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MUDDY HYPERPYCNAL FLOWS AND ORGANIC-RICH SHALES. THE UPPER JURASSIC-LOWER CRETACEOUS VACA MUERTA FORMATION, NEUQUÉN BASIN, ARGENTINA

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Abstract

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.
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
*e-mail: czavala@gcsargentina.com
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.
... In spite of the common occurrences of pelagites/hemipelagites, muds can be also eroded, transported and deposited by flows in both marine and non-marine settings from coast to deep-water (Kineke and Sternberg, 1995;Nishida et al., 2013;Azhikodan and Yokoyama, 2018;Pierce et al., 2018). When suspended sediment concentration (SSC) reaches 10g/L, such flows can be considered as fluid mud flows and have been reported to be resuspended by waves and tides or driven directly by hyperpycnal river discharges (Traykovski et al., 2000;Ichaso and Dalrymple, 2009;Zavala and Arcuri, 2016;Otharán and ...
... Organic matter enrichment constitutes a process balanced by the effects of dilution by inorganic components, production and preservation of organic matter (Tyson, 2005), and the question of which of these processes represents the main driving factor constitutes an extensive debate (Hedges and Keil, 1995;Sageman et al., 2003;Katz, 2005;Macquaker et al., 2010). Recently, many studies underscored the contribution of sediment-density flows in trapping organic matter towards deep, fine-grained depositional environments (Könitzer et al., 2014;Lash, 2016, DeReuil andBirgenheier, 2019), in addition to studies from the Vaca Muerta Formation (Otharán and Zavala, 2018). The present analysis shows low TOC content associated with buoyant plume (minor massive fine mudstone in facies F5) and fine-grained hyperpycnal flows (facies F6b) deposits in the bottomset, and with mudbelt (facies F9a) deposits in the foreset, suggesting that, dilution of sediment-starved, hemipelagic basinal areas constitutes the main controlling factor for organic matter enrichment in the study area. ...
Article
The Upper Jurassic-Lower Cretaceous Vaca Muerta Formation at the Picún Leufú area constitutes bottomset and foreset marine deposits comprising open bay, siliciclastic basin and mixed carbonate-siliciclastic shelf facies. Detailed sedimentological, ichnological and sequence stratigraphic analysis of six stratigraphic sections allows establishing two depositional sequences. Depositional Sequence 1 (DS1) begins with lowstand eolian deposits included in the Quebrada del Sapo Formation, and continues with thin, retrograding, transgressive open bay facies of the Vaca Muerta Formation encompassing coastal sand sheet and dune, marginal marine, bay margin and distal bay facies. Above the transgressive deposits, the regressive hemicycle of DS1 consists of bottomset deposits representing a siliciclastic basin facies association. Lobe and lobe fringe hyperpycnal flow sedimentation alternated with hemipelagic basinal deposition in an oxygen-deficient environment. A relative sea-level fall generated a sequence boundary, which coincides with an angular unconformity that marks the base of Depositional Sequence 2 (DS2). This sea-level fall triggered the formation of an extensive lowstand channel-fill and lobe complex at the base of the slope, followed by retrogradational lobe facies during the subsequent transgression. The regressive hemicycle of DS2 represents foreset mixed carbonate-siliciclastic shelf facies, forming a slope mud belt and slope sand bodies. Near the top, sedimentation in a foreset-topset transition resulted in the accumulation of bioturbated mixed slope and sandy shoal deposits. Sand bars and lagoonal facies occur on top of all sections and are truncated by a sequence boundary. The combined analysis provides insights into the sedimentary processes affecting bottomset and foreset deposition, underscoring the role of wave-influenced hyperpycnal flows, and the effect on organic matter dilution in one of the most important unconventional reservoirs from Argentina.
Thesis
Full-text available
The Vaca Muerta Formation is a fine-grained marine stratigraphic unit accumulated during the Late Jurassic-Early Cretaceous in the Neuquén Basin, Argentina. This contribution presents a sedimentological and stratigraphic analysis of Vaca Muerta´s shales emerged from the integration of regional and detailed outcrop research and subsurface data gathered from the study of different well cores. The fieldwork comprised, on the one hand, the description of 7 stratigraphic sections of the Early Tithonian-Early Valanginian interval widely distributed across the western region of the Neuquén province and southern Mendoza province. The sedimentological and sequence-stratigraphic analysis of the different stratigraphic sections allowed integrating them into a regional N-S oriented correlation panel covering an area of 340 km. Five composite depositional sequences were recognized (GS1-GS5). They represent the evolution of a complex mixed shelf/ramp depositional system comprising from basinal/slope facies (central area) to ramp (northern area) and mixed-shelf to continental deposits (southern area). Depositional sequences are represented by thin, organic-rich (up to 10 % TOC), mudstone dominated transgressive cycles, whereas regressive cycles are commonly thicker and mainly composed of organic-lean, carbonate and mixed (carbonate/siliciclastic) facies. On the other hand, aiming at studying the transport and accumulation processes of Vaca Muerta´s organic-rich shales, hand specimen samples of early-diagenetic carbonate concretions were collected from the organic-rich basal condensed section deposited in basinal settings. Concretion samples were thoroughly examined (mm- μm scale) integrating the observation of macroscopic polished samples, thin sections and scanning-electron microscope analyses. The evidence found inside concretions suggest an origin related to muddy underflows, possibly triggered by slope failures on the western margin of the basin. The bedload and suspended-load transport of mud within these muddy underflows would have resulted in the accumulation of graded event mudstone beds exhibiting ripple lamination. Petrographic analysis demonstrates that muddy underflows would have been important mechanisms for reworking the seafloor and redistributing mud in basinal settings. Furthermore, muddy underflows would have been effective processes for organic matter concentration in organic-rich mudstone strata (up to ≈ 6 % TOC). The subsurface studies comprised the sedimentological analysis of seven well cores of the Vaca Muerta Formation, representing a total core data of 387 m. The descriptive facies analysis at centimeter scale allowed the recognition of a distally steepened mixed ramp system. Sediment delivery to basinal settings was mainly controlled by muddy underflows triggered by different mechanisms. Deposition from muddy underflows would have interacted with fallout processes from buoyant plumes and marine snow from the water column. The stacking pattern of the studied core deposits shows ≈ 0.5 to 3 m-thick parasequences building up prograding/retrograding parasequence sets of high-order depositional cycles (10-20 m thick).
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