpreted from seismic in the deep-water deposits of Lake Tanganyika and similar units within similar facies in the Reconcavo Basin of Brazil. Direct analogue links between the Red Sea and the South Atlantic are less strong, mainly because marine incursions, and salt deposition, occurred at a later structural stage in the South Atlantic. The onset of salt deposition in the Gulf of Suez-Red Sea partly salt filled, rapidly turned to fully-marine deposition. The earliest part of the post-rift sag phase in the South Atlantic is represented by laterally extensive lacustrine source rocks. Salt deposition in the South Atlantic occurred only after full development of the sag phase.
The Aptian salt deposits of the South Atlantic Ocean extend for 250 km offshore and underlie the continental rise off Brazil and Angola in 4000 m water depth. These salt deposits do not occur south of the Walvis Ridge and Rio Grande Rise, and apparently lie on oceanic and continental crust. Geological evidence and an Aptian-Middle Barremian reconstruction of the South Atlantic suggests the pre-Aptian South Atlantic was closed at both ends, thus favouring widespread salt deposition on oceanic and continental crust. Three models of halite deposition are discussed briefly.
Wide Triassic evaporite outcrops run parallel to the strike of Hellenide thrust faults. They crop out along the leading edges of thrust sheets emplaced as part of the foreland-migrating fold-and-thrust belt. The evaporites acted as the plane of decollement for thrust-sheet emplacement over much of the External Hellenides. Plio-Quaternary diapirs occur locally within the broad outcrops of Triassic evaporite. They are the results of local mobilization of evaporite after its emplacement. -Author
Sedimentary basins can be classified according to their structural genesis and evolutionary history and the latter can be linked to petroleum system and play development. We propose an approach in which we use the established concepts in a new way: breaking basins down into their natural basin cycle division, then defining the characteristics of each basin cycle (including the type of petroleum systems and plays they may contain) and comparing them with similar basin cycles in other basins, thereby providing a means to learn through a greater population of (perhaps not immediately obvious) analogues. Furthermore, we introduce the use of the trajectory plot as a new tool in such an analysis. This methodology has been applied to the West African South Atlantic marginal basins between Cameroon and Angola, and we demonstrate that the similar tectonostratigraphic evolution of the individual basins along this margin has led to the development of similar types of petroleum systems and play (level)s. Consequently, we can make analogue comparisons among these basins in order to evaluate and predict the presence of potential, yet undiscovered, hydrocarbon accumulations in less well explored parts of the margin.
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