Stacked crevasse splays in the semi-arid Huesca fluvial fan (Ebro Basin, Spain)

Abstract

Low-net-to-gross floodplain stratigraphy contains thin-bedded crevasse splays that may have tough gas reservoir potential. Floodplain deposits are abundant in the distal part of dryland fluvial fans fringing endorheic basins, such as existed in the Permo-Triassic North and Central Atlantic margins. Outcrops of the Miocene Huesca fluvial fan (Ebro Basin, Spain) serve as an analogue to subsurface floodplain intervals. Horizontally-laminated clays and fine silts are dominant, whereas low-sinuous fluvial channels constitute only a fraction of the total stratigraphy. Crevasse splays are common and frequently occur in stacks of up to two metres thick. This results from aggradation of the active channel belt when sufficient accommodation space is available, forming an heterogeneous elevated fringe around the active channel. Lateral amalgamation, vertical stacking, and interaction with sand-rich channel fill deposits significantly increase the connected reservoir volume of the laterally-extensive crevasse splays. The thickness of the stacked intervals can be used as a proxy to determine the channel lag thickness of their feeder channel. This enables estimations of the connectivity in low-net-to-gross floodplain intervals to be made in absence of any direct well penetrations of the fluvial channels.