Eastern Brazilian Margin Research Articles

Interactions between Clays and Carbonates in the Aptian Pre-Salt Reservoirs of Santos Basin, Eastern Brazilian Margin

The giant Pre-salt reservoirs represent most of the oil production in Brazil. The main Aptian sag reservoirs were deposited in a unique and highly complex hyper-alkaline lacustrine setting. These deposits are essentially constituted by fascicular and spherulitic calcite precipitated in a magnesian clay matrix (stevensite, kerolite, and saponite/hectorite). Although vital for understanding the origin and main reservoir quality control, the genesis and interactions of clays and carbonates are still poorly constrained. The detailed petrographic description was focused on 812 thin sections from five wells drilled in the Santos Basin Aptian Barra Velha Formation, combined with cathodoluminescence, UV epifluorescence, and X-ray diffraction analyses. The main syngenetic processes were the deposition of finely laminated peloidal and ooidal Mg-clays, the formation of fascicular calcite crusts on the sediment–water interface, and the redeposition of these materials as intraclasts. Abundant clay peloids engulfed in syngenetic shrubs indicate that calcite and clay precipitation was concomitant, though with highly variable rates. Eodiagenetic phases include matrix-replacive and -displacive spherulites and fascicular shrubs; matrix-replacive blocky calcite and dolomite; lamellar carbonates filling matrix shrinkage pores; and microcrystalline calcite, dolomite, and silica replacing the Mg-clay matrix. The preferential dolomitization and calcitization of peloidal layers were most likely due to their higher permeability and larger specific surface. Matrix-replacive saddle dolomite, macrocrystalline calcite, and dawsonite are interpreted as mesodiagenetic or hydrothermal phases after significant matrix dissolution. Unraveling the processes of the formation and alteration of the carbonates and clays and their interactions in the Pre-salt deposits is essential for constraining the depositional and diagenetic conditions in their unique environments and their diagenetic overprinting and for decreasing the exploration risks and increasing the production of those extraordinary reservoirs.

Three-dimensional characterization of the Búzios Field reveals implications on the tectonostratigraphic evolution of the Santos Basin using combined seismic attributes and well-logs analysis

The Búzios Field is one of the most prolific petroleum fields along the Eastern Brazilian Margin. Despite significant research conducted restrictedly on the pre-salt reservoirs, few studies have focused on the three-dimensional characterization of the entire Búzios Field area. Therefore this work aims to spatially characterize a volume along the Búzios Field area using 3D seismic volume interpretation, seismic attributes, and well-log data. The combined use of these tools allowed the understanding of several parameters concerning the tectonostratigraphic evolution of the Santos Basin. Within the mapped area, we identified five units: Syn-Rift I, Syn-Rift II, Post-Rift I, Post-Rift II, Drift I, and Drift II. The 3D model reveals significant features such as the depth variation of the Syn-Rift I unit and the heterogeneous morphology of the Post-Rift II unit. The Syn-Rift I unit is deeper towards the N-NW and shallower towards the S-SE due to a regional half-grabens system. This system impacts the pre-salt geometry and thickness, as structural highs and lows are relevant factors in sediment accommodation. The Post-Rift I unit has a predominantly regular geometry and thickness but tends to thicken towards the S-SE and exhibits slight downward flexure towards the NE in the southeast of the Búzios Field. In three dimensions, the heterogeneous morphology of the Post-Rift II unit is evidenced by the variable thickness and irregular topography, accounting for the intense halokinesis in the Santos Basin. Above the evaporite layer, the thickness of the Drift I unit is highly conditioned by salt morphology, with thicker and deformed packages in areas of thinner salt section. On the other hand, the Drift II unit shows relatively constant thickness with minor variations near fault zones. Comparisons with other fields within the Santos Basin (Tupi and Sapinhoá/Lapa) reveal differences in layer thickness, mobilization, and rotation of the syn-rift blocks. The Post-Rift I layer (Barra Velha Fm.) and the Post-Rift II package (Ariri Fm.) are thinner and less mobilized in the Búzios Field area. Moreover, the syn-rift blocks are less rotated, and the internal reflectors have a lower inclination in the study area when compared to these other fields. Therefore, this research concludes that the lower mobilization of salt, the lower thickness of the post-rift package, and the minor syn-rift block rotation observed in the Búzios Field area are associated with the relative distance from the continental margin and, consequently, with the amount of displacement generated by the South Atlantic Rift. In this case, the Búzios Field is located in a more proximal portion than the Tupi and Sapinhoá/Lapa fields, exhibiting features suggestive of a lower displacement.

Upper maastrichtian palynology and paleoenvironment of the Urucutuca Formation, Almada basin (BA), Brazil

Hydrocarbon exploration involving the Urucutuca Formation is taking place in several basins located mainly in the central segment of the eastern Brazilian margin, including the Almada Basin. Relatively rich accumulations of organic material and promising prospects, especially for gas, have been discovered. The Brazilian National Petroleum Agency (ANP) considers the Urucutuca Formation as a speculative play exploratory (Urucutuca-Urucutuca) in deep and ultra-deep areas of the Almada Basin, including others.The focus was on improving the understanding of the sediments of the Urucutuca Formation and establishing a palynostratigraphic outline for the upper cretaceous strata, which still need to be updated. Therefore, integrated palynological, palynofaciological, and organic-geochemical (total organic carbon -TOC) investigation methods were carried out. For this purpose, 250 m of well 1-SST-01, drilled in the onshore part of the Almada Basin in Bahia State, were analyzed using 26 core samples along the Urucutuca Formation.By typifying palynofaciological variations based on organic matter content in conjunction with palynological and geochemical data, it was possible to distinguish three palynofacies with different characteristics of the marine influence environment. Palynofacies 1, the amorphous organic matter (AOM) group, predominates with proportions up to 54.1 and a TOC of 0.75%. The high phytoclasts percentage (70%) and a TOC content of 1% characterize the Palynofacies 2. Palynofacies 3, on the other hand, has the highest content of marine palynomorphs (40%).The analysis of total organic carbon yielded values ranging from 0.04% to 1.85%, which were heterogeneously distributed throughout the borehole.The palynological succession showed a dominance of representatives of continental origin. However, marine elements (dinoflagellates and microforaminiferal inner tests) are present throughout the section.Two expressive occurrences of dinoflagellate cysts are evident throughout the section. In the basal and middle portions, quantity and diversity suggest transgressive events or at least sea-level fluctuations. Biostratigraphically, the unit could be placed in the Tricornites elongatus (P-470) Biozone of the Crassitricolporites brasiliensis (P-450) Superzone and assigned to the Eomaastrichtian. These biostratigraphic units are consistent with the proposed zonation for other basins of the Brazilian Atlantic margin.Palynofacies and palynological data allowed the observation of variations in a predominantly platform paleoenvironment, sometimes distal and sometimes more proximal, with possible transgressions.

Petrogenetic insights on tephritic magmatism from Davis Bank (South Atlantic Ocean - Vitória-Trindade Ridge (VTR), Brazil): The role of a CO2-enriched mantle source

Davis Bank is one of the largest physiographic features in the Vitória-Trindade Ridge (VTR), which is located offshore of the Eastern Brazilian margin, midway between the coastline and the South Atlantic mid-ocean ridge. The VTR is related to a linear volcanic track of ca. 1200 km in length, composed of several volcanic seamounts that are lithologically characterized by nephelinitic/tephritic alkaline rocks. The Davis Bank lies between the parallels 20°29′ and 20°32′ S and the meridians 29°17′ and 29°21′ W, and is located southeast of the Abrolhos Volcanic Complex (AVC), at the middle of the volcanic ridge, far from the Brazilian coast. This work focuses on the 40Ar/39Ar age determination of the rocks recovered from the seamount and characterization of Sr–Nd isotopes, mineral chemistry and whole-rock composition, aiming to access information about physical properties of magmas, source composition, magmatic evolution of the volcanic ridge, and the regional tectonic interpretation in the context of other seamounts along the Vitória-Trindade Ridge (VTR). Davis Bank presents 40Ar/39Ar age of ca. 21 Ma and is in an oceanic lithosphere of ca. 100 km thickness, which represents a recharging area in the oceanic lithospheric mantle for CO2-metasomatism enrichment. It is associated with mantle anomalies caused by a hotspot or mantle plume that was influenced by the E-W direction of leaking transform fracture zones in the South Atlantic Ocean. Age constraints indicate decreasing ages eastward of the AVC, favoring the hotspot hypothesis for the generation of magmatic rocks in the Vitória-Trindade Ridge.

New insights on the Late Quaternary evolution of Abrolhos reefs, Eastern Brazilian Shelf

The Abrolhos shelf, in the eastern Brazilian margin, encompasses the largest reef province in the South Atlantic, characterised by low coral diversity and a high level of endemism. Knowledge gaps about how the Abrolhos coastal reefs evolved, during the post LGM and MIS 6/MIS 5, favoured a paradigm that is revisited in the light of new data. Here, new data from two new, long boreholes (20 and 50 m-long) are presented in order to investigate and provide new insights into the evolution of two shallow reefs on the Abrolhos Shelf during the late Pleistocene and Holocene epochs. Results show that Holocene and Pleistocene reefs grew on highly turbid waters with a significant terrigenous sediment input. Corals and CCA assemblages were identified as main framework builders of Abrolhos inner-shelf reefs since the Pleistocene. No clear shallowing upward sequence of reef builders was observed, which differs from other reef sequences all over the world. In terms of Holocene reef growth, Pedra Grande and Coroa Vermelha reefs turned on roughly coevally at different depths and with different coral assemblages. Pedra Grande reef started growing circa 6.8 kyr BP in a water depth approximately 10 m deeper than Coroa Vermelha, which started growing around 6.4 kyr BP, but Coroa Vermelha turned off at around 4 kyr BP, while Pedra Grande kept growing until around 1.2 kyr BP. This novel reef stratigraphy data provided a new insight on the evolution of the Abrolhos coastal reefs, showing that different environmental and topographic controls could have acted spatially in different ways. Coroa Vermelha reef seems to have started its growth as a result of a topographic high control. Pedra Grande reef seems to be connected to a broader submerged reef setting and started to grow in deeper water depths. The presence of three reef arcs in Abrolhos is proposed, including a submerged reef arc.

Obliquity Influence on Low‐Latitude Coastal Precipitation in Eastern Brazil During the Past ∼850 kyr

AbstractPaleoclimate records from tropical South America typically show precession‐paced variability in rainfall, caused by insolation‐driven changes in the South American Monsoon System, however this mechanism may not be responsible for hydroclimate change outside of the core monsoon domain, such as in the coastal zone of tropical eastern Brazil. Our findings are based on a ∼850 kyr‐long multiproxy record from a marine sediment core collected from the eastern Brazilian margin that represents the longest continuous record of South American hydroclimate to date. Utilizing the ln (K/Al) chemical weathering proxy from the core, we determine that past hydroclimate change in the coastal zone was primarily modulated by obliquity forcing. We demonstrate that high obliquity is associated with an increase in the boreal summer interhemispheric insolation contrast which decreases the zonality of the southern trade winds and reduces moisture advection to the coastal zone. Based on the long‐term coherence between the ln (K/Al) record and benthic δ13C records from the Atlantic during Marine Isotope Stages 16–13, we infer that an increase in the strength of the overturning circulation, caused by Northern Hemisphere high‐latitude forcing, may have produced surface cooling in the western tropical South Atlantic which led to reduced moisture advection to the coastal zone. We suggest that this mechanism may have also caused the amplification in millennial‐scale variability in the coastal hydroclimate system, which began after the decoupling of the coastal hydroclimate system from obliquity forcing.

Environmental controls on holocene reef development along the eastern brazilian margin

Environmental controls on holocene reef development along the eastern brazilian margin

Morphology and sedimentology of the shelf-upper slope transition in the Abrolhos continental shelf (east Brazilian margin)

In mixed terrigenous-carbonate shelves, terrigenous sediment input may influence changes in slope morphology and sedimentation. Moreover, sea level changes and antecedent geology can also play an important role in defining depositional patterns for the shelf-slope system. This work investigates contrasting depositional and geomorphological patterns in the Abrolhos shelf-slope system, eastern Brazilian margin, South Atlantic. We have combined existing bathymetric datasets with bottom imagery and samples collected at the Johnson-Sea-Link manned submersible along two Abrolhos slope sites, which are separated by almost 2° of latitude. Our dataset includes seafloor sediments, single-beam echosounder, and 20 h of video recording. Dives ranged from 90 to 620 m water depth. Results showed that the north and south Abrolhos shelf-slope systems are distinct in terms of their morphology and sedimentology. The depositional pattern in the northern upper slope is typically carbonate dominated, with the occurrence of carbonate mounds around 500 m deep, small biogenic structures around 200 m deep, interspersed with wackstones and packstones. The upper slope is characterized by a steep limestone wall at 120 m deep that resembles a shelf-edge reef, followed by extensive rhodolith beds at the shelf edge and outer shelf. The southern upper slope shows a sigmoidal profile with a gentle gradient. Mixed siliciclastic–carbonate fine sedimentation dominates from 300 to 700 m deep. On the upper slope, fine carbonate sedimentation predominates, and a biogenic reef structure is observed in a water depth around 120–130 m. The shelf edge and outer shelf are also characterized by extensive rhodolith beds. Main findings indicate that a volcanic rock complex is an important antecedent geology that may influence the setting up of distinct sedimentary regimes and sediment input to the slope. The geometry of the antecedent geology controls the paleotopography forming a gradient towards the south. This, combined with sea-level changes and Rio Doce riverine sediment input, can explain the contrasts between the northern and southern shelf-upper slope depositional settings. Here, for the first time, the presence of possible Last Glacial Maximum or early deglaciation stage shelf-edge reef systems is reported for the Abrolhos Bank.

Deposition, diagenesis and reservoir potential of non-carbonate sedimentary rocks from the rift section of Campos Basin, Brazil

A new petrographical study was performed on the rift section of the Lagoa Feia Group, Lower Cretaceous of the Campos Basin, eastern Brazilian margin. The primary constituents of the analysed rocks are siliciclastic and volcaniclastic grains, stevensite ooids and peloids, and bioclasts of bivalves and ostracods. This study focused on the clastic, stevensitic and hybrid rocks, as previous studies were limited to the bioclastic rudstones and grainstones that constitute the producing reservoirs. The rift sedimentation was mostly intrabasinal, with extrabasinal contribution concentrated close to half-graben border faults. The mixture of rounded volcanic fragments with angular quartz, feldspars and plutonic fragments in the sandstones and conglomerates indicates recycling of early rift epiclastic deposits, combined with first-cycle sediments eroded from uplifted plutonic basement blocks. Stevensitic ooids and peloids, formed in shallow, alkaline lacustrine environments, were mixed throughout the rift section with bivalve and ostracod bioclasts, and with the clastic sediments. Gravitational redeposition was promoted by intense and recurrent tectonism along the rift margins. The main diagenetic processes in clastic sandstones and conglomerates and hybrid arenites are cementation and grain replacement by smectite, zeolites, calcite and dolomite, mechanical compaction and dissolution of feldspars, volcanic fragments and bioclasts. Stevensitic arenites experienced early cementation and replacement of ooids and peloids by quartz, calcite and dolomite, or intense compaction of stevensitic grains in uncemented areas. Volcaniclastic sandstones and conglomerates with smectite rims, remnant intergranular porosity and grain dissolution may constitute fair hydrocarbon reservoirs. Stevensitic and hybrid arenites with dissolution of stevensite grains, bioclasts and calcite cement may also constitute reservoirs, although with potential quality limited by the poor connection of their pore systems. An understanding of the controls on the depositional and diagenetic evolution of the dominantly intrabasinal, gravitationally redeposited, rift succession will contribute to new exploration strategies for the Campos Basin.

Bathymetric and regional changes in benthic macrofaunal assemblages on the deep Eastern Brazilian margin, SW Atlantic

Deep-sea continental slopes have valuable mineral and biological resources in close proximity to diverse, undersampled and fragile marine benthic ecosystems. The eastern Brazilian Continental Margin (19.01°S to 21.06°S, 37.88°W to 40.22°W) is an important economic region for both fishing and oil industries, but is poorly understood with respect to the structure of the soft-sediment benthic fauna, their regional distribution and their bathymetric patterns. To identify spatial and temporal patterns of benthic macrofaunal assemblages on the slope (400 to 3000m), the Espirito Santo Basin Assessment Project (AMBES, coordinated by Cenpes-Petrobras) sampled 42 stations across the Brazilian Eastern Slope during both Summer 2012 and Winter 2013. We found a significant decrease in macrofaunal abundance at the 400m isobath along the slope near the northern region of the Espirito Santo Basin, suggesting benthic responses to upwelling events towards the south in Campos Basin and southern Espirito Santo Basin. The taxonomic diversity and assemblage composition also changed significantly across depth zones with mid-slope peaks of diversity at 1000–1300m. In general, macrofaunal assemblages were strongly related to slope depth, suggesting a strong influence of productivity gradients and water mass distribution on this oligotrophic margin. Sediment grain size was marginally important to macrofaunal composition on the upper slope. In general, macrofaunal assemblages on the slope of Espirito Santo Basin are similar to other areas of the SE Brazilian margin, but regional changes in response to productivity and depth need to be considered for management strategies in the face of increasing economic activities off-shore.

Shelf morphology as an indicator of sedimentary regimes: A synthesis from a mixed siliciclastic–carbonate shelf on the eastern Brazilian margin

Modern shelf morphology is the result of the interplay between short and long term sedimentary processes. The relation between rates of sediment supply/carbonate growth and accommodation space creation will not only control coastal transgression and regression, but will also define the shelf sedimentary regimes acting to shape the seabed. Herein, shelf morphology and sedimentology are investigated in order to discuss how these characteristics can be representative of distinct sedimentary regimes. The study area is the eastern Brazilian shelf where coastal transgression and regression coexist with the most important coral reef system of the South Atlantic. A compilation of existing published and unpublished data was carried out in order to produce morphological and faciological maps and compare the mapped features with high-resolution seismic and sonographic data. The results show three major regions or morphological compartments: Abrolhos Shelf, Doce River Shelf and the Paleovalleys Shelf. In terms of shelf sedimentary domain, rhodolith beds predominate over the outer shelf along the entire area, coralline reefs are present along the northern Abrolhos inner shelf and a significant terrigenous mud deposit is observed associated to the Doce River adjacent inner shelf beds. The rest of the shelf is composed by bioclastic or terrigenous mud sand and gravel. Terrigenous sedimentation is always restricted to the shoreface or inner shelf shallower areas and carbonate sands and gravels are predominant elsewhere. The Abrolhos shelf shows two distinct sectors; the northern area is a typical mixed sediment environment that has a supply regime along the coast/shoreface, mainly due to longshore transport and a carbonate regime along the inner and outer shelf. The southern shelf morphology and sedimentation are controlled by the antecedent topography and is typically a accommodation regime shelf with associated rhodolith beds. The Doce river shelf is a supply regime environment with the formation of a 5–8 m thick regressive deposit with downlapping clinoforms. Southward from the Doce river shelf, a significant shift in sedimentary regime is observed as the morphology becomes very irregular with associated hardbottoms and unfilled paleovalleys. This sector of the shelf (Paleovalley shelf) is characterized by an accommodation regime. The interpretation shows that the entire study area can be defined as a mixed sedimentation shelf, showing supply and accommodation regimes. Shelf morphology worked as an indicator of these changes. Carbonate/terrigenous deposition during a highstand/regressive phase coeval along the eastern Brazilian shelf, either laterally and across shelf. This lateral/along coast variation in sediment supply and carbonate production leads to distinct lateral facies and geometry. These spatial changes in morphology and facies, with coexistence of carbonate and siliciclastic sedimentation, are very important for the correlation and interpretation of the geological record, especially stratigraphic surfaces and sequence units.

Cenozoic folding in the Cumuruxatiba basin, Brazil: An approach to the deformation trigger by the Abrolhos magmatism

The Cumuruxatiba basin is located in the central portion of the eastern Brazilian margin surrounded by Cenozoic magmatic highs that belong to the Abrolhos Magmatic Complex. This basin was formed by rifting, in the Neocomian followed by thermal subsidence during late Cretaceous like other basins along the Eastern Brazilian margin. In the Cenozoic, the Abrolhos magmatism took place as sills and dykes intruded the sedimentary section, primarily during the Paleogene. In that time, there was a strong NS contractional deformation in the basin represented by folds related to reverse faults coeval with Abrolhos magmatism activity. The structural restorations of regional 2D seismic sections revealed that most of the contractional deformation was concentrated at the beginning of the Cenozoic with maximum peak at the Eocene (up to 33% of total shortening and rate of 6 km/Ma). The Post-Eocene period was marked by a decrease in the strain rate that continues to the present day (around 4 km/Ma to less than 1). 3D structural modelling exhibited a major, well-developed E–W to NE–SW fold belt that accommodated most of the contractional Cenozoic deformation between Royal Charlotte and Sulphur Minerva magmatic highs. Volcanic eruptions and magmatic flows from the Abrolhos complex resulted in differential overburden on edge of the basin, acting as a trigger for halokinesis and the subsequent formation of fault-related folds. In general, such structures were developed close to adjacent magmatic highs, commonly exhibiting vergence towards the centre of the basin. Some magmatic features formed coeval with Cenozoic syn-deformation sediments clearly indicate that Abrolhos magmatism activity and contractional deformation development were associated. The study of the thickness variation of the syn-deformation section in relation to fault-related folds on deformation maps and maximum strain diagrams revealed that most folds were activated and re-activated several times during the Cenozoic without a systematic kinematic pattern. This lack of systematic deformation might be related to the variation of the magmatic pulse activity of adjacent magmatic highs resulting in a complex interference pattern of Cenozoic folds. These structural interpretations of the timing of fault-related folds that are potential Cenozoic traps in the Cumuruxatiba basin play a fundamental role in petroleum systems and exploration of low-risk hydrocarbon prospects.

RESERVOIR QUALITY ASSESSMENT AND PETROFACIES OF THE LOWER CRETACEOUS SILICICLASTIC, CARBONATE AND HYBRID ARENITES FROM THE JEQUITINHONHA BASIN, EASTERN BRAZIL

A study of the rift to early drift phase, fluvial, deltaic and shallow‐marine Lower Cretaceous (Upper Aptian to Lower Albian) siliciclastic sandstones, hybrid arenites and calcarenites from the Jequitinhonha Basin, eastern Brazilian margin, has shown that the distribution of diagenetic alterations and of related reservoir quality evolution can be constrained within a sequence stratigraphic framework. Description of cores, wireline logs, thin sections and petrophysical (porosity and permeability) analyses were integrated in order to investigate the controls on patterns of diagenesis in these rocks, and hence of reservoir quality distribution. The results of this study will be relevant to the exploration of rift and early drift successions in Atlantic‐type passive margin settings. The study suggests that depositional facies and detrital composition impact the diagenetic evolution and the characterization and prediction of reservoir quality.Reservoir quality assessment was based on the concept of reservoir petrofacies, which is defined in terms of the main attributes affecting reservoir quality including depositional structures, textures, primary composition, diagenetic processes and products, and pore types. This paper demonstrates the practical use of the reservoir petrofacies concept in the evaluation of the controls on porosity and permeability, as well as on seismic and log signatures.

Mean Circulation, Seasonal Cycle, and Eddy Interactions in the Eastern Brazilian Margin, a Nested ROMS Model

A nested configuration of the Regional Ocean Modeling System (ROMS) is used to study the seasonal circulation patterns and mesoscale activity of the Eastern Brazilian Margin (EBM). The EBM encompass an oligotrophic and bathymetrically complex zone in the NW South Atlantic from 8°S to 20°S. Sea-level anomaly data are used to validate the model. Analysis of the mean circulation reveals that the EBM is dominated by seasonal and spatial dynamics of the southward Brazil Current (BC) and the northward North Brazil Undercurrent (NBUC), as well as their connection to the South Equatorial Current (SEC) dynamics. The EBM can be divided in three dynamic provinces, which are seasonally connected either by the permanent main flow or by mesoscale process. In the northern province, from 8°S to 13°S, the NBUC is the major permanent feature and the BC is just a thin flow, confined to the top few meters. As it moves southward, the BC gets deeper and stronger. In the middle province, from 13°S to 16°S, the dominance of the top 0–100-m circulation is seasonally alternated between the southward BC flow and the northward NBUC flow. In the southern province, from 16°S to 20°S, the BC appears as a dominating surface feature. While on the top (0–100 m) the main current presents a pronounced seasonal and spatial variability, on subsurface waters (100–500 m) the NBUC connects the EBM continuously. Finally, analysis of the regional simulation reveals well-defined cyclonic and anticyclonic eddies. They detach from the main flow and translate along the domain throughout the year. The translation patterns are associated with the seasonal variability of the main EBM flow, with anticyclonic mesoscale features translating southward and mesoscale cyclonic features translating northward. On their pathway, these features may come very close to the margin, interacting with the near-shelf flow.

Provenance of deep-water reservoir sandstones from the Jubarte oil field, Campos Basin, Eastern Brazilian Margin

The provenance of the Maastrichtian deep-water reservoir sandstones from the Jubarte oil field (Campos Basin, eastern Brazil), was studied using an integrated approach that included quantitative petrography, conventional heavy mineral analysis, garnet mineral chemistry and zircon geochronology. The reservoirs are predominantly coarse, poorly-sorted sandstones with feldspathic composition derived from uplifted basement terrains. The fourth- and fifth-order depositional sequences analyzed show no major variations in composition or in provenance through time. However, variations in apatite:tourmaline presents potential to be used as a parameter for sandstone correlation within the field. The composition of heavy minerals indicates derivation from high temperature and low-to-medium pressure aluminous metapelitic rocks, from granites and subordinate mafic rocks, derived from the Cabo Frio Tectonic Domain and the Oriental terrain of Ribeira orogen, characterizing a supply route from SW to NE. The low ZTR index, as well as the absence of low-grade stable heavy minerals and of metasedimentary rock fragments, suggest that by the end of Cretaceous all supracrustal, low-grade terrains had already been totally eroded, and that plutonic, infracrustal rocks were exposed, similarly to the present situation. The integration of the compositional and textural aspects suggests a tectonically active source area, located relatively close to the basin. The active tectonic uplift of the basement terrains provided a large volume of coarse, poorly-sorted sediments carried directly from mountain streams and alluvial fans to deep water. The high frequency shifts in apatite:tourmaline values support a direct derivation of the deep-water sandstones from adjacent alluvial systems with short but differential exposure to weathering. The voluminous deposition of such coarse-grained and immature turbidites under the general transgressive conditions of the Maastrichtian indicates a tectonic, rather than eustatic control on their deposition, with important implications for the exploration of similar reservoirs in Campos Basin and in similar basins of eastern Brazilian margin.

RIFTING AND MAGMATISM ASSOCIATED WITH THE SOUTH AMERICA AND AFRICA BREAK UP

Based on about 380 K/Ar dating of basic and alkaline magmatic rocks from Brazilian sedimentary basins and the Argentinean Atlantic margin it is possible to interpret the evolution of the rifting that caused the South America and Africa break-up. This process started in the Triassic and took place northwestward of the Foz do Amazonas Basin, on the equatorial Brazilian continental margin, and reached the San Julian and North Malvinas basins, in the southern portion of South America. From this latter region up to the Espírito Santo Basin, rifting took place between the Jurassic and Neocomian whereas transcurrence caused by clockwise rotation of South America on the equatorial margin generated the Potiguar Basin. The available information suggests that the final break-up of Africa and South America occurred along the eastern Brazilian margin, between the Cumuruxatiba/Mucuri and Pernambuco basins, during the Cenomanian/Turonian. The resistance offered by the São Luis/West African and São Francisco/Congo cratons promoted the huge magmatic manifestations which occur in the Brazilian Paleozoic intracratonic basins (Acre, Solimões, Amazonas, Parnaíba and Paraná basins) as well as in the interior Tacutu Rift. Hot spots turned out during the drift phase, as registered by the Mecejana (CE)/Rocas Atoll/Fernando de Noronha Archipelago Alignment and Poços de Caldas/Cabo Frio Alkaline Rocks Alignment that persists up to the Vitória/Trindade Chain.

Rifting and magmatism associated with the south America and Africa break up

Based on about 380 K/Ar dating of basic and alkaline magmatic rocks from Brazilian sedimentary basins and the Argentinean Atlantic margin it is possible to interpret the evolution of the rifting that caused the South America and Africa break-up. This process started in the Triassic and took place northwestward of the Foz do Amazonas Basin, on the equatorial Brazilian continental margin, and reached the San Julian and North Malvinas basins, in the southern portion of South America. From this latter region up to the Espirito Santo Basin, rifting took place between the Jurassic and Neocomian whereas transcurrence caused by clockwise rotation of South America on the equatorial margin generated the Potiguar Basin. The available information suggests that the final break-up of Africa and South America occurred along the eastern Brazilian margin, between the Cumuruxatiba/Mucuri and Pernambuco basins, during the Cenomanian/Turonian. The resistance offered by the Sao Luis/West African and Sao Francisco/Congo cratons promoted the huge magmatic manifestations which occur in the Brazilian Paleozoic intracratonic basins (Acre, Solimoes, Amazonas, Parnaiba and Parana basins) as well as in the interior Tacutu Rift. Hot spots turned out during the drift phase, as registered by the Mecejana (CE)/Rocas Atoll/Fernando de Noronha Archipelago Alignment and Pocos de Caldas/Cabo Frio Alkaline Rocks Alignment that persists up to the Vittoria/Trindade Chain.

Reservoir geophysics in deep and ultradeep water in the Campos Basin

Petrobras has been producing from more than 400 oil fields in many geologic settings along the eastern Brazilian margin. These reservoirs vary in geometry, size, and internal character. They contain original oil in place of 52.3 billion bbl and total oil reserves of 9.8 billion bbl; about 28% are in deep water (400–1000 m) and 45% in ultradeep water (over 1000 m).

General evolution of the eastern Brazilian continental margin

Brazil has one of the world’s largest series of basins along its continental margin, and past exploratory activity resulted in a very large data set along diverse segments of the whole margin. The main objective of this article is to review the general tectonostratigraphic evolution of the Eastern Brazilian Margin and discuss the main phases and their structural styles and depositional megasequences, including the prerift, synrift, transitional, and postrift phase strata.

Reservoir Architecture of Deep-Lacustrine Sandstones from the Early Cretaceous Recôncavo Rift Basin, Brazil

Synrift, lacustrine sedimentation took place along the entire eastern Brazilian margin in response to the Neocomian breakup of Gondwana. In the Reconcavo basin, a deep lake was developed and filled mainly by a thick (>2000 m) succession of dark-colored, organic-rich mudstones and sandstones, and by subordinate, oncolite- and Ostracod-rich carbonates. In the Fazenda Balsamo oil field, near the northeastern border fault margin of Reconcavo basin, deep-lacustrine reservoirs comprise a succession up to 424 m thick composed mainly of laminated (lower section) and massive (upper section), medium- to fine-grained sandstones. Their porosities and permeabilities typically range from 14 to 23%, and 100 to 500 md, respectively. The sandy reservoirs and interbedded source-rock mudstones comprise eight transgressive successions (mostly 20-120 m thick) bounded by basin-wide, mudstone marker beds. The average duration (about 83,000 yr) of each succession suggests that climatically driven lake level fluctuations may have modulated the cyclic, deep-lacustrine sedimentation in the study area; however, the interaction of changes in lake level with tectonic activity defined the dominant type of reservoir facies and the geometry and position of sand bodies. Laminated sandstones form sandstone bodies that are 600-1200 m wide, 1.5-4.5 km long, and up to 46 m thick, which fill northeast-oriented, fault-bounded troughs. These rocks probably were deposited by long-duration, sand-rich density underflows preferentially developed during rising lake level or highstands. These rocks may reflect an increasingly humid climate and more powerful and sediment-loaded influent streams. The more intense fault activity during the deposition of the lower reservoir section would have axially focused density underflows coming from the northeastern end of the basin. Massive sandstones comprise turbidite channel-fills that are 100-800 m wide, more than 2 km long, and up to 38 m thick. Fault activity diminished during the deposition of the upper reservoir section, allowing the cutting and filling of northwest-oriented channels by turbidity currents derived from the nearby border fault margin, preferentially during falling lake level or lowstands. Despite the local erosion associated with some turbidite channels, no unconformity or widespread erosion surface can be recognized within the studied succession. The relatively rapid and continuous tectonic subsidence along the eastern border fault margin of the Reconcavo basin provided virtually unlimited accommodation space for the aggradational stacking of climatically controlled, deepening-upward successions.