Sedimentology, Ichnology and Sequence Stratigraphy of Black Shales from the Upper Jurassic - Lower Cretaceous Vaca Muerta Formation, Neuquén Basin, Argentina
During the past decades, the study of fine-grained successions has received considerable attention, mainly due to the groundbreaking work of various mudstone sedimentologists who suggested bottom transport processes as plausible mechanisms for mud sedimentation. Moreover, an increased interest has been growing due to the extensive development of unconventional shale reservoirs in USA. In this context, the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation from the Neuquén Basin, Argentina, stands as a valuable shale reservoir for both industry and academic research that can be used to expand our knowledge on ancient fine-grained, organic-rich depositional environments. This Formation is the most important unconventional shale reservoir in South America, with exceptional characteristics for oil and gas extraction. It is well exposed in central western Argentina and contains a fair amount of core data that can be used to evaluate sedimentary processes and trace fossils. The present study analyzed sedimentologic, ichnologic and sequence stratigraphic datasets of the Vaca Muerta Formation to construct a robust depositional model and compare the results with other studies in order to test previous models and propose new ideas. For this purpose, outcrops from a basin margin location (Picún Leufú area), as well as outcrops (Yesera del Tromen) and cores (wells 1 to 9) from the basin centre (where most of the unconventional development is currently active), were analyzed in detail. In the basin margin location (Picún Leufú area), ten sections were logged in the Kimmeridgian to Tithonian succession. The interval encompasses two third-order sequences formed in continental environments and a mixed carbonate-siliciclastic, shelf-margin clinoform system. At the base, the Quebrada del Sapo Formation represents sandstone, conglomerate and mudstone of eolian and fluvial origin. On top, marine carbonaceous, mixed to calcareous mudstone and sandstone of the Vaca Muerta (bottomset and foreset of the clinoform) and Picún Leufú (topset) formations occur. At the base, the clinoform system consists of a thin, retrograding, open bay facies association comprising beach, bay margin, proximal bay and distal bay facies. Above, a siliciclastic basin facies association is constituted by basin, hyperpycnal lobe, lobe fringe, channel-fill complex and overbank facies, developed at the bottomset of the clinoform system. In the foreset and topset, slope mud belt, mixed slope, slope sand bodies, sandy shoal, sand bar complex and lagoonal facies are part of the foreset facies association. This analysis challenged the previous paradigm of a catastrophic transgression at the onset of deposition of the Vaca Muerta Formation. Catastrophic rates of sea-level rise (metres per day) are incompatible with the existence of a basal, retrograding, marginal marine succession suggesting a transition between continental and marine facies. A comparison to modern analogues of marine flooding over dunes indicates that rates of sea-level rise similar to Holocene ones (mm to cm per year) are plausible and may have generated a rapid, yet not catastrophic transgression. Hence, rapid transgressions represent an alternative to “catastrophic” and “normal” scenarios of marine flooding over eolian deposits. The Late Jurassic represents a non-glacial time, and the global sea-level maximum highstand pre-dated the Vaca Muerta transgressive event, indicating that part of the sea-level rise has to be attributed to tectonic/thermal subsidence and compaction. In addition, the combined analysis provides insights into sedimentary processes affecting bottomset and foreset of a fine-grained clinoform system, highlighting the role of wave-influenced hyperpycnal flows associated with higher fluvial input in Picún Leufú and their effect on diluting organic matter content. In the basin centre (Yesera del Tromen and cores from wells 1 to 9), the succession was represented by the Tithonian to Valanginian, mixed carbonate-siliciclastic, shelf-margin clinoform system of the Vaca Muerta (bottomset and foreset) and Quintuco (topset) formations. The succession mostly comprises mixed to calcareous mudstone and bioclastic to intraclastic wackestone developed in four third-order sequences (Sequence 1 to 4). Sequence 1 and 2 start with a transgressive, marginal marine facies association comprising beach and open bay facies. Above, the bottomset and lower foreset of the clinoform shows starved, anoxic, dysoxic and oxic basin facies grouped into the basin facies association, and crinoid-rich, bioclastic- and silt-rich drift, and mixed drift and fluid mud facies, included in the drift facies associations. In the foreset, distal, mid, proximal, and fluid mud-rich slope facies occur as part of the slope facies associations. The slope association evolves from dominantly hemipelagic facies in the older, low-angle clinoforms, to mostly fluid mud-rich slope facies due to higher foreset angles in younger clinoforms. Sequence 3 and 4 are poorly cored, yet low- and high-energy outer ramp facies of the outer ramp facies association with subordinate basin and slope facies associations can be observed. This analysis provides several insights into the understanding of the Vaca Muerta Formation and other ancient fine-grained depositional environments. First, climate and consequent basin circulation were found to control sediment partitioning along the clinoform. Warmer climates triggered equatorward migration of the arid belt, humid conditions and estuarine circulation, producing high TOC and siliciclastic content due to anoxia and enhanced riverine input, respectively. In contrast, cooler climates associated with a poleward migration of the arid belt generated arid conditions and anti-estuarine or weakened estuarine circulation, triggering intense contour currents at deep waters. Low TOC and high carbonate content is recorded, due to high burndown of organic matter under upper dysoxic to oxic conditions, and enhanced shelf export of carbonate sediments by cascading. Organic matter content was sourced from the water column (pelagic), indicating that bottom currents and sediment-gravity flows dilute the TOC content of these deposits. Second, the trace-fossil analysis shows an ichnofauna differing from the classic Chondrites and Zoophycos ichnoguild recorded in organic-rich mudstone successions. In contrast, cryptobioturbation, pellet-rich ichnocoenoses, and biodeformational structures are abundant in this formation. This characteristic can be explained by bioturbation in a food-rich benthic environment, which precluded specialized feeding and the development of tiered communities. Limited oxygenation is deemed responsible for reduced bioturbation index, penetration depth, burrow size, and ichnodiversity. Moreover, biodeformational structures in mudstone are associated with soupy to very soft substrates, whereas a change to soft and stiffgrounds during ash deposition enhanced preservation of discrete trace fossils. This last relationship indicates that tuff deposits and rock fabric can be used to evaluate bioturbation in homogeneous, fine-grained successions in order to claim biogenicity of structures or delineate bottom water oxygenation. Finally, the characteristics of this Formation (low bioturbation intensity due to oxygen deficiency and low foreset angles hampering gravity flow transport) allowed the delineation of contourite deposits. Contourite deposits (drifts) can be differentiated from sediment-gravity flow deposits by their evidence of long-term transport, low sediment concentration, and long-term oxygen introduction at the sea-floor. Furthermore, contourites are host to a particular trace-fossil assemblage that was controlled by food (deposited at the surface or in suspension), oxygenation (increasing oxygen levels), hydrodynamic energy (precluding biogenic reworking during higher energy events) and water turbidity (allowing suspension feeding). The basin-wide increase in oxygenation generated by contourites indicates that these were produced by a wind- and thermohaline-driven circulation system of deep waters that was intensified during arid and cooler climates, which were times of enhanced cascading of dense, shelf waters. The trace fossil data supports the existence of high bioturbation intensity in contourite deposits, yet indicate that sedimentary structures can be preserved in muddy contourites when different stress factors are combined.
Cryptobioturbation, Transgression, Hyperpycnites, Contourites, Mudstone, Unconventional Reservoirs
Doctor of Philosophy (Ph.D.)