Properties of Faults in Subsiding Sedimentary Basins - Examples from the Haltenbanken Offshore Norway and the North Sea
Knut Bjørlykke1, Lars Grande2 and Arild Kjeldstad3
1 University of Oslo, Oslo, Norway
2 Norwegian Geotechnical Institute,
3 Shell Norway
The properties of faults in sedimentary basins depend very much on the burial depth at the time of faulting and the subsequent burial (temperature) history. Deformation occurring at relatively shallow depth (<1 Km) will normally not result in extensive grain breakage
(cataclasis) because the normal stresses during shearing are too low.
Experimental shearing of sand show that shearing at effective stresses below 5 MPa there is limited grain fracturing except in coarse grained and weak lithic sands.
This is well illustrated in faults in Jurassic reservoirs in the North Sea and Haltenbanken, Offshore Norway. Most of the faulting occurred during the Late Jurassic prior to the Base Cretaceous Unconformity and Middle Jurassic reservoir sands were then inn most cases buried to less than 500-600 meter depth.
In soft sediments the horizontals stress can not exceed the vertical stress and will then be limited to about 5 MPa. This is in most cases not enough to cause much grain breakage, but the shearing tend to produce a dense grain packing and a strain hardening in sands with a low clay content.
Deep reservoir rocks at Haltenbanken contain deformation (shear) bands with little grain crushing, but a dense interlocking grain fabric causing strain hardening.
Despite about 5 km present burial depth there is relatively little quartz cement in parts of the reservoir due to the effect of chlorite coatings inhibiting quartz overgrowth. This is true also of many shear-bands because the formation of the chlorite coatings post-dated the shear deformation. The permeability of the shear bands is in this case reduced mostly by dens grain packing and to a lesser extent by cementation and some permeability is therefore retained..
Fracturing of sand grains due to the overburden stress after the shear deformation is however common. The strain hardening caused by shearing in sandstones at shallow to moderate burial depth result in many small fault planes each with small offsets. The flow properties of fault zones therefore depends both on the permeability of the shear band and their distribution in a fault zone which may be broad and diffuse to accommodate to total offset.
Studies of exposed faults in Cretaceous sediments at Bornholm, Denmark show more grain fracturing, probably because faulting occurred at depth between1-2 Km. Fault zones are here more continuous and well defined. This is probably due to more fine grained material causing less strain hardening which can cause the zone of deformation to shift.
Quartz cementation may at temperatures exceeding 80-100°C develop fault zones into nearly perfect seals in the absence of clay coatings. Shearing and fracturing of grains after the development of clay coating may cause selective quartz cementation along fault planes.
In subsiding basins faults will nearly always represent reduced permeability compared to the adjacent sediments and are not likely to be conduits for fluid flow even if they may not be perfect barriers. Renewed tectonic deformation is not likely to follow previous faults and fractures because of cementation during burial.
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