Velha Formation Research Articles

The challenging carbonates from the Pre-Salt reservoirs offshore Brazil: facies, palaeoenvironment and diagenesis

The Santos Basin in offshore Brazil belongs to the major carbonate reservoir province known informally as the Pre-Salt. A detailed analysis of cores, plugs, sidewall samples and thin-sections from two wells in a sector of this basin was undertaken, on an interval corresponding to the upper part of the Lower Cretaceous Barra Velha Formation. Several facies and subfacies types have been classified independently from existing terminologies for these carbonate rocks. The specific features of the studied material are discussed and comparison to other nomenclatures is made. The facies include, namely: laminated argillaceous limestones; in-situ grown carbonates with a range of singular morphologies (of debatable origin), from laminar crust, stromatolitic-like fabric to robust or delicate shrubby structures; intraclastic grainstones/packstones, whose clasts may be mostly remnants of shrubs or differ; spherulitic grainstones/packstones; locally, oncoidal grainstones/rudstones and packstones/floatstones.These rocks are interpreted as depositional and diagenetic or essentially diagenetic facies. The latter are regarded as originally similar to some of the former ones but now "disguised", because they were differently impacted by diagenesis processes over different time periods. Recognizing these effects is relevant, because they produce different reservoir facies characteristics and quality. The best potential reservoirs are some of the depositional and the mostly diagenetic facies.The palaeoenvironmental setting of the studied facies fits into a shallow, low-gradient lake. Charophyte and other possible, unidentified algae, albeit extremely rare, are documented for the first time. The facies pattern shows some cyclicity, suggestive of both water depth and chemistry changes.If the interpretations in this study are correct, they (i) reinforce existing datasets to improve understanding of these facies; (ii) raise caution concerning some facies previously considered as separate depositional products, which in fact may instead result from diagenetic modifications; (iii) stress the relevance of microfacies analysis and diagenetic petrography to help deciphering how diagenetic modifications impact on reservoir facies properties and quality; and (iv) add new issues to the naming and textural classification problems of the Brazil Pre-Salt carbonates.

Seismic pattern classification integrated with permeability-porosity evaluation for reservoir characterization of presalt carbonates in the Buzios Field, Brazil

This paper presents the application of an unsupervised neural network to classification of seismic facies based on a unique combination of stratigraphic and structural seismic attributes. This classification was integrated with statistical analysis of the porosity and permeability of the seismic facies which permitted the advanced reservoir characterization of the presalt carbonate reservoirs of the Barra Velha Formation in the Buzios Field, Santos Basin. This advanced approach is required due to the high degree of heterogeneity and complexity these reservoirs, which directly impacts on their porosity and permeability. Preconditioning of the seismic data was essential for filtering random and structurally oriented noise prior to the generation of seismic attributes. Four seismic attributes; acoustic impedance, rms amplitude, local flatness and principal dip component, were selected as the inputs for the unsupervised classification based on their capacity for differentiating between the observed seismic patterns within the Barra Velha Formation. Principal component analysis was then performed to decrease redundancy of the input seismic attribute data prior to classification. The analysis of amplitude seismic features as well as of the classification results allowed the identification of three different seismic patterns within the study area: build-ups, debris and aggradational/progradational carbonate platforms. The seismic attribute of principal dip component proved to be crucial for distinguishing between carbonate build-ups and debris seismic facies. Our results demonstrated that the build-up and debris seismic facies are commonly occur aligned with faults, display higher porosity and permeability and as such they are inferred to be the best reservoirs.

DEFINIÇÃO ESTRATIGRÁFICA DA FORMAÇÃO MISSÃO VELHA, BACIA DO ARARIPE

A Formação Missão Velha corresponde aos depósitos fluviais do intervalo Neojurássico-Eocretáceo na Bacia do Araripe, depositada no contexto da Depressão Afro-Brasileira no Início de Rifte do Gondwana Ocidental. A Formação Missão Velha destaca-se com relação às demais unidades da Bacia do Araripe, sobretudo, pela ocorrência de uma assembleia fossilífera de troncos silicificados de coníferas. Embora os depósitos da Formação Missão Velha e das outras unidades do Andar Dom João nas demais bacias interiores do Nordeste de modo geral não apresentem feições diagnósticas de tectonismo e magmatismo ativo, uma série de investigações na última década desafiaram a obviedade em busca do entendimento das condições do início do rifte através de novos métodos comparativos e interpretativos. Tendo em vista estes recentes avanços e observando a necessidade de uma revisão da unidade à luz desses novos dados, a fim de facilitar a comunicação entre os autores, o presente trabalho determina de forma objetiva através da análise da evolução de conhecimentos e delimitação, condições tectonoestratigráficas, a caracterização geológica e paleontológica da Formação Missão Velha. Concluiu-se que a unidade apresenta uma discordância interna e que suas sequências inferior e superior apresentam distinção de sentido de paleocorrentes que mudam de prioritariamente S-SE para NW; associação de fácies que mudam de depósitos fluviais com planícies de inundação intercaladas para depósitos de fundo de canal de alta energia com presença de troncos silicificados. Por sua vez, a atribuição dos troncos ao táxon Dadoxylon-Araucarioxylon deve ser desencorajada pois estão em desacordo com o Código Internacional de Nomenclatura Botânica.

The mantle, CO2and the giant Aptian chemogenic lacustrine carbonate factory of the South Atlantic: Some carbonates are made, not born

AbstractDuring the Aptian (Cretaceous), in what is now the South Atlantic, the largest chemogenic (abiotic) carbonate factory so far identified in the Phanerozoic geological record developed as a vast hyper‐alkaline lake system. This covered at least 330 000 km2, producing carbonates, locally over 500 m thick, in what are now the offshore Santos and Campos basins (Brazil), and Kwanza Basin (Angola). Current evidence supports the view that almost all of this carbonate was chemogenic in origin, precipitated from hyper‐alkaline, shallow lake waters, probably by evaporation. This unit, best documented from offshore Brazil and known as the Barra Velha Formation (Santos Basin) and the Macabu Formation (Campos Basin), consists of just two basic carbonate components, millimetre to centimetre sized crystal shrubs and spherulites. These are commonlyin situbut can also be reworked into a range of detrital facies. Demonstrable microbialites are generally rare. These carbonates are associated with Mg silicates (as clays) which had a profound influence not only on the textural development of thein situcarbonates, but also on their diagenesis. The dissolution of the clays produced much of the porosity in these limestones, which are the hosts for multi‐billion barrel oil fields. The source of the carbonate was most likely from metasomatic alteration of mafic rocks, such as continental flood basalts related to Atlantic opening, with some contribution from much older continental basement. Clear evidence that serpentinization of possible exhumed mantle is lacking but mantle CO2is likely to have been a critical factor in determining the composition of the fluids from which the carbonates formed and the high alkalinities of the lake waters.

Estratigrafia da Bacia do Araripe: estado da arte, revisão crítica e resultados novos

A Bacia do Araripe constitui-se em bacia rifte originada pelo rompimento do Supercontinente Gondwana, conhecida como a bacia interior de registro sedimentar mais completo na Região Nordeste do Brasil. O manuscrito apresenta o estado da arte da estratigrafia da Bacia do Araripe enfocando sua evolução no tempo, nas sequências estratigráficas com base em superfícies de discordâncias, em novos resultados e faz ainda revisão crítica das classificações estratigráficas propostas com vistas à elaboração de classificação integrada. As primeiras propostas estratigráficas foram elaboradas de forma muito abrangente e simplificada. Com o tempo, foram realizados estudos mais minuciosos corroborando uma melhor e mais refinada compartimentação dos pacotes sedimentares presentes. A década de 1980 foi imprescindível para as produções científicas inerentes à geologia da bacia em função do interesse no potencial petrolífero das bacias brasileiras. Não obstante, ainda hoje a bacia é motivadora de inúmeras pesquisas determinantes para um melhor entendimento da geologia local e regional. A Bacia do Araripe tem cinco sequências maiores: Paleozoica, representada pela Formação Cariri; Início de Rifte, constituída pela Formação Brejo Santo e parte inferior da Formação Missão Velha; Rifte, formada pela parte superior da Formação Missão Velha e por toda a Formação Abaiara; e Pós-Rifte, separada em duas sequências — pós-rifte I, constituída pelas formações Barbalha, Crato, Ipubi e Romualdo, e pós-rifte II, caracterizada pelas formações Araripina e Exu. Novos resultados estão incorporados às formações Missão Velha e Abaiara, cuja distinção é demonstrada aqui; também com relação às formações Barbalha e Cariri, novos elementos foram reunidos. Este trabalho foca a viabilidade dos dados, as interpretações propostas conforme as normas estratigráficas e a proposição de uma classificação estratigráfica sequencial.

Silicification and dissolution features in the brazilian Pre-salt Barra Velha formation: impacts in the reservoir quality and insights for 3D geological modeling

Silicification and dissolution features in the brazilian Pre-salt Barra Velha formation: impacts in the reservoir quality and insights for 3D geological modeling

Geological process modeling and geostatistics for facies reconstruction of presalt carbonates

The presalt reservoirs of the Barra Velha Formation are complex and heterogeneous. Their deposition was controlled by a mixture of physical and chemical processes, which consequently affected the distributions of facies and variations in porosity and permeability. In this paper we present a presalt facies reconstruction of reservoirs within the Barra Velha Formation of the Búzios Field. This was achieved through the use of an innovative workflow that integrates geological process modeling with a truncated Gaussian simulation geostatistical algorithm with trends. Our results show that the structural highs are dominated by carbonate build-ups/mounds architecture seismic patterns which are dominated by spherulites and its intercalation with shrub sediments culminating in spherulitites and shrubby carbonate lithologies. Reworked sediments and facies are concentrated in regions dominated by debris seismic patterns and lake bottom seismic facies corresponding to laminites also occur within the study area. In chronostratigraphic terms the Barra Velha is muddier at the base transitioning to in-situ carbonates in the middle section, before becoming muddier again at the top. The western portion of the Búzios Field is mostly composed by shrubby carbonates and reworked facies whilst spherulitities are more abundant to the east intercalated with shrubby carbonates. Some uncertainties remain in relation to the parameters used in the proposed methodology. Nonetheless, the proposed workflow was effective for facies reconstruction of the presalt carbonates allowing not only a better understanding of sedimentological processes but also accurately distributing facies throughout the study area.

Sedimentology and petrophysical analysis of pre-salt lacustrine carbonate reservoir from the Santos Basin, southeast Brazil

Lacustrine carbonate deposits mark the rift-sag sedimentation stage of the so-called pre-salt section of the Campos and Santos Basins, in the Southeast Brazilian continental shelf. These salt-sealed carbonate reservoirs form deep-water, giant and prolific oil fields that are currently responsible for approximately 70% of the Brazilian oil and gas production. In the Santos Basin, the sag stage carbonates are represented by the Aptian Barra Velha Formation, characterized by exotic textural and compositional features whose origin, development, and correlation with petrophysical properties have been broadly discussed in the last decade. A geological and petrophysical study has been performed based on core samples, plugs, and thin-sections of a well in the Barra Velha Formation from the eastern Santos Basin. The analyzed core interval was grouped into six depositional facies constituted by the association of five key textural components: (1) crystal shrubs; (2) calcite spherulites; (3) hybrid forms; (4) mud-grade carbonates and dolomite; and (5) intraclasts. The relationship between the depositional framework and the poroperm system, as evidenced by petrographic image analysis and CT-scan data, is highly complex. The depositional control over porosity is erratic, mainly due to the extensive diagenetic and possible hydrothermal overprint. Dolomite cementation, dolomitization, pervasive silicification, and dissolution processes are the main primary porosity modifiers. Petrophysical analysis indicates that intraclastic and spherulitic grainstones–rudstones, affected by matrix dissolution, may present the best reservoir properties, in certain intervals, even surpassing the shrubby framestones, which are widely regarded as the best reservoir facies. This study seeks to contribute to an emerging body of research about the Brazilian pre-salt lacustrine carbonates, broadening the geological knowledge about these deposit, while providing new data to improve hydrocarbon forecast and recovery.

Pore typing using nuclear magnetic resonance, an example with samples from cretaceous pre-salt lacustrine carbonates in the Santos Basin, Brazil

The Brazilian sub-salt play carbonate reservoirs present highly complex and heterogeneous pore systems, making it difficult to define their petrophysical properties, which is essential for the reservoir characterization. Nuclear magnetic resonance (NMR) has emerged as an effective tool in the petrophysical characterization of carbonates, both on the core and log scales. This work uses plug samples from the Barra Velha Formation in the Santos Basin, SE Brazil, which is characterized by unconventional lacustrine carbonate deposits dominated by shrub-like forms and spherulitic allochems. The porosity types were defined based on NMR laboratory analysis associated with petrographic descriptions and high-resolution X-ray computed tomography. From the NMR results it is possible to define irreducible water saturation and calculate porosity and permeability for each sample. The results obtained through the resonance technique showed high correlation with the porosity and permeability values obtained by gas injection. Additionally, the interpretation of the data enabled the division of samples into porous and tight categories and the definition of groups with characteristics of tight, good and excellent reservoirs. The interpreted data contributes to a better understanding of the petrophysical properties of these complex and heterogeneous samples.

Environmental conditions of deposition of the Lower Cretaceous lacustrine carbonates of the Barra Velha Formation, Santos Basin (Brazil), based on stable carbon and oxygen isotopes: A continental record of pCO2 during the onset of the Oceanic Anoxic Event 1a (OAE 1a) interval?

This study presents new stable carbon and oxygen isotope data from Lower Cretaceous lacustrine carbonate rock samples recovered from a well drilled in the Santos Basin, offshore southeast Brazil. These samples represent a record of a continental environment just prior to the opening of the South Atlantic Ocean and the ultimate break-up of Gondwanaland. The geochemical data, along with carbonate mineralogy, indicate repeated cycles of lake level variation that could be attributed to climatic oscillations. Despite the absence of correlations between δ13C and δ18O values, facies analysis and the isotopic and mineralogical data suggest that lake hydrology was essentially closed for most of the depositional interval studied here. The existence of persisting trends of nearly constant δ13C values with a spread in δ18O values though, suggests long water residence times in the palaeolake, equilibrium between atmosphere and lake water CO2, as well as significant evaporation of water. The overall geological model that emerges unveils a more comprehensive picture of the depositional conditions that favoured the continuity of a significant carbonate factory in the middle of the Gondwanan continent, corroborating previous studies that suggested the lasting existence of a large and somewhat shallow endorheic lake in the area during the Early Cretaceous. As a result of this recorded trend strongly suggesting equilibrium between lake waters DIC (dissolved inorganic carbon) reservoir and atmospheric CO2, the data are most consistent with lacustrine deposition rather than precipitation of travertine, contrasting with some suggestions for the genesis of the carbonates of the Barra Velha Formation. Finally, this apparent equilibrium with the atmosphere likely left a preserved record in the continental carbonates of the final stages that preceded a major global environmental disturbance associated with an increase in atmospheric CO2, known for this time as the Oceanic Anoxic Event (OAE) 1a. If this is correct, it also helps to put further time constraints on this studied interval, which should not be younger than Barremian age, and to provide a regional continental perspective on a global event.

Facies classification and patterns of lacustrine carbonate deposition of the Barra Velha Formation, Santos Basin, Brazilian Pre-salt

Abstract The lacustrine carbonate rocks of the giant Pre-salt oil-fields in the Santos and Campos Basins differ in a number of important aspects from classic lake deposits. Prior work has described distinctive and unusual features of deposition, early diagenesis and reservoir quality with in these rocks that defy classification using established schemes. In addition to their unusual textures, debate continues as to how to interpret the vertical stacking pattern of facies. A new facies classification scheme is proposed based on the relative abundance of three end-member components: mud (micrite and clay minerals), calcite spherulites and fascicular calcite shrubs. Although focused on the main textural and mineralogical features of the rocks as seen today, the proposed scheme takes into account the relative contribution of depositional and early diagenetic components, sediment mineralogy and the degree of dissolution. Application of this scheme offers insight into depositional conditions, whilst also linking to reservoir quality. Basic (elementary) cycles can be defined using this classification in combination with the variation in the abundance of detrital grains and the chemical composition. Three possible models are suggested to interpret the origin of the basic cycle within the Barra Velha Formation in the context of variation in climate, lake level and water chemistry. Integration of data from different sources, including petrographic, core description and chemical data for the basic cycle, offers improved process-based understanding of metre-scale facies patterns and provides a foundation for understanding larger scale vertical variations within and between unconformity-bound sequences.

The textural evolution and ghost matrices of the Cretaceous Barra Velha Formation carbonates from the Santos Basin, offshore Brazil

The Aptian lacustrine carbonates of the Barra Velha Formation, offshore Brazil, have undergone a highly unusual paragenetic and diagenetic history and present significant terminological problems if current textural classifications are applied. Existing limestone classifications invoke the concept of textural maturity, whereby the ratio of matrix to grain is a critical property and can be used to infer energy levels and the degree of transport prior to deposition. In the Barra Velha Formation, some critical carbonate grains grew in a matrix which later dissolved, congruently, producing what appears to be primary intergranular porosity. In addition, apparent boundstone textures occur, not due to organic trapping or binding, but ones which can be shown to be diagenetic, and were also related in part to the former presence of labile matrices. Thus, applying some terms in existing classifications can lead to false interpretations, and to avoid misinterpretations criteria for identifying ghost (former) matrices are provided.

Presalt reservoirs of the Santos Basin: Cyclicity, electrofacies, and tectonic-sedimentary evolution

Presalt reservoirs of the Santos Basin accounted for more than 50% of Brazilian hydrocarbon production in the first two months of 2019. Its most important reservoirs are found in the Barra Velha Formation; thus understanding its genesis and geologic history is essential. This formation is composed of carbonates deposited in an alkaline lacustrine environment with a multiplicity of facies from boundstones and grainstones to mudstones. We have performed a sedimentary analysis based on the integration of sidewall core (SWC) samples and well logs from two wells coupled with seismic patterns discrimination to characterize the tectonic and depositional evolution of the Barra Velha Formation in a sector of the Outer High of the Santos Basin in this study. Our method initially consisted of the evaluation of well logs, aiming to identify shallowing and flooding upward cycles of the second and third orders for the paleoenvironmental conceptualization upper rift and sag phases. Then, we defined electrofacies through the integration of SWCs, gamma ray, and acoustic impedance logs using the crossplot approach. Finally, we described seismic patterns throughout the study area and correlated with results from well data analysis. Therefore, the Barra Velha Formation was subdivided into three zones: the lower, intermediate, and upper zones. Well 1 has a facies association characteristic of a proximal and stable environment during deposition, whereas well 2 initially presents in a distal environment sedimentation that evolves to a more proximal setting. This fact evidenced differences with respect to base-level variations indicating compartmentalization within the lacustrine environment of the upper rift and sag phase that corroborated with the detailed seismic pattern interpretation of the study area.

Modelling the impact of depositional and diagenetic processes on reservoir properties of the crystal-shrub limestones in the ‘Pre-Salt’ Barra Velha Formation, Santos Basin, Brazil

Abstract The Cretaceous Barra Velha Formation (‘Pre-Salt’) of the Santos Basin, Brazil, includes lacustrine crystal-shrub limestones which represent important hydrocarbon reservoirs. Neither single nor multi-phase flow behaviour of these unusual fabrics has been modelled to date. Here we quantify the impact of depositional and diagenetic processes on selected reservoir properties of the crystal-shrub limestones using highly flexible 3D synthetic pore-scale models that can explore parameter space. Rather than producing models validated to actual reservoir properties, this seeks to establish trends and identify emergent clustering behaviour. Shrubby aggregates nucleate from a surface to form upward and outward-growing, branching, fascicular optical calcite crystal fans that are diverse in size and form. The model allows exploration of the impact of primary growth parameters: number of bifurcations (or branching), shrub height, angle of cone (or fan) inflation as well as subsequent inter-shrub cementation and dissolution, on resultant porosity, permeability, and multiphase flow. Simulations show that depositional parameters, as well as two major diagenetic processes - cementation and dissolution - control porosity and permeability. In some cases, these trends diverge considerably from widely-referenced Kozeny-Carman (KC) porosity-permeability relationships. The angle of shrub inflation produces data clusters that traverse KC bands. By contrast, the number of bifurcations and the degree of dissolution follows KC trends, so providing guidance for potential rock type subdivision of similar lithologies. On this basis we propose that the crystal-shrub limestones in the Barra Velha Formation can be subdivided into three broad petrophysical reservoir rock types based on the degree of shrubby aggregate bifurcation and extent of dissolution. The trends explored here can be used to link conceptual and process-based geological models to data, so enabling more accurate prediction and upscaling of heterogeneous reservoir properties and their reservoir-scale distribution.

†Ellimma longipectoralis sp. nov. (Teleostei: Clupeomorpha: †Ellimmichthyiformes) from the Aptian of the Santos Basin, southeastern Brazil

We described herein †Ellimma longipectoralis sp. nov., a new paraclupeid fish from the Aptian laminites of the pre-salt section in the Barra Velha Formation, Santos Basin, southeastern Brazil. This new species is represented by a single articulated specimen with elongate body and expanded pectoral region. It differs from the other nominal species of †Ellimma (†E. branneri and †E. cruzae) mainly by having a long pectoral fin (with 10–11 fin rays, reaching the seventh pleural rib), a complex arrangement of the supraorbital sensory canal in the frontal, predorsal scutes ornamented with straight ridges, and edentulous jaws and palate. This is a new occurrence of †ellimmichthyiforms in the Early Cretaceous and in offshore sedimentary basin, extending their geographical distribution southwards in the Western Gondwana.

Evaporitic carbonates in the pre-salt of Santos Basin – Genesis and tectonic implications

Abstract Since the first discoveries of oil accumulations in the carbonate reservoirs of the Lower Cretaceous Barra Velha Formation of the Santos Basin, their genesis has been the subject of intense discussion. Different depositional models have been proposed, with some authors advocating a microbial origin, while others an abiotic origin as travertines or travertine-like rocks. This search for a modern carbonate analogue for these unusual rocks may have disregarded an intriguing alternative: an evaporative model supported by our petrographical and geochemical data. Evaporitic sediment patterns in endorheic basins, in a depositional framework of brine drawdown, are dominated by ephemeral salt pan hydrology. Bedded salt crusts form a stack of subaqueous aligned crystal beds at the sediment-water interface; salt grows diagenetically within saline and mud layers beneath the dry pan surface; finally laminated sediments are deposited. We observe a similar cyclic pattern in the pre-salt core we described: beds of shrub-like crystals and diagenetic spherulites growing in magnesium clay matrix are followed by laminites. The model we propose attempts to contribute solving the puzzle of Pre-Salt carbonate deposition by integrating sedimentology and geochemistry with basin tectonics. Our observations and basin tectonics point to the exciting possibility that the sag phase carbonates of the Santos-Campos basin were deposited as the initial/transitional stage of a closed sub-sea-level evaporite basin whose rising salinities culminated in the deposition of halite and bittern salts (carnallite, sylvite, tachyhydrite). Basinwide evaporites are singular events in the geological record. They form when a giant sub-sea-level basin becomes a sump of continental, marine and hydrothermal fluids, producing unexpected brine compositions in unusual depositional settings present in ancient evaporitic basins but absent from most modern ones. The novelty in this paper is that we apply to pre-salt carbonate deposition the sedimentology, hydrology and geochemistry of ancient evaporitic basins. With this concept in mind, we propose a geochemical model for the unusual pre-salt carbonate precipitation of the Santos Basin. Regarding the Walvis-Rio Grande volcanic high as a topographic barrier during the Aptian, we suggest that the main sources of calcium to the Barra Velha paleolake were hydrothermal brines, mostly from infiltrating seawater that reacted at depth with the basalts of the volcanic barrier. Hyperextension of the lithosphere, with a shallow Moho, increased geothermal gradients to levels favorable for the generation of hydrothermal CaCl2 brines, a process facilitated by the already elevated Ca++ ion and low sulfate concentration of the Cretaceous ocean. These CaCl2-rich fluids would pass through the volcanic barrier into a sub-sea-level paleolake in the rift basin. The high salinity paleolake would become saturated in calcium bicarbonate/carbonate as internally drained NaHCO3-bearing alkaline waters, formed by physical/chemical weathering of penecontemporaneous basaltic rocks, mix with the CaCl2-bearing hot brines percolating through the barrier, producing a hybrid brine (2NaHCO3 + CaCl2 = Ca(HCO3)2 + 2NaCl). Such a depositional system, integrating the geochemical-depositional controls of sag phase carbonates with those of the South Atlantic evaporites, has important implications for unravelling the genesis of these exotic deposits.

Unsupervised seismic facies classification applied to a presalt carbonate reservoir, Santos Basin, offshore Brazil

Mapping of seismic and lithological facies is a very complex process, especially in regions with low seismic resolution caused by extensive salt layers, even when only an exploratory view of the distribution of the reservoir facies is required. The aim of this study was to apply multi-attribute analysis using an unsupervised classification algorithm to map the carbonate facies of an exploratory presalt area located in the Outer high region of the Santos Basin. The interval of interest is the Barra Velha Formation, deposited during the Aptian, which represents an intercalation of travertines, stromatolites, grainstones and spherulitic packstones, mudstones, and authigenic shales, which were deposited under hypersaline lacustrine conditions during the sag phase. A set of seismic attributes, calculated from a poststack seismic amplitude volume, was used to characterize geological and structural features of the study area. We applied k-means clustering in an approach for unsupervised seismic facies classification. Our results show that at least three seismic facies can be differentiated, representing associations of buildup lithologies, aggradational or progradational carbonate platforms, and debris facies. We quantitatively evaluated the seismic facies against petrophysical properties (porosity and permeability) from available well logs. Seismic patterns associated with the lithologies helped identify new exploration targets.

Facies architecture of the fluvial Missão Velha Formation (Late Jurassic–Early Cretaceous), Araripe Basin, Northeast Brazil: paleogeographic and tectonic implications

Abstract Sedimentological analysis of the Missão Velha Formation (Araripe Basin, northeast Brazil) is the aim of this paper through detailed facies analysis, architectural elements, depositional systems and paleocurrent data. The main facies recognized were: (i) coarse-grained conglomeratic sandstones, locally pebbly conglomerates, with abundant silicified fossil trunks and several large-to-medium trough cross-stratifications and predominantly lenticular geometry; (ii) lenticular coarse-to-medium sandstones with some granules, abundant silicified fossil wood, and large-to-medium trough cross-stratifications, cut-and fill features and mud drapes on the foresets of cross-strata, (iii) poorly sorted medium-grained sandstones with sparse pebbles and with horizontal stratification, (iv) fine to very fine silty sandstones, laminated, interlayered with (v) decimetric muddy layers with horizontal lamination and climbing-ripple cross-lamination. Nine architectural elements were recognized: CH: Channels, GB: Gravel bars and bed forms, SB: Sand bars and bedforms, SB (p): sand bedform with planar cross-stratification, OF: Overbank flow, DA: Downstream-accretion macroforms, LS: Laminated sandsheet, LA: Lateral-accretion macroforms and FF: Floodplain fines. The lithofacies types and facies associations were interpreted as having been generated by alluvial systems characterized by (i) high energy perennial braided river systems and (ii) ephemeral river systems. Aeolian sand dunes and sand sheets generated by the reworking of braided alluvial deposits can also occur. The paleocurrent measurements show a main dispersion pattern to S, SE and SW, and another to NE/E. These features imply a paleodrainage flowing into the basins of the Recôncavo-Tucano-Jatobá.

Mawsoniid remains (Sarcopterygii: Actinistia) from the lacustrine Missão Velha Formation (Lower Cretaceous) of the Araripe Basin, North-East Brazil

Mawsoniids are a lineage of extinct fresh/brackish water coelacanth fishes, common in Cretaceous Godwanan deposits of South America, North and West Africa and Madagascar. Here we formally describe mawsoniid remains from the fluvio-lacustrine Missão Velha Formation (Lower Cretaceous) of the Araripe Basin, North-East Brazil. The examples from the Missão Velha Formation are here described as Mawsonia cf. gigas, based mainly on elements of the lower jaw and opercular series. The occurrence of M. cf. gigas in the Missão Velha Formation expands the stratigraphic and geographical ranges of the type species.

Diagenetic pathways linked to labile Mg-clays in lacustrine carbonate reservoirs: a model for the origin of secondary porosity in the Cretaceous pre-salt Barra Velha Formation, offshore Brazil

Abstract The lacustrine carbonate reservoirs of the South Atlantic host significant accumulations of chemically reactive and Al-free Mg-silicate minerals (e.g. stevensite, kerolite and talc). Petrographic data from units such as the Cretaceous Barra Velha Formation in the Santos Basin suggest that Mg-silicate minerals strongly influenced, and perhaps created, much of the observed secondary porosity. The diagenetic interactions between reactive Mg-silicate minerals and carbonate sediments are, however, poorly known. Here we develop a conceptual model for the origin of secondary porosity in the Barra Velha Formation guided by considerations of the chemistry that triggers Mg-silicate crystallization, as well as the geochemical and mineralogical factors that act as prerequisites for rapid Mg-silicate dissolution during early and late diagenesis. We conclude that sub-littoral zones of volcanically influenced rift lakes would have acted as the locus for widespread Mg-silicate accumulation and preservation. Organic-rich profundal sediments, however, would be especially prone to Mg-silicate dissolution and secondary porosity development. Here, organic matter diagenesis (especially methanogenesis) plays a major role in modifying the dissolved inorganic carbon budget and the pH of sediment porewaters, which preferentially destabilizes and then dissolves Mg-silicates. Together, the sedimentological, stratigraphic and geochemical predictions of the model explain many enigmatic features of the Barra Velha Formation, providing a novel framework for understanding how Mg-silicate–carbonate interactions might generate secondary porosity more broadly in other lacustrine carbonate reservoirs across the South Atlantic.