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

Abstract

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.Past work has established that faults may act as barriers to fluid flow in siliciclastic hydrocarbon reservoirs and therefore represent challenges for production. In the recent past, the concepts of fault growth through segment linkage and overlap have received a great deal of attention (Peacock &

Sequential growth of deformation bands in carbonate grainstones in the hangingwall of an active growth fault: Implications for deformation mechanisms in different tectonic regimes

“…An argument against this would be that one would expect to see more scatter in orientations if the structures grew in a perturbed stress field (cf. Fossen et al 2005;Rotevatn et al, 2007); however, this is not always be the case and there are examples where structures growing between overlapping faults follow a more systematic pattern like the one seen in the current study (e.g. Peacock and Parfitt, 2002;Faure Walker et al, 2009).…”

Conceptual and numerical models of a tectonically-controlled geothermal system: a case study of the Euganean Geothermal System, Northern Italy

“…Peacock and Sanderson 1994;Trudgill and Cartwright 1994;Peacock and Sanderson 1995;Long and Imber 2011), and in general interaction zones of faults, are potential sites of hydrothermal fluid flow because they are characterized by high stress, which increases the local fracturing and permeability (Curewitz and Karson 1997;Ferrill and Morris 2001;Rotevatn et al 2007). The EGF coincides with the breached relay ramp of the SVFS (Figs 2C and 2D).…”

Dynamic investigation of the effect of a relay ramp on simulated fluid flow: geocellular modelling of the Delicate Arch Ramp, Utah

“…Relay ramps may, therefore, exercise significant controls on fluid flow across faults in producing hydrocarbon reservoirs. However, some studies have documented that overlapping faults commonly are associated with the formation of a complex and anomalously wide fault damage zone (linking damage zone of Kim et al 2004), which may well envelope the entire relay zone (Davatzes & Aydin 2003;Kim et al 2004;Fossen et al 2005;Ciftci & Bozkurt 2006;Rotevatn et al 2007). Sub-seismic structural damage in the relay ramp may have profound effects on fluid conductivity along the ramp (e.g.…”

“…assess the effects of sub-seismic structural damage on fluid flow within a relay ramp by the application of a welldocumented field analogue (Rotevatn et al 2007); 2. test these effects dynamically by the means of reservoir modelling and fluid flow simulation of the outcrop example; 3. assess the overall ability of relay ramps to act as cross-fault conduits for fluid flow based on results from the fluid flow simulation.…”