Strain Distribution and Consequences for Relay Ramp Flow Properties: Structural Analysis and Flow Simulation

Atle Rotevatn, Jonny Hesthammer, and Haakon Fossen
University of Bergen, Bergen, Norway

In clastic reservoirs offshore Norway, soft-linked overlapping faults forming relay ramps have in the recent past frequently been promoted as conductors for fluid flow across fault zones that would otherwise be sealing. Some work has been attributed to understanding relay ramps on a seismic scale. However, much work is needed to resolve the issues concerning internal deformation and distribution of strain within such structures, and their consequences for fluid flow.

In Arches National Park, SE Utah, a structural study was undertaken to investigate the nature of small scale deformation structures and their distribution within a relay ramp affecting the porous eolian Entrada Sandstone of the Jurassic San Rafael Group. The study disclosed a systematic distribution of mainly cataclastic deformation bands forming an intricate and very continuous network across the ramp. The deformation bands display dominantly fault-parallel and diagonal orientations across the ramp.

Flow simulation studies are currently being undertaken in order to investigate the effect these deformation bands would have on fluid flow in a petroleum reservoir during production.

Preliminary results indicate that caution should be exercised when dealing with relay ramps in a reservoir setting. Although it may be tempting to regard relay ramps in clastic reservoirs as passages for fluid flow when planning wells, variables such as lithology and distribution of strain will control flow properties across such structures.

Thus, relay ramps require individual consideration, and attention must be paid to sedimentology, style of deformation and strain distribution for proper well planning and reservoir management.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005