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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
Carlos Zavala1, German Otharan1,2 and Mariano Arcuri1
1 GCS Argentina. www.gcsargentina.com
2 CONICET
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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