HR: 1340h
AN: T53C-1441    [Abstracts]
TI: Controls on the Permeability of Faults in Clay-free Sands and Sandstones
AU: Fisher, Q
EM: quentin@rdr.leeds.ac.uk
AF: School of Earth and Environment, Leeds University, Leeds, LS2 9JT United Kingdom
AU: * Casey, M
EM: m.csey@earth.leeds.ac.uk
AF: School of Earth and Environment, Leeds University, Leeds, LS2 9JT United Kingdom
AB: The microstructural and fluid flow properties (permeability and threshold pressure) of >2000 predominantly normal faults from >100 petroleum reservoirs in structurally diverse settings have been analysed to elucidate how faults control fluid flow in sands and sandstones. The presence of cements within dilatant faults has been used as an indication as to whether or not they acted as conduits for fluids. Faults formed in poorly lithified sediments are rarely mineralised. Instead, syn-sedimentary faults have the same microstructure and permeability as their protolith. Faults formed in poorly lithified sediments under higher effective stress conditions (>5 MPa) tend to experience cataclasis and a permeability reduction. Mineralisation is generally only found within faults that formed while the reservoir was being uplifted or when faulting occurred after considerable cementation. Formation of fault-related conduits during uplift is consistent with the concepts of critical state soil mechanics. That is, the rock is on the more compacted side of the critical state line and dilates during deformation, with accompanying peak strength and strain softening. The formation of fault-related conduits in many reservoirs can be predicted by combining a simple numerical model for quartz cementation with results from rock deformation experiments. Rocks deposited slowly under high geothermal gradients may fault to produce conduits for fluid flow at shallow depth, when porosities are high. Faulting of rocks deposited rapidly under low geothermal gradients is unlikely to result in the formation of conduits until very deep burial when porosities and the fluid content of sediments is low. The deformational behaviour of porous rocks is very similar to that of soils, in that it is strongly influenced by the microstructure and the normal or mean stress. The dependence on mean stress allows some differences between the fracture behaviour on normal faults and thrusts to be distinguished. For normal faults, σ_1 is vertical and determined by the overburden pressure and σ_3 is reduced by a tectonic extension. This results in a reduction of the mean stress and a tendency for dilatant brittle failure, for a given microstructure. For thrust faults, σ_3 is vertical and determined by overburden pressure and σ_1 is increased by tectonic compression giving an increased mean stress and a tendency for failure by compactive, cataclastic failure. Thus, for the same microstructure, thrust faults are more likely to be barriers to fluid flow and normal faults are more likely to be conduits.
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: Fall Meeting 2005