Abstract

The Huesca fluvial fan (Miocene, Ebro Basin, Spain) contains a low-gradient, mixed-load fluvial system. A detailed outcrop study of its meandering river deposits shows that the preservation of elongate channel-floor sandstone ribbons is common and that these deposits create a continuous along-stream sand-to-sand connectivity between successive crescent-shaped sandy point-bar accumulations on both sides of the channel. The combined appearance of the sandstone resembles a string of beads consisting of a thin, sinuous ribbon with thick and wide protuberances on either side. The studied meandering river sandstone bodies are laterally amalgamated and vertically stacked with a net-to-gross (N/G) ratio of about 40%. They occur in 1–1.5-km (0.62–1-mi)-wide, northeast–southwest-oriented elongate meander belts occupying paleochannels. Beyond these belts, the sandstone is limited to isolated bodies with a very low N/G ratio.

A generic model of the string-of-beads geometry, based on the outcrop data analysis, showed a significant increase of bulk rock volume for the connected string of beads compared with the model of isolated point bars.

The outcrop results demonstrate the potential for channel-floor sandstone bodies to be preserved in a low-gradient, mixed-load fluvial system and their importance in connecting point-bar units in an along-stream direction. We recommend that fluvial reservoir architecture modeling programs include a function that allows the connectivity between channel-floor and point-bar architectural elements. This may greatly impact the estimated reservoir volumes and recovery factors in primary and secondary production as well as influence the sweep efficiency of enhanced recovery technologies.

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