CONICET | Buscador de Institutos y Recursos Humanos

To develop a 3D geologic model for the Ventana-Vacas Muertas oil field in the Cuyo Basin (western Argentina), we studied the fluvial architecture of the Río Blanco Formation (RBF) along and across the excellent exposures of the Potrerillos area. The RBF has been divided into three informal sections. The lower one is composed of a progradational systems tract in which Gilbert-type or homopycnal deltaic clinoforms prograded upon lacustrine shales. Mouth bar deposits and distributary channel deposits are located at the top of this deltaic system. The middle and upper sections of the RBF are characterised by repeated cycles of amalgamated channel-fill deposits (L) and aggradational flood plain deposits (H). Based on the vertical arrangement of lithofacies, primary sedimentary structures and geometry of sandstone/conglomerate bodies, a mixed-load fluvial system is proposed. In comparison with the classic conceptual models, the fluvial system of the RBF shows more frequent downstream and lateral accreting gravelly macroforms, and a sharp transition between the channel-fills and the flood-plain deposits. Fluvial channels are composed of tractional deposits produced by both downstream and lateral accreting gravel bars and sand bars. The sharp vertical transition form channel facies to fine-grained flood plain facies suggests avulsive processes for channel abandonment. As shown by the coarsening upward arrangement at their base, fluvial channels evolved from previously formed crevasse channels and splays. From the sequence stratigraphic viewpoint, the middle section of the RBF may be defined as a backstepping systems tract in which thick narrow and wide ribbon channel- fill deposits (L1) evolve to flood-plain mudstones and thin crevasse-channel deposits (H1), as the result of a progressive increase in accommodation controlled by both tectonic subsidence and a gradual decrease in coarse-grained clastic supply. In the upper section of the RBF, an aggradational systems tract is defined. It is composed of two main cycles of channel amalgamation (L2 and L3) - flood plain mudstones associated to minor channel fills and splay lobes (H2, H3). This cyclical arrangement suggests a repeated variation in accommodation associated to changes in tectonic subsidence and clastic supply. In the RBF, large variations in channel interconnectedness are related to the development of different sequential tracts. Sections characterized by low accommodation and high values of the avulsion/subsidence ratio show the highest linear interconnectedness of channel fills, ribbons, and sand and gravel sheets.

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