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Coquinas are a sedimentary rock composed mainly of shells and their fragments with a carbonate or siliciclastic matrix. These rocks exhibit a high range of porosity and permeability values that are related to depositional and diagenetic processes. A series of coquina reservoirs were discovered in the Campos and Santos Basins in the eastern margin of Brazil and the understanding of the petrophysical properties of these rocks is essential for reservoir production. In this work, samples from the Morro do Chaves Formation (Sergipe–Alagoas Basin) and the Itapema Formation (Santos Basin) were selected to identify the porosity vs permeability relationship according to the biofabric characteristics and porosity control. The biofabric textures where classified into four groups according to taphonomic patterns and diagenetic modification: (i) well-sorted and preserved biofabric, (ii) well-sorted and altered biofabric, (iii) unsorted and preserved biofabric, and (iv) unsorted and altered biofabric. The porosity control varies in all groups from depositional to diagenetic with distinct pore types, pore sizes, and pore connectivity that are located in different global hydraulic elements. The analysis of the pore space was carried out through laboratorial measurements (gas injection) and by images from high-resolution computed tomography from plug samples, resulting in a pore network model where values of pore radius, pore throat radius, and coordination number were obtained. The best values of porosity and permeability are related to well-sorted altered biofabrics, whereas the lowest values, either with unsorted biofabrics, with low pore connectivity, or biofabrics that had the porosity reduced by compaction and cementation.

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.

The Barremian–Aptian Morro do Chaves Formation, NE Brazil, was deposited in lacustrine environments during the rifting between the African and South American plates. This formation contains shell beds (coquinas) interbedded with mudstone and sandstone deposits. Shell concentrations may be used to identify paleontological and paleoenvironmental data that allow to interpret the depositional energy, paleoenvironmental conditions and the accumulation history of the formation. The following features of the shell beds were analyzed: (i) taphonomic aspects (sorting, fragmentation, abrasion, rounding and orientation of the shells), (ii) sedimentological aspects (grain packing, abundance of shells, type of matrix and sedimentary structures) and (iii) stratigraphic aspects (bed dimension and shape, bounding surfaces). Two siliciclastic lithofacies (clay mudstones with mud cracks and laminated sandy mudstones) and four carbonate lithofacies (wackestones, packstones, grainstones and rudstones) are described based on the textural features. The carbonate facies were successively divided into six taphofacies (T1 to T6) with the help of an artificial neural Self-Organizing Map (SOM) and evaluated by non-metric multidimensional scaling (NMDS), based on taphonomic features of the carbonate lithofacies. Taphofacies and lithofacies formed at lacustrine margins. Clay mudstone layers with mud cracks and without shells fragments are interpreted as distal parts of alluvial fans or fluvial distributary systems at the edges of the lake system. Rudstones and packstones (taphofacies T1, T2, and T3) with fragmented and oriented shells, wave ripples and horizontal lamination correspond to upper shoreface deposits. Rudstones, packstones and wackestones (taphofacies T4, T5 and T6) that display shells with low signs of fragmentation and abrasion and laminated clay mudstone are interpreted as lower shoreface deposits. A depositional model of the coquinas of the Morro do Chaves Formation is proposed based on the characteristics and distribution of the taphofacies. Moreover, the use of an embedded Markov Chain enabled the relation of the different upper and lower shorefaces in a preferential sequence of deposits of shallowing- and deepening-upward cycles.

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.

The Itapema Formation in the Santos Basin is described as a carbonate coquina interval, a type of sedimentary rock composed mainly by shells and their fragments. These rocks have different textural characteristics related to depositional and diagenetic processes, which influence the pore system and consequently the permeability. Understanding how the different facies are distributed and their relationship with petrophysics is essential to preview the quality of a reservoir. In this study, we classify the shell beds of the Itapema Formation through taphonomic characteristics, applying the concept of taphofacies, where taphonomic patterns such as orientation of shells, degree of packing, fragmentation of shells, abrasion, rounding, and sorting (type of grains) are used to differentiate shell concentrations. Six taphofacies (Tf-1 to Tf-6) were described in a core interval of 12 m, which corresponds to the Itapema Formation. Taphofacies Tf-1 and Tf-2 are well-sorted and unsorted grainstones/rudstones with parallel-oriented and densely packed shells with a braded valves. Taphofacies Tf-3 and Tf-4 are well-sorted and unsorted grainstones/rudstones with oblique oriented shells, generally well-preserved but often with high shell dissolution. Taphofacies Tf-5 and Tf-6 are well-sorted and unsorted grainstones/rudstones with randomly oriented shells and densely packed deposits. In general, all the well-sorted taphofacies are composed mainly of shells and their fragments, do not show mud matrix or even shell fragments smaller than 0.2 mm, the size of valves varies from 0.5 to 5 mm and shell fragments are the smallest components. The unsorted taphofacies contain grains smaller than 0.2 mm, most of them peloids, and very small shell fragments. Through the analysis of thin sections, x-ray tomography (CT), high resolution tomography (H-CT) and Nuclear Magnetic Resonance (NMR) it was possible to quantify the porosity in the described taphofacies and classify them according to Choquette and Pray (1970) and Ahr (2008). In well-sorted taphofacies primary pores are preserved; the total porosity varies from 9% to 21% in thin sections, and mainly consists of interparticle, intraparticle, and moldic pores and vugs. The control of porosity varies from depositional to hybrid 1, according to Ahr (2008), and pores are generally connected. In unsorted taphofacies, preserved primary porosity is uncommon and most pores are moldic and vugs. The total porosity in this facies varies from 2 to 15%, although almost all values are between 10% and 15%. The control of porosity varies from hybrid 1 to diagenetic, based on Ahr (2008), and the connectivity varies. Despite these textural differences between well-sorted and unsorted taphofacies, in general, the porosity is good and varies between 10% and 25%. The permeability in well-sorted taphofacies varies from 0.3 to 8 D, the high values being associated with a connected porous system, whereas in unsorted taphofacies the permeability varies from 0.01 to 0.4 D due to the occurrence of isolated vugs and moldic pores. The well-sorted coquinas were interpreted as deposits influenced by storm-induced currents and waves where the energy was sufficient to winnow the finer grains and preserve the primary pores. The taphonomic characteristics indicate tempestites to shoreface deposits, whereas in unsorted taphofacies the energy was not sufficient to remove finer grains generally associated with distal tempestites and backshore deposits.

The recent increase in the use of X-ray microtomography (I¼-CT) for reservoir rock characterization can be explained by numerous factors, such as its non-destructive nature, higher spatial resolution and 3D pore space visualization, which were explored in this work to evaluate the pore system of coquinas, a potential reservoir rock mainly composed of shells and their fragments. However, most of the recent studies have not considered an association between petrophysical parameters extracted via I¼ -CT and coquina facies. For this reason, this work had the goal to characterize the pore types, quantify total porosity, obtain the porosity profile, analyze the pore and pore-throat size distribution, as well as to extract additional petrophysical parameters of different taphofacies from Morro do Chaves Formation coquinas, Sergipe-Alagoas basin. The results haven shown that taphofacies from shallow sub-environment under normal conditions (group T2) and deeper sub-environment under storm influence (group T5) are better in terms of reservoir quality. Nevertheless, rocks from storm influence, deeper sub-environments are more likely to represent a good reservoir, since its pore system is predominantly dominated by moldic pores, which are originated during eogenetic phase, while rocks from shallow normal conditions have pores dominantly generated during telogenesis. Additionally, I¼-CT derived data such as coordination number and pore and pore-throat sizes could also be used to explain differences in absolute permeability in the studied rocks. Nevertheless, our data suggests that coquinas have a multiscale pore system and finer imaging scales are indispensable for more accurate petrophysical characterization . Keywords : Coquinas, I¼-CT, petrophysics. RESUMO. A crescente utilizacao da microtomografia de raios-X (I¼-CT) visando a caracterizacao das rochas reservatorio pode ser explicada por diversos fatores, como sua natureza nao-destrutiva, alta resolucao espacial e visualizacao 3D do espaco poroso, que sao propriedades exploradas nesse trabalho para avaliar o sistema poroso de coquinas, uma potencial rocha reservatorio composta principalmente por conchas e seus respectivos fragmentos. Entretanto, a maioria dos estudos recentes nao tem associado os parâmetros petrofisicos com as facies de coquinas. Por essa razao, esse trabalho buscou realizar a caracterizacao dos tipos de poros, quantificar a porosidade total, obter o perfil de porosidade, analisar a distribuicao do tamanho de poros e gargantas, assim como extrair parâmetros adicionais de diferentes tafofacies das coquinas da Fm. Morro do Chaves, Bacia de Sergipe-Alagoas. Os resultados mostraram que as tafofacies dos subambientes rasos sob condicoes normais de deposicao (grupo T2) e dos subambientes profundos sob influencia de tempestades (grupo T5) sao melhores em termos de qualidade de reservatorio. No entanto, as rochas de subambientes profundos sob influencia de tempestades sao mais provaveis de representarem bons reservatorios, pois seu sistema poroso e predominantemente dominado por poros moldicos, que sao originados durante a fase eogenetica, enquanto que as rochas depositadas no subambiente raso em condicoes normais possuem poros gerados durante a telogenese. Adicionalmente, dados derivados da I¼-CT, como numero de coordenacao e tamanho de poros e gargantas, tambem poderiam ser usados para explicar as diferencas em permeabilidade absoluta nas rochas estudadas. Entretanto, nossos dados sugerem que as coquinas possuem um sistema poroso multiescalar e o imageamento em escalas mais finas para uma caracterizacao petrofisica mais acurada e indispensavel. Palavras-chave: Coquinas, I¼-CT, petrofisica

Orientador: Alexandre Campane Vidal Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Geociências Resumo: As coquinas da Formação Morro do Chaves são consideradas rochas análogas aos depósitos de coquinas da Bacia de Campos, essas rochas foram formadas no Cretáceo Inferior e possuem idades entre Barremiano-Aptiano. Nos últimos anos, devido a descoberta de reservatórios em coquinas no pré-sal, diversos estudos estão sendo realizados para compreender como essas rochas se formaram e como se distribuem ao longo de uma bacia. Essas rochas carbonáticas são constituídas principalmente por conchas e possuem diferentes características texturais de acordo com o ambiente em que foram formadas. A concentração, preservação e distribuição das conchas possuem um significado importante na assinatura do sistema deposicional, ao analisar a sua estrutura é possível identificar as condições ambientais e energéticas que levaram a sua formação. A base para essa análise consiste na observação de algumas características: I ¿ tafonômicas, como articulação, seleção, fragmentação, abrasão, arredondamento e orientação das conchas; II - sedimentológicas, através de estruturas sedimentares, empacotamento de grãos, abundância de conchas e tipo de matriz; III ¿ estratigráficas, sendo elas extensão lateral das camadas, tamanho das camadas, sua geometria e contato físico entre camadas. Analisando as coquinas contidas na Formação Morro do Chaves foi possível identificar diferentes fácies deposicionais, dessa forma, um modelo de tafofácies foi concebido. Cada um dos ambientes apresentam características distintas em aspectos tafonômicos e estratigráficos envolvendo wackestones, packstones, grainstones e rudstones, que se sobrepõe de acordo com a oscilação nível de água do lago. Foi possível identificar diferentes fácies que envolvem coquinas depositadas em ambientes localizados acima do nível de onda de tempo bom e coquinas de tempestade, apresentando características distintas quanto ao estado de preservação e disposição de valvas como também a ocorrência de estruturas sedimentares e os tipos de contatos entre suas camadas. Coquinas de tempo bom apresentam altas taxas de fragmentação, abrasão e empacotamento, enquanto coquinas de tempestade apresentam sinais moderados a baixo de fragmentação, abrasão e arredondamento. A segunda etapa desse trabalho consiste na análise de perfil de raios gama. Utilizando um gamaespectrômetro portátil foi possível levantar um perfil vertical do comportamento das rochas aflorantes, identificando ciclos deposicionais que relacionam coquinas e folhelhos, onde é possível identificar ciclos de shallowing-upward indicando transições de wackestones para rudstones. O perfil levantado foi comparado com dois poços do Campo de Furado localizados próximos a pedreira, onde foi identificada a distribuição lateral das camadas e com isso gerar um modelo 3D com a distribuição dessas rochas Abstract: The coquinas of Morro do Chaves Formation are considered analogous to coquinas deposits in the Campos Basin, this rocks were formed during the same geological period in the Lower Cretaceous from Barremian to Aptian. In recent years, due the discovery of reservoirs of coquinas in the pre-salt of Campos Basin, several studies are being conducted to understand how these rocks were formed and how they are distributed. These carbonate rocks are mainly formed by shells, and have different textural characteristics according to the depositional environment in which they were formed. The concentration, preservation and rocks shells distribution have an important significance in the signature of depositional system, to analyze its structure is possible to identify the environmental and energy conditions that led to its formation. The basis for this analysis is the observation of some characteristics, such as: I - Taphonomic as selection, fragmentation, abrasion, rounding and orientation of the shells; II - sedimentological, through sedimentary structures, grain packing, plenty of shells and type of matrix; III - stratigraphic, lateral extension of the layers, layers size, geometry and physical contact between layers. Analyzing coquinas contained in the Morro do Chaves Formation was possible to identify different depositional facies and thus a taphofacies model was designed. Each environment has distinct characteristics involving wackestones, packstones, grainstones and rudstones, which overlaps according to the oscillating water level of the lake. It was possible identify different facies involving coquinas deposited in environment localized above the level of fair-weather wave base and storm coquinas, showing different characteristics about the state of preservation and arrangement of valves as well as the occurrence of sedimentary structures and layers contacts. Fair weather coquinas displays high signals of fragmentation, abrasion and packing, whereas storm coquinas show a moderate and low signals of fragmentation, abrasion. The second stage of this work is the profile analysis of gamma rays. Using a portable gamma spectrometer was possible build a vertical profile of the behavior of outcropping rocks, identifying depositional cycles that relate coquinas and shales, where you can identify cycles of shallowing upward showing wackestones transitions to rudstones. The raised profile was compared with two wells of Furado Field located near quarry, where we identified the lateral distribution of the layers and thus generate a 3D model with the distribution of these rocks Mestrado Geologia e Recursos Naturais Mestre em Geociências CNPQ 132487/2014-4