Pre-salt Sequence Research Articles

Crustal extension and necking at central Campos rifted margin: The Marlim Detachment System

Detachment faults play a crucial role in accommodating crustal thinning and controlling basin architecture in extensional tectonic settings. This study focuses on the characterization of extensional structures at the Necking Domain of the central Campos Rifted Margin, southeast Brazil. Based on seismic reflection and magnetic data we interpreted a system of long offset detachment faults, here named as the Marlim Detachment System. This system is characterized by three distinct large-displacement detachment faults which systematically root at subhorizontal intracrustal reflectors, interpreted as middle crust ductile shear zones, indicating a depth-dependent accommodation of extensional strain. The detachment fault systems resulted in highly tilted upper crustal blocks, which lead to intense erosion of the footwall and the pre-salt sequences eastward. The large horizontal displacement of the hanging wall blocks by most of the detachment faults created deep and wide half-grabens resulting in thick syn-rift sedimentary wedges. The basement structural framework seems to reflect a northward strike-rotation from NE-to NS-direction of the fault systems. That rotation promoted the segmentation of the Marlim Detachment System and points out an oblique-slip extensional kinematics during rifting. We propose that the observed tectonic style at the Necking Domain of central Campos is the result of its interaction with the Vitória-Colatina Transfer Zone to the north and the Araruama Transfer Zone to the south, configuring the Marlim area as a structurally transition zone. The tectonostratigraphic analysis suggests a migration of the deformation from continent to ocean from Hauterivian to Aptian, and a polyphase and/or tardi-tectonic deformation of the Necking Domain in the central Campos Rifted Margin.

Diagenetic products, settings and evolution of the pre-salt succession in the Northern Campos Basin, Brazil

Abstract Despite all the effort made towards an understanding of the sedimentary, tectonic and diagenetic evolution of the pre-salt sequence and the Pre-Salt reservoirs of the Northern Campos Basin (East Brazil), two knowledge gaps have yet to be filled: (1) a detailed study of diagenesis in the crystalline basement and rift phases; and (2) the timing of diagenetic events that affected the pre-salt succession. In this study, samples from these geologic units were analysed for mineral composition and paragenetic evolution, fluid temperature and salinity, stable isotope compositions and laser ablation inductively coupled plasma mass spectrometer derived U–Pb ages of carbonate phases. The U–Pb ages of replacive and vein-filling cements reveal three tectono-diagenetic events, named the Barremian–Aptian (BADE, 125–117 Ma), the Albo–Cenomanian (ACDE, 103–98 Ma) and the Campanian–Maastrichtian (CMDE, 83–70 Ma). Each phase is characterized by distinct minerals, precipitation temperatures and burial conditions. The hydrothermal qualifier, identified by the temperature contrast between fluid and host rock, was initially high during BADE, then diminished over time (through ACDE) until it achieved equilibrium with the host rocks during CMDE. Diagenetic events are not coeval with magmatism but do coincide with known regional tectonic events described in the literature and are interpreted to be the result of increasing intraplate stresses. Multidisciplinary studies that include diagenetic events constrained by geochronological data will certainly lead to more robust conceptual geologic models, and, therefore, to a more reliable management of resources and strategies such as enhanced oil recovery, carbon capture, utilization and storage, and drinking water.

Characterization of subaerial lava flows of the presalt strata in the Santos Basin based on basic well logs

It is hard to identify volcanic morphologies using well logs to support technical decisions during well drilling and the acquisition of geological information. Therefore, this study was carried out on the volcanic sequence present in the Presalt interval of the Santos Basin, Brazil, which is referred to as the Camboriú Formation. This section was described in a previous study using image well logs as a succession of subaerial lava flows formed mainly by compound pahoehoes, sheet pahoehoes and rubbly pahoehoes. Based on sidewall core samples and petrophysical, geochemical and mineralogical reports, we identified five electrofacies from gamma ray, resistivity, sonic, density and neutron logs, representing patterns for identifying lava flows in subaerial sequences. In addition, we applied a porosity quantification method that resulted in a method for obtaining information from the matrix and presented coherent values in facies that did not present amygdalae or breccias and inferring altered zones from variations in resistivity and density logs or from cross-plots. Patterns of well log responses were defined for each type of lava flow present in the studied well, as was the inference about the most favorable facies for reservoirs. This methodology, which uses basic well logs from basalts, is unprecedented in the Presalt sequence and can be easily used to evaluate this exploratory frontier because of its close association with subaerial volcanism.

Diagenetic Products, Settings and Evolution of the Presalt Succession in the Northern Campos Basin, Brazil

Abstract Despite all the effort made towards an understanding of the sedimentary, tectonic, and diagenetic evolution of the presalt sequence and the Pre-Salt reservoirs of the North Campos Basin (East Brazil), two knowledge gaps have yet to be filled: 1) a detailed study of diagenesis in the crystalline basement and rift phases, and 2) the timing of diagenetic events that affected the presalt succession. In this study, samples from these geologic units were analysed for mineral composition and paragenetic evolution, fluid temperature and salinity, stable isotope compositions, and LA-ICP-MS derived U-Pb ages of carbonate phases. The U-Pb ages of replacive and vein-filling cements reveal three tectono-diagenetic events, named Barremian-Aptian (BADE, 125-117 Ma), Albo-Cenomanian (ACDE, 103-98 Ma), and Campanian-Maastrichtian (CMDE, 83-70 Ma). Each phase is characterised by distinct minerals, precipitation temperatures, and burial conditions. The hydrothermal qualifier, identified by the temperature contrast between fluid and host rock, was initially high during BADE, then diminished over time (through ACDE) until it achieved equilibrium with the host rocks during CMDE. Diagenetic events are not coeval with magmatism but do coincide with known regional tectonic events described in the literature and are interpreted to be the result of increasing intraplate stresses. Multidisciplinary studies that include diagenetic events constrained by geochronological data will certainly lead to more robust conceptual geologic models, and therefore, to a more reliable management of resources and strategies such as Enhance Oil Recovery (EOR), Carbon Capture, Utilisation and Storage (CCUS), and drinking water. Supplementary material at https://doi.org/10.6084/m9.figshare.c.7103900

Aptian flood basalts in Bacalhau oil and gas field: petrogenesis and geodynamics of post-rift tholeiites in the pre-salt sequence of Santos Basin, Brazil

Aptian flood basalts in Bacalhau oil and gas field: petrogenesis and geodynamics of post-rift tholeiites in the pre-salt sequence of Santos Basin, Brazil

Hydrocarbon accumulation conditions and exploration targets of the Sinian Dengying Formation in the Southeastern Sichuan Basin

No breakthrough has been made in petroleum exploration of the Sinian Dengying Formation in the southeastern Sichuan Basin as the major factors controlling the hydrocarbon accumulation of the Dengying Formation have not been well established, including the development period and distribution of the platform margin, the effectiveness of the source-reservoir combination and the reliability of the Cambrian pre-salt structural trap. Based on the interpretation of seismic data and hydrocarbon-source correlation, the distribution of the platform margin of the Sinian Dengying Formation in the southeastern Sichuan Basin was mapped. Moreover, the gas source rocks of the Dengying Formation were clarified according to geochemical characteristics of fluid inclusions, and the pre-salt structures were further confirmed by seismic forward modeling and experiments of prestack depth migration. Results showed that the sedimentary facies of the 4th member of Dengying Formation in the southeastern Sichuan Basin was platform margin of gentle slope type. The platform margin reservoir of the Dengying Formation and the Lower Cambrian source rocks constitute a favorable source - reservoir combination. The carbon isotope characteristics of gas in fluid inclusions indicate that some of the oil/gas in the Dengying Formation were contributed by the Doushantuo Formation source rocks. As the difference in seismic velocities between carbonate rocks and gypsum/salt beds may cause a reduction in the amplitude of pre-salt structures, the low-amplitude structures identified on current seismic profiles are underestimated instead of overestimated. Accordingly, the Sinian structural traps are confirmed to develop in the deep pre-salt sequences in the low-steep anticline belt, southeastern Sichuan Basin. On the basis of the above new understanding, we propose two types of hydrocarbon migration modes for the Sinian Dengying Formation in southeastern Sichuan Basin, including lateral migration of hydrocarbons generated from the Cambrian Qiongzusi Formation and vertical migration of hydrocarbons derived from the Sinian Doushantuo Formation. Lithologic traps at the platform margin and pre-salt structural traps within the platform are favorable targets for natural gas exploration.

Integrated bio- and chemo-stratigraphy for Early Cretaceous strata offshore Gabon: Additional constraints on the timing of salt deposition and rifting of the South Atlantic

Early Cretaceous rift basins of the incipient South Atlantic have been the focus of intense hydrocarbon exploration and production activities and host some large oil accumulations in sections predating an interval of major salt deposition, particularly in the central segment of the South Atlantic. Understanding the timing (and associated uncertainties) of source rock and reservoir deposition and their relationship with rift evolution is critical for successful exploration. However, there are still many unresolved issues and data gaps regarding the precise age and duration of salt deposition. Better chronological constraints are particularly needed to determine the timing of deposition of Pre-Salt reservoirs and the primary evaporites, as well as the secondary phase of halokinensis that resulted in variable reservoir sealing potential. To help address this gap, stable carbon isotope (δ13C) records from bulk organic matter and insoluble kerogen were generated for the Early Cretaceous salt and Pre-Salt intervals from two exploration wells offshore of Gabon. The bulk organic δ13C stratigraphies for the two wells were then integrated with palynological and ostracod biostratigraphy and placed within a sequence stratigraphic and regional tectonic framework, providing new constraints on the timing of rift lake evolution and salt deposition. The good correlation between the offshore Gabon δ13C record with other published sections calibrated to the current Geologic Time Scale as well as other regional sections from NE Brazil, supports the reliability of our new Gabon δ13C record. Several δ13C excursions are identified in the Pre-Salt sequence and are correlated with the Valanginian Weissert event and Early Aptian δ13C event(s). Salt deposition on the Gabon margin is interpreted to have occurred during an interval straddling the Early-Late Aptian boundary (∼118.4–116.8 Ma). These findings are comparable with other published estimates for salt deposition from northeast Brazil but differ from published estimates from the Campos-Santos basins; the latter are critically discussed. This study provides an important stratigraphic dataset for offshore Gabon and contributes to the ongoing debate regarding the timing of rifting and salt deposition in the Early Cretaceous of the South Atlantic passive margin system.

Crustal structures and salt tectonics on the margins of the western Algerian Basin (Mediterranean Region)

We present an overview of the crustal architecture of the continental margins of the oceanic Algerian Basin in the westernmost Mediterranean Sea. During the Cenozoic, and with a variable oblique convergence between the African and Eurasian plates, the Western Mediterranean Sea has experienced thinning and extension behind a tight orogenic arc formed by the Betics, Rif, and Tell Cordilleras. This study is focused on the structural style affecting the Messinian salt layer, which is mostly restricted to the deep domains of the Algerian Basin, where it is floored by a thin oceanic crust of probable Miocene age.Using deep-penetrating seismic profiles and wells from offshore western Algeria to southeastern Spain, we have analyzed the crustal structures affecting the domains close to the oceanic-continent transition on the three margins of the western Algerian Basin. Since the Early Miocene, active shortening in the Tell-Atlas domain has accommodated most of the plate convergence in the basin, whereas the Alboran margin in the west and the Iberian margin in the north experienced eastward and southward crustal extension and thinning, respectively, accompanied by volcanism. The Algerian margin in the south shows incipient thrusting of African continental crust over oceanic crust. This shortening occurred since at least the Late Miocene, also promoting decoupling and contraction of the deep, sub-horizontal Messinian salt layer. The salt exhibits diapir squeezing and suprasalt folding, whereas the presalt sequence preserves partially-inverted half-grabens. Salt tectonic processes along the northern and western margins of the Western Mediterranean Basin show contrasting structural styles formed by narrow extensional and transtensional domains with gentle salt anticlines.This region shows therefore a somewhat unusual salt-tectonic style, departing from the gravity-driven model typical of continental margins that contain an initial continuous, gently-dipping salt layer. In the Algerian Basin, salt is mostly restricted to deep water domain floored by oceanic crust, so it does not participate in significant gravity-driven deformation. Instead, Messinian salt and the suprasalt sequences underwent significant shortening along the southern margin, simultaneous with thick-skinned extension involving the Messinian evaporites in the northern and eastern margins.

The Volcanic Outer High of Campos Rifted Margin in Southeast Brazil: The structure and formation of SEMA

The SEMA (South East Magnetic Anomaly) is a high amplitude magnetic lineament in the most distal part of the Campos Rifted Margin in Southeast Brazil. It runs continuously from south to north along 85 km reaching 500 nT in deep waters. The aim of this paper is to investigate the structure, stratigraphy and nature of the SEMA, and to discuss its significance for the evolution of the margin. Based on seismic, magnetic and gravity analysis, we interpret the SEMA as a volcanic outer high. Its formation was prior to the salt migration oceanwards, probably earlier to the breakup, constraining a time-relative formation in the Late Aptian - Early Albian. It is bounded by a basinward dipping, large-scale fault that created a huge depocenter continentwards. The Volcanic Outer High of the SEMA marks the transition to a distinct tectonostratigraphic compartment and salt domain, characterized by intense halokinesis, significant thinning and modification of the pre-salt sequences, and increasing presence of volcanic and intrusive rocks. It forms a long volcanic ridge in the external boundary of the Campos Rifted Margin, which may have influenced the paleo-circulation. Moreover, its understanding could provide new insights into the breakup process.

Petrogenesis and geodynamics of Eocene alkaline intrusions in the pre-salt sedimentary sequence of Santos Basin, Brazil

Lamprophyres and phonolite intrusions were sampled in the so-called Pre-Salt sequence of well 1-BRSA-905-RJS in northern Santos Basin nearby the Cabo Frio Structural High in SE Brazil. These alkaline rocks indicate the existence of a strongly silica-undersaturated trend. However, fractional crystallization modeling as well as Sr-Nd-Pb-Hf isotope data shows that the alkaline lamprophyres are not suitable parental compositions to the evolved felsic phonolitic magmas. The Sr-Nd-Pb-Hf isotope compositions of the lamprophyric magmas represent mixing of EM1 and Trindade plume mantle components. The EM1 is possibly related with the melt- and/or fluid-induced metasomatic processes on the subcontinental lithospheric mantle (SCLM) that occurred beneath the South American Platform during the Neo-Proterozoic-Cambrian subduction processes associated with the final stages of the Gondwana amalgamation, as recorded by the TDM ages (601-499 Ma) of the studied intrusions in Santos. 40Ar/39Ar analysis presented integrated age of 41.06±0.02Ma and 38.62±0.02Ma for phonolite and lamprophyre samples, respectively. A “plateau” age assignment for the true crystallization age of the sample could not be obtained due to the discordance in the ages of individual steps. This discordance was attributed to loss of radiogenic 40Ar that can happen in nature due to reheating events or metasomatism. An interaction between the Trindade plume head and SCLM is proposed for the geodynamics of Santos Basin at 40 Ma. The presence of an anomalously hot mantle represented by the deflected Trindade plume head beneath Santos at the Eocene is supported by the petrological and geochronological data. Therefore, it may be relevant to consider high geothemal flow regimes in basin evolution models proposed for Santos during the Eocene.

Structural control and geochronology of Cretaceous carbonate breccia pipes, Crato Formation, Araripe Basin, NE Brazil

The laminated limestones of the Crato Formation, Araripe Basin (NE of Brazil) have been investigated, as they are analogous to facies of the carbonate reservoirs of the pre-salt sequence of the Brazilian marginal basins. The Early Cretaceous Crato laminites comprise a complex range of structures that may play an important role in the porosity and permeability distribution in carbonate reservoirs. This work aims to (1) understand the control of a boundary fault zone (the Triunfo Fault, Lower Cretaceous) on the development in carbonate breccia pipes of the Crato Formation, and (2) constrain the timing of brittle deformation, based on field observations, microstructural investigation and U–Pb carbonate geochronology. The breccia pipes are observed surrounding both normal and strike-slip faults. The brittle deformation superimposed on the pipes is characterized by brecciation and veining that show multiples phases of mineralization. Our results propose the circulation of fluid along the pre-existing faults and pipes, these fluids are responsible for polyphasic deformation in the pipes. We report two U–Pb ages from carbonate-vein mineralization of 94.9 ± 3.8 Ma (Cenomanian to Turonian) and 80.2 ± 3.0 Ma (2σ) (Campanian), which provide time constraints on the brittle reactivations of the Triunfo Fault Zone, during the post-rift phase of the Araripe Basin.

The tectono-magmatic and subsidence evolution during lithospheric breakup in a salt-rich rifted margin: Insights from a 3D seismic survey from southern Gabon

In this study, we present an interpretation of a 3D depth-migrated dataset imaging the Ocean Continent-Transition (OCT) of the southern Gabon margin. Located in an area where salt is not strongly overflowing the OCT, it allows the geometrical relationships of salt deposition and its relative timing to breakup to be determined. The OCT is formed by extensional propagators floored by exhumed mantle limited by either fault-bounded upper-crustal or pre-salt sediments and/or magmatic additions. Based on our interpretations, we conclude that the pre-salt sequence was deposited over a wide area spanning crustal thinning to the onset of hyperextension and exhumation. The relative timing of salt deposition is constrained by the observations that detachment faults break away in the pre-salt sequence and that some of these faults also strongly offset the base salt and are linked to the creation of accommodation space in the salt. This is direct proof that salt was deposited during mantle exhumation. Salt deposition is interpreted to have occurred at variable depths. Evaporites formed approximately 1 km below sea level in the north, above a 4–6 km thick pre-salt sequence, and as deep as 2 km in the south. In this area, salt is interpreted to directly overlie exhumed mantle, but whether it was deposited on mantle or juxtaposed during exhumation remains more debatable. Space created in the faulted outer trough was filled partly by autochthonous salt and possibly also by downslope flow. Rifting continued after the cessation of salt deposition, followed by magmatic additions in the outer high and ultimately the onset of seafloor spreading and the formation of Penrose oceanic crust.

Recognition of the pre-salt regional structure of Kwanza basin, offshore in West Africa, derived from the satellite gravity data and seismic profiles

Abstract Kwanza basin, located on the west coast of Africa and the east side of the South Atlantic Ocean, has the potential for deep-water oil and gas exploration. Previous studies have shown that the pre-salt system within the area has high potential for oil and gas storage. However, due to the shielding effect of the evaporating salt rock during the Aptian period, the quality of seismic reflection profiles of the pre-salt layers is poor. This means that the pre-salt sequences, the main fault, the scale and distribution pattern of the rift are not clear. To clarify the pre-salt regional structure pattern and further guide pre-salt exploration, we carried out a series of analyses and target processing of seismic and gravity data. Further, combining other available geological and lithology data as well as a tectonic model, we put forward a new understanding of the pre-salt structure of Kwanza basin. The research shows that the Kwanza basin can be divided into three uplift belts below the salt layer, which are distributed in the NW–SE trending direction. The three key profiles illustrate the distribution of uplift and depression in detail. The explained structural highs distributed in the outer Kwanza basin may be related to oil and gas reservoir. This study could provide the geophysical basis for the re-interpretation of the pre-salt seismic sequence, the strategic selection of pre-salt oil and gas and the next exploration deployment.

Regional geological analysis of the southern deep Gulf of Mexico and northern Yucatán Shelf

Abstract We discuss the sedimentary and structural evolution, and oil possibilities, of a Late Jurassic-Early Cretaceous major salt detached extensional–contractional province in the Yucatán sub-basin at the southern Gulf of Mexico using 2D seismic reflection profiles and well data from the Deep Sea Drilling Project (DSDP). The main basement trends are a central depression that accommodates the pre-salt sequences, and a long ridge where the continental–oceanic crust transition is found. Along this ridge, the geometry of the outer trough was defined by mapping the base salt. The base salt changes from steeply dipping on the Campeche Escarpment to irregular-horizontal on the limit with the recognized outer trough. The extensional–contractional system was divided into three subdomains with: (1) tilted blocks; (2) growth faults and rollover anticlines; and (3) diapirs and fold and thrust. We regionally identify the highest sedimentation stages through the Tertiary, the possible main directions of sediment supply to the basin and their relationship to Neogene diapirism. The two structural traps formation times interpreted in the area are: (a) Late Jurassic-Early Cretaceous traps associated with seaward salt translation; and (b) middle Miocene–early Pliocene traps related to diapiric growth in the outer trough, triggered by sediment load derived from the north of the Gulf of Mexico and from SE Mexico, where the Chiapanecan Orogeny was developed. The integration of the results of this study with the evidence of some elements and processes of the petroleum system allow us to propose that the area may represent an important oil province.

Extensional models for the development of passive‐margin salt basins, with application to the Gulf of Mexico

AbstractRecent subsalt petroleum discoveries associated with rifted‐margin salt basins have piqued interest in the presalt geology of the Gulf of Mexico margin. Available subsurface data does not uniquely constrain the subsalt geometry, so creating an interpretation of the crustal architecture requires the application of geological models for crustal extension and breakup. However, published interpretations of the nature of the transition from continental rifting to seafloor spreading range from magma‐rich to magma‐poor. To address this uncertainty, we present 2D forward kinematic models for crustal configurations generated by diverse models (symmetric extension, depth‐dependent extension, and volcanic extension). Through a series of conceptual balanced cross‐sections grounded in a ~600 km 2D seismic line from the NE Gulf of Mexico, we demonstrate the implications of each model for the limit of oceanic crust, basement morphology, crustal architecture, and hydrocarbon prospectivity. We discuss evidence for the dominant crustal processes, including geodynamic factors and structural and stratigraphic observations. Based on our observations and the geologic history, we favour an asymmetric, magma‐poor to ‐intermediate margin interpretation for the NE Gulf of Mexico, but suggest that the degree of volcanic input and width of the ocean‐continent transition zone may vary along strike. The along‐strike variability highlights the importance of understanding all potential presalt crustal configurations, their key features, and their implications. With increased data availability on the presalt geology in the Gulf of Mexico the relevance of these scenarios can be assessed, allowing development of comprehensive geodynamic and tectonic models of the margin and consideration of petroleum system elements in the presalt sequence.

Geodinámica de los márgenes conjugados del Atlántico Sur central: implicaciones para el potencial de hidrocarburos

Using a new margin restoration model for the central South Atlantic we highlight margin-opposed rift basin subsidence characteristics, reconcile important facies relationships and consider the associated processes responsible for contrasting basinward transitions to oceanic crust. We interpret these parameters as diagnostic of strain evolution during rifting. Pre-salt subsidence patterns and sequence isopachs of late syn-rift and early post-rift sequences are symptomatic of these complex extension behaviours. Inherited basement fault trends partition extensional strain during stretching with resultant rift-related structural styles recording the heterogeneity of the pre-rift crust. Correspondingly the seismic and gravity expression of the transition from continental crust to oceanic crust differs systematically along the length of the ‘conjugate’ basins. The presence of a conspicuous coast parallel positive linear gravity anomaly, referred to as the ‘terminal horst’, defines the basinward extent of attenuated continental crust in the more symmetrical Campos Basin and its interpreted conjugate. In the ‘asymmetrical’ Santos and Benguela Basins the ocean–continent transition at the present-day OCB (ocean–continent boundary) becomes equivocal with little gravity expression. Consequently the relationship of primary salt basin edges to the basinward extent of crustal extension appears more complex. Pre-existing crustal and lithospheric mantle scale heterogeneities are considered to impart a first-order control on whole crust deformation and ultimately ‘sag’ basin development. These structurally defined heterogeneities are believed to partition crustal strain and juxtapose endmember mechanisms of pure-shear and simple shear deformation styles as recorded by the complex distribution of syn-rift subsidence patterns within the pre-salt basins of the central South Atlantic. These conclusions challenge the necessity for invoking dominantly depth-dependent processes as a mechanism to explain apparent pre-salt, syn-rift subsidence anomalies. The complex interplay of structure and stratigraphy with an overlay of palaeoclimatic models provides important insights into hydrocarbon play fairways along the margin. In particular the impact on reservoir and source rock distribution and heat flow are discussed.

Scale-relationships and geometry of normal faults reactivated during gravitational gliding of Albian rafts (Espírito Santo Basin, SE Brazil)

Three-dimensional (3D) seismic-reflection data from SE Brazil are used to investigate the geometry and significance of normal faults reactivated during gravitational gliding of Albian rafts. Late Cretaceous gravitational instability in the study area led to the fragmentation of Albian strata in individual rafts separated by listric (roller) faults and associated sub-basins (Stage 1). Renewed gravitational gliding caused Albian rafts to be translated downslope until salt welds were formed and Albian strata became grounded over pre-salt sequences (Stage 2). Diachronous grounding promoted the reactivation of faults and folds, which at places developed into pop-up structures and broad anticlines sub-parallel to the strike of individual rafts (Stage 3). In this paper we present a new methodology to identify and date reactivated (i.e. shortened) fault families. Statistical analyses of six main fault families confirm that fault reactivation was diachronous in the study area, and its cessation depended on: i) the complete grounding of Albian rafts or, ii) gravitational stabilisation of the rafts imposed by sediment loading and cessation of tectonic tilting of the slope. The results of this study show that tectonically generated pulses of compression and uplift, commonly related to the Andean Orogeny, are not the only mechanisms deforming post-salt overburden units in proximal extensional-dominated regions of the southeast Brazilian margin. This is a crucial piece of information for geodynamic reconstructions and hydrocarbon exploration, as the close control of Albian rafts on fault reactivation induced a continuum of overburden deformation in the Espírito Santo Basin, rather than discrete episodes of inversion.

Role of halokinesis in controlling structural styles and sediment dispersal in the Santos Basin, offshore Brazil

Abstract Salt–sediment interplay in the Santos Basin is investigated integrating seismic interpretation, kinematic restoration and analogue modelling. Deformation within the post-salt sequence results from thin-skinned gravitational gliding and spreading, driven primarily by halokinesis, greatly affected by massive sediment inflows. The impressive landward-dipping listric Cabo Frio Fault controls the major depocentres updip, whereas salt-cored folds accommodate downdip shortening. Sediment supply from confluent directions creates a complex interference pattern of superposed folds with intervening polygonal minibasins. A new structure is identified (termed the ‘Ilha Grande Gravitational Cell’), a linked system of updip extension and downdip contraction detached on salt, comprising the Cabo Frio Fault and minibasins. It moves to the SE, with eastern and western borders suggesting lateral gradients of slippage. This thin-skinned feature results from the differential load imposed by a thick prograding wedge over the ductile salt and is independent of pre-salt structures. The post-salt sequence moves basinwards due to halokinesis, thereby changing position relative to the pre-salt sequence, which implies that any present-day correspondence between pre- and post-salt structures may not attest to linkage in the past. Application of kinematic restoration techniques allows the true position and geometry of the key elements through time, improving petroleum systems assessment.

Hyper-extended crust in the South Atlantic: in search of a model

ABSTRACT The discovery of giant hydrocarbon reservoirs in the pre-salt sequence of the deep-water Brazilian rifted margin together with the new acquisition of high-quality reflection and refraction seismic surveys across many rifted margins worldwide has attracted the interest of industry and researchers to deep-water rifted margins. For the first time, the new data sets enable the imaging and description of the pre-salt structures, which indicate that deep-water rifted margins are very different from what classical models had predicted thus far. Instead of the expected fault-bounded basins and a sharp ocean–continent boundary, the new data suggest the existence of a sag basin lying on hyper-extended crust with little indication for brittle high-angle faulting, a transitional domain between continental and oceanic crust showing neither characteristics of oceanic nor continental material, and very asymmetrical distal conjugate rifted margins. These observations raise significant doubts on the validity of the classical concepts used in rheology, mechanics and isostasy to explain extensional systems leading to seafloor spreading. They also require new concepts and more data in order to understand how these rifted margins evolved in time and space. This has important implications for the exploration and evaluation of petroleum systems in the frontier areas of hydrocarbon exploration. In this study we publish two multi-channel seismic sections across the Angola and conjugate Brazilian rifted margins that we consider as ‘type’ sections for hyper-extended magma-poor rifted margins in the South Atlantic. The aim of this study is to discuss various possible interpretations and models to explain the high-resolution seismic images presented in this paper.

ブラジルカンポス堆積盆地の石油システム評価

The Campos Basin, offshore Brazil, is a typical passive margin basin in the western South Atlantic, and the most prolific basin with about forty producing fields including deep-water giant fields. The Campos Basin is currently responsible for more than 80% of Brazilian oil production.The hydrocarbon accumulations are distributed throughout the stratigraphic column of the basin from Neocomian to Miocene. The most important oil accumulations in the basin are associated with deep-water fans distributed in the stratigraphic column from Cenomanian to Miocene. Almost all the hydrocarbon accumulations discovered to date are sourced mainly from Barremian to early Aptian lacustrine shales of the Lagoa Feia Formation in the pre-salt sequence. The oil pooled in the post-salt sequence migrated through a system associated with pre-salt normal faults, “salt windows” in the Aptian evaporite, listric faults and regional unconformities.The play fairway map was made based on considering the basement and the salt development, the fault distributions and the kitchen area. This map presented that the Campos Basin was divided into five structural zones. These zones could be closely related to the hydrocarbon potential. It was evaluated that the eastern part of Zone I and Zone IIa along the present day shelf edge provided more favorable condition for the hydrocarbon accumulation than other zones. The oil of Marlim Sul field is trapped in the blanket turbidites that pinch out toward the northwest landward. The thick mud deposits derived from the muddy turbidites was considered to be the important factor for establishing the stratigraphic trap.