Connectivity of fluvial point-bar deposits: An example from the Miocene Huesca fluvial fan, Ebro Basin, Spain

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. Marinus E. Donselaar received his M.Sc. degree in geology from the University of Utrecht and his Doctor of Science degree in reservoir geology from Delft University of Technology, both in the Netherlands. He worked at the Comparative Sedimentology Division at the University of Utrecht. Since 1987, he has been a lecturer in sedimentology at Delft University of Technology. His research interests are in the modeling of fluvial and barrier island reservoir architecture. Irina Overeem has a Ph.D. from the Faculty of Civil Engineering and Geosciences, Delft University of Technology, Netherlands, where she was an assistant professor in geological modeling from 2005 to 2007. Her research focuses on the numerical modeling of fluviodeltaic processes. Over the last six years she has held a position at Institute of Arctic and Alpine Research (INSTAAR), and she is presently working at the Community Surface Dynamics Modeling Facility at the University of Colorado.