3D Reservoir-Scale Study of Eocene Confined Submarine Fans, South-Central Spanish Pyrenees

Pickering, K.T., Dept of Geological Sciences, University College-London, Gower Street, London WC1E 6BT U.K., and Jordi Corregidor, Institut Cartogràfica de Catalunya, Barcelona, Spain

Abstract

The Ainsa Deep Water (drilling) Project was designed as an integrated outcrop-subsurface study of sand-rich submarine fans, in order to use the data for quantitative reservoir-scale characterization, leading to improved reservoir performance prediction and simulation.

The Eocene (lower/lower middle Lutetian) Ainsa clastic systems, north-central Pyrenees, represent cross-sections mainly through the proximal parts of highly-confined and linear lower-slope and basin-floor submarine fans. The sections are generally subparallel or approximately perpendicular to palaeoflow. Detailed field mapping and outcrop logging has been used in the definition of three discrete fan systems, the Ainsa I, II and III fans. These cross-sections record the early stages of fan development leading to several phases of major sediment bypass, channel back-fill, and finally fan abandonment. Initial fan growth was marked by the infill of complex seafloor topography, by relatively muddy turbidity currents immediately above sediment slide and debris flow deposits. Detailed correlations between sections reveal both the complexity of lateral changes in facies-associations and fan environments.

The Ainsa fans are interpreted to represent sedimentary response to cycles of tectonically induced base-level change in the source area for the sediments, e.g., phases of increased tectonic activity linked to periods of more active compression-uplift in the hinterland. The duration of these cycles has been probably in the order of 0.1-1 Myr. In planform, it is possible to map the progressive west southwest migration of each successive fan system, in response to the movement of the thrust-and-fold belt, and to observe the confining effects of subtle seafloor topography. The results of this study have wide applicability, including an improved understanding of the architectural elements and stacking patterns in confined sand-rich fa n systems, for example in slope basins where the confining slopes cause stratigraphic pinch-out and act as an important local source of sediment slides and debris flows.