Los campos Matachines, localizados en la subcuenca de Girardot (cuenca del valle superior del Magdalena), presentan una morfología antiformal con su eje dispuesto en rumbo submeridianal que involucra secuencias tectosedimentarias multifásicas en las que pueden reconocerse geometrías de hemigraben asociadas con las secuencias mesozoicas que han sido reactivadas por al menos tres fases andinas. Las superficies de estratificación son planos mecánicamente activos contribuyentes de los flexodeslizamientos que genera el plegamiento de secuencias multilitológicas. En los campos Matachines, la estratificación contribuye a mejorar las características petrofísicas de sus reservorios al incorporar una importante participación de discontinuidades que complementan la conectividad de los fluidos. Así, desde sus posiciones subhorizontales se vincula con los planos de fracturas subverticales dispuestos preferencialmente en dirección ENE-WSW, generados por los esfuerzos compresionales horizontales. En particular, estos efectos se magnifican sobre el flanco occidental y charnelar de la estructura anticlinal, donde exhiben una mayor participación en la conectividad de los pozos productivos. En los reservorios considerados naturalmente fracturados, la correcta evaluación técnico-económica 4D de la calidad y disposición de las discontinuidades es fundamental para determinar las mejoras petrofísicas que determinan. En este sentido, muchas metodologías habitualmente aplicadas en el estudio de pozos a partir de núcleos o imágenes reconocen, además de distintos tipos de fracturas, la presencia de superficies de estratificación. Sin embargo, se las evalúa separadamente, lo que no incide estadísticamente en las consideraciones petrofísicas que contribuyenan evaluar su porosidad y permeabilidad. Se discuten los aspectos genéticos y morfológicos de las superficies de estratificación que contribuyen a aumentar la potencialidad petrofísica de los reservorios, cuyos valores primarios de permeabilidad y porosidad suelen ser limitados, para sostener expectativas económicas satisfactorias de ciertos recursos en actividades de exploración o producción de hidrocarburos.
The Matachines field, located in the Girardot subbasin, Upper Magdalena Valley (Valle Superior del Magdalena – VSM) basin, has a morphology characterized by antiforms arranged along a N–S axis. These antiforms involve multipha-se tectono-sedimentary sequences with half-graben geometries associated with Mesozoic sequences reactivated by at least three Andean phases. Bedding surfaces are mechanically active planes that contribute to the flexural slip that is generated by the folding of multilithologic sequences. In the Matachines field, the bedding planes help to improve the petrophysical characteristics of the reservoirs by incorporating a significant number of discontinuities, which comple-ment the fluid connectivity. Thus, the subhorizontal planes are linked to subvertical fractures preferentially arranged in the ENE–WSW direction, generated by horizontal compressive stress. In particular, these effects are magnified on the western flank and hinge zone of the anticline structure, where they contribute greatly to the connectivity of the production wells. In naturally fractured reservoirs, the correct 4D technical and economic evaluation of the quality and arrangement of discontinuities is essential to determine their actual contribution to improving the petrophysical properties of such reservoirs. Accordingly, many methods often used to study wells from core samples or images identify, along with different types of fractures, the presence of bedding surfa-ces, although they evaluate them separately, so they overlook petrophysical factors that affect the porosity and permeability of the wells. The present study discusses the genetic and morphological aspects of bedding surfaces that enhance the petrophysical potential of reservoirs, which usually have limited primary values of permeability and porosity, to meet the economic expectations of specific resources in hydrocarbon exploration and/or production activities.
Revista de la Asociación Geológica Argentina, Vol. 77 Núm. 3 (2020)
Rosselia socialis were studied in the Ordovician Áspero Formation, in order to explore their sedimentary and diagenetic fingerprint in the substrate. These trace fossils are found forming crowded Rosselia ichnofabrics, described for the first time in pre-Quaternary strata of Argentina. We identified three microstructures corresponding to the central shaft complex, the burrow lining and the host rock of a trace fossil assigned to a terebellid polychaete. The infill of the central shaft complex represents downwards advection of surficial deposits located close to the burrow opening: fine-grained fecal mounds, and sandy mounds and lag deposits of manipulated, non-ingested material. Abundant phylosillicates in the central shaft complex and burrow lining evidence mechanical selection of particles with high specific surface area by the tracemaker. The fine-grained composition and multilayered organically bound structure of the burrow lining generate an impermeable and reinforced burrow, which combined with crowding grants physical and chemical stability to its inhabitants. This is especially advantageous in the high energy environments with shifting substrates where crowded Rosselia ichnofabrics are typically found. The central shaft complex and burrow lining are enriched in secondary iron minerals with respect to the host rock. Mineralized bacterial structures in the burrow lining evidence biologically induced precipitation of iron oxides and possibly sulphides. This coupled with the distribution of iron minerals in the burrow lining and central shaft complex suggests the occurrence of early diagenetic processes of organic matter decomposition and precipitation of authigenic iron minerals in Rosselia burrows, as observed in modern terebellid polychaetes.
La anisotropía de susceptibilidad magnética (AMS) es un método preciso para conocer la fábrica magnética y permite conocer la ubicación espacial de los esfuerzos regionales en áreas deformadas. La relación entre el elipsoide de AMS y su coaxialidad con el elipsoide de esfuerzos y deformación es conocida, donde el eje menor de AMS (Kmin) coincide con la dirección del esfuerzo principal σ1. A partir de la medición de la susceptibilidad magnética y la interpretación de la fábrica magnética interna de modelos análogos de arena, se puede hacer una aproximación sobre la distribución de los esfuerzos en zonas de deformación. Aquí presentamos los resultados de tres modelos análogos con diferentes mineralogías magnéticas. Los modelos se realizaron con el objetivo de evaluar la susceptibilidad magnética en diferentes posiciones del modelo y calcular el elipsoide de AMS y la relación entre la fábrica magnética y la mineralogía magnética, para comparar cuáles de las diferentes mineralogías son las mejores para los modelos posteriores.
Anisotropy of magnetic susceptibility (AMS) is a precise method to know the magnetic fabric and allows to know the spatial location of regional stresses in deformed areas. The relationship between the ellipsoid of AMS and its coaxiality with the ellipsoid of stress and deformation is known, where the minor axis of AMS (Kmin) coincides with the direction of the main stress σ1. From the measurement of magnetic susceptibility and the interpretation of the internal magnetic fabric of analogous sand models, an approximation can be made to the knowledge of the distribution of stress in deformation zones. Here we present the results of three analogues models with different magnetic mineralogies. They were performed with the objective of evaluating the magnetic susceptibility in different positions of the model and calculate the AMS ellipsoid and the relationship between the fabric obtained and the magnetic mineralogy, to compare which of the different mineralogies are the best to subsequent models.
This work focuses on the transition from the Las Leoneras to Lonco Trapial Formations, in the lower part of the Early Jurassic succession of the incipient rift phase of the Cañadon Asfalto Basin (western part of Chubut Province – Patagonia, Argentina). Twenty lithofacies have been identified and grouped into seven facies associations on the basis of field characteristics (sedimentological and lithological) and optical microscope analysis, from two localities representing proximal and distal locations in the basin. The spatial relationship between all the lithofacies provided a four‐dimensional reconstruction of the palaeoenvironmental evolution, showing how the original, clastic sedimentation in alluvial/fluvial and lake environments was modified by short‐lived volcanic events during three volcanic cycles, and how the environment reacted after the input of huge amounts of volcaniclastics. Progradation of small deltas and subaqueous lobes, retrogradation caused by rising lake levels, and frequent erosion of valleys were typical processes in this environment. When explosive volcanism began, the original tectono‐climatic control on sedimentation was replaced by the volcanic control, and the volcanically‐forced sedimentation broke the equilibrium among production, delivery and accumulation of sediments. The nature of the volcanic eruptions and the different propensity of volcanic lithofacies to produce particles of different size and types (lithics, crystals and glass) are also analyzed. The role of volcanism in the production and transport of great volumes of sediments across sedimentary systems needs to be carefully re‐examined, and the analysis on the variability in the composition of volcaniclastic deposits must take into account that volcaniclastic particle types may not simply reflect a linear deepening in the dissection of magmatic arcs through time but are often controlled by the style of the eruptions and the lithological variation of the volcanic products.
Although the subject “Oil and Gas” is not usually present in the Secondary School Curriculum, it is of interest to students due to its obvious relationships with everyday life, and has been selected to design a series of teaching resources. Not only natural sciences such as Geology, Biology and Chemistry are embedded in the formation and accumulation of petroleum, but also con-stant references to History, Geography and Technology are present when linking together its discovery, exploitation, industrial-ization and use. The materials being produced cover three categories: a reference text to be available in printed and PDF file versions, a portfolio of teaching resources and a website. The first two are described and exemplified. The portfolio also in-cludes examples of published materials, which contain conceptual errors proposing their use to explore misconceptions if used at the beginning of the presentation of the subject in class, or to identify levels of understanding if used at the end.
Jorge G. Lozano, Alejandro Tassone, Emanuele Lodolo, Marco Menichetti, María E. Cerredo, Donaldo M. Bran, Federico Esteban, Juan P. Ormazabal, Luca Baradello, Juan F. Vilas
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AndGeo vol.45 no.3 Santiago 2018
El lago Yehuin, una cuenca elongada de rumbo ONO-ESE localizada en la faja plegada y corrida externa de los Andes Fueguinos, ocupa una depresión compartimentada originada a lo largo de un segmento del sistema de fallas sinistrales del lago Deseado. Este trabajo describe un primer relevamiento geofísico llevado a cabo en el lago. Los datos de sísmica monocanal de alta resolución, integrados con información geológica de los alrededores del lago Yehuin, permitieron: (i) producir un mapa de la batimetría completa del lago, (ii) reconstruir la superficie del basamento del lago, y (iii) analizar la geometría, distribución y espesor del relleno sedimentario. Se reconocieron dos subcuencas dentro del lago Yehuin: una subcuenca oeste de 7,5 km de largo, con una profundidad máxima de 118 m; una subcuenca este de 7,2 km de largo y una profundidad máxima de 80 m. Ambas subcuencas están limitadas por un conjunto de fallas normales que cortan a una serie de corrimientos de vergencia NE. Se identificaron tres unidades sismo-estratigráficas en el registro sísmico: (1) una unidad inferior con geometría acuñada interpretada como depósitos de remoción en masa; (2) una unidad intermedia gruesa (de hasta 120 m) de origen glaciolacustre e irregularmente distribuida en la cuenca del lago; (3) una unidad superior lacustre delgada (<10 m) que cubre la cuenca entera. El lago Yehuin se considera una cuenca neógena e origen tectónico que fue luego afectada por depositación glaciaria y glaciolacustre. Se han interpretado morenas sumergidas dentro del lago Yehuin que se correlacionan con los arcos morrénicos en tierra y permiten completar el camino recesivo de los lóbulos de hielo Ewan y Fuego. Se propone un fuerte control estructural no solo para la formación del lago Yehuin, sino también para las rutas generales de los brazos norte del Paleoglaciar Fagnano.
Lago Yehuin, a WNW-ESE elongated basin located in the outer fold-and-thrust belt of the Fuegian Andes, occupies a compartmented structural depression originated along a segment of the left-lateral Lago Deseado fault system. This paper describes the first geophysical survey performed within the lake. New acquired high-resolution single-channel seismic data, integrated with geological information in the surroundings of the Lago Yehuin, allowed to: (i) produce a complete bathymetric map of the lake, (ii) reconstruct the basement surface of the lake, and (iii) analyze the geometry, distribution, and thickness of the sedimentary infill. Two sub-basins were recognized within Lago Yehuin: A western sub-basin, 7.5 km long, with a maximum depth of 118 m; an eastern sub-basin, 7.2 km long with a maximum depth of 80 m. Both sub-basins are limited by a set of normal faults which overprint NE-verging thrusts. Three seismostratigraphic units have been identified in the seismic records: (1) a lower unit with wedged geometry interpreted as a mass flow deposits; (2) a thick (up to 120 m) intermediate unit of glacio-lacustrine nature and irregularly distributed in the Yehuin basin; (3) a thin (generally <10 m) upper lacustrine unit which drapes the entire basin. Lago Yehuin is considered a Neogene basin generated by strike-slip tectonics that was later affected by glacial and glacio-lacustrine deposition. Interpreted submerged ridge moraines within Lago Yehuin are correlated with onland moraine arcs built by the complete recessional paths of Fuego and Ewan ice lobes. A significant structural control is proposed not only for the formation of Lago Yehuin, but also for the general paths of the northern arms of the Fagnano palaeo-glacier.
Constanza Rodriguez Piceda, Pablo Franceschinis, Mónica Escayola, Augusto Rapalini
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Revista de la Asociación Geológica Argentina, Vol 75, No 4 (2018)
Estudios sedimentológicos, de magnetofábrica y paleomagnéticos fueron realizados en la Formación La Pedrera (Grupo Santa Victoria) de edad ordovícica temprana en la Sierra de Mojotoro (Salta) para contribuir al conocimiento de la posición paleogeográfica de Pampia durante el Paleozoico temprano en relación a Gondwana. Se coleccionaron muestras orientadas en 12 sitios distribuidos en tres localidades. Estos afloramientos consisten en depósitos cuarcíticos de plataforma media a proximal. Los estudios de anisotropía de susceptibilidad magnética sugieren una fábrica depositacional en una de las localidades, mientras que en las otras dos, una cierta influencia tectónica. El estudio paleomagnético permitió determinar una componente magnética característica portada por hematita. Se obtuvo un polo paleomagnético para el Grupo Santa Victoria en estas localidades (340,4°E 38,3°N A95=8,8°) cuya posición sugiere que ninguna de las tres localidades sufrió rotaciones tectónicas andinas o anteriores significativas. Sin embargo, presenta una paleolatitud más baja que la esperada según los polos de referencia de similar edad para Gondwana. Esta diferencia no se debería a errores de inclinación por compactación. El polo del Grupo Santa Victoria discrepa con los polos obtenidos para el Grupo Mesón (Cámbrico Medio a Superior) y la Formación Santa Rosita (Ordovícico basal), al norte de la zona de estudio. Estas discrepancias pueden adjudicarse a la posible presencia de rotaciones tectónicas andinas según ejes verticales en dichas localidades o bien indicar que el movimiento de Pampia a lo largo del margen del Cratón del Río de la Plata ya habría concluido o estaba próximo a concluir en el Ordovícico temprano.
Leandro C. Gallo, Ernesto O. Cristallini, Marcela Svarc
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AGU Journals Advancing Earth and Space Science, 05 November 2018
The fitting of a plane to data points is essential to the geosciences. However, it is recognized that the reliability of these best fit planes depends upon the point set distribution and geometry, evaluated in terms of the eigen‐based parameters derived from the moment of inertia analysis. Despite its significance, few studies have addressed the uncertainties of the analysis, which can adversely affect the reproduction of results one of the cornerstones of scientific endeavor. Aiming to contribute toward the neglected issue of the moment of inertia precision, we have developed a bootstrap resampling scheme to empirically discover the distribution of uncertainties in the orientation of best fit planes. Dispersion of the bootstrapped normal vectors to the best fit plane is regarded as a measure of precision, evaluated with the maximum angular distance from the optimal solution. This rationale was tested using Monte Carlo‐generated samples covering a comprehensive range of shape parameters to assess the dependence between eigen parameters and their inherent bias. Our results show that the oblateness of the point cloud is a robust parameter to assess the reliability of the best fit plane. Given this, the method was then applied to a publicly available lidar data set. We argue that georeferenced point clouds with an oblateness parameter greater than 3 and 1.5 may be placed at 95% confidence levels of 5° and 10°, respectively. We propose using these values as thresholds to obtain robust best fit planes, guaranteeing reproducible results for scientific research.
Leal Pablo R.; Remesal Marcela; Salani Flavia M.; Cordenons Pablo D.
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Acta geológica l i l loana 28 (1) , 2016
En este trabajo se describen las texturas y la mineralogía de la secuencia volcánica de edad eocena que aflora entre Bajada del Diablo y Marra-Có, Chubut. La misma se compone de basaltos alcalinos donde se reconocen tres niveles texturalmente distintos. Los niveles superiores se caracterizan por la presencia de minerales secundarios entre los que destacan amígdalas rellenas de ceolitas con un notorio predominio de agregados aciculares de natrolita. Esta especie representa no solo el último pulso de relleno sino también el más importante. El estado de alteración de los minerales primarios y la paragénesis secundaria que rellena a las cavidades de los basaltos evidencian el desarrollo de un proceso de alteración hidrotermal con temperaturas inferiores a los 100°C y baja actividad de SiO2.
In this contribution we describe the textures and the mineralogy of eocene volcanic sequences from Bajada del Diablo and Marra-Có, Chubut province. They are composed of alkali basalt with three different sections. The two upper ones are characterized by the occurrence of secondary minerals among which amygdules filled by fibrous aggregates of natrolite are the most common ones. This species not only represents the last mineral precipitation but also the most important one. The alteration assemblages of primary minerals as well as the secondary assemblages, that fill each basalts’ cavities, suggest a very low-grade hydrothermal alteration with temperatures lower than 100°C and low silica activity.
Cordenons Pablo D.; Remesal Marcela B.; Salani Flavia M.; Cerredo Maria E.; Colombo Fernando; Guereschi Alina B.
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Acta geológica lilloana 28 (1), 2016
Las Vulcanitas Trayén Niyeu (VTN) son representantes de un inusual magmatismo calcoalcalino mioceno relacionado con el estadio postplateau de la Provincia Magmática Somún Curá (PMSC), de carácter mayormente alcalino. Cubren un área de 650 km2 en el margen noroeste del Macizo Nordpatagónico, determinando una zona de transición entre el ambiente de retroarco y el antepaís. Estudios texturales y de microsonda electrónica sobre una andesita permitieron establecer que la paragénesis en equilibrio químico con este líquido está compuesta por En78-80 y An43-52, mientras que individuos de composición An15-19 no se encuentran en equilibrio, desarrollando en grado variable un reborde de textura celular esponjosa de composición An30-38. A su vez, los cambios texturales observados en los minerales en equilibrio como la textura celular “boxy” en la plagioclasa cálcica y el reborde de clinopiroxeno en el ortopiroxeno, probablemente hayan estado relacionados al descenso de temperatura asociado a la disolución y recristalización de la plagioclasa sódica. En este marco, la evolución del magmatismo del cerro Trayén Niyeu debe involucrar procesos en los que interactúan magmas con propiedades físicas y/o químicas contrastantes, tales como mezcla de magmas por inyección, recarga o convección.
“Identification of disequilibrium paragenesis by electron microprobe analysis of the andesitic facies of Trayén Niyeu Volcanites, Somún Curá plateau, Extra-andean Patagonia”. The Trayén Niyeu Volcanites TNV) represent an unusual miocene calc-alkaline magmatism related to the postshield stage of the Somún Curá Magmatic Province, of mainly alkaline nature. They develop a 650 km2 mesa at the north-western margin of the North Patagonian Massif, defining a back-arc to foreland transition zone. Textural and EMPA studies on TNV andesitic facies allowed us to establish that the equilibrium paragenesis with this liquid is of En78-80 and An43-52 composition, whereas individuals of composition An15-19 are not in equilibrium, developing a more calcic (An30-38) rim with spongy cellular texture of varying intensities. The growing of a boxy cellular texture rim on plagioclase and a clinopyroxene epitactic overgrowth on the orthopyroxene, probably result from the temperature drop associated to the dissolution and recrystallization of the sodic plagioclase. In this regard, TNV’s magmatic evolution must involve processes that account for the interaction of magmas of contrasting physicochemical properties, such as magma mixing by injection, chamber replenishment or convection.
