ISC2018, Québec City
MUDDY HYPERPYCNAL FLOWS AND ORGANIC-RICH SHALES. THE
UPPER JURASSIC – LOWER CRETACEOUS VACA MUERTA
FORMATION, NEUQUÉN BASIN, ARGENTINA
G. Otharán1, 2, 3, C. Zavala1, 2,*
1GCS Argentina SRL. Interna 1320, 8000 Bahía Blanca, Argentina.
2Departamento de Geología, Universidad Nacional del Sur, San Juan 670, 8000 Bahía Blanca, Argentina.
3CONICET, Rivadavia 1917, Ciudad Autónoma de Buenos Aires, Argentina
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.