The Onshore Northeast Brazilian Rift Basins: An Early Neocomian Aborted Rift System: ABSTRACT

The northeast Brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the South Atlantic. These basins represent the locus of intersection of the Southern and Equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases. Similar data are not found in its counterpart in Africa, especially for the Neocomian. These Early Cretaceous rift basins of northeast Brazil illustrate key three‐dimensional geometries of intracontinental rift systems, mainly controlled by the basement structural framework. During the main rift phase (Syn‐rift II, Neocomian‐early Barremian) extensional deformation was distributed over three main rift axes: (1) the Gabon‐Sergipe Alagoas (GSA) trend, (2) the Recôncavo‐Tucano‐Jatobá (RTJ) trend and (3) the Cariri‐Potiguar (CP) trend. During this phase, extensional deformation jumped west from the easternmost basins (GSA trend) to a series of NE trending intracratonic basins (RTJ and CP trends), characterized by a set of asymmetric half grabens separated by basement highs, transfer faults, and/or accommodation zones. These basins are typically a few tens of kilometers wide and trend NE‐SW, roughly perpendicular to the main extension direction during the Neocomian. Preexisting upper crustal weakness zones, like the dominantly NE‐SW trending shear zones of the Brazilian/Pan‐African orogeny, controlled the development of intracrustal listric normal faults. Internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework. The megashear zones of Pernambuco (Brazil) and Ngaundere (Africa) seem to have behaved like a huge accommodation zone, accommodating extensional deformation along the RTJ/GSA trends with simultaneous extension along the CP trend. During the late Barremian (Syn‐rift phase III), a significant change in rifting kinematics occurred, when the CP trend was aborted and major rifting initiated at the Equatorial branch. During the Aptian, while the Equatorial branch and Benue trough (Africa) experienced the main rift phase, the RTJ trend was aborted and the GSA trend developed a transitional phase between the rift and drift stage. The GSA trend and the offshore Potiguar basin represent the site of continued evolution into passive margin basins following the main rift episode.

From a geometric analysis of the fault pattern in the Reconcavo basin, Brazil, supported by a reinterpretation of the early opening history of the South Atlantic Ocean, it is inferred that the basin formed as a result of Valanginian (Early Cretaceous) motion on a major N40°E-striking left-lateral transform fault located offshore between Salvador and Recife. This left-lateral motion was due to the location of the Valanginian pole of South American-African plate rotation within northern Brazil, at 2.5°S, 45.0°W, rather than farther north as interpreted previously. Left-lateral movement along the inferred transform created three fault sets: (1) a predominant set striking N30°E ± 20° (1σ); and two lesser developed sets striking (2) N13°W ± 6° and (3) N37°W ± 12° (1σ). Sets (1) and (3) are interpreted, respectively, as Riedel and conjugate Riedel shears (with an extensional component) to the N40°E-trending left-lateral transform fault. Set (2) formed as an extensional fault system. Intersections of these three sets subdivide the basin into triangular and diamond-shaped blocks. The Reconcavo basin continues northward in northeastern Brazil into the Tucano and Jatoba basins. These basins collectively form a north-south-trending, mega half-graben within continental crust. Geohistory curves for Early Cretaceous units in the Reconcavo basin indicate that the syn-tectonic Valanginian shales of the lacustrine Candeias Formation began to generate hydrocarbons during the earliest subsequent deposition of the Ilhas Formation (?Hauterivian). Because diamond-shaped structural traps had formed earlier during the Valanginian, hydrocarbons generated in the Candeias Formation are believed to have then migrated into the Sergi Formation (?Upper Jurassic) reservoir units in structurally juxtaposed fault blocks. During Barremian to Aptian time, deposition of fluvial, lacustrine, and alluvial sediments of the Ilhas and Sao Sebastiao Formations occurred. Rejuvenation as normal faults of earlier formed strike-slip and normal faults occurred in the latest Aptian (106 Ma) when the pole of South American-African plate rotations jumped to 41.3°N, 43.8°W. This pole jump caused extension along the previously formed transform margin between Salvador and Recife and the abandonment of rifting in the Reconcavo basin. Rejuvenated faults tapped earlier filled Sergi Formation reservoirs, which then leaked earlier reservoired hydrocarbons up these fault planes into higher reservoirs in the Ilhas and Sao Sebastiao Formations. This structural history thus explains the frequency distribution and orientation of faults in the basin, Sergi Formation production from corners of diamond-shaped fault blocks, and production from stratigraphically and structurally higher reservoirs (Ilhas and Sao Sebastiao Formations) in rollover folds and stratigraphic traps charged by second-phase faulting.

Relative role of accommodation zones in controlling stratal architectural variability and facies distribution: Insights from the Fushan Depression, South China Sea

Provenance analysis of the Araripe intracontinental basin, northeast Brazil – Routes for proto-Atlantic marine incursions in northwest Gondwana

Continental rifts are segmented along their lengths into 50 to 100 km‐long extensional basins, suggesting a genetic relationship with regularly segmented oceanic rifts. To investigate the spatial and temporal development of along‐axis segmentation in youthful continental rifts, field, remote sensing, and K‐Ar geochronology studies were conducted in four Western rift (East Africa) basins. Volcanism within the Rungwe region began in late Miocene time prior to or concurrent with the development of high‐angle border faults bounding the then isolated Karonga and Rukwa basins. The Usangu and Songwe border fault segments located between the Rukwa and Karonga basins developed in mid‐Pliocene and Pleistocene time, respectively. Within the northern Karonga basin, approximately 2–4 km of WSW‐ENE directed “thick‐skinned” extension is estimated both from extrapolation of late Pleistocene slip rates to 5 Ma and from surface fault geometries. Differential strains between these extensional basins of different ages are transmitted to adjoining segments by NW and ENE striking oblique‐slip transfer faults within comparatively high‐strain accommodation zones. Throws along transfer faults also accommodate 2 km (minimum) variations in depth to basement beneath adjacent basins, whereas monoclines and ramps accommodate differential uplift along the flanks of basins. The observed geometry of en echelon border fault segments linked by shorter oblique‐slip transfer faults is similar to patterns predicted in numerical models of en echelon normal fault interactions [e.g., Aydin and Pollard, 1982]. Eruptive centers for alkali olivine basalt and phonolite flows within the Rungwe region coincide with the tips of en echelon border fault segments and with high‐angle transfer faults linking discrete border fault segments. The locations of eruptive centers appear to be controlled by faults, and these centers generally have propagated northward along transfer faults during Plio‐Pleistocene time. Historic eruptions near late Miocene centers located at the southern end of the transfer fault system, however, argue against a northward migration of the magma source. Thus, an along‐axis propagation of “thick‐skinned” lithospheric extension links once isolated basins and produces an along‐axis segmentation of this youthful rift. Propagation of magmatism along transfer faults linking en echelon border faults locally contributes to the regular segmentation.

This study applied the Pearson correlation coefficient and principal component analysis as tools for unsupervised qualitative petroleum system evaluation techniques. A total of 252 oil samples (32 features per sample) representative of two Brazilian sedimentary basins (Recôncavo and Potiguar) were used to classify them according to their respective degrees of maturation and origin. The large initial set of variables comprises data on δ13C composition, saturate, aromatic, polar compound fractions, and the techniques reduced biomarkers to the most important variables, maintaining the global pattern of variance. The results were efficient in discriminating different petroleum systems from lacustrine, marine, and mixing sources, as observed in the studied accumulations from the Lower Cretaceous sediments of the Recôncavo and Potiguar basins. The methodology proved to be very useful to vene better characterize the petroleum systems. This methodology can be applied to analyze a large amount of oil samples, using simple software and spending relatively less time.

The available information on the Cretaceous-Tertiary magmatism in Brazilian marginal basins is synthesized, aiming at a comparison with similar events that affected the emerged adjacent areas. The Eocretaceous pre-Aptian basaltic volcanism is more common in the basins from Espirito Santo to the Sduth, where it can be correlated with dikes and lava flows of the Serra Geral Formation. It is also present in Northeast Brazil, at the southern part of the submerged Potiguar Basin. At the extreme south of Brazil, the presence of acid magmatism in the Parana Basin allows one to suppose the existence of similar compositions in the poorly known Pelotas Basin. The quiescence of magmatism during Aptian-Albian time is a common characteristic of almost all Brazilian marginal basins, as well as in the emerged continental area, but rocks belonging to this age are present in the Ceara Basin and in the Cabo area (PE). Neocretaceous-Eocene magmatism of alkaline trend is present in the emerged region of the Cabo Frio Magmatic Lineament. Possibly it can be extended to the submerged margin. Eocene volcanism is very important at the Cabo Frio Platform and south of Campos Basin. The islands situated along the Santos BasirTs margin are composed by feldspathoid-bearing alkaline rocks, the emplacement of which is related to the Rio de Janeiro Lineament. The Tertiary volcanism in Northeast Brazil lasted until Miocene, in the Potiguar Basin and emerged region, but in South-Southeast Brazil it seems to have ceased after Eocene. The Fernando de Noronha Fracture Zone extended up to the coast, allowing the onset of Oligocene alkaline volcanism. The Remanche Fracture Zone extends up to the Ceara Basin and the Sao Luis Craton, and controls the acid magmatism observed in the Atlântico and Ceara highs. The Vitoria-Trindade Fracture Zone also seems to extend to the continent, as suggested by the structures and basic dikes of the Fundao Suite. The basins situated between the Ceara State and the Marajo Island are amagmatic, as well as the Reconcavo Basin, in part because they have been installed on rigid crust of Sao Luis and Sao Francisco cratons. It is suggested that the faster continent withdrawal in the South as compared to the Northeast Brazil associated to different rift processes could have been the cause for cessation of volcanism only after the Miocene in the last region.

This paper presents qualitative and quantitative interpretations of geophysical data in the onshore western part of Potiguar sedimentary basin, NE Ceara. These geophysical data were performed in order to understanding tectonostratigraphy relationships involving these part of Potiguar Basin based on determination of main geophysical lineaments, geometry and depth of sources and the important determinations of geophysical domains. The study area shown structural lineaments partitioning characterized by lineaments in the NE-SW direction, E-W and NW-SE inflexions. The spatial arrangement of geophysical fields is related to the Brasiliano lineaments that occur in the region. Gravity modeling over the profiles shows the geometry of the study area which has a graben feature that may be associated with the accumulation of hydrocarbons such as the Fazenda Belem oil field. Introduction The Potiguar basin internal geometry shows a preferencial ENE-WSW direction, showing asymmetric grabens separated by internal basement highs. The structural framework of Potiguar basin is a result of changes that occurred during evolution in rift and drift stages and recent magmatism. (Bertani et al., 1990). The study area is inserted in the western portion of the Potiguar basin in its portion onshore and within the boundaries of the Ceara state. The region where is inserted the study area has a positive gravimetric anomaly and a significant magnetic anomaly (Pedrosa et al., 2010). This peculiar structure is not well defined and is near Fazenda Belem oil field. Regional Geological Context The Potiguar basin is inserted in the Precambrian basement complex of northeastern Brazil named Borborema Province (Almeida et al., 1981). This basin is part of a series of small to medium rift basins which form the Northeast Brazilian Rift System and its origin is linked to the South Atlantic opening in the Cretaceous (Matos 1992). The main stratigraphic units in the study area are, from base to top: 1) the Paleoproterozoic basement represented by supracrustal sequence as mica schist and quartzites of the Santarem Formation into Oros zone; 2) In the southeast of study area outcrop sandstones of the Acu Formation and limestones of the Jandaira Formation, which are into the Apodi Group; 3) most of the area is covered by sandstones and conglomerates of the Barreiras Formation; e 4) recents sediments associated with alluvial sediments and coastal Aeolian sediments along the entire coast (Cavalcante et al., 2003) (Figure 1). The Jaguaribe lineament or shear zone (JL) was inferred from structural data from Cavalcante et al., (2003). The Ponta Grossa-Fazenda Belem lineament (PGFBL) was determinate by Sousa (2002), who identified strongly deformed sector of Barreiras Formation, which alignement has coincided with Brasiliano shear zones that outcrop in the south of study area. Figure 1: Simplified geological map of the study area. JL: Jaguaribe lineament; PGFBL: Ponta Grossa-Fazenda Belem lineament.(Modified from Cavalcante et al., 2003). Geophysical datasets Airborne magnetic data The aerogeophysical survey provided by the Brazilian Petroleum National Agency (ANP) is named the Potiguar Basin Project. This dataset was undertaken between 1986 and 1987 and covered an area of 44,600 km2 with N20°W oriented flight lines and 2.0 and 4.0 km spacing between these lines. The nominal flight height was 500 m and the sampling rate 100 m. The aeromagnetic data were corrected for diurnal and main component of the geomagnetic field variations IGRF (International Geomagnetic Reference Field). These data were then interpolated into 500 m regular grid by the bidirectional method in order to generate the anomalous field. After this procedure, several filtering techniques were applied to improve the signal/noise relationship and to highlight specific features of the magnetic sources.

Comparison of the Tanganyika, Malawi, Rukwa and Turkana Rift zones from analyses of seismic reflection data

Stratigraphy and architecture of the fault bounding the sedimentary basins developed along the Eastern Tyrrhenian margin provide information on the kinematic of faults transversal to the Apennine chain. To understand how changes in geodynamic processes control the structural evolution of transverse faults in the thrust belt-backarc hinge zone, we performed the interpretation of a 2D strictly spaced seismic data set, tied to stratigraphic data of exploration wells and onshore data constrains, and a 3D basin analysis.On the basis of geometry and age of the basin infill, we dated three events of fault activity and a complex kinematics of Pliocene-Quaternary transverse faults. The first tectonic phase produced the oldest normal faults developed along the Latium margin. These faults, active between 5.1 and 3.2 Ma (MPL2-MPL3 and MPL4 succession), bound sedimentary basins filled by a Transgressive/Regressive succession made up of sands, silts and clays. They gradually migrated from the NE- trending (transversal to the Apennine chain), Pliocene in age, toward the NW-trending (parallel to the Apennine chain) during the Lower Pleistocene (1.8 MA, MPL6 succession). The displacement along these normal faults was transferred, or relayed, from one to the next one along accommodation zones, corresponding to transfer faults. Accommodation zones along major bounding structures are sites of intra-basinal highs, characterized by thinner sedimentary covers. The transfer faults, orthogonal to the normal faults, offset the basin depocenters. Whereas a positive inversion structure located near a transfer fault deforms the central basin rift. During the Lower Pleistocene the transform faults are transversal to the Apennines chain. These latter developed from the Latium margin to the Campania Margin.The third phase of the development of the transverse faults corresponds to a second episode of rifting of the Eastern Tyrrhenian margin. This event is linked to the activity of NE-trending normal faults, during the Middle Pleistocene since the 0.7 Ma, producing half-grabens and a deepening of the basement in the northwestern part of the Campania Plain and in Naples Bay. The stratigraphic succession architecture records the tilting of the fault block. During the middle Pleistocene along the Campania margin the transverse faults were reactivated as normal faults.The great variability in the tectonic evolution of the Tyrrhenian margin has been interpreted as strictly related to the complex and rapid geodynamic evolution of the area during Pliocene-Quaternary times: Pliocene slab retreat of the Adria plate, followed by Pleistocene growth of a Subduction-Transform-Edge-Propagator (STEP) fault along the northern margin of the Ionian slab.

Comparison of the Tanganyika, Malawi, Rukwa and Turkana Rift zones from analyses of seismic reflection data

Three representative basins in the Western rift system of East Africa are bordered along one side by high‐angle normal faults with 2‐ to 5‐km throws (border faults). In plan view ∼100‐km‐long systems of linear border faults form curvilinear border fault segments bounding the East Kivu, West Kivu, and Rusizi basins. The opposite sides of these asymmetric basins are bounded by lower relief faulted monoclines or en echelon ramps. The largely unfaulted rift flanks have been uplifted 2 km above the 1.3‐km‐high East African plateau, with uplift narrowing basins during Quaternary time. Maximum estimates of ∼E‐W crustal extension within basins are less than 25% (< 16 km), and planar border faults may penetrate the crust. The East Kivu and West Kivu basins are linked across the rift valley by a horst that serves as a hinge for subsidence in both basins. The westward tilted East Kivu and eastward tilted Rusizi border fault segments are linked along the rift by oblique‐slip transfer faults that also accommodate along‐axis differences in elevation. Upper Miocene‐Recent eruptive volcanic centers within the comparatively high‐strain interbasinal region (accommodation zone) generally coincide with the tips of border fault segments and transfer faults. The orientations of Miocene‐Recent dip‐slip and oblique‐slip faults show little correlation with Precambrian shear zones or foliations in metamorphic basement. Differences between the East Kivu, West Kivu, and Rusizi basins in the age of initial faulting, subsidence, and age/composition of volcanic products suggest that border fault segments developed diachronously and propagated along the length of the rift. This along‐axis border fault propagation and the crosscutting geometry of transfer faults contribute to the segmentation of the Western rift valley.

This paper presents a Bayesian approach to evaluate remaining potential of oil and gas in the Reconcavo basin, Brazil. The purpose is to test a new Bayesian weighting methodology and quantify the favorability for the existence of new fields in the basin. The methodology implies organization of petroleum system data in descriptive models with which results from drilling are manipulated statistically, including analysis of geologic factors that are spatially correlated with both producing and dry areas. In the first stage of modeling, the essential elements (reservoir, seal, and overburden rocks) that control the fundamental processes of generation, expulsion, migration, and entrapment of petroleum accumulation are defined throughout integration of previously published data. The petroleum accumulation models of Reconcavo basin comprise generation from Neocomian shale rocks of the Gomo Member (Candeias Formation), vertical migration along extensional and transfer faults, and accumulation in tilted horsts with Upper Jurassic prerift reservoirs (Sergi Formation) or in Neocomian turbidite reservoirs in stratigraphic/combined traps (Candeias and Marfim formations). Probability distributions and weights are then calculated through Boolean operations among producing and dry areas and each diagnostic criterion evaluated through descriptive models such as source bed thickness, onset of organic maturation, presence or absence of faults and structural blocks, reservoir thickness, and seal distribution. The final stage of evaluation consists of spatial integration of raster maps that are weighted according to their necessity and sufficiency conditions, the results being presented as favorability maps. The characterization of favorable areas and their comparison with known fields suggest that such a Bayesian approach can contribute to the understanding of petroleum systems as a practical approach that considers the spatial nature of exploration variables.

Architecture and early evolution of the Oslo Rift

Six major tectonic-depositional sequences, reflecting rift and passive margin evolution, variously characterize the filling of Brazilian coastal margin basins: (1) Late Jurassic prerift, (2) Early Cretaceous tectonic rift, (3) Early Cretaceous quiescent stage (evaporitic or calcilutitic), (4) middle Cretaceous initial drift carbonate platform, (5) Late Cretaceous platform/deltaic progradational and deep marine retrogradation, and (6) Tertiary main passive margin progradation. Habitat of oil discovered to date meets two regional geologic conditions: (1) in tectonic rifts known to have basin core of starved, lacustrine shales, and (2) in basins which developed a quiescent phase during the transition from tectonic rift to passive margin. Two major plays characterize the central core rifts, including (I) underlying prerift sediments in fault contact with the central core, and (II) sublacustrine fans overlying the central core. These plays, typified in the Reconcavo basin, constitute about half the recoverable oil found to date. A structurally related variation of type II play and a third regional play exist where the quiescent condition occurred, including reservoirs of the rift below evaporitic or calcilutitic regional seals and carbonate platform and turbidite reservoirs in the passive margin above the quiescent episode. The subevaporitic-calcilutitic subplay is prominent where overlying regional seals are structurally unmodified, contains about 15% of the discovered oil, and has typical development in the Sergipe (evaporitic) and Potiguar (calcilutitic) basins. Where regional seals of the quiescent phase have been mobilized, structurally modified, or cut by subsequent submarine canyons, carbonate platform and turbidite reservoirs of the overlying passive-margin fills are the prominent play (type III). Thi play, with typical development in Campos basin, in Mosqueiro low in Sergipe basin, and onshore Espirito Santo basin accounts for 35% of the discovered oil. Exploration implications of the established plays are: (1) source is from tectonic or quiescent stage fill (Aptian or older); (2) structural integrity of the quiescent stage seals is critical to oil migration; and (3) tectonic rifts are productive when a core of deep lacustrine shales was developed. End_of_Article - Last_Page 510------------