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Germán Otharán
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Germán Otharán
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The Vaca Muerta Formation is a fine-grained marine stratigraphic unit accumulated during the Late Jurassic-Early Cretaceous in the Neuquén Basin, Argentina. This contribution presents a sedimentological and stratigraphic analysis of Vaca Muerta´s shales emerged from the integration of regional and detailed outcrop research and subsurface data gathered from the study of different well cores. The fieldwork comprised, on the one hand, the description of 7 stratigraphic sections of the Early Tithonian-Early Valanginian interval widely distributed across the western region of the Neuquén province and southern Mendoza province. The sedimentological and sequence-stratigraphic analysis of the different stratigraphic sections allowed integrating them into a regional N-S oriented correlation panel covering an area of 340 km. Five composite depositional sequences were recognized (GS1-GS5). They represent the evolution of a complex mixed shelf/ramp depositional system comprising from basinal/slope facies (central area) to ramp (northern area) and mixed-shelf to continental deposits (southern area). Depositional sequences are represented by thin, organic-rich (up to 10 % TOC), mudstone dominated transgressive cycles, whereas regressive cycles are commonly thicker and mainly composed of organic-lean, carbonate and mixed (carbonate/siliciclastic) facies. On the other hand, aiming at studying the transport and accumulation processes of Vaca Muerta´s organic-rich shales, hand specimen samples of early-diagenetic carbonate concretions were collected from the organic-rich basal condensed section deposited in basinal settings. Concretion samples were thoroughly examined (mm- μm scale) integrating the observation of macroscopic polished samples, thin sections and scanning-electron microscope analyses. The evidence found inside concretions suggest an origin related to muddy underflows, possibly triggered by slope failures on the western margin of the basin. The bedload and suspended-load transport of mud within these muddy underflows would have resulted in the accumulation of graded event mudstone beds exhibiting ripple lamination. Petrographic analysis demonstrates that muddy underflows would have been important mechanisms for reworking the seafloor and redistributing mud in basinal settings. Furthermore, muddy underflows would have been effective processes for organic matter concentration in organic-rich mudstone strata (up to ≈ 6 % TOC). The subsurface studies comprised the sedimentological analysis of seven well cores of the Vaca Muerta Formation, representing a total core data of 387 m. The descriptive facies analysis at centimeter scale allowed the recognition of a distally steepened mixed ramp system. Sediment delivery to basinal settings was mainly controlled by muddy underflows triggered by different mechanisms. Deposition from muddy underflows would have interacted with fallout processes from buoyant plumes and marine snow from the water column. The stacking pattern of the studied core deposits shows ≈ 0.5 to 3 m-thick parasequences building up prograding/retrograding parasequence sets of high-order depositional cycles (10-20 m thick).
Germán Otharán
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The accumulation of organic-rich mudstones was largely associated to low energy depositional environments with anoxic bottom waters, where mudstone deposition was mainly related to gradual and continuous mud fallout from dilute buoyant plumes. Based on this, basin inner zones were considered as unsuitable environments for source rock deposition due to the generally low concentration of OM resulting from both low inputs and production, adding an intense OM degradation during its transit time through the water column. Nevertheless, recent detailed sedimentological analyses in a variety of unconventional shales have revealed that the participation of fallout processes is probably subordinated to other still poorly known depositional processes, opening a new paradigm for source rocks origin. In Argentina, the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation is composed of organic-rich mudstones and carbonates dominated by type II kerogen representing South America's main unconventional reservoir. New cores and excellent outcrops provide a great opportunity to study the depositional history of this unit. These deposits were previously interpreted as accumulated by fallout deposition in a quiet and anoxic deep marine environment. However, recent studies revealed that the Vaca Muerta Formation is a highly heterogeneous stratigraphic unit accumulated by different and poorly know depositional processes. In fact, the formation displays distinct lithofacies alternating at centimeter to millimeter scale having variable organic matter content (up to 14% TOC), features that influence the reservoir quality and performance. High resolution sedimentological analysis were performed on relatively uncompacted intervals preserved in early diagenetic calcareous concretions collected from the basal deposits of the Vaca Muerta Formation in basinal settings. Evidences found in concretions suggest a deposition related to fluid mud flows instead of the classic model of "normal fallout". The triggered mechanisms for the origin of the recognized fluid mud flow deposits are mainly associated to direct river discharges during flood events. Each flood event would be capable of generating quasi-steady muddy hyperpycnal flows that may be sustained for days, weeks, or even months. These long-lasting events would be able to transfer significant volumes of organic matter and fine-grained sediments for long distances towards distal basinal settings. The erosion capacity of muddy hyperpycnal flows enables the incorporation of intrabasinal components (e.g. marine microfossils, carbonate mud, type II OM) which are transported together with the primary extrabasinal sedimentary load (e.g. detrital mud, micas, plant debris). The rapid and direct basinward transfer of OM by hyperpycnal flows would have avoided its dilution and degradation in coastal marine environments. Finally, the arrival of extinguishing hyperpycnal flows to the basin inner zones would have provided a fast deposition and burial of the OM, favoring its long term preservation. Therefore, muddy hyperpycnites would have a great potential for the accumulation of type II-III source rocks. The future understanding of the complexity of fluid mud flows and their internal stacking pattern will be crucial to identify long-term exploitable intervals in unconventional oil/gas plays.
Germán Otharán
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RESUMEN El rol de los flujos fluidos de fango en la acumulación de fangolitas bituminosas. Formación Vaca Muerta (Tithoniano temprano-Valanginiano temprano), Cuenca Neuquina, Argentina. Durante los últimos años, el avance de técnicas no convencionales de explotación de hidrocarburos puso especial interés en obtener una mejor comprensión de los procesos sedimentarios que controlan la acumulación de materiales finos con abundante contenido orgánico. Bajo este escenario, en Argentina el principal foco de estudio ha sido puesto en la Formación Vaca Muerta, dadas las excelentes propiedades que presenta esta unidad como reservorio no convencional de hidrocarburos. En esta contribución se presenta un estudio integrado de afloramiento y subsuelo orientado al análisis sedimentológico de la Formación Vaca Muerta. El estudio de afloramiento se focalizó en el análisis de procesos de sedimentación de materiales finos en cortes pulidos de concreciones carbonáticas. Por otro lado, el estudio de subsuelo permitió abordar un análisis sedimentológico de alta resolución en testigos corona provenientes de ocho pozos exploratorios de la Formación Vaca Muerta. Las evidencias encontradas en los intervalos analizados permitieron reconocer una gran variedad de depósitos asociados a flujos fluidos de fango. Su origen se relaciona a dos procesos principales: flujos hiperpícnicos fangosos de larga duración (origen extracuencal) y flujos de fango intracuencales asociados a procesos de resedimentación. Los flujos de fango intracuencales serían más frecuentes en sectores próximos al quiebre de la plataforma, generando depósitos con bajo contenido orgánico. Por otro lado, los flujos hiperpícnicos fangosos de origen extracuencal habrían actuado como excelentes medios de transferencia de sedimentos finos y materia orgánica desde áreas emergidas hasta zonas internas de la cuenca, atravesando relieves de muy baja pendiente. En su trayectoria cuenca adentro, habrían incorporado material intracuencal previamente depositado creando depósitos de origen mixto. La rápida transferencia de fango rico en materia orgánica (tipo II y III) hacia sectores internos de la cuenca habría permitido alcanzar un soterramiento rápido y eficiente, evitando su exposición en el fondo marino. Este proceso habría favorecido la preservación de fangos bituminosos en sectores internos de la Cuenca Neuquina. Palabras clave: flujos fluidos de fango (extracuencales, intracuencales), preservación de la materia orgánica, fangolitas bituminosas
Germán Otharán
added a research item
The accumulation of organic-rich mudstones was largely associated to low energy depositional environments with anoxic bottom waters, where mudstone deposition was mainly related to gradual and continuous mud fallout from dilute buoyant plumes. Based on this, basin inner zones were considered as unsuitable environments for source rock deposition due to the generally low concentration of OM resulting from both low inputs and production, adding an intense OM degradation during its transit time through the water column. Nevertheless, recent detailed sedimentological analyses in a variety of unconventional shales have revealed that the participation of fallout processes is probably subordinated to other still poorly known depositional processes, opening a new paradigm for source rocks origin. In Argentina, the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation is composed of organic-rich mudstones and carbonates dominated by type II kerogen representing South America's main unconventional reservoir. New cores and excellent outcrops provide a great opportunity to study the depositional history of this unit. These deposits were previously interpreted as accumulated by fallout deposition in a quiet and anoxic deep marine environment. However, recent studies revealed that the Vaca Muerta Formation is a highly heterogeneous stratigraphic unit accumulated by different and poorly know depositional processes. In fact, the formation displays distinct lithofacies alternating at centimeter to millimeter scale having variable organic matter content (up to 14% TOC), features that influence the reservoir quality and performance. High resolution sedimentological analysis were performed on relatively uncompacted intervals preserved in early diagenetic calcareous concretions collected from the basal deposits of the Vaca Muerta Formation in basinal settings. Evidences found in concretions suggest a deposition related to fluid mud flows instead of the classic model of "normal fallout". The triggered mechanisms for the origin of the recognized fluid mud flow deposits are mainly associated to direct river discharges during flood events. Each flood event would be capable of generating quasi-steady muddy hyperpycnal flows that may be sustained for days, weeks, or even months. These long-lasting events would be able to transfer significant volumes of organic matter and fine-grained sediments for long distances towards distal basinal settings. The erosion capacity of muddy hyperpycnal flows enables the incorporation of intrabasinal components (e.g. marine microfossils, carbonate mud, type II OM) which are transported together with the primary extrabasinal sedimentary load (e.g. detrital 10
Germán Otharán
added 2 research items
La Formación Vaca Muerta (Tithoniano – Valanginiano temprano) se compone mayoritariamente por sedimentitas marinas de grano fino ampliamente distribuidas en la Cuenca Neuquina. Si bien en áreas centrales incluye depósitos turbidíticos (Miembro Huncal), son escasas las menciones de turbiditas hacia el sur de la Dorsal de Huincul. Se relevaron cinco secciones estratigráficas (QS-01 – QS-05) de un intervalo arenoso reconocido en los afloramientos de la Formación Vaca Muerta en la Quebrada del Sapo. A partir de la correlación estratigráfica de estas secciones pueden reconocerse una serie de clinoformas progradantes al noreste, las cuales muestran una fuerte variación de espesor a lo largo de su extensión. El análisis de facies sugiere un origen turbidítico para estos depósitos, cuya acumulación parece estar vinculada a procesos de resedimentación en un medio de talud sujeto a frecuentes deslizamientos gravitacionales, de modo semejante al modelo propuesto para las turbiditas que componen el Miembro Huncal. Sin embargo, estas turbiditas son tithonianas, mientras que aquellas del Miembro Huncal corresponden al Berriasiano. Datos regionales indican que la acumulación de la Formación Vaca Muerta en esta área se habría desarrollado sobre un marcado relieve preexistente. La presencia de truncaciones de bajo ángulo sugiere la posible existencia de actividad tectónica sinsedimentaria durante el Tithoniano en áreas de la Dorsal de Huincul. Palabras clave: Quebrada del Sapo, procesos de resedimentación, deslizamientos gravitacionales, actividad tectónica sinsedimentaria.
La fuerte compactación mecánica que sufren los sedimentos limo-arcillosos durante la diagénesis dificulta el estudio de su historia depositacional. Sin embargo, existen capas cementadas y concreciones carbonáticas exentas de compactación que conservan un registro excepcional de las condiciones de los sedimentos al momento de su depositación. Al realizar cortes pulidos de estos intervalos en muestras de afloramiento de la Formación Vaca Muerta en los perfiles Río Neuquén y Arroyo Mulichinco (provincia del Neuquén), se reconocieron abundantes y repetidas superficies de erosión/bypass relacionadas a una sedimentación episódica. Se realizó un análisis descriptivo de facies de alta resolución, reconociéndose un total de 5 facies sedimentarias con claras vinculaciones genéticas: mudstones gruesas masivas (MGm; MGc); mudstones gruesas con ripples de arcilla (MGr); sets mudstones gruesas-mudstones finas bandeados (MGFb); mudstones gruesas con hummocks (MGh) y mudstones finas masivas (MFm). Su origen se relaciona a corrientes de turbidez que transportan materiales silicoclásticos-carbonáticos de grano fino y restos vegetales como carga de lecho y carga en suspensión turbulenta. Las corrientes de turbidez tendrían una composición textural variable y comportamiento anisótropo, factores que gobernarían el desarrollo de las diferentes facies. Asimismo, las fluctuaciones de flujo asociadas a eventos de larga duración (flujos casi-estacionarios), producirían importantes y repetidos cambios rítmicos en la arquitectura de las facies resultantes. La ignición de estos flujos posiblemente esté relacionada a inundaciones catastróficas que generaron importantes descargas fluvio-deltaicas de naturaleza hiperpícnica. Estos sistemas habrían actuado como excelentes medios de transferencia de materiales finos y materia orgánica desde áreas emergidas hasta ambientes marino distales.