It is important to understand the occurrence of bedding-parallel veins in the Vaca Muerta Formation because this helps to predict their presence away from well controls so that they can ultimately be incorporated in reservoir simulators and hydraulic fracturing modelers. Given that their occurrence has a significant impact on the propagation of hydraulic fractures, their spatial distribution will help to select sweet spots for unconventional resource development. In this study, we try to identify the key parameters that control bedding-parallel veins. Therefore, detailed sedimentological core descriptions were performed on 10 different wells, including total organic carbon measurements at a spacing of 0.5 cm and several degrees of maturity ranging from 0.7% to 1.8% vitrinite reflectance. Through a comparison between bedding-parallel vein localization and sedimentological descriptions, we built a statistical method to identify key parameters controlling the localization of such veins within the Vaca Muerta Formation. We show that bedding-parallel veins are primarily located at facies boundaries (70%) rather than in homogeneous facies (30%). The major rheological discontinuities, such as ashbeds and calcitic concretion boundaries, as well as organic-rich facies have a significant impact on the localization of both bedding-parallel veins. The total organic carbon seems to influence the generation of bedding-parallel veins by locating these fractures in organic-rich areas with more than 2 wt. % in total organic carbon. We also found a correlation between the degree of maturity of the source rock within a same sequence stratigraphy and the (1) number of bedding-parallel veins and (2) thickness of the bedding-parallel vein, suggesting a strong link between the generation of hydrocarbons during the burial of the rock with the generation and distribution of bedding-parallel veins.
The Great Glen Fault trends NNE–SSW across northern Scotland. According to previous studies, the Great Glen Fault developed as a left-lateral strike-slip fault during the Caledonian Orogeny (Ordovician to Early Devonian). However, it then reactivated right-laterally in the Tertiary. We discuss additional evidence for this later phase. At Eathie and Shandwick, minor folds and faults in fossiliferous Jurassic marine strata indicate post-depositional right-lateral slip. In Jurassic shale, we have found bedding-parallel calcite veins (‘beef’ and ‘cone-in-cone’) that may provide evidence for overpressure development and maturation of organic matter at significant depth. Thus, the Jurassic strata at Eathie and Shandwick accumulated deeper offshore in the Moray Firth and were subject to Cenozoic exhumation during right-lateral displacement along the Great Glen Fault, as suggested by previous researchers. Differential sea-floor spreading along the NE Atlantic ridge system generated left-lateral transpressional displacements along the Faroe Fracture Zone from the Early Eocene to the Late Oligocene ( c . 47–26 Ma), a period of uplift and exhumation in Scotland. We suggest that such differential spreading was responsible for reactivation of the Great Glen Fault. Indeed, left-lateral slip along the Faroe Fracture Zone is compatible with right-lateral reactivation of the Great Glen Fault.
Located in the south-west Pacific, New Caledonia is an archipelago with a complex geodynamic history. Grande Terre, the main island, is covered by more than 25% of regolith developed on ultramafic rocks. These peridotites, naturally enriched in metallic elements (Fe, Al, Ni, Co, Cr), constitute most of the southern part of New Caledonia. Since approximately 27 Ma, these rocks have undergone weathering (under a humid tropical climate) and erosion. This combination has led to the formation of the fluvio-lacustrine system, by accumulation of sediments eroded from local summits in depressions and valleys. The distribution of the fossil fluvio-lacustrine system in New Caledonia has been mapped by Folcher N. (2016). The sedimentary filling of this system is essentially of fluvial origin, with rare lacustrine occurrences. Its structure is complex and some sedimentary levels show abnormally high accumulations of Trace Metallic Elements (Ni, Co, Cr). Thus, the METAL FLAP thesis project aims to understand how the geodynamic history, paleoenvironmental evolution, and diagenetic processes within the fluvio lacustrine formation influence the dynamics of metallic elements and their potential transfer to the modern fluviatile system? In one of the sub-basins of the system (the basin of the pirogue river), the relationships between geochemistry and structures have been studied at the edges of two types of objects: horizontal indurated beds and vertical ferruginous partitions. The first results suggest that these two types of structures, which have different formation origins, strongly impact the distribution of elements in sediments (enrichment in Cr, depletion in Fe, etc.). The vertical partitions are thought to be related to fluid circulation in basin fractures, while the formation of horizontal indurated layers is believed to originate from fluctuations in groundwater levels. The quantification of the contribution of each phenomenon to the dynamics of elements is still poorly defined but constitutes one of the major focuses of this study. Furthermore, a detailed analysis of the system’s architecture and a thorough sediment characterization, coupled with the study of the few fossils present in the formation, shed light on the paleoenvironmental history of the region during deposition. This also provides assistance in understanding the geological processes affecting the formation of the fluvio-lacustrine system, as well as the dynamics of metallic elements in the land-sea continuum. This fossil continental system in the southern region is one of the least studied formations in the area, despite the wealth of information it holds about the geodynamic history of New Caledonia during the last post-obduction cycle.
Dans les bassins sedimentaires petroliers. il est habituel de trouler des filons. paralleles a la stratification ct composes de fibres
de calcite. Ces filons. les beef sont abondants dans les roches-meres. surtout quand elles ont atteint la fenetre de generation de lhuile.
C'es tle cas notamment des bassins de Wessex (Angleterre) ct Neuquen (Argentine). ou les fibres de calcite renferment des inclusions
d·huile, de gaz. ou de bitume. Nous interpretons ces filons comme resultanl d'une fracturation hydraulique naturelle. due a une surpression de fluides durant la generation d·huile.
Dans le Bassin parisien. notamment en Lorraine. certains geologues (Marcel Denaeyer et Pierre Maubeuge) avaient mis en evidence
a l'affleurement des filons de calcite a structure conique (cone in cone). Surtout dans des pelites toarciennes (Schistes canon). mais
aussi dans d'autres niveaux plus anciens (triasiques) ou plus recents (bajociens ou oxfordiens). Nous avons verifie la presence de filons
de beef dans (1) des niveaux triassiques a Bainville et (2) des schistes carton liasiques a Gelaucourt.. Dans d'aulre endroits. il existe des veines de calcite fibreuse. mais leur forme reste moins ty pique du bee! ou alors s'apparente a des fossiles bivalves (Trichites).
Abstract Understanding shale petrophysical parameters is of interest due to its direct implications as cap rocks for CO 2 or hydrogen storage, waste depositions, and as unconventional reservoirs. The generation and propagation of natural and induced fracture networks in such rocks is highly dependent on the mechanical behavior linked to several sedimentological parameters, as lithological discontinuities or bioturbation. This study is focused on a different sedimentological parameter that consists of trace fossils and their implication on the generation of fluid-assisted fractures, called bedding-parallel veins. In the Austral-Magallanes Basin, Southern Patagonia, Argentina, both geological features, Skolithos Ichnofacies (doomed pioneers trace fossils) and bedding-parallel veins, are numerous, especially at the top of the turbiditic bodies. The trace fossils exhibit U-shaped vertically oriented burrows composed of clean sandstone, partially cemented by calcite, and a spreite in the central part with heterogenous laminated siltstone. Bedding-parallel veins are composed of calcite fibers with some pyrite grains and bitumen. They are located on the top of the trace fossils along the lithological discontinuity between the turbiditic bodies and the impermeable shales. On their surfaces, a radial pattern starts growing from the trace fossils. Moreover, the number of bedding-parallel veins is dependent on the bioturbation intensity. With this study, we infer that trace fossils represent ichnological mechanical discontinuities (IMD) that have a key role in the generation and development of bedding-parallel veins. By correlation, we also suggest that these geological features must be thoroughly studied, especially regarding their potential for the development of induced fracturing networks.
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