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The Namibian and Brazilian southern South Atlantic petroleum systems: are they comparable analogues?

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Abstract

Tectonic reconstructions made across the southern South Atlantic Ocean indicate a diversity of rift and drift basin characteristics on the conjugate margins that define them as different stratigraphic and structural entities. In terms of petroleum systems, the basins are not as unlike as some characteristics suggest. Given the lack of significant hydrocarbon discoveries to date south of the Walvis Ridge, doubts have been cast on the presence in this area of the prolific Lower Cretaceous lacustrine and marine source rock systems, which are well known in the Greater Campos Basin and offshore Angola. Oils and condensates from the basins south and north of the Walvis Ridge exhibit geochemical similarities suggesting that comparable source rock systems are present in both areas. The condensate geochemical analysis results from the Kudu Field in Namibia are compared with oils from marine and lacustrine sources in Brazil, indicating that the Kudu condensates are derived from at least two different source rocks. These results suggest that the underexplored basins offshore Namibia contain thermally mature Lower Cretaceous lacustrine and marine source rocks, offering a new frontier for petroleum exploration in Africa's southern South Atlantic.
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    
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   
  
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
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   






 

    
   
      

         

     


     


     
      
    
   
  



       

  
       
      
    
   
   
       

      
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
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    

   
   
   
  
   
     
   
   
   
   
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
    
   
   



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
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       


     

    
    
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
   
     
   



    


    



     
    
   
   
    

    
  

     
     
      
   
      


     

    
     
     


    
    
    
    
        
        
     
  
    
      

      
      


      

   

 


     
    
      
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
      

 
     
  
 

      

          

 
     
 
      
     


    
 
     



     
   
     
      

     

  
     
      
     
   
      


     







     


    


    
 
    
    
    
     
     
     
     
    
      
     


   
      
     
    
      
    
      


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
  
    


      

      
    

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  

     

     
     
     

     

   
      
    


     
      
     
    
       
 

             
    

    
       


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 

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    
    
      
    
    
 
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   
    
     
      
    
  
     
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   
   
    
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     
     
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      
   
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   
  
   
     
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
     

   
    



ACROBAT (PDF) SYNOPSIS ON ALL AVAILABLE PROJECTS ARE DOWNLOADABLE FROM ENVOI’S WEB SITE:
www.envoi.co.uk
ACTIVE PROJECTS
1. JUAN de NOVA (S.E. AFRICA): Offshore (exploration) Under Offer
2. SENEGAL: Offshore (exploration) Under Offer
3. UGANDA: Onshore (exploration)
4. GEORGIA: Onshore (exploration / appraisal & production) Under Offer
5. SLOVENIA / HUNGARY: Onshore Pannonian Basin (appraisal / development)
6. HUNGARY: Onshore (exploration) Under Offer
7. ITALY: Onshore (corporate sale / development asset package) Under Offer
8. NORTH WEST EUROPE: Onshore / Offshore (corporate sale) Under Offer
9. UK (Onshore): Cheshire Basin / East Midlands (CBM exploration) Under Offer
10. UK (Onshore): Weald Basin (exploration)
11. PHILIPPINES: Offshore (exploration)
12. NW AUSTRALIA: Offshore Carnarvon Basin (appraisal, development & upside exploration)
13. UK (Onshore): East Midlands(rehabilitation /development)
FOR MORE INFORMATION CONTACT:

November 2011
Mike Lakin
Tel: +44 (0)20 8566 1310
Email: mail@envoi.co.uk
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... There are many divergent South Atlantic sag basin interpretations in recent literature (Mello et al., 2012, andLentini et al., 2010). New basin exploration requires diagnostic criteria to differentiate presalt sag geometries from other nonmarine or deep-water settings. ...
... A seismic interpretation of published Luderitz Basin 2D (Mello et al., 2012) and the comparable southern Walvis Basin geoseismic section highlight differences between presalt margins and the southern margins ( Fig. 16 and Fig. 4, respectively). SDRs converge at the Namibian Basin margin and indicate subaerial to shallow water environments. ...
... Diachronous reopening suggests a pre-existing line of weakness (anisotropy) facilitated new rifting. This rifting was also asymmetric in character, with the conjugate margin in Brazil varying considerably in its sedimentary and subsidence history (Mello et al., 2013). Due south of the volcanic Walvis Ridge, the basins offshore Namibia were not affected by the restricted marine conditions created by it during the Albian-Aptian, and thus no salt is encountered . ...
... prolonged post-rift thermal subsidence or thermal sag(Bray et al., 1998). Syn-rift structures are timed to the Neocomian-Barremian with sag basin development during the Barremian-Aptian(Mello et al., 2013). Asymmetric rifting has resulted in significant variability in the sedimentary and subsidence history between the conjugate margins with this reflected in the nature of hydrocarbons discovered in these areas(Mello et al., 2011). ...
Preprint
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An understanding of the subsurface thermal regime is beneficial to many disciplines, including petroleum and geothermal exploration, carbon capture and storage (CCS) and nuclear waste sequestration. This project developed and tested a new methodology for determining subsurface temperature using a non-invasive approach based on the velocity information derived from seismic reflection data. By solving a one-dimensional steady state approximation of Fourier’s Law, it is possible to determine a bulk thermal gradient as a function of depth, enabling the determination of temperatures across an entire volume using this methodology, termed reflection seismic thermometry. There are two principal components to this methodology, requiring 1) a bulk thermal conductivity structure and 2) heat flow and/or temperature data to condition the model. The first component uses an empirical velocity to thermal conductivity transform whilst the second uses sparse temperature data from boreholes or a bottom simulating reflector (BSR) to derive the shallow thermal regime and heat flow. The thermometry workflow has been applied to three case studies; in the Lüderitz Basin, offshore Namibia; the Blake Ridge, offshore USA; and the North Viking Graben (NVG) in the North Sea. In the frontier Lüderitz Basin, a BSR was identified and used to derive heat flow of 60-70 mW m-2. The Aptian source rock interval here was shown to presently be in the gas generative window. On Blake Ridge borehole velocities and a BSR were used to determine heat flow (43-56 mW m-2) and subsurface temperatures. Finally, methodology validation was conducted in the North Sea Basin using a high-resolution 3D full waveform inversion (FWI) velocity dataset calibrated with 141 wells. Forward models of subsurface temperatures were calibrated against the borehole temperatures, with inverse modelling used to derive heat flow at km scale lateral resolution. The availability of a fast track velocity volume for this area allowed comparison with the FWI derived thermal model results. It was found that stacking velocities were lower than well and FWI velocities, leading to overprediction of subsurface temperature. Modelling the temperature profile for CCS well 31/5-7 showed bottom hole temperature (BHT) within 6 °C of recorded BHT. With application and verification of the method in different basins, the versatility of the work conducted is demonstrated. It is envisioned that this technique opens avenues for the seismic characterisation of thermal regime in disparate settings and varied disciplines.
... In accord with classical rifting models, this sediment package has been mostly interpreted to be deposited during the post-rift thermal subsidence. Thus, break-up in this basin has been interpreted to occur either before or very early in the Aptian (Models 1 and 2 in Fig. 2, Mohriak et al., 2008;Blaich et al., 2013;Kumar et al., 2012;Mello et al., 2012;Marcano et al., 2013Miranda et al. 2013;Rowan 2014, Quirk andRüpke, 2018). Some authors, however (e.g., Model 3, Figure 2, Karner andGamboa, 2007 andHuismans andBeaumont, 2011), interpret this sequence to be synrift and its subhorizontal appearance, a consequence of greater extension in the mantle and lower crust than in the upper crust during active rifting. ...
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This paper presents the time and space evolution of crustal deformation and their respective sedimentary infill of the 600 km wide, asymmetric conjugate rifted margin of Santos -Benguela basins. Based on a geoseismic transect obtained with interpretation of long-offset reflection seismic and tied by wells, we interpret six main synrift unconformities, corresponding to different deformation phases processed from Valangianian to Early Albian. Confined by these unconformities, sedimentary growths with progressively young relative ages toward the boundary with the oceanic crust are interpreted as evidence of oceanward rift migration. Combination of these information with crustal structure derived from long offset reflection seismic illuminating the deep crust of Santos-Benguela Conjugate Margin, resulted in a complete view of sedimentary infill, internal compartments, and crustal structure. These data were used to guide a dynamic model of rifting that results in a simulated lithospheric section. We show that the margin architecture can be explained by a combination of an early, protracted phase of distributed deformation, followed by basinward rift migration. Distributed deformation lasted from Valanginian to early Aptian (135-117 Ma), initiating with isolated lakes that later coalesced into a wide basin-scale lake (> 450 km). From Mid Aptian to Early Albian (117-110 Ma), rift migration formed the main structural compartments and unconformities, as well as the distal hinge zones we observe today in the seismic lines. During this time, the inner proximal margins were left behind to thermally subside, whereas outer proximal and distal margins were tectonically active. Coexistence of these two processes explain the enigmatic simultaneous formation of proximal sag-like geometries, with late synrift accumulation of an up to 3 km thick salt layer, with tectonically active faults in the distal margin, promoting crestal blocks uplift that could explain the deposition of late Aptian, shallow water pre-salt carbonate rocks. Supplementary material at https://doi.org/10.6084/m9.figshare.c.5819879
... Decoupling of the salt layers together with the tectonic activity and diapirism results in the geometric incompatibility of surface structures with depth and made a complicated geological interpretation for the structural evolution of a given region. The presence of such layers in areas hosting high hydrocarbon resources and their influence on petroleum systems has attracted the attention of many researchers (Alsop et al., 2012;Jackson and Hudec, 2017;Mello et al., 2013). ...
Article
Salt tectonics can influence the petroleum systems via the development and distribution of ductile migration pathways through the structural and stratigraphic traps. In this study, the effects of base-salt relief on the development of salt décollement horizon and the salt-influenced structures were investigated in the eastern part of the Persian Gulf, NE of the Afro-Arabian Plate. For this purpose, data from hydrocarbon exploration wells and seismic lines of the region have been used. To set the position in a time of the main stratigraphic and acoustic markers, and to compare well and seismic data, the processing of synthetic seismogram and the conversion of logs in the time domain has been performed for the selected well. Interpretation of seismic sections revealed the occurrence of a NE-SW trending horst structure and development of a natural model of the salt tectonics on its overburden of the Oligocene to Miocene sedimentary layers. The presence of Oligo-Miocene saliferous beds as a décollement has provided suitable conditions for overburden sediments gravitational sliding and development of thin-skinned extension which started during the Miocene time and it was evolved up to the present time. This condition controlled the geometry of salt-influenced structures in the eastern parts of the Persian Gulf and resulted in the formation of special structural features namely hereafter as Curly Bracket Structures in the overburden.
... Paik, 2005;Seard et al., 2013;He et al., 2015;Chidsey et al., 2015;Gong et al., 2017;Adiya et al., 2017;Roche et al., 2018). Lacustrine microbialites have been linked to economically significant petroleum systems, especially prolific in the Lower Cretaceous "pre-salt" systems found offshore Brazil and Angola (Awramik and Buchheim, 2012;Wasson et al., 2012;Mello et al., 2013;Muniz and Bosence, 2015;Gomes et al., 2020) and the Eocene Green River Formation within the Uinta Basin in Utah, USA (Bradley, 1924;Loewen et al., 1999;Leggitt and Cushman, 2001;Leggitt et al., 2007;Seard et al., 2013;Chidsey et al., 2015;Hurst et al., 2018). The Limagne Basin, located in the French Massif Central, shares numerous geodynamical (e.g., a normal fault system framework linked to a rifting system and volcanic activity; Davison, 2007) and sedimentological (diverse and abundant lacustrine microbialites; Bertrand-Sarfati et al., 1966;Donsimoni and Giot, 1977;Freytet, 2000;Wattinne et al., 2003;Roche et al., 2018) characteristics with the South-Atlantic "pre-salt" basins and therefore represents an exceptional analogue. ...
... Paik, 2005;Seard et al., 2013;He et al., 2015;Chidsey et al., 2015;Gong et al., 2017;Adiya et al., 2017;Roche et al., 2018). Lacustrine microbialites have been linked to economically significant petroleum systems, especially prolific in the Lower Cretaceous "pre-salt" systems found offshore Brazil and Angola (Awramik and Buchheim, 2012;Wasson et al., 2012;Mello et al., 2013;Muniz and Bosence, 2015;Gomes et al., 2020) and the Eocene Green River Formation within the Uinta Basin in Utah, USA (Bradley, 1924;Loewen et al., 1999;Leggitt and Cushman, 2001;Leggitt et al., 2007;Seard et al., 2013;Chidsey et al., 2015;Hurst et al., 2018). The Limagne Basin, located in the French Massif Central, shares numerous geodynamical (e.g., a normal fault system framework linked to a rifting system and volcanic activity; Davison, 2007) and sedimentological (diverse and abundant lacustrine microbialites; Bertrand-Sarfati et al., 1966;Donsimoni and Giot, 1977;Freytet, 2000;Wattinne et al., 2003;Roche et al., 2018) characteristics with the South-Atlantic "pre-salt" basins and therefore represents an exceptional analogue. ...
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The Limagne Basin (Massif Central, France) originated during a major, European-scale, extensive event (European Cenozoic Rift System), which led to the formation of several rift systems in the foreland of the Alps between the Upper Eocene and Pliocene. A fluvio-lacustrine system emplaced in the basin and resulted in a mixed carbonate-siliciclastic sedimentation in which microbial and metazoan buildups occupy an important place. However, microbial deposits are not exclusive to the Cenozoic history of the Limagne Basin; nowadays, in the basin, they still form in association with thermal spring systems. A fieldtrip was carried out in the Limagne Basin as part of the Microbialites: formation, evolution and diagenesis (M-Fed) meeting (October 2019). The objective of this excursion was to assess the diversity of modern and fossil (Chattian to Aquitanian) microbial sediments and structures in three prime locations (the Jussat and Chadrat outcrops and the Grand Gandaillat quarry). A detailed description of the morphologies and fabrics of the buildups and their associated biotic components can be used to discuss the spatio-temporal distribution pattern. Different margin models are proposed based on the changes in the distribution, morphology and size of the microbial and metazoan-rich deposits through time. The Jussat outcrop offers novel perspectives to unravel the evolution of the lacustrine/palustrine cycles over time and to establish a long-term paleoenvironmental history of the western margin of the basin during the Aquitanian. These cycles are composed of (i) lacustrine sedimentation comprising microbial and metazoan buildups and organic matter-rich marls reflecting a period of high accommodation, and (ii) palustrine deposits made of mudstones and clayey paleosoils, indicative of a period of low accommodation. It is suggested that climatic, tectonic, volcanic and local parameters (physiography, substrate) control the deposition of the buildups in each of the different cycles. In addition, the modern microbial mats of the Sainte-Marguerite and La Poix outcrops offer an opportunity to approach the controlling processes at the origin of the mineralization involved in the formation of the microbialites and their preservation in the fossil record.
... Although not the case for the GRF, marine incursions can periodically supply seawater to some lacustrine systems, which is documented in lacustrine petroleum systems in China (e.g., Hu et al., 2015;Wei et al., 2018) and lacustrine facies of Cretaceous rift petroleum systems in Brazil and West Africa (Mello et al., 2013). Intervals of the BCF hosting the saline lacustrine biomarker signature may represent intervals of basin isolation from the ocean. ...
Article
The Paleoproterozoic Barney Creek Formation, which is currently interpreted as a restricted, deep marine paleoenvironment, plays a disproportionate role in our understanding of Proterozoic ocean chemistry and the rise of complex life. The Barney Creek Formation hosts several unusual biomarker features, specifically its methylhopane and carotenoid signatures. Herein, we demonstrate that the saline lacustrine Eocene Green River Formation shares a similar distribution of methylhopanes and carotenoids, which is characteristic of saline lacustrine organic matter more generally. These distinct methylhopane and carotenoid patterns are not observed together in marine organic matter of any geologic age. These results imply a saline lacustrine depositional environment for the Barney Creek Formation, which agrees with earlier but now abandoned depositional models of this formation. As a result, models of Proterozoic ocean chemistry and emergence of complex life that rely on a marine Barney Creek Formation should be reexamined. Alternatively, if Paleoproterozoic marine biomarker signatures resemble those of younger saline lacustrine systems, then this must be recognized to accurately interpret geologic biomarker and paleoenvironmental records.
... The thick basalt layers along the continental margin have been penetrated by dozens of exploratory boreholes that drilled the pre-salt rift sequences in shallow and deep waters (Mohriak et al. 2008;Fornero et al. 2019). The southernmost basins in the South Atlantic are characterized by thick wedges of seaward-dipping reflectors that have been drilled in West Africa and southern Brazil (Mello et al. 2013). The younger post-rift magmatic events are dated as Late Cretaceous to Paleogene, and have been registered in the Pelotas, Santos, Campos, Espírito Santo, and offshore Bahia basins (Geraldes et al. 2012). ...
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The Brazilian continental margin includes several volcanic islands, submerged volcanic seamounts, and a unique non-volcanic archipelago located in a transform segment of the Equatorial South Atlantic. The mechanism of formation of these islands is related to post-breakup magmatic episodes dated as Late Cretaceous to Pleistocene. Diverse Late Cretaceous to Paleogene alkaline magmatic episodes are registered in southeast Brazil, resulting in igneous plugs onshore and volcanic structures offshore. The Abrolhos Volcanic Complex in eastern Brazil is characterized by several volcanic features on the continental shelf, including small islands that expose Paleogene sedimentary layers interbedded with volcanic sequences. The adjacent Vitória-Trindade Chain extends to oceanic crust forming basaltic to alkaline seamounts that outcrop at the Trindade Archipelago, the easternmost islands in Brazil with the youngest volcanic eruptions. The Fernando de Noronha lineament in northeast Brazil is characterized by Neogene alkaline igneous plugs. The small islets in the São Pedro—São Paulo archipelago, located near the mid-Atlantic ridge, are formed by exhumed mantle rocks related to compressional episodes a transform fault zone. The Rio Grande Rise in southern Brazil is characterized by shallow Paleogene seamounts and a large oceanic plateau probably related to subaerial spreading centers formed in the Late Cretaceous. Multiple mechanisms are responsible for the origin and evolution of the volcanic islands offshore Brazil in continental, transitional, and oceanic crust settings, including volcanic build-ups, leaking fracture zones, and hotspots. Some of the islands might be related to mantle plume activity, as indicated by comparisons with modern mantle plume analogues in the South Atlantic.
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The ionizable molecular composition of 36 lacustrine oil samples and 29 marine oil samples from different off-shore Brazilian basins have been screened by ultra-high resolution and accuracy Fourier transform ion cyclotron resonance mass spectrometry with electrospray ionization in the negative ion mode [ESI(-)-FT-ICR MS]. The negative mode has been demonstrated successfully to characterize source rock because it favors the detection of heteroatomic compounds such as nitrogen-, sulfur-, and oxygen-containing (NSO) which reveal information about geochemical environmental. ESI(-)-FT-ICR MS data was processed with a new approach which unify information from chemometrics tool and petroleomics conventional data processing. Novel diagrams were proposed that use variable important to projection (VIP), such as molecular formula obtained, via partial least squares-discriminant analysis (PLS-DA) to plot in diagrams, such as those that display double bond equivalence (DBE) vs number carbon atom (NCA) and H/C ratio vs molecular mass (MM) trends. This new approach is advantageous since it is able to verify and compare all samples at once, obtaining more selective variables, such as molecular formula, and with more confidence because the figures of merit are acquired by statistical tests from PLS-DA, whereas it also provides the usual information from petroleomics tools such DBE, NCA, H/C ratio, MM and molecular classes. This new approach revealed significant differences in the molecular distribution of heteroatomic components for N-containing and O-containing molecules (N class and O class) allowing clear separation of all samples in two groups of marine or lacustrine crude oils.
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Intraplate Continental Margins comprise fully rifted areas of continents and cratons recording the tectonic events associated with plate breakup and subsequent continental drift. In contrast to extensional basins, this tectonism post-dates the initial stages of continental rifting of tectonic plates and is specifically associated with the breakup and ‘drifting’ of continents. Seismic and outcrop data from eight (8) Intraplate Continental Margins are reviewed in this work, and tectonic-subsidence models are compiled to their large-scale structural and depositional variability. Specific geological aspects that enhance the hydrocarbon potential of Intraplate Continental margins include: a) syn-rift strata that are strikingly thicker, and presumed older, than previously assumed on vintage seismic profiles, b) an important control of basement type and fabric on tectonic subsidence and subsequent reactivation after continental breakup is achieved, and c) widespread extensional collapse accompanies the continental breakup process, generating a phase of important sediment bypass into relatively deeper, distal depocentres. There is evidence on seismic data that the continental breakup process per se overprints older syn-rift structures to create a complex mosaic of uplifting and subsiding basins that may not coincide in location and extent with early syn-rift depocentres. This character has the potential of generating vast amounts of reservoir strata during tectonically active periods, at the same time enhancing the influx of organic matter to basins distant from point-sources of sediment during continental breakup. As a corollary, a classification of breakup sequences is presented in this work and a new case-study from South California is developed to acknowledge the varied hydrocarbon-generating settings of Intraplate Continental Margins when continental breakup is established.
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Petroleum geochemical data and burial history, thermal, and fluid flow models show that the southern Lower Congo Basin contains dual petroleum systems. The older petroleum system includes a presalt type I lacustrine source (Barremian Bucomazi Formation time equivalent) that is present but not well understood in the deeper offshore Lower Congo Basin. The lacustrine synrift grabens are generally obscured by salt on seismic profiles, and there have been few well penetrations except on the shelf. However, based on models of burial history, thermal evolution, and fluid flow, we have found that the lacustrine sequence matures offshore and that migration is focused updip toward the shelf where the reservoired oils correlate to this source. A younger petroleum system consists of type II marine clastic shales of the Cenomanian-Turonian (Upper Cretaceous) labe Formation that have charged the overlying Tertiary sandstones in deep-water areas. These turbidite reservoir sandstones, although charged primarily by the Cretaceous Iabe marine clastic source, show some evidence of mixing with early oils from the Tertiary Landana Formation. Mixing of oils between the two petroleum systems is limited to those areas where the Loeme Salt has evacuated sufficiently to allow migration through an assumed permeable salt weld.
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Crude oils of nonmarine source can be distinguished from those of marine shale source and from oils originating in marine carbonate sequences by using a battery of geochemical parameters, as demonstrated with a sample suite of nearly 40 oils. A novel parameter based on the presence of C30 steranes in the oil was found to be a definitive indication of a contribution to the source from marine-derived organic matter. A second novel parameter based on monoaromatized steroid distributions was effective in helping to distinguish nonmarine from marine crudes and can be used to gauge relative amounts of higher plant input to oils within a given basin. Sterane distributions were similarly useful for detecting higher plant input but were less effective than monoaromatize steroid distributions for making marine versus nonmarine distinctions. Concentrations of high molecular-weight paraffin can also be effective nonmarine indicators but are influenced by maturation and biodegradation processes. Certain algal-derived nonmarine oils may show little high molecular-weight paraffin response. Oils from carbonate sources (with a few exceptions) can be distinguished by having low pristane-phytane ratios, low carbon preference indexes, and high sulfur contents. Gammacerane indexes and carbon isotope ratios of the whole crude are not effective in distinguishing these types of environmental differences on a global basis.
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Satellite-derived gravity anomalies were used to map the location and distribution of rift subbasins comprising the Campos and Santos Basins of the southeastern Brazilian continental margin. Free-air and crustal Bouguer gravity anomalies define several features. A negative-positive gravity gradient along the southeastern Brazilian margin correlates generally with termination of oceanic fracture zones, the boundary of synrift evaporites, and an abrupt change in anomaly trends from east-west to margin-parallel. The gravity gradient thus defines the location of the ocean-continent boundary and suggests that much of the São Paulo Plateau is underlain by thinned continental crust. Second, a major offshore tectonic hinge zone, consisting of a series of short-segment, en echelon, high-standing blocks subparallel to the Brazilian margin demarcates the western limit of significant continental extension. The Badejo High of the Campos Basin is part of this hinge zone trend. The Serra do Mar and Serra da Mantiqueira mountains represent an onshore hinge zone. Third, a series of major rift subbasins exist seaward of both the Campos and Santos hinge zones. These have limited along-strike continuity, implying that synrift lake communication, water chemistry, and possibly source quality and preservation were restricted to each subbasin. Extension between west Africa and Brazil occurred during the Early Cretaceous as a series of rift pulses that culminated in the initiation of sea floor spreading. The Congo-Cabinda margin of west Africa and the Camamu-Almada margin of eastern Brazil are characterized by a common tripartite rift history: Berriasian-Hauterivian, Hauterivian-middle Barremian, and late Barremian-early Aptian. Early depth-independent, broadly distributed, and increasingly focused brittle deformation (rift phases I and II) was replaced by depth-dependent deformation dominated by plastic thinning of the lower crust and lithospheric mantle (rift phase III). The nonmarine part of the Lagoa Feia Formation correlates with rift phase II while the "transitional" part is associated with rift phase III. The intervening pre-Alagoas unconformity is equivalent to the pre-Chela unconformity on the Congo margin. The possibility of a mid-crustal detachment beneath the Brazilian margin active during rift phase III has profound implications for the hydrocarbon maturation history of the margin. Seismic reflection data from the São João da Barra Low (Campos Basin) have helped define an early synrift depositional package that likely equates with rift phase I.