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The importance of fluid mud flows in the accumulation of thick mudstone successions: Examples from the Jurassic – Cretaceous Vaca Muerta Formation, Neuquén Basin, Argentina

Abstract

During the last years, the study of fine grained sedimentary deposits received a growing attention due to the advance in the development of shale reservoirs, leading out to global multidisciplinary detailed geological surveys on a variety of unconventional resource shales. In addition, recent studies of present day processes and depositional sub-environments provided novel information about the mechanisms that control the distribution and accumulation of mud, contributing to a gradual paradigm shift in mudstone sedimentology. Even though, the current knowledge of the depositional history of these rocks is strongly limited because of the occurrence of post-depositional processes (highly mechanical compaction, strong weathering) that often disrupt the rock primary fabric. Nevertheless, uncompacted intervals of fine grained deposits are often preserved on early cemented beds and calcareous concretions. These early diagenetic bodies often preserve an exceptional record of primary features allowing their study by making macroscopic polished slabs. In Argentina, the Upper Jurassic – Lower Cretaceous Vaca Muerta Formation is composed of shale deposits up to 1200 meters thick, representing the main unconventional reservoir of South America. The widespread excellent outcrops of this unit in the Neuquén Basin thrust and fold belt provides a great opportunity to study the depositional history of thick mudstone successions. For a long time, these deposits were described as a monotonous mudstone succession with high TOC accumulated by fallout processes in quiet and anoxic deep marine environments. However, recent sedimentological research and reservoir characterization revealed that the Vaca Muerta Formation is a highly heterogeneous stratigraphic unit, displaying centimetre to millimetre scale lithofacies variations with fluctuating organic matter content, features that often influence on the reservoir quality and performance. In order to achieve production goals and reduce the exploitation risk it is really important to properly identify those intervals having the highest potential as unconventional resources (sweet spots). Preliminary detailed sedimentological studies focused on understanding the origin of Vaca Muerta shales suggest the common occurrence of bottom turbulent fluid mud flows carrying out fine grained sediments and organic debris as bedload and suspended load. These evidences are against the traditional model of “normal fallout” previously assumed as the main depositional mechanism for the accumulation of mud. The future understanding of the complexity of these fluid mud flows and their internal stacking pattern will be crucial to identify long-term exploitable organic rich levels. In this webinar the authors present a synthesis of the main working paradigms, some recent achievements and perspectives on the evolution of shale sedimentology.
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