Floodplain deposits are abundant in low-gradient dryland river systems, but their contribution to connected reservoir volumes has not yet been fully acknowledged due to their poor detectability with typical wireline log suites and relatively-lower reservoir quality. This study presents an analysis of stacked crevasse splays in the distal part of the Miocene Huesca fluvial fan (Ebro Basin, Spain). Vertical stacking of crevasse splays implies local aggradation of the active channel belt. Lateral amalgamation of crevasse splays created an elevated rim around their feeder channel, raising its bankfull height. Subsequent crevasse splays were deposited on top of their predecessors, creating sand-on-sand contact through incision and further raising the active channel belt. This process of channel-belt super-elevation repeated until an upstream avulsion occurred. Amalgamated crevasse splays constitute connected reservoir volumes up to ~10 7 m 3. Despite their lower reservoir quality, they effectively connect channel deposits in low net-to-gross fluvial stratigraphy, and hence, their contribution to producible volumes should be considered. Unswept intervals of amalgamated crevasse splays may constitute a secondary source of natural gas. Their interval thickness can serve *Revised manuscript with no changes marked Click here to view linked References as a proxy for feeder-channel dimensions, which can in turn be used to estimate the degree of stratigraphic connectivity.

SUMMARYThe subsurface of the West European gas province contains up to several hundred meters thick continuous Upper Rotliegend and Lower Triassic mud rock sequences which have to date been labelled as nonreservoir 'waste zone'. The mud rock formed as fluvial floodplain deposits in a semi-arid climate. The sequences contain thin-bedded porous and permeable sandstone beds of crevasse-splay origin. A core study of Triassic deposits in the West Netherlands Basin shows that the sandstone beds are nested in up to 2-m-thick sand-prone heterolithic stacks with net-to-gross up to 0.5. Because of the heterolithic nature and small bed thickness the sand-prone intervals are not detected by the gamma-ray log. An outcrop analogue study of fluvial sediments in the Altiplano Basin of Bolivia shows that the crevasse-splay sediments amalgamate laterally to extensive sand sheets with surface areas of several square kilometres. Combination of the core and outcrop study suggests that the nested sandstone beds may constitute secondary plays with economically interesting gas reservoir volumes that may help postpone the end of field-life in mature production areas.