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Avaliação do potencial da Formação Cunga (Bacia do Kwanza, Angola) como reservatório não-convencional: implicações no armazenamento de dióxido de carbono
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A região de Cabo de São Brás, localizada na parte ocidental do onshore da Bacia do Kwanza em Angola, é representada por uma sequência sedimentar da Formação Cunga constituída por intercalações de rochas carbonatadas e siliciclásticas ricas em matéria orgânica, a qual apresenta uma espessura de cerca de 100 m e que aflora numa faixa ao longo de mais de 40 km no litoral angolano. O estudo procurou avaliar o potencial da Formação Cunga como rocha-mãe, assim como reservatório não-convencional para a sequestração/armazenamento de dióxido de carbono (CO2). No decurso do desenvolvimento deste trabalho foi efetuada uma campanha de amostragem perfazendo um total de 49 amostras. A composição mineralógica, cenário paleoambiental e potencial petrolífero e económico são apresentados com base em dados de deteção remota, difração de raios-X (DRX), petrologia e geoquímica orgânica e isotérmicas de sorção. Neste trabalho foram utilizadas as técnicas spectral angle mapper e análise da componente principal através de dados espectrais das bandas do sensor Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). A análise espectral implicou a identificação dos minerais calcite, caulinite, moscovite e ilite na região do infravermelho de ondas curtas, enquanto que a aplicação das bandas da região do infravermelho termal resultou na identificação do quartzo. Os resultados destes métodos foram validados com os dados de campo e análise dos dados de DRX. As amostras de campo submetidas a DRX indicaram que a sequência analisada, localizada na região de Cabo de São Brás, é Avaliação do Potencial da Formação Cunga (Bacia do Kwanza) como Reservatório Não-convencional constituída por duas importantes fases minerais, a calcite e o quartzo, além de outros minerais menos abundantes, tais como moscovite, clinoptilolite, albite e microclina. A análise das palinofácies estabeleceu uma distribuição quantitativa e qualitativa da matéria orgânica particulada exibindo três associações principais de palinofácies e ambientes de deposição. As palinofácies da associação CSB-1, dominadas por palinomorfos, e da associação CSB-2, representadas predominantemente por matéria orgânica amorfa, sugerem uma deposição em ambientes neríticos externos a oceânicos, prevalecendo para a associação CSB-1 condições oxidantes, e para a associação CSB-2 condições redutoras. Uma mudança de deposição externa/oceânica para um ambiente nerítico interno distal foi observada nas palinofácies da associação CSB-3, apresentando uma concentração elevada de fitoclastos opacos, o que sugere uma deposição sob um ambiente fortemente oxidante. Utilizando as ferramentas de análises da geoquímica orgânica (pirólise Rock-Eval), a avaliação do potencial petrolífero da sequência analisada aponta para rochas potencialmente geradoras de hidrocarbonetos, uma vez que os teores em carbono orgânico total (COT) são superiores a 0,5 %, incluindo intervalos com valores significativamente elevados de 7,52 %. O potencial residual de geração de hidrocarbonetos é, normalmente, representado pelos valores do pico S2, que nas amostras estudadas são de uma forma geral superiores a 5 mg HC/g rocha, correspondendo a um potencial petrolífero bom a excelente. O tipo ou qualidade da matéria orgânica comummente determinado através da relação índice de hidrogénio (IH) versus índice de oxigénio (IO) e juntamente com os dados das palinofácies da sequência analisada sugerem a presença de cerogéneos do tipo II/III, II e III. A determinação da maturação térmica da matéria orgânica foi efetuada a partir dos valores de temperatura máxima (Tmax) e índice de produção (IP), os quais sugerem que as amostras se encontram imaturas em relação à janela de geração de petróleo. O estudo das propriedades de sorção em rochas ricas em matéria orgânica foi utilizado para analisar a capacidade de armazenamento das diferentes amostras. As amostras utilizadas neste estudo apresentam diferentes capacidades de armazenamento de gás CO2, as quais variam entre 17,04 scf/ton e 64,11 scf/ton. No presente caso de estudo, as diferentes capacidades de armazenamento encontram-se intimamente relacionadas com o tipo de matéria orgânica identificada.
Neste sentido, a maior capacidade de armazenamento é identificada nas amostras com o conteúdo em vitrinite mais elevado em comparação com outros grupos de macerais, liptinite e inertinite. Este estudo permitiu concluir que a Formação Cunga, na Bacia do Kwanza, na região de Cabo de São Brás, possui características de reservatório de shale gas, que pode ser considerado um recurso a ser potencialmente explorado pela aplicação de técnicas de recuperação avançada de hidrocarbonetos com a injeção de CO2. Assim sendo, o presente estudo permitiu concluir que alguns dos níveis sedimentares da Formação Cunga na região de Cabo de São Brás apresentam um bom potencial como reservatório não-convencional para o armazenamento de CO2. Os resultados obtidos representam um avanço científico significativo no conhecimento paleoambiental da Bacia do Kwanza em Angola, bem como a definição dos horizontes temporais de interesse económico para o sector petrolífero do país.
The Cabo de São Brás region, located in the western part of the onshore Kwanza Basin in Angola, is represented by a sedimentary sequence of the Cunga Formation consisting of intercalations of organic-rich carbonate and siliciclastic rocks, which have a thickness of approximately 100 m, outcropping in a strip more than 40 km long Angolan coast. The study main goal is to assess the potential of the Cunga Formation as a source rock, as well as an unconventional reservoir for carbon dioxide (CO2) sequestration/storage. During the development of this work, a sampling campaign was carried out with the collection of 49 samples. The mineralogical composition, paleoenvironmental scenario and petroleum and economic potential are presented based on remote sensing, X-ray diffraction (XRD), organic petrology and geochemistry and sorption isotherms. In this work, spectral angle mapper techniques and principal component analysis were used through spectral data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor bands. Spectral analysis involved the identification of calcite, kaolinite, muscovite and illite minerals in the short-wave infrared region, while the application of thermal infrared bands resulted in the identification of quartz. The results of these methods were validated with field data and XRD data analysis. Samples submitted to XRD indicated that the sequence analyzed, located in the Cabo de São Brás region, is constituted by two important mineral phases, calcite and quartz, in addition to other less abundant minerals, such as muscovite, clinoptilolite, albite and microcline. The analysis of palynofacies established a quantitative and qualitative distribution of particulate organic matter showing three main associations of palynofacies and depositional environments. The palynofacies of the CSB-1 association, dominated by palynomorphs, and of the CSB-2 association, predominantly represented by amorphous organic matter, suggest a deposition in neritic environments external to oceanic, with oxidizing conditions prevailing for the CSB-1 association, and for the association CSB-2 reducing conditions. A shift from external/oceanic deposition to a distal internal neritic environment was observed in the palynofacies of the CSB-3 association, presenting a high concentration of opaque phytoclasts, suggesting a deposition under a strongly oxidizing environment. Using the analysis tools of organic geochemistry (Rock-Eval pyrolysis), the assessment of the petroleum potential of the analyzed sequence points out to rocks potentially generating hydrocarbons, since the contents of total organic carbon (TOC) are higher than 0.5 %, including significantly elevated ranges of 7.52 %. The residual potential for hydrocarbon generation is normally represented by the values of the S2 peak, which in the studied samples are generally above 5 mg HC/g rock, corresponding to a good to excellent petroleum potential. The type or quality of the organic matter, commonly determined through the hydrogen index (HI) versus oxygen index (OI) ratio and together with the palynofacies data, is kerogen of type II/III, II and III. The thermal maturation of the organic matter was determined from maximum temperature values (Tmax) and production index (PI), which suggest that samples are immature in relation to the oil generation window. The study of sorption properties in organic-rich rocks was used to analyze the storage capacity of different samples. The samples used in this study have different CO2 gas storage capacities, which vary between 17.04 scf/ton and 64.11 scf/ton. In the present case study, the different storage capacities are closely related to the quality of organic matter identified. So, the greatest storage capacity is identified in the samples with the highest vitrinite content compared to the other groups of macerals, liptinitis and inertinitis. This study allowed to conclude that the Cunga Formation, in the Kwanza Basin, in the Cabo de São Brás region, has characteristics of a shale gas reservoir, which can be considered a resource to be potentially exploited by applying advanced hydrocarbon recovery techniques with the CO2 injection. Therefore, this study allowed to conclude that some of the sedimentary levels of the Cunga Formation in the Cabo de São Brás region have a good potential as an unconventional reservoir for CO2 storage. The obtained results represent a significant scientific advance in the paleoenvironmental knowledge of the Kwanza Basin in Angola, as well as the definition of time horizons of economic interest for the country's oil sector.
The Cabo de São Brás region, located in the western part of the onshore Kwanza Basin in Angola, is represented by a sedimentary sequence of the Cunga Formation consisting of intercalations of organic-rich carbonate and siliciclastic rocks, which have a thickness of approximately 100 m, outcropping in a strip more than 40 km long Angolan coast. The study main goal is to assess the potential of the Cunga Formation as a source rock, as well as an unconventional reservoir for carbon dioxide (CO2) sequestration/storage. During the development of this work, a sampling campaign was carried out with the collection of 49 samples. The mineralogical composition, paleoenvironmental scenario and petroleum and economic potential are presented based on remote sensing, X-ray diffraction (XRD), organic petrology and geochemistry and sorption isotherms. In this work, spectral angle mapper techniques and principal component analysis were used through spectral data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor bands. Spectral analysis involved the identification of calcite, kaolinite, muscovite and illite minerals in the short-wave infrared region, while the application of thermal infrared bands resulted in the identification of quartz. The results of these methods were validated with field data and XRD data analysis. Samples submitted to XRD indicated that the sequence analyzed, located in the Cabo de São Brás region, is constituted by two important mineral phases, calcite and quartz, in addition to other less abundant minerals, such as muscovite, clinoptilolite, albite and microcline. The analysis of palynofacies established a quantitative and qualitative distribution of particulate organic matter showing three main associations of palynofacies and depositional environments. The palynofacies of the CSB-1 association, dominated by palynomorphs, and of the CSB-2 association, predominantly represented by amorphous organic matter, suggest a deposition in neritic environments external to oceanic, with oxidizing conditions prevailing for the CSB-1 association, and for the association CSB-2 reducing conditions. A shift from external/oceanic deposition to a distal internal neritic environment was observed in the palynofacies of the CSB-3 association, presenting a high concentration of opaque phytoclasts, suggesting a deposition under a strongly oxidizing environment. Using the analysis tools of organic geochemistry (Rock-Eval pyrolysis), the assessment of the petroleum potential of the analyzed sequence points out to rocks potentially generating hydrocarbons, since the contents of total organic carbon (TOC) are higher than 0.5 %, including significantly elevated ranges of 7.52 %. The residual potential for hydrocarbon generation is normally represented by the values of the S2 peak, which in the studied samples are generally above 5 mg HC/g rock, corresponding to a good to excellent petroleum potential. The type or quality of the organic matter, commonly determined through the hydrogen index (HI) versus oxygen index (OI) ratio and together with the palynofacies data, is kerogen of type II/III, II and III. The thermal maturation of the organic matter was determined from maximum temperature values (Tmax) and production index (PI), which suggest that samples are immature in relation to the oil generation window. The study of sorption properties in organic-rich rocks was used to analyze the storage capacity of different samples. The samples used in this study have different CO2 gas storage capacities, which vary between 17.04 scf/ton and 64.11 scf/ton. In the present case study, the different storage capacities are closely related to the quality of organic matter identified. So, the greatest storage capacity is identified in the samples with the highest vitrinite content compared to the other groups of macerals, liptinitis and inertinitis. This study allowed to conclude that the Cunga Formation, in the Kwanza Basin, in the Cabo de São Brás region, has characteristics of a shale gas reservoir, which can be considered a resource to be potentially exploited by applying advanced hydrocarbon recovery techniques with the CO2 injection. Therefore, this study allowed to conclude that some of the sedimentary levels of the Cunga Formation in the Cabo de São Brás region have a good potential as an unconventional reservoir for CO2 storage. The obtained results represent a significant scientific advance in the paleoenvironmental knowledge of the Kwanza Basin in Angola, as well as the definition of time horizons of economic interest for the country's oil sector.