Tough Gas Reservoirs in Fluvial Crevasse Splay Sandstones

Abstract

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 stratigraphic intervals may constitute a secondary source of fossil energy to prolong the gas supply in the future. A recent re-perforation test in a depleted well successfully produced 30 Mm³ gas from such an interval at an investment cost of only 50 k€. Production of these fine-grained, low-permeable reservoirs has so far been hampered by the economic risks related to the uncertainties in their size, shape, spatial distribution and reservoir properties. This research focusses on fluvial crevasse splay reservoir sandstone in the West Netherlands Basin, the Netherlands. The thin-bedded sandstones are abundant in floodplain mudrock intervals, but may be undetectable on gamma-ray logs and have previously been discarded as part of non-reservoir zones. Cores, when available, only provide a spatially limited view and do not allow for three-dimensional, high-resolution characterisation of such intervals. In this study, modern-day fluvial systems in the Altiplano Basin, Bolivia, and outcrops of the Cenozoic Ebro and Tremp-Graus Basins, Spain, serve to establish the reservoir architecture of crevasse splay complexes. Combined with well-log and core data, these are used to assess the reservoir potential of crevasse splay deposits as secondary reservoirs. Results show that thin-bedded sand sheets locally constitute over 50% of fluvial floodplain intervals. Individual crevasse splays have surface areas of up to several square kilometres and thicknesses ranging from centimetre to decimetre scale. Intervals of vertically-stacked and laterally-amalgamated crevasse splays reach up to several metres in thickness and form large, interconnected volumes. Despite their grain size typically not exceeding very-fine sand, subsurface core plugs show porosity values up to 15% and permeabilities ranging from 0.01 to 10 milliDarcy. Estimates of the potential gas initially in place are in the order of tens of Mm³, which makes them a potential target for tough gas production as secondary reservoirs.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015