Reservoir Architecture of Fluvial Crevasse Splay Deposits

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. However, to date, production of these fine-grained, low-permeable reservoirs has been hampered by the economic risks related to the uncertainties in their size, shape, spatial distribution and reservoir properties. This research focusses on crevasse splay sediment bodies in fluvial floodplain intervals using data from Cenozoīc outcrop analogues from the Ebro and Tremp-Graus Basins in Spain and contemporary fluvial systems in the Altiplano Basin, Bolivia. The study aims to develop a thorough understanding of the underlying depositional mechanisms and their autogenic and allogenic controls. Furthermore, the resulting reservoir architecture is analysed and compared to observations made in the Triassic subsurface of the West Netherlands Basin, the Netherlands. Results show that individual crevasse splays have surface areas up to several square kilometres and thicknesses ranging from centimetre to decimetre scale. Their grain-size distribution comprises two main constituents: (1) a coarser component, deposited in a waning flow regime, and (2) a finer component, settled out from suspension in a standing body of flood water. Crevasse splays appear as vertically-stacked and laterally-amalgamated sheets in intervals up to several metres in thickness. These intervals show sand-on sand contact near their apex (levee breach point) and may be incised by the main fluvial channel after successful avulsion, forming large, interconnected volumes. Intervals of nested crevasse splays, typically overlying well developed paleosols, appear periodically both in outcrop and in the subsurface, indicating some level of allogenic control by climatic or base-level variations and providing a tie point for basin-scale correlation. This work will be used to assess the potential of crevasse splay sandstones as secondary reservoirs and to establish a predictive workflow for the development of such reservoirs in the Triassic, U. Permian and Carboniferous subsurface of the Netherlands.

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