Super bounding surface development in a periodically flooded aeolian system: the Avilé Member (Hauterivian) in central Neuquén Basin, Argentina

Abstract

Several studies have highlighted the importance of the characterisation of the wide variety of bounding surfaces that develop during the accumulation of aeolian successions, both for understanding the system evolution and for reservoir characterisation in term of baffles/barriers development. This is especially important for large-scale, regionally extensive surfaces that bound different episodes of aeolian accumulation (supersurfaces) as their significance in the evolution of an erg system or their influence in the heterogeneity of a reservoir can be more significant. The record of the Avile Member of the Agrio Formation (Hauterivian) in central Neuquen Basin is characterized by the alternation of fluvial and aeolian deposits that result in a relatively thin (~30 m thick) but complex unit that constitutes an excellent reservoir in the subsurface of the basin. Aeolian deposits are represented by simple/ complex dunes and fewer dry interdunes, while fluvial deposits are dominated by amalgamated sandy channels in updip sections and unconfined heterolithic flood deposits in more distal areas. Extensive, horizontal bounding surfaces within the aeolian section have been previously interpreted as deflation supersurfaces (Veiga et al. 2002). In order to define the regional character of these bounding surfaces and the processes behind their development, discontinuous outcrops across a 40 km transect were analysed. This transect is almost parallel to the main wind and fluvial transport direction which is mainly in a NNE direction. In updip sections towards the south, aeolian sequences are erosively truncated by fluvial channel deposits. Fluvial sediments are slightly coarser and show medium scale trough cross-bedding. The erosive relief of the bounding surface between these deposits can be locally important suggesting a period of erosive degradation of the original aeolian relief. In downdip areas, fluvial erosion of aeolian deposits is insignificant and water-lain mudstones and heterolithic deposits conformably cover extensive horizontal surfaces. In both cases, subsequent aeolian deposits migrate across sharp, horizontal surfaces, with no evidence of interdigitation. Even when the development of these surfaces can be linked to episodes of fluvial flooding, their nature and evolution represent a complex interaction of processes in terms of sand supply and availability within the system. While in updip sections the bounding surface can be related to the flooding of an aeolian system, in downdip sections this flooding is neither erosive, nor restricted to interdune areas and was developed after a period of important deflation. This indicates that flooding of updip areas might have had a strong impact in sand availability, promoting undersaturated winds and deflation in downdip areas. This mechanism is more important is systems that share a same fluvial and aeolian transport direction as changes in the upper reaches of the fluvial system have direct impact in sand supply and availability. Tracking of individual surfaces across the study area was difficult suggesting that their development and preservations is also influenced by local factors. However, their processes and character remain relatively constant throughout the study area highlighting their importance when predicting and characterising the behaviour of these deposits as hydrocarbon reservoirs in the subsurface.