Opening-mode fractures—joints and veins—are widespread structures in sedimentary rocks even in slightly deformed and flat-lying sequences. Understanding the growth and connectivity of fractures in low permeability sandstone reservoirs is essential for optimal hydrocarbon exploitation. In a linear elastic fracture mechanics framework, it is generally assumed that fractures widen in aperture while they propagate in length or height. However, it is also conceivable that a phase of proportional aperture to length or height growth is followed by a phase of aperture growth with relatively slow or arrested tip propagation. Slow propagation relative to aperture opening can occur by non-elastic deformation processes or if the material elastic properties change over time. Fracture propagation in length or height can be halted by material strength heterogeneities. To test for concurrent length versus aperture growth of these fractures, I reconstructed the crack-seal opening history for multiple cement bridges sampled at different distances from the tip of three opening-mode fractures in Travis Peak Sandstone of the SFOT-1 well, East Texas. Crack-seal cement bridges have been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement that bridges the fractures. Crack-seal cement textures were imaged using a scanning electron microscope with a cathodoluminescence detector, and the number and thickness of crack-seal cement increments determined. Trends in crack-seal increments number and thickness are consistent with fast initial fracture propagation relative to aperture growth, followed by a stage of slow propagation and pronounced aperture growth. Cumulative fracture opening displacement based on palinspastic reconstruction of two cement bridges was compared to analytical solutions for a stationary and a propagating fracture aperture as a function of position relative to the fracture tip in an elastic medium. Based on this comparison, I conclude that the crack-seal cement record reflects largely the phase of dominant aperture growth and subcritical fracture propagation under constant loading stress. / text

Orientador: Peter Christian Hackspacher / Banca: Luiz Sergio Amarante Simões / Banca: Nelson Angeli / Banca: Carlos José Archanjo / Banca: Marcos Egydio da Silva / Resumo: A zona de cisalhamento Itapetim (ZCI) desenvolveu foliação e lineação nos diferentes tipos de rochas, milonitos a ultramilonitos. Muitos dos tipos de microestruturas observadas na ZCI preservam evidências de condições metamórficas de alto grau e baixo grau de temperatura. O padrão das orientações preferenciais dos eixos [c] de quartzo, obtida a partir de agregados policristalinos deformados, mostra dois diferentes tipos de simetrias, guirlandas do tipo I e simples. A análise da textura sugere partição da deformação com cisalhamento coaxial e não coaxial na ZCI. Um veio de quartzo mineralizado em ouro encaixado em rocha ultramilonítica, constitui a zona de precipitação hidrotermal. O veio é bandado por segmentos de turmalina, o qual inclui agregados de grãos de quartzo maciço entre as bandas. Os grãos de quartzo mostram contatos poligonais e restritos limites intumescentes. O último sugere recristalização dinâmica por migração de limite de grão e os grãos poligonais (microestrutra em espuma) são melhor explicados por annealing. Os padrões de eixos [c] de quartzo obtido no veio aurífero é idêntico ao observado nos ultramilonitos da ZCI. Este padrão é interpretado como uma textura preservada formada durante o cisalhamento não coaxial. A estrutura bandada é interpretada como o resultado de variações da pressão de fluidos e a formação microfraturas, associadas a eventos de crack-seal. Mecanismos de crack-seal foram ativos antes ou durante a deformação da ZCI. As feições microestruturais suportam evidências para a última possibilidade. / Abstract: The Itapetim shear zone(ISZ) developed both foliation and lineation in the different rock types, from mylonites to ultramylonites. Most of the microstructure types observed in the ISZ preserve evidence of high temperature metamorphic conditions. The quartz [c]-axis, of deformed polycrystalline aggregates, show two distinct symmetries, type I crossed girdles and single girdles. The geometrical analysis of texture, suggest non-coaxial shear for deformation in ISZ. A gold mineralized quartz vein hosted in ultramylonitic rocks, constitutes the hydrothermal precipitation zone. The vein is banded by tourmiline segments, which include massive quartz grains aggregates between the bands. The quartz grains show polygonal contacts and minor intumescent limits. These latter suggest grain boundary migration recrystallization and the polygonal grains seem to be best explained by the annealing after stopping the shear deformation. The quartz [c]-axis obtained in the auriferous vein shows the same single girdle pattern observed in the ultramylonites of the ISZ. This pattern is interpreted as a preserved texture formed during the shear deformation. The banded structure is interpreted as the result of variations of the fluid pressure and the formation of microfractures, associated the crack-seal events. Crack-seal mechanisms can have been active previous or during the deformation of the IZS. The microstructurals features supports evidences for the last possibility. / Doutor

Les études géophysiques identifient une partition entre segments à comportement sismique et segments à comportement asismique le long des grandes failles actives californiennes. Il a été proposé que la présence des serpentinites le long de certains segments soit une cause potentielle d'un fluage asismique à ces endroits. Afin d'améliorer nos connaissances sur les mécanismes de déformation des serpentinites dans ce type de contexte, nous avons réalisé une étude des structures de déformation naturelles dans ces roches, collectées dans le réseau de failles actives de San Andreas (Californie). Le couplage de plusieurs techniques (microscope pétrographique, MEB, MET, microsonde) a été nécessaire pour préserver les informations au cours des changements d'échelles d'observations. Les serpentinites hors des zones cisaillées offrent des indices de leur histoire, antérieurement à la tectonique actuelle. L'étude détaillée des veines a permis, d'une part, une bonne caractérisation des textures optiques complexes classiquement rencontrées dans ces roches et, d'autre part, une meilleure compréhension des mécanismes de croissance des serpentines dans les interstices et les fractures ouvertes. Des effets capillaires pourraient faciliter la cristallisation dans des interstices de l'ordre du micron en permettant la mise en place d'un transfert de matière par diffusion en phase aqueuse depuis l'encaissant vers la veine. Ce mécanisme a été proposé pour expliquer la formation de veines fibreuses à ouverture lente ou saccadée (crack-seal). En revanche, les fractures ouvertes seraient nourries grâce à un transport advectif des éléments au sein d'un fluide. La précipitation serait alors permise par la chute de la solubilité du soluté au contact d'un encaissant relativement plus froid. La cinétique de croissance apparaît comme un facteur déterminant sur les conditions d'occurrence relative des différents types de serpentines représentés (lizardite et chrysotile), ce qui relativise le rôle des facteurs thermodynamiques classiques (pression et température). La déformation des serpentinites dans les failles naturelles, caractérisées par un taux de déformation lent et la présence de fluides, est très localisée, soit dans une zone de gouge, soit sur un plan de faille brillant. La déformation y est accommodée, respectivement, par un mécanisme de fluage par dissolution-cristallisation (glissement stable) et des épisodes de glissement fragiles instables (magnitudes faibles, M<0). Cela aboutit à un comportement globalement asismique des serpentines à l'échelle des failles actives.

Made available in DSpace on 2014-06-11T19:32:21Z (GMT). No. of bitstreams: 0 Previous issue date: 2003-08-15Bitstream added on 2014-06-13T20:23:37Z : No. of bitstreams: 1 almeida_hl_dr_rcla.pdf: 806140 bytes, checksum: 7940276ecd62a931d89c15464ccc97db (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / A zona de cisalhamento Itapetim (ZCI) desenvolveu foliação e lineação nos diferentes tipos de rochas, milonitos a ultramilonitos. Muitos dos tipos de microestruturas observadas na ZCI preservam evidências de condições metamórficas de alto grau e baixo grau de temperatura. O padrão das orientações preferenciais dos eixos [c] de quartzo, obtida a partir de agregados policristalinos deformados, mostra dois diferentes tipos de simetrias, guirlandas do tipo I e simples. A análise da textura sugere partição da deformação com cisalhamento coaxial e não coaxial na ZCI. Um veio de quartzo mineralizado em ouro encaixado em rocha ultramilonítica, constitui a zona de precipitação hidrotermal. O veio é bandado por segmentos de turmalina, o qual inclui agregados de grãos de quartzo maciço entre as bandas. Os grãos de quartzo mostram contatos poligonais e restritos limites intumescentes. O último sugere recristalização dinâmica por migração de limite de grão e os grãos poligonais (microestrutra em espuma) são melhor explicados por annealing. Os padrões de eixos [c] de quartzo obtido no veio aurífero é idêntico ao observado nos ultramilonitos da ZCI. Este padrão é interpretado como uma textura preservada formada durante o cisalhamento não coaxial. A estrutura bandada é interpretada como o resultado de variações da pressão de fluidos e a formação microfraturas, associadas a eventos de crack-seal. Mecanismos de crack-seal foram ativos antes ou durante a deformação da ZCI. As feições microestruturais suportam evidências para a última possibilidade. / The Itapetim shear zone(ISZ) developed both foliation and lineation in the different rock types, from mylonites to ultramylonites. Most of the microstructure types observed in the ISZ preserve evidence of high temperature metamorphic conditions. The quartz [c]-axis, of deformed polycrystalline aggregates, show two distinct symmetries, type I crossed girdles and single girdles. The geometrical analysis of texture, suggest non-coaxial shear for deformation in ISZ. A gold mineralized quartz vein hosted in ultramylonitic rocks, constitutes the hydrothermal precipitation zone. The vein is banded by tourmiline segments, which include massive quartz grains aggregates between the bands. The quartz grains show polygonal contacts and minor intumescent limits. These latter suggest grain boundary migration recrystallization and the polygonal grains seem to be best explained by the annealing after stopping the shear deformation. The quartz [c]-axis obtained in the auriferous vein shows the same single girdle pattern observed in the ultramylonites of the ISZ. This pattern is interpreted as a preserved texture formed during the shear deformation. The banded structure is interpreted as the result of variations of the fluid pressure and the formation of microfractures, associated the crack-seal events. Crack-seal mechanisms can have been active previous or during the deformation of the IZS. The microstructurals features supports evidences for the last possibility.