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Abstract: High Resolution Sequence Stratigraphy and Reservoir Architecture of Proximal Alluvial Deposits: The Buntsandstein Facies of Central Spain

Olivier Lerat, Guy Desaubliaux, Francois Verdier, Yves Mathieu, Francois Lafont, Pascal Houel, Emmanuelle Wicquart

The Buntsandstein facies outcrops along a 12 km long, 150 m thick cuesta near Ayllon (Central Spain). The outcrop study is based on vertical sedimentological sections and continuous photo paneling, and demonstrates the presence of two depositional systems : an alluvial fan system in the lower half of the outcrop, and a straight and braided river system in the upper part of the outcrop.

This overall evolution is probably related to base-level fall to base-level rise cycle, in which the reservoir architecture is linked to genetic units stacking pattern:

- during the base-level fall, the alluvial fan is prograding over sand flat and sandy alluvial plain deposits. Coarse and pebbly proximal sandsheets are interbedded with finer reddish distal deposits. Reservoirs units are laterally continuous, but shaly alluvial plain deposits constitute vertical permeability barriers,

- during base-level stillstand, erosive channels and sandsheets are vertically amalgamated. Reservoirs units are laterally continuous and vertically connected,

- during the base-level rise, alluvial fan deposits are overlapped by straight river deposits. Reservoirs units are laterally connected but silty argillaceous alluvial plain horizons are preserved, and

- at the end of the base-level rise, braided and straight river deposits are amalgamated. Fully connected, these reservoirs units have a very large lateral extension.

A lithofacies database is compiled on this outcrop, and variograms, horizontal and vertical proportion curves are completed. Each stage of the base-level cycle is then quantitatively characterized by a specific heterogeneity pattern. The outcrop study will improve the prediction of reservoir extension and architecture in subsurface gas storage of the Paris basin.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France