--> Sedimentologic and Stratigraphic Characterization of Mixed-Energy Deltas: Example From the Lajas Formation (Argentina)

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Sedimentologic and Stratigraphic Characterization of Mixed-Energy Deltas: Example From the Lajas Formation (Argentina)

Abstract

Mixed-energy deltas are key hydrocarbon-bearing depositional environments that show complex morphologies and architectures caused by the interplay of river, waves, and tidal currents. However, methods for extracting the signature of the individual processes from the stratigraphic architecture are poorly developed, often hampering reliable subsurface predictions. Through an analysis of facies, paleocurrents, and stratigraphic stacking-patterns in the Jurassic Lajas Formation we (1) show how to separate the signals of wave, tide, and river currents; (2) illustrate strong tidal reworking in the distal reaches of deltaic systems; (3) discuss the implications of tides for the evolution of mixed-energy systems and their reservoir heterogeneities. This work can improve exploration in areas such as the Neuquén Basin, North Sea, and Norwegian Continental Shelf where mixed-energy systems are present. The Lajas Formation, a sand-rich, mixed-energy deltaic system in the Neuquén Basin (Argentina) presents an exceptional example of process variability at different scales. Tidal signals are located in the delta-front, subaqueous platform, and distributary channel deposits. Tidal currents vigorously reworked the delta front producing intensely cross-stratified, sheet-like, sandstone units and this could not have been predicted from knowledge of the coeval proximal reaches of the regressive delta. The wave signals occur mainly in the shelf or lateral shoreface deposits. The fluvial signals increase in abundance proximally and above major sequence boundaries, but are always mixed with the other processes. Recognition of the mixed-energy character of a deltaic system is essential because the abrupt lateral facies changes in the Lajas system (even in a few hundred meters) suggest major impact on reservoir heterogeneities and quality. Furthermore, fluvial-dominated deposits tend to be coarser-grained, poorly sorted, and with a high concentration of plant and wood fragments; fluvial-dominated mouth bars are likely to have an aspect ratio of approximately 2:1, with tidal currents reworking sediments only during low-discharge periods and creating possible horizontal and vertical permeability barriers. Tide-influenced and wave-influenced deposits are finer-grained and better sorted. The interplay of different processes produced a final stratigraphic record much more complex than the simple coarsening-upward trends of river- and wave-dominated delta fronts.