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Geological Society, London, Special Publications; 2007; v. 270; p. 55-71;
DOI: 10.1144/GSL.SP.2007.270.01.04
© 2007 Geological Society of London

Outcrop studies

Are relay ramps conduits for fluid flow? Structural analysis of a relay ramp in Arches National Park, Utah

Atle Rotevatn, Haakon Fossen, Jonny Hesthammer, Tor E. Aas & John A. Howell

Centre for Integrated Petroleum Research, University of Bergen, Allégaten 41, 5007 Bergen, Norway (e-mail: atle.rotevatn{at}cipr.uib.no)

Relay ramps associated with overlapping faults are commonly regarded as efficient conduits for fluid flow across potentially sealing intra-reservoir fault zones. The current study demonstrates that structural heterogeneity in the often anomalously wide damage zone of relay ramps may represent potential baffles to intra-ramp fluid flow. A network of ramp-parallel, ramp-diagonal and curved cataclastic deformation bands causes compartmentalization of the ramp studied in Arches National Park, Utah. Harmonic average calculations demonstrate that, although single deformation bands have little or no effect on effective permeability, the presence of even a very small number of low-permeable deformation band clusters could reduce along-ramp effective permeability by more than three orders of magnitude. Thus, although relay zones may maintain large-scale geometric communication, the results of this study demonstrate that caution must be exercised when considering relay ramps as fluid conduits across sealing faults in a production situation. Although relay ramps clearly represent effective migration pathways for hydrocarbons over geological time, the extent to which they conduct fluids in a production situation is more uncertain. Quantitative approaches include adjusting the transmissibility multipliers for faults in reservoir models to allow for increased cross-fault flow. If, however, the effect of internal structural heterogeneity is not taken into consideration, this type of adjustment may lead to gross overestimation of the effect of relay ramps. Sedimentology, stratigraphy, burial history and deformation mechanisms are some of the controlling factors for the formation of such structural heterogeneities.





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