The role of floodplain aggradation in building low-gradient dryland river stratigraphy

The Northwest European gas province is a mature area in which the production of gas from conventional reservoirs is declining. Unconventional tough gas reservoirs in low-net-to-gross fluvial stratigraphy may constitute a secondary source of fossil energy to prolong the gas supply in the future. However, as the reservoir potential of floodplain intervals has been under-acknowledged in conventional hydrocarbon exploration, there is limited published research on the accumulation of overbank deposits and their preserved stratigraphy. This research focusses on floodplain aggradation in low-gradient semi-arid fluvial fans on the fringe of endorheic basins, aiming to develop a thorough understanding of overbank deposition within the framework of fluvial system evolution. To this end, field work data from the modern-day Río Colorado fluvial system, Altiplano Basin, Bolivia, and the Miocene Huesca fluvial fan, Ebro Basin, Spain, are combined with subsurface data from the gas-prone Triassic deposits of the West Netherlands Basin. The study aims to establish quantitative spatial relations between preserved facies types, so as to provide characterisation of fluvial plays and the assessment of their reservoir potential. Results show that floodplain deposits constitute the majority of preserved stratigraphy in the distal part of the studied fluvial fans. Floodplain aggradation occurs by levee growth and subsequent crevasse splay breakthrough and sediment redistribution, effectively elevating the floodplain around the active channel. In a low-gradient fluvial system, the resulting lobate geometry causes a significant decrease in channel gradient. The gradual filling of local depressions on the floodplain adjacent to the aggrading channel is mirrored by the evolution of crevasse channels which may develop gradient inversion and backflow phenomena in their proximal reaches. The consequent reduction in transport capacity causes an upstream increase in avulsion proneness, favouring a steeper river profile onto a lower-lying part of the floodplain. The resulting stratigraphy comprises thin but laterally extensive fine-grained beds that may be connected through amalgamation, vertical stacking, and interaction with heterolithic channel fill deposits. Subsurface data show porosities up to 15% and permeability ranging from 0.1 to 10 milliDarcy, qualifying these deposits as potential targets for tough gas production.