Reservoir Potential of Thin-bedded Sandstone in Continental Mudrock Successions - The Search for Hidden Treasures

Abstract

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.