Igneous sill and finger emplacement mechanism in shale-dominated formations: a field study at Cuesta del Chihuido, Neuquén Basin, Argentina

Seismic reflection data and field observations have revealed the presence of voluminous igneous sill complexes emplaced into organic-rich shale formations in sedimentary basins worldwide. Damage and structures associated with sills have major implications for fluid flow through basins. Constraining the distribution of these structures requires a good understanding of the sill emplacement mechanism. However, most mechanical models of sill emplacement assume elastic host behaviour, whereas shale is expected to deform inelastically. This contradiction calls for new field observations to better constrain sill emplacement mechanisms. In this paper, we report on detailed field observations of spectacularly exposed fingers and a sill emplaced in shale at Cuesta del Chihuido, in the Neuquén Basin, Argentina. Exceptional outcrop conditions allow detailed descriptions of both (1) the entire cross-section of the intrusions, and (2) the deformation structures accommodating intrusion propagation in the host rock. All intrusions exhibit irregular, blunt or rectangular tips. The structures accommodating the tip propagation are systematically compressional, including reverse faults, folding and imbricate thrust system. Our observations suggest that the studied intrusions have propagated by pushing the host rock ahead, as a viscous indenter. Our observations suggest that the viscous indenter model is probably a dominant mechanism of sill emplacement in shale.Facultad de Ciencias Naturales y Muse

Maturation of the Vaca Muerta and Agrio Formations due to the emplacement of magmatic intrusive complexes in the southern Men-doza region of the Neuquén Basin

La existencia de complejos intrusivos en cuencas sedimentarias ha sido documentada en varias cuencas en distintas partes delmundo. Generalmente se emplazan en rocas ricas en materia orgánica, produciendo la maduración de la misma. Además, el emplazamientopuede generar vías de migración de fluidos, estructuras que pueden ser posibles trampas de hidrocarburos, y pueden serreservorios fracturados. Este trabajo se focalizará en el impacto térmico y la consecuente generación de hidrocarburos producidospor el emplazamiento de un complejo intrusivo. Para ello se escogió el sector surmendocino de la Cuenca Neuquina, dónde existengran cantidad de intrusiones ígneas emplazadas en las rocas generadoras del Grupo Mendoza (Formaciones Vaca Muerta y Agrio)y en sedimentitas continentales del Grupo Neuquén. Se realizó la identificación de intrusivos mediante datos de sísmica y pozos. Seconfeccionó una sección estructural de la zona de estudio, para luego realizar los modelos termales. Los resultados de las modelizacionesmuestran que el emplazamiento de intrusivos genera la maduración de las Formaciones Vaca Muerta y Agrio, especialmentedonde se produce una clusterización de los intrusivos. Éste efecto está regulado por la temperatura de la roca de caja al tiempo delemplazamiento, la edad del emplazamiento y el solapamiento de las intrusiones. Se concluye que en la región surmendocina de laCuenca Neuquina, las Formaciones Vaca Muerta y Agrio se encuentran maduradas por el impacto térmico de un complejo intrusivo,pudiendo llegar a generar cantidades considerables de hidrocarburos.The existence of intrusive-complexes in sedimentary basins has been documented in many basins all around the world. Generally, they are emplaced in organic rich rocks, producing their maturation. Also, the emplacement can generate fluid's migration conduits, structures that can be hydrocarbon traps, and fractured reservoirs. This paper will focus on the thermal impact and the consequent hydrocarbon generation produced by the emplacement of an intrusive-complex. For that aim, we have chosen the southern Mendoza area of the Neuquén Basin, where a great quantity of igneous intrusions are emplaced in the source rocks of the Mendoza Group (Vaca Muerta and Agrio Formations), and in continental sedimentites of the Neuquén Group. The identification of the intrusives was made based on seismic and borehole data. We produced a structural section of the study area, which was used afterwards to make the thermal models. The results of our modellings show that the emplacement of intrusives generates the maturation of the Vaca Muerta and Agrio Formations, especially where a clustering of the intrusions is produced. This effect is regulated by the temperature of the host rock at the time of emplacement, the age of the emplacement, and the clustering of intrusions. We conclude that in the southern Mendoza region of the Neuquén Basin, the Vaca Muerta and Agrio Formations were matured because of the thermal impact of an intrusive complex, being able to generate considerable amount of hydrocarbons.Fil: Spacapan, Juan Bautista. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”; Argentina. YPF - Tecnología; ArgentinaFil: Palma, Joaquín Octavio. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. YPF - Tecnología; ArgentinaFil: Rocha Vargas, Marcelo Emilio. YPF - Tecnología; ArgentinaFil: Leanza, Hector Armando. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”; ArgentinaFil: D'Odorico, Alejandro. YPF - Tecnología; ArgentinaFil: Rojas Vera, Emilio Agustin. YPF - Tecnología; ArgentinaFil: Manceda, René Enrique. YPF - Tecnología; ArgentinaFil: Galland, Olivier. University of Oslo; NoruegaFil: Medialdea, Adrián. YPF - Tecnología; ArgentinaFil: Cattaneo, Diego Matias. YPF - Tecnología; Argentin

Structural signatures of igneous sheet intrusion propagation

The geometry and distribution of planar igneous bodies (i.e. sheet intrusions), such as dykes, sills, and inclined sheets, has long been used to determine emplacement mechanics, define melt source locations, and reconstruct palaeostress conditions to shed light on various tectonic and magmatic processes. Since the 1970’s we have recognised that sheet intrusions do not necessarily display a continuous, planar geometry, but commonly consist of segments. The morphology of these segments and their connectors is controlled by, and provide insights into, the behaviour of the host rock during emplacement. For example, tensile brittle fracturing leads to the formation of intrusive steps or bridge structures between adjacent segments. In contrast, brittle shear faulting, cataclastic and ductile flow processes, as well as heat-induced viscous flow or fluidization, promotes magma finger development. Textural indicators of magma flow (e.g., rock fabrics) reveal that segments are aligned parallel to the initial sheet propagation direction. Recognising and mapping segment long axes thus allows melt source location hypotheses, derived from sheet distribution and orientation, to be robustly tested. Despite the information that can be obtained from these structural signatures of sheet intrusion propagation, they are largely overlooked by the structural and volcanological communities. To highlight their utility, we briefly review the formation of sheet intrusion segments, discuss how they inform interpretations of magma emplacement, and outline future research directions.Facultad de Ciencias Naturales y Muse

Igneous sill and finger emplacement mechanism in shale-dominated formations: a field study at Cuesta del Chihuido, Neuquén Basin, Argentina

Seismic reflection data and field observations have revealed the presence of voluminous igneous sill complexes emplaced into organic-rich shale formations in sedimentary basins worldwide. Damage and structures associated with sills have major implications for fluid flow through basins. Constraining the distribution of these structures requires a good understanding of the sill emplacement mechanism. However, most mechanical models of sill emplacement assume elastic host behaviour, whereas shale is expected to deform inelastically. This contradiction calls for new field observations to better constrain sill emplacement mechanisms. In this paper, we report on detailed field observations of spectacularly exposed fingers and a sill emplaced in shale at Cuesta del Chihuido, in the Neuquén Basin, Argentina. Exceptional outcrop conditions allow detailed descriptions of both (1) the entire cross-section of the intrusions, and (2) the deformation structures accommodating intrusion propagation in the host rock. All intrusions exhibit irregular, blunt or rectangular tips. The structures accommodating the tip propagation are systematically compressional, including reverse faults, folding and imbricate thrust system. Our observations suggest that the studied intrusions have propagated by pushing the host rock ahead, as a viscous indenter. Our observations suggest that the viscous indenter model is probably a dominant mechanism of sill emplacement in shale.Fil: Spacapan, Juan Bautista. Universidad Nacional de La Plata; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Fundación YPF; ArgentinaFil: Galland, Olivier. University of Oslo; NoruegaFil: Leanza, Hector Armando. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Planke, Sverre. University of Oslo; Noruega. Volcanic Basin Petroleum Research; Norueg

Control of strike-slip fault on dyke emplacement and morphology

Strike-slip faults are commonly assumed to influence magma transport and emplacement in the Earth’s crust. However, direct observations of magma conduits within strike-slip faults are lacking. Here we provide some of the first detailed field observations of dykes emplaced within strike-slip faults in the Neuquén Basin, Argentina. We show how fault planes within strike-slip fault zones affect the emplacement of dyke offshoots, resulting in complex dyke morphology. Our study also emphasizes the importance of pre-existing strike-slip fault array on the development of dyke swarms, showing that orientations of dyke swarms may not systematically relate to the principal tectonic stress axes.Fil: Spacapan, Juan Bautista. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de La Plata; ArgentinaFil: Galland, Olivier. University of Oslo; NoruegaFil: Leanza, Hector Armando. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina. Universidad Nacional de La Plata; ArgentinaFil: Planke, Sverre. University of Oslo; Noruega. Volcanic Basin Petroleum Research; Norueg

Integrated source rock evaluation along the maturation gradient. Application to the Vaca Muerta Formation, Neuquén Basin of Argentina

The Vaca Muerta Formation (Tithonian–early Valanginian) is the main source rock in the Neuquén Basin and the most important unconventional shale resource in South America. In the present study, organic geochemistry, electron microscopy and basin and petroleum system modelling (BPSM) were combined to evaluate source rock properties and related processes along a transect from the early oil (east) to the dry gas (west) window. The unit is characterized by high present-day (1%–8% average) and original (2%–16% average) total organic carbon contents, which increase towards the base of the unit and basinal (west) settings. Scanning electron microscopy shows that organic pores derived from the transformation of type II kerogen. Isolated bubble pores are typical of the oil window, whereas bubble and densely distributed spongy pores occur in the gas stage, indicating that the maturity gradient exerts strong control on organic porosity. Organic geochemistry, pressure and porosity data were incorporated into a 2D basin petroleum system model that includes the sequential restoration of tectonic events and calculation of compaction trends, kerogen transformation, hydrocarbon generation and estimation of pore pressure through geologic time. The W–E regional model extends from the Agrio Fold and Thrust Belts to the basin border and allows us to evaluate the relationship between thermal maturity and timing of hydrocarbon generation from highly deformed (west) to undeformed (east) regions. Modelling results show a clear decrease in maturity and organic matter (OM) transformation towards the eastern basin margin. Maximum hydrocarbon generation occurred in the inner sectors of the belt, at ca. 120 Ma; long before the first Andean compression phase, which started during the Late Cretaceous (ca. 70 Ma). Miocene compression (15–7 Ma) promoted tectonic uplift of the inner and outer sectors of the belt associated with a reduction in thermal stress and kerogen cracking, as well as massive loss of retained fluids and a decrease in pore pressure. The OM transformation impacted (a) the magnitude of effective porosity associated with organic porosity development, and (b) the magnitude and distribution of pore pressure within the unit controlled by hydrocarbon generation and compaction disequilibrium. BPSM shows a progressive increase in effective porosity from the top to the base and towards the west region related to the original organic carbon content and maturity increasing along the same trend. Overpressure intervals with high organic carbon contents are the most prone to develop organic pores. The latter represent favourable sites for the storage of hydrocarbons in the Vaca Muerta Formation.Fil: Spacapan, Juan Bautista. YPF - Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Comerio, Marcos. YPF - Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Brisson, Ignacio. YPF - Tecnología; ArgentinaFil: Rocha, Emilio. Pluspetrol S.a.; ArgentinaFil: Cipollone, M.. YPF - Tecnología; ArgentinaFil: Hidalgo, Juan Carlos. Schulmberger GmbH; Alemani

Fracture networks in shale-hosted igneous intrusions: Processes, distribution and implications for igneous petroleum systems

Igneous intrusions in sedimentary basins can influence basin-scale fluid flow and petroleum systems in various ways. They may act as barriers, preferential pathways or even reservoirs for fluids. The fracture networks of intrusions usually represent the main control of their hydraulic properties. However, our understanding of different fracturing mechanisms and their quantitative effect on fracture network properties remains limited, and good field examples are sparse. Here, we present a comprehensive field study from the Neuquén Basin, Argentina, using a reservoir-scale outcrop of a sill complex emplaced in organic-rich shale, which constitutes a direct analogue of oil-producing fractured igneous reservoirs. We provide field evidence of various fracturing mechanisms affecting the fracture network, including cooling joints, bituminous dykes, hydrothermal veins, and tectonic faults. Using high-resolution digital fracture network quantification, we then tie these fracture mechanisms to spatial variations of fracture orientation, intensity and connectivity. Our results indicate that all observed fracture types are involved in hydrocarbon migration and/or storage. Bitumen of very high thermal grade within the intrusions implies migration of hydrocarbons into the sills in a destructive high-temperature environment. Importantly, bitumen dykes and faults locally alter the fracture network, creating zones of strongly increased fracture intensity and connectivity and therefore improved reservoir properties

From field analogues to realistic seismic modelling: a case study of an oil-producing andesitic sill complex in the Neuquén Basin, Argentina

Interpretation of seismic data has played a major role in recent advances in the studies of igneous sill complexes. Seismic modelling studies based on field analogues represent a promising tool to close the scale gap between observations from outcrops and seismic data and support seismic interpretation. Virtual outcrop models are commonly used to include high-resolution geological structures in models of seismic-scale field analogues. However, realistic seismic modelling requires not only detailed structural input, but also well-constrained elastic properties and an adequate seismic modelling technique. Here, we present a seismic modelling study of oil-producing andesitic sills in the Neuquen Basin, Argentina, which implements all modelling elements at high accuracy by combining virtual outcrop models, well data and a 2(3)D filtering method. Our results indicate that the modelled seismic signatures of intrusive bodies observed in field analogues are characterized by frequency-dependent interference and strong amplitude variations owing to highly variable elastic properties of both host rock and sills. We demonstrate that detailed waveform patterns observed in real seismic data can be linked to intrusive bodies below the traditionally assumed limit of resolution via realistic seismic modelling. This illustrates how an integrated modelling approach based on field analogues can aid seismic interpretation.Facultad de Ciencias Naturales y Muse

From field analogues to realistic seismic modelling: a case study of an oil-producing andesitic sill complex in the Neuquén Basin, Argentina

Interpretation of seismic data has played a major role in recent advances in the studies of igneous sill complexes. Seismic modelling studies based on field analogues represent a promising tool to close the scale gap between observations from outcrops and seismic data and support seismic interpretation. Virtual outcrop models are commonly used to include highresolution geological structures in models of seismic-scale field analogues. However, realistic seismic modelling requires not only detailed structural input, but also well-constrained elastic properties and an adequate seismic modelling technique. Here, we present a seismic modelling study of oil-producing andesitic sills in the Neuquén Basin, Argentina, which implements all modelling elements at high accuracy by combining virtual outcrop models, well data and a 2(3)D filtering method. Our results indicate that the modelled seismic signatures of intrusive bodies observed in field analogues are characterized by frequencydependent interference and strong amplitude variations owing to highly variable elastic properties of both host rock and sills. We demonstrate that detailed waveform patterns observed in real seismic data can be linked to intrusive bodies below the traditionally assumed limit of resolution via realistic seismic modelling. This illustrates how an integrated modelling approach based on field analogues can aid seismic interpretation

Structure, emplacement mechanism and magma-flow significance of igneous fingers ? Implications for sill emplacement in sedimentary basins

Field and seismic observations show that numerous sills exhibit lobate morphologies. Each lobe corresponds to a distinct igneous segment exhibiting a finger-like shape, the long axis of which is commonly interpreted as a magma-flow indicator. Robust understanding of the emplacement mechanisms of finger-shaped sills, and direct observations supporting finger orientation as magma-flow indicator are lacking. In this paper, we present the results of detailed structural mapping on an exceptional, easily accessible 1-km long outcrop in the Neuquén Basin, Argentina, that exhibits a sill, its contacts and the structures in the finely layered sedimentary host rock. We show that the sill is made of distinct segments that grew, inflated or coalesced. We also demonstrate that the fingers were emplaced according to the viscoelastic fingering or viscous indenter models, with no field evidence of tensile elastic fracture mechanism as commonly assumed in mechanical models of sill emplacement. We identified new structural criteria at the intrusion's contacts for inferring magma flow direction during the magma emplacement. Our small-scale structural observations carried out on a seismic-scale outcrop have the potential to considerably aid the structural interpretation of seismic data imaging igneous sills, i.e. to fill the standard gap between outcrop-scale field observations and seismic-scale geophysical data