Preservation mechanisms of aeolian sequences in the central Neuquén Basin (Argentina): implications for basin evolution and reservoir characterization

VEIGA, G.D.

Zaragoza

Congreso; 28th IAS Meeting Zaragoza 2011; 2011

International Association of Sedimentologists

Regardless of the precise mechanisms involved in the incorporation of aeolian sequences in the rock record,preservation is always controlled by external forcing factors. Therefore, the correct identification of these mechanisms may reveal key information about the formation of sedimentary successions with aeolian sequences. In order to characterize preservation mechanisms and to unravel the main external forcing factors controlling the accumulation of aeolian sequences in the central Neuquén Basin, two units were studied: the Avilé Mb(Hauterivian) and the Lower Troncoso Mb (Barremian). These units represent excellent hydrocarbon reservoirs in the subsurface and share many sedimentological features. Both units crop out in the central portion of the basin, are composed of non-marine deposits (mainly fluvial and aeolian) that sharply overlie marine shales/carbonates and are covered also by marine facies (shales/evaporites). They have been interpreted as lowstand wedges accumulated during low-order relative sea-level falls that could have lead to the complete desiccation of the basin. The Avilé Mb is characterized by a 30 m-thick aeolian succession dominated by tabular, large-scale cross sets associated with the development of aeolian dunes. These deposits intercalate with horizontally laminated,well sorted sandstone beds or heterolithic intervals composed of mudstones and fine-grained sandstones. These are interpreted as aeolian sandsheets and distal flood deposits, respectively. The contact between dune and sandsheet/flood units is always sharp and horizontal, suggesting the development of several supersurfaces. The aeolian deposits are overlain by amalgamated channel-fill deposits across an erosion surface, indicating that the aeolian section was well below the regional baseline of erosion by the end of the accumulation of this unit. The aeolian deposits of the Troncoso Mb represent a discontinuous interval that can be up to 35 m thick. Aeolian facies are dominated by large-scale cross-bedded sandstones representing dune deposits. Laterally, horizontally laminated facies indicate the development of dry/damp interdunes. A key point in this case is that most of the aeolian deposits are related to the preservation of original dune topography, even when some reworking associated with the final flooding of the erg occurred at the top. This suggests that preservation was only possible due to the sudden disappearance of the aeolian sequence below sea level. The different preservation mechanisms recorded in the studied units indicate that the internal evolution of these systems was controlled by different external factors, which can be crucial when building detailed sequence stratigraphic schemes. It can also have implications in the subsurface characterization of these units, as the presence of barriers/baffles related to episodes of non-aeolian accumulation and supersurfaces could add complexity to what can be seen as simple reservoirs.