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The Potential of Fluvial Crevasse Splays as (Secondary) Reservoirs

Abstract

Production from conventional Permo-Triassic reservoirs in the Northwest European gas province is in decline. Mature areas like this may benefit from the re-evaluation of low net-to-gross continental stratigraphy, which has previously been discarded as non-reservoir. Earlier studies have shown that thin-bedded fluvial floodplain deposits comprise sand and silt. Laterally-extensive crevasse splays amalgamate through compensational stacking and aggradation, combining to connected bulk rock volumes up to several tens of Mm3. Exhibiting porosities of up to 15% and permeabilities ranging from 0.1 to several mD, these deposits have tough gas reservoir potential. This research investigates the (secondary) reservoir potential of fluvial crevasse splays. To this end, the modern-day Río Colorado fluvial system (Altiplano Basin, Bolivia) and the Miocene Huesca fluvial fan (Ebro Basin, Spain) were used as analogues in order to build high-resolution reservoir-architecture models. These models were populated with petrophysical properties derived from gas-prone intervals in the Southern Permian Basin and West Netherlands Basin. Several production mechanisms were tested through dynamic modelling, so as to determine to what degree crevasse splays could contribute to overall production. The results show that crevasse splays effectively connect fluvial channel and bar sandstones that were previously thought to be isolated by floodplain fines. Crevasse splays contribute to the production of these conventional fluvial reservoirs, despite their lower reservoir quality. Hence, estimations of producible volumes in these deposits may have been underestimated. The economic development of amalgamated crevasse-splay deposits as secondary reservoirs is largely dependent on the proximity of existing infrastructure, as producible volumes do not justify large capital expenditure. In settings where low-cost redevelopment is possible, secondary crevasse-splay reservoirs may add up to several percent to producible volume estimates.