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Previous issue date: 2005-07-01 === Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico === The structural framework of the sedimentary basins usually plays an important role in oil prospects and reservoirs. Geometry, interconectivity and density of the brittle features developed during basin evolution could change the permo-porous character of the rocks involved in generation, migration and entrapment of fluid flow. Once the structural characterization of the reservois using only sub-surface data is not an easy task, many studies are focused in analogous outcrops trying to understand the main processes by which brittle tectonic is archieved. In the Santana do Acara? region (Cear? state, NE Brazil) a pack of conglomeratic sandstone (here named CAC) has its geometry controlled mainly by NE trending faults, interpreted as related to reactivation of a precambrian Sobral Pedro II Lineament (LSP-II). Geological mapping of the CAC showed a major NE-SW trending synform developed before its complete lithification during a dextral transpression. This region was then selected to be studied in details in order of constrain the cretaceous deformation and so help the understanding the deformation of the basins along the brazilian equatorial margin. In order to characterize the brittle deformation in different scales, I study some attributes of the fractures and faults such as orientation, density, kinematic, opening, etc., through scanlines in satellite images, outcrops and thin sections. The study of the satellite images showed three main directions of the macrostructures, N-S, NE-SW and E-W. Two of theses features (N-S and E-W) are in aggreement with previous geophysical data. A bimodal pattern of the lineaments in the CAC?s basement rocks has been evidenciated by the NE and NW sets of structures obtained in the meso and microscale data. Besides the main dextral transpression two others later events, developed when the sediments were complety lithified, were recognized in the area. The interplay among theses events is responsible for the compartimentation of the CAC in several blocks along within some structural elements display diferents orientations. Based on the variation in the S0 orientation, the CAC can be subdivided in several domains. Dispite of the variations in orientations of the fractures/faults in the diferents domains, theses features, in the meso and microscopic scale, are concentrated in two sets (based on their trend) in all domains which show similar orientation of the S0 surface. Thus the S0 orientation was used to group the domains in three major sets: i) The first one is that where S0 is E-W oriented: the fractures are oriented mainly NE with the development of a secondary NW trending; ii) S0 trending NE: the fractures are concentrated mainly along the trend NW with a secondary concentration along the NE trend; iii) The third set, where S0 is NS the main fractures are NE and the secondary concentration is NW. Another analized parameter was the fault/fracture length. This attribute was studied in diferent scales trying to detect the upscale relationship. A terrain digital model (TDM) was built with the brittlel elements supperposed. This model enhanced a 3D visualization of the area as well as the spatial distribution of the fault/fractures.
Finally, I believe that a better undertanding of the brittle tectonic affecting both CAC and its nearby basement will help the future interpretations of the tectonic envolved in the development of the sedimentary basins of the brazilian equatorial margin and their oil reservoirs and prospects, as for instance the Xar?u field in the Cear? basin, which subsurface data could be correlated with the surface ones === Grande parte dos prospectos e reservat?rios de petr?leo tem o seu arcabou?o estrutural como um fator de grande import?ncia. A geometria, interconectividade e densidade dos elementos da tect?nica fr?gil (falhas, fraturas, etc.), t?m grande influ?ncia no car?ter permo-poroso do meio e, por conseguinte, no fluxo de fluidos. Tendo em vista as dificuldades encontradas para a caracteriza??o da deforma??o fr?gil, unicamente com dados de subsuperf?cie, v?rios estudos est?o sendo direcionados ao detalhamento das por??es emersas das bacias e de seus substratos, em busca de an?logos da deforma??o. Na Regi?o de Santana do Acara? aflora um corpo aren?tico conglomer?tico (CAC) cuja geometria ? controlada por falhas, principalmente de trend NE, interpretadas como decorrentes da reativa??o do Lineamento Sobral Pedro II (LSP-II). A fim de caracterizar a deforma??o fr?gil em diferentes escalas, estudou-se os atributos do fraturamento tais como: orienta??o, densidade, cinem?tica, abertura, etc., atrav?s de scanlines em imagens de sat?lite, afloramentos e se??es delgadas. O estudo das imagens de sat?lite mostrou que as macroestruturas da ?rea apresentam tr?s dire??es preferenciais sendo elas N-S, NE-SW e E-W. As dire??es N-S e E-W s?o compat?veis com movimento de blocos observados por estudos gravim?tricos. O estudo do CAC mostrou que ele apresenta uma estrutura sinformal alongada na dire??o NE-SW, fruto de uma tect?nica transpressional dextral desenvolvida enquanto o corpo ainda n?o estava completamente litificado. Foi evidenciada a exist?ncia de duas outras fases de reativa??o, ocorridas quando o CAC j? se encontrava totalmente litificado. A tect?nica fr?gil compartimentou o CAC em blocos, cujo basculamento, gera varia??es na orienta??o de alguns de seus elementos (S0, por exemplo). Com base na varia??o da orienta??o de S0, o CAC pode ser subdividido em v?rios dom?nios. Do ponto de vista da orienta??o do fraturamento/falhamento os estudos na meso e microescala mostraram que as rochas do embasamento do CAC possuem um padr?o de lineamentos basicamente bimodal (NE e NW), enquanto no CAC observou-se que, embora haja uma distribui??o do fraturamento em varias dire??es, em todos os dom?nios, a concentra??o ao longo de dois trends espec?ficos nos dom?nios que apresentam as mesmas orienta??es de S0 ? marcante, caracter?stica e distinta. Os diversos dom?nios s?o agrupados em tr?s conjuntos, onde o conjunto que apresenta S0 praticamente NW mostra um fraturamento principal com trend NE e secund?rio NW, enquanto que o conjunto apresentando S0 com dire??o NE, mostra uma maior concentra??o dos lineamentos ao longo dos trend NW e secundariamente com trend NE. No conjunto de dom?nios com S0 praticamente N-S, o trend principal do fraturamento ? NE e o secund?rio NW. Outros atributos do fraturamento, tal como o seu comprimento, foi tamb?m analisado e comparado em diversas escalas buscando verificar se existia uma rela??o de upscale. O desenvolvimento de um modelo digital de terreno, com as estruturas fr?geis superpostas, propiciou uma vis?o tridimensional da regi?o estudada. O entendimento da deforma??o fr?gil que atingiu o corpo aren?tico-conglomer?tico (CAC) da regi?o de Santana do Acara? (CE) e seu substrato reveste-se de import?ncia pela presen?a de reservat?rios fraturados nas bacias da margem equatorial brasileira (Campo de Xareu na bacia do Cear?, por exemplo), cujos dados podem ser confrontados com os de superf?cie
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