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Análise e correlação litostratigráfica de sondagens nos Grés de Silves da Bacia Lusitânica
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Este trabalho tem como objetivo estabelecer correlações entre as unidades litostratigráficas presentes nos poços exploratórios para a pesquisa de hidrocarbonetos, que atravessaram ou atingiram a Formação de Grés de Silves (datadas do Triásico, na Bacia Lusitânica), com as unidades definidas no bordo Este da Bacia. Estas correlações tornam possível indicar as condições ambientais de deposição dessas unidades litostratigraficas atravessadas por esses mesmos poços. Relativamente à Formação de Grés de Silves, esta, é datada do Triásico Superior ,e corresponde ao início da primeira fase de rifting, subsidência da bacia (Kullberg, et al. 2006) ,e consequente intenso estiramento crustal (Pimentel & Pena dos Reis, 2016). As unidades de Silves e da Dagorda (esta última da base do Jurássico Inferior, Hetangiano) marcam o início da sedimentação na bacia, e são sobrepostas pela Formação de Coimbra (Jurássico Inferior, Sinemuriano). É importante referir que o grupo de Silves foi inicialmente estudado por Palain (1976), e mais tarde foram definidas as formações de Conraria, Castelo Viegas, e a Formação de Pereiros (Rocha et al, 1996). As unidades atravessadas pelos poços analisados, foram comparadas com as sequências aflorantes no bordo Este da bacia, descritas e interpretadas por Palain (1976) e mais recentemente por Soares et al. (2012). Estes trabalhos foram cruciais para a elaboração desta dissertação, pois forneceram informação litológica e de litofácies (granularidade, estruturas sedimentares, rolamento e calibração dos grãos) utilizadas para comparar com as unidades atravessadas pelos poços , e para nelas definir termos correlacionáveis. Com intuito de tentar compreender os ambientes deposicionais das unidades litostratigráficas atravessadas pelos poços estudados na bacia Lusitânica, foram definidos 3 termos para agrupar as litologias: o termo I, o termo II e o termo III, respetivamente da base para o topo; estes termos foram definidos através de uma síntese litológica das unidades atravessadas pelos 9 poços apresentados (4 no Onshore e 5 no Offshore). Estes termos informais, foram então comparados com as formações do trabalho de Soares, e com os termos clássicos do trabalho de Palain (1976), contribuindo assim para a sua interpretação paleoambiental, paleogográfica e paleodeposicional. Foi possível perceber que o termo I é equivalente à Formação de Penela indicada no trabalho de Soares (2012) e à base do termo B1 do trabalho de Palain (1976); enquanto o termo II corresponde à Formação de Castelo Viegas e ao topo do termo B1 de Palain (1976); e por último, o termo III corresponde à base da Formação de Pereiros e ao termo B2+C1 de Palain (1976). Concluiu-se então que, o termo I tem características que indicam que o seu ambiente deposicional seja de um sistema fluvial meandriforme. Relativamente ao termo II, este termo não só é bastante heterogéneo em toda a sua espessura, sendo possível distinguir dois ambientes da base para o topo na sua maioria, como também está presente em todos as unidades atravessadas pelos poços analisados. Na base, o ambiente de deposição indica um sistema fluvial entrançado. No entanto variou no topo para um sistema fluvial meandriforme com sinuosidade variável. No termo III o ambiente foi claramente mais evaporitico, embora com influências terrígenas, apontando para a existência de Sabhkas ou Playas com acarreios terrígenos distais. A subsidência e preenchimento muito variáveis, evidenciam uma topografia irregular com blocos rodados, resultante da movimentação coeva das falhas tardi-hercínicas do substrato.
The goal of this thesis is to establish correlations between lithostratigraphic units studied in hydrocarbons exploratory wells that reached or went through the Grés de Silves Formation (dated back to Triassic, on Lusitanian Basin), and units defined in the Eastern border of the basin. These correlations make possible to indicate environmental conditions of deposition of these lithostratigraphic units that go through the wells. Regarding Grés de Silves formation, this formation is dated from the Upper Triassic which corresponds to the beginning of the first stage of rifting of the basin (Kullberg, et al. 2006) and subsequent intense crustal stretching/thinning (Pimentel & Pena dos Reis, 2016). Silves and Dagorda units (being the latter of the early Lower Jurasic - Hetangian) define the beginning of the basin sedimentation and are overlapped by the Coimbra Formation units (Lower Jurassic, Sinemurian). It is important to mention that Silves’ group was initially studied by Palain (1976) and later its Formations were defined as Conraria, Castelo Viegas, and Pereiros (Rocha et al, 1996). The units analyzed in the wells were compared with the sequences that outcrops in the Eastern border of the basin, previously defined and interpreted by Palain (1976) and more recently by Soares and colleagues (2012). These two studies were crucial to this thesis because both provided lithofacies and lithological information (such as granularity, sedimentary structures, grains’ roundness and calibration) used to compare with the units in the wells and further define correlations. In order to try to understand the environmental conditions of the sedimentary deposition of all the units studied in the wells, 3 informal terms were defined based on similar lithologies: Term I, II and III, from the base to the top, respectively. These terms were defined through a lithological summary of the units crossed by the 9 studied wells (4 onshore and 5 offshore). These informal terms were then compared with the formations defined in Soares et al. (2012) and the classical terms of Palain (1976), contributing to its paleoenviromental and paleogeographic interpretations. It was possible to understand that Term I corresponded to the Penela Formation defined by Soares et al. (2012) and also to the base of B1 Term defined by Palain (1976); while Term II corresponded to Castelo Viegas Formation and also to the top of B1 Term defined by Palain (1976); lastly, Term III corresponded to the base of Pereiros Formation as well as B2+C1 Term from Palain (1976). It was concluded that Term 1 shows characteristics that indicate a depositional environment with a system of meandering fluvial channels. Regarding Term II, this term not only showed heterogeneity within its thickness, but also an evolution from a system of braided fluvial channels at the base, to a system of a meandering fluvial channels at the top. In Term III, the environment was clearly more evaporitic although with terrigeneous influences, pointing to the existence of Sabhkas or Playas with a far away terrestrial input. The large variability on subsidence and filling showed evidences of an irregular topography with rotated blocks, as a result of the reactivation of tardi-hercynean faults of the basement.
The goal of this thesis is to establish correlations between lithostratigraphic units studied in hydrocarbons exploratory wells that reached or went through the Grés de Silves Formation (dated back to Triassic, on Lusitanian Basin), and units defined in the Eastern border of the basin. These correlations make possible to indicate environmental conditions of deposition of these lithostratigraphic units that go through the wells. Regarding Grés de Silves formation, this formation is dated from the Upper Triassic which corresponds to the beginning of the first stage of rifting of the basin (Kullberg, et al. 2006) and subsequent intense crustal stretching/thinning (Pimentel & Pena dos Reis, 2016). Silves and Dagorda units (being the latter of the early Lower Jurasic - Hetangian) define the beginning of the basin sedimentation and are overlapped by the Coimbra Formation units (Lower Jurassic, Sinemurian). It is important to mention that Silves’ group was initially studied by Palain (1976) and later its Formations were defined as Conraria, Castelo Viegas, and Pereiros (Rocha et al, 1996). The units analyzed in the wells were compared with the sequences that outcrops in the Eastern border of the basin, previously defined and interpreted by Palain (1976) and more recently by Soares and colleagues (2012). These two studies were crucial to this thesis because both provided lithofacies and lithological information (such as granularity, sedimentary structures, grains’ roundness and calibration) used to compare with the units in the wells and further define correlations. In order to try to understand the environmental conditions of the sedimentary deposition of all the units studied in the wells, 3 informal terms were defined based on similar lithologies: Term I, II and III, from the base to the top, respectively. These terms were defined through a lithological summary of the units crossed by the 9 studied wells (4 onshore and 5 offshore). These informal terms were then compared with the formations defined in Soares et al. (2012) and the classical terms of Palain (1976), contributing to its paleoenviromental and paleogeographic interpretations. It was possible to understand that Term I corresponded to the Penela Formation defined by Soares et al. (2012) and also to the base of B1 Term defined by Palain (1976); while Term II corresponded to Castelo Viegas Formation and also to the top of B1 Term defined by Palain (1976); lastly, Term III corresponded to the base of Pereiros Formation as well as B2+C1 Term from Palain (1976). It was concluded that Term 1 shows characteristics that indicate a depositional environment with a system of meandering fluvial channels. Regarding Term II, this term not only showed heterogeneity within its thickness, but also an evolution from a system of braided fluvial channels at the base, to a system of a meandering fluvial channels at the top. In Term III, the environment was clearly more evaporitic although with terrigeneous influences, pointing to the existence of Sabhkas or Playas with a far away terrestrial input. The large variability on subsidence and filling showed evidences of an irregular topography with rotated blocks, as a result of the reactivation of tardi-hercynean faults of the basement.
Descrição
Tese de mestrado, Geologia (Estratigrafia, Sedimentologia e Paleontologia) Universidade de Lisboa, Faculdade de Ciências, 2020
Palavras-chave
Bacia Lusitânica Grupo de Silves Triássico Superior Ambientes deposicionais Teses de mestrado - 2020