conjugate divergent margins an introduction the main objective of this book is to provide a global overview of divergent margins based on geological and geophysical interpretation of sedimentary basins along the south central and north atlantic conjugate margins from plate tectonics and crustal scales to a more detailed description of stratigraphical and structural elements that are responsible for petroleum plays these themes are complemented by geodynamic concepts based on physical and numerical models and by comparisons with present day embryonic margins which are succinctly discussed in some papers

the namibian and brazilian southern south atlantic petroleum systems are they comparable analogues 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 cretac eous 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 lacus trine and marine source rocks offering a new frontier for petroleum exploration in africa s southern south atlantic

enrichment of diamondoids and 13c in condensate from hudson canyon us atlantic abstract gas condensate migrated vertically to upper jurassic and lower cretaceous reservoir rocks in the hudson canyon hc area of the baltimore canyon trough us atlantic condensate samples are enriched in diamondoids 4742 4774 ppm and 13c 13c 23 7 to 24 6 pdb several lines of evidence suggest that the main source rock is within the middle to lower jurassic section of hudson canyon condensates from hudson canyon are similar to condensates generated by the upper jurassic smackover formation of the us gulf coast a laminated lime mudstone with shale facies the petrologic fabric of the lime mudstone favors early generation of diamondoids hydrogen rich kerogen and clay catalysts are concentrated within thin laminations of depositional origin separated by thicker barren carbonate layers

seismic stratigraphy and subsidence analysis of the southern brazilian margin campos santos and pelotas basins abstract the campos santos and pelotas basins have been investigated in terms of 2d seismo stratigraphy and subsidence the processes controlling accommodation space e g eustacy subsidence sediment input and the evolution of the three basins are discussed depositional seismic sequences in the syn rift barremian to the drift holocene basin fill have been identified in addition the subsidence uplift history has been numerically modeled including i sediment flux ii sedimentary basin framework iii relation to plate tectonic reconfigurations and iv mechanism of crustal extension although the initial rift development of the three basins is very similar basin architecture sedimentary infill and distribution differ considerably during the syn rift sag to the drift basin stages after widespread late aptian early albian salt and carbonate deposition shelf retrogradation dominated in the campos basin whereas shelf progradation occurred in the santos basin in the tertiary these basin fill styles were reversed since the paleogene shelf progradation in the campos basin contrasts with overall retrogradation in the santos basin in contrast long term cretaceous paleogene shelf retrogradation and intense neogene progradation characterize the pelotas basin its specific basin fill and architecture mainly resulted from the absence of salt deposition and deformation these temporally and spatially varying successions were controlled by specific long term subsidence uplift trends onshore and offshore tectonism in the campos and santos basins affected the sediment flux history distribution of the main depocenters and occurrence of hydrocarbon stratigraphic structural traps this is highlighted by the exhumation and erosion of the serra do mar serra da mantiqueira and ponta grossa arch in the hinterland as well as salt tectonics in the offshore domain the pelotas basin was less affected by changes in structural regimes until the eocene when the andean orogeny caused uplift of the source areas flexural loading largely controlled its development and potential hydrocarbon traps are mainly stratigraphic

eo alpine metamorphism in the uppermost unit of the cretan nappe system petrology and geochronology the uppermost unit of the cretan nappe system consists of ophiolites on the top and an ophiolitic melange at the base

comments on petrogeneses and the tectonic setting of columbia river basalts abstract data on major and a few trace element contents for specimens of five formational scale units of the columbia river basalt group are reviewed and compared with those for other flood basalts and the chyulu basalts of kenya flows of the grande ronde formation which constitutes more than two thirds by volume of the columbia river basalt group in several respects resemble calc alkaline rocks and they plot in the field for magmas erupted at destructive plate margins on a th hf ta diagram these features of grande ronde flows may be related to the anomalous setting of the columbia river basalt group as a whole calc alkaline characteristics are exhibited by other flood basalts but rarely to the degree shown by grande ronde flows few if any of the columbia river basalts represent primary magmas rather they generally have been affected by multi stage fractionation and perhaps also by interaction with lower crustal rocks their compositions also may reflect partial melting of fragments of subducted lithospheric slabs

application of tetracyclic polyprenoids as indicators of input from fresh brackish water environments abstract c30 tetracyclic polyprenoids tpp are most prominently observed in samples derived from low salinity i e fresh to brackish lacustrine environments and are generally present in low levels in samples derived from saline i e marine and saline lacustrine environments a near shore facies of the chonta formation peru that has no marine palynomorphs but abundant chlorococcalean green algal nonmarine palynomorphs has high levels of tpp suggesting green algae or chlorophyta are a possible source for the tpp compounds the ratio between a c30 tpp compound and 27 norcholestanes is useful for assessing this nonmarine algal input moderate elevations of tpp above what is common in marine derived samples were found in ostensibly marine source rocks and oils from certain basins of western and northern south america middle magdalena colombia maracaibo venezuela and trinidad basins this is likely due to transport from the nonmarine to the marine environment because of an influx of fresh water into the near shore marine environment alternatively oils from these basins may have inputs from near shore shallow marine algae with chemistry similar to that found in lacustrine settings the tpp ratio in conjunction with other environmental indicators such as 4 methyl steranes or hopane sterane ratio is useful for differentiating marine and nonmarine influences in pre salt oils and source rocks of west africa the tpp ratio used with other environmental indicators gammacerane c29 steranes c30 steranes and age diagnostic biomarkers dinosteranes 4 methylsteranes can be useful in differentiating among nonmarine source facies for example in the turpan hami basin china permian saline lacustrine and jurassic lacustrine deltaic facies can be discriminated

deformation of the continental crust the legacy of mike coward this special publication in memory and celebration of the work of professor mike coward is about the deformation of the continental lithosphere the collected papers discuss geometry structural principles processes and problems in a wide range of tectonic settings and thereby reflect the breadth of coward s interests they encompass the evolution of precambrian basement gneiss terrains the geometry and evolution of thrust systems basement involvement and structural inheritance in basins syn orogenic extension salt tectonics the implication of structural evolution on hydrocarbon prospectivity and structural controls on mineralization examples are drawn from the lewisian and moine thrust belt of nw scotland the italian apennines nw himalayas the cyclades oman zagros mountains colombian cordillera carpathians north sea offshore brazil regional studies of the irumide belt central africa taurus mountains turkey greater south america and from the witwatersrand basin of south africa and the antler orogeny of sw usa

application of 24 norcholestanes for constraining source age of petroleum abstract two c26 sterane ratios the 24 nordiacholestane ratio ndr and the 24 norcholestane ratio ncr are useful for assessing petroleum source age values for these parameters generally increase in the cretaceous when diatoms were rapidly radiating and further in the oligo miocene due to widespread high latitude deposition of diatomaceous sediments sources with shale lithologies tend to exhibit the highest ratios while carbonate and terrigenous rich lithologies generally show lower ratios on the north slope of alaska elevated ratios are observed for cretaceous hrz oils and mixed oils containing cretaceous input compared to older triassic and jurassic derived oils tertiary sourced oils from romania and bulgaria have much higher ratios relative to jurassic sourced oils in the black sea region oleanane was observed in comparable levels in some black sea tertiary and jurassic oils and source rocks ndr and ncr coupled with other age diagnostic indicators such as oleanane and dinosterane provide powerful tools for differentiating age of source and organic facies for oils from the black sea and sub andean basins of peru and ecuador respectively oils derived from sources deposited in low tropical latitudes tend to have little diatom input and as a consequence do not display high values regardless of age

relationship between petroleum composition and depositional environment of petroleum source rocks 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

south atlantic volcanic margins the early cretaceous south atlantic continental break up and initial sea floor spreading were accompanied by large scale transient volcanism emplacing the parana etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the torres arch abutment plateau on the north namibia margin we interpret four main tectono magmatic crustal units 1 oceanic crust 2 thickened oceanic crust covered by huge seaward dipping wedges 3 a c 150 km wide break up related rift zone partly covered by the dipping wedges and 4 thicker continental crust partly deformed by palaeozoic extension east of the early cretaceous rift similar settings also characterize other south atlantic margin segments we infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the south atlantic volcanic margins comparison with other volcanic margins demonstrates in spite of local and regional differences gross similarities in tectono magmatic style crustal units and dimensions

the sources of mycenaean gypsum the sources of gypsum used as an interior building material in the bronze age palaces of crete some buildings at akrotiri in thera and in the mainland palaces at mycenae and tiryns have been attributed by archaeologists to geological deposits in crete itself on mainland greece or in the ionian islands of zakynthos and kephallenia isotopic analyses of sulphur and strontium have been used to characterize the greek gypsum deposits and to investigate the provenance of gypsum excavated at knossos hagia triadha akrotiri mycenae and tiryns these analyses show that central cretan gypsum deposits of pliocene age supplied the minoan palaces akrotiri on thera and the palace at mycenae in contrast tiryns obtained its gypsum from triassic gypsum deposits either on crete or in the ionian islands or on mainland greece

organic geochemical characterisation of depositional palaeoenvironments of source rocks and oils in brazilian marginal basins abstract an investigation of bulk isotopic and molecular features of a wide selection of source rocks and oils from brazilian marginal basins ranging from lower cretaceous to tertiary in age provides a number of organic geochemical criteria that characterise and distinguish their depositional palaeo environments the methods employed include evaluation of organic carbon contents rock eval pyrolysis vitrinite reflectance measurements determination of carbon isotope ratios elemental and visual kerogen analysis assessment of oils and solvent extracts by liquid and gas chromatography and quantitative biological marker investigations using gc ms for aliphatic hydrocarbons and uv vis spectrophotometry for metalloporphyrins selected data were evaluated using principal component analysis the results enable the recognition and differentiation of seven depositional regimes lacustrine freshwater lacustrine saline water marine evaporitic marine carbonate marine deltaic marine highly anoxic with a predominance of calcareous mudstone lithology and marine anoxic with predominance of siliciclastic lithology diagnostic features in the biological markers from these depositional environments include the carbon number distributions of n alkanes pristane phytane ratios absolute concentrations of carotane higher acyclic isoprenoids c 30 hopane and steranes and the abundance of the latter relative to 4 methylsteranes the occurrence and abundance of particular biological markers such as gammacerane tricyclic and tetracyclic terpanes 18 h oleanane 28 30 bisnorhopane and 25 28 30 trisnorhopane and the relative abundances of nickel and vanadyl porphyrins

a new protocol for the chemical characterisation of steatite two case studies in europe the shetland islands and crete steatite has been one of the most difficult materials of archaeological interest to characterise by physico chemical methods of analysis with a view to identifying its origin this paper presents a new protocol for the chemical characterisation of steatite based on the analysis by icp ms in tandem with inaa of rare earth elements ree and some transition metals in geological steatite from several outcrops on the shetland islands and on crete the major advance over earlier work has been the secure determination of ree at low concentrations the results have demonstrated the ability to discriminate chemically three of the known viking age sources on shetland on crete although the situation is complex four sources in the centre of the island have been partially differentiated this study has also shown that first mineralogical analysis by xrd of steatite provides valuable supplementary characterisation data and second strontium isotope analysis has promising potential for the sourcing of steatite

rifted margins ductile deformation boudinage continentward dipping normal faults and the role of the weak lower crust the stunningly increased resolution of the deep crustal levels in recent industrial seismic profiles acquired along most of the world s rifted margins leads to the unraveling of an unexpected variety of structures it provides unprecedented access to the processes occurring in the middle and lower continental crust we present a series of so far unreleased profiles that allows the identification of various rift related geological processes such as crustal boudinage ductile shear and low angle detachment faulting and a rifting history that differs from the classical models of oceanward dipping normal faults the lower crust in rifted margins appears much more intensely deformed than usually represented at the foot of both magma rich and magma poor margins we observe clear indications of ductile deformation of the deep continental crust along large scale shallow dipping shear zones these shear zones generally show a top to the continent sense of shear consistent with the activity of continentward dipping normal faults cdnf observed in the upper crust this pattern is responsible for a migration of the deformation and associated sedimentation and or volcanic activity toward the ocean we discuss the origin of these cdnf and investigate their implications and the effect of sediment thermal blanketing on crustal rheology in some cases low angle shear zones define an anastomosed pattern that delineates boudin like structures the maximum deformation is localized in the inter boudin areas the upper crust is intensely boudinaged and the highly deformed lower crust fills the inter boudins underneath the boudinage pattern controls the position and dip of upper crustal normal faults we present some of the most striking examples from the margins of uruguay west africa south china sea and barents sea and discuss their implications for the time temperature history of the margins

oil and gas habitats of the south atlantic introduction the south atlantic hydrocarbon province defined for the purposes of this volume the south atlantic comprises the atlantic ocean south of a direct line from tangier morocco to the guyana venezuela border and north of a direct line from cape town south africa and cape horn argentina at the end of 1997 this region contained 4 3 of the world s total discovered oil reserves and 2 4 of the world s total discovered gas reserves petroconsultants 1998 on the basis of the 1997 figure six countries brazil argentina nigeria gabon congo brazzaville and angola host 96 3 of the region s oil reserves and 92 1 of the region s gas reserves figure 1 illustrates the geographical distribution of the reserve splits table 1 gives the details one hundred years of discoveries voluminous basin margin asphalts in west africa cote d ivoire to angola argentina brazil and surinam attracted the serious attention of entrepreneurs from the late 1800s as discussed in more detail by dolan shallow wells were drilled soon afterwards on seep locations though with little commercial success more systematic explorations began on both margins in the 1930s and in 1941 the first large discovery the candeias field in the reconcavo basin of brazil was made the reserves of this field are now placed at 120 mmbo success did not materialize in west africa until 1955 when the small benfica field was discovered in the kwanza basin of angola thereafter following the first giant field discovery in the niger delta in 1958 west african reserves rose

geochemical and biological marker assessment of depositional environments using brazilian offshore oils abstract a combined geochemical and molecular characterization of a wide selection of oils from the major brazilian offshore basins has been undertaken the elemental sulphur nickel and vanadium and bulk oapi and 13c properties of each sample been considered together with its molecular composition determined using liquid and gas chromatography and quantitative biological marker investigations using gas chromatography mass spectrometry for alkanes the results reveal significant differences in the chemical features of the various oils which enable them to be divided into five groups the distinction of the groups appears to reflect differences in the depositional environment of the source rocks of the oils each group is correlated tentatively with source rocks laid down in a specific depositional regime namely lacustrine freshwater lacustrine saline water marine evaporitic marine carbonate or marine deltaic the diagnostic features that allow this classification are the relative abundance and carbon number distributions of n alkanes pristane phytane ratios sulphur nickel and vanadium contents carbon isotope data the absolute concentrations of hopanes and steranes and their abundance relative to 4 methylsteranes and also the occurrence and abundance of several specific biological markers including 18 h oleanane gammacerane carotane tricyclic terpanes higher acyclic isoprenoids 28 30 bisnorhopane and 25 28 30 trisnorhopane this investigation shows the value of a combined geochemical and molecular approach in the assessment of the palaeoenvironment of deposition of the source rocks which gave rise to the oils