Xiaolong Sun, Juan Alcalde , Enrique Gomez-Rivas , Amanda Owen, Albert Griera , Juan Diego Martín-Martín , David Cruset , Anna Travé 

Fluvial fans represent one of the dominant sedimentary systems at the active margins of non-marine foreland basins. The Puig-reig anticline at the north-eastern margin of the Ebro Foreland Basin (SE Pyrenees, Spain) exposes continuous outcrops of late Eocene-early Oligocene fluvial deposits, from proximal to medial fluvial fan environments. The proximal deposits, located in the northern limb of the anticline, especially in the northwest zone, are characterised by conglomerates with minor interbedded sandstones, which present thick and wide sheet-like geometries with unscoured or scoured basal surfaces. These are interpreted to be the deposits of unconfined flash floods and wide-shallow channel streams. The medial deposits, covering the rest of the anticline, consist of interbedded beds of conglomerates, sandstones and claystones, deposited from braided channel streams and overbanks. Distal deposits are found towards the south, beyond the anticline, and are characterised by sandstone and clay deposits of terminal lobes or lacustrine deltas and interdistributary bays. This study assesses the impact of the primary depositional characteristics, diagenesis and deformation of the most heterolithic portion of the system, with implications for the understanding of folded fluvial reservoirs. Diagenetic processes, mainly mechanical compaction and calcite cementation, resulted in overall low matrix porosity, with limited relatively higher porosity developed in sandstone lithofacies in the medial deposits. Deformation associated with thrusting and fold growth resulted in the formation of abundant fractures, with relatively higher fracture intensities observed in sandstone lithofacies in the anticline crest. This study shows that post depositional processes can both improve and diminish the reservoir potential of basin proximal fluvial deposits, by the development of open fracture networks and by compaction-cementation, respectively. The comparison of the Puig-reig anticline with other similar settings worldwide shows that foreland basin margin locations can be potential areas for effective reservoirs, even in the case of low matrix porosity.