Reactivation Of Pre-existing Faults Research Articles

Modelling of structural complexity in sedimentary basins: The role of pre-existing faults in thrust tectonics

Analogue and numerical models have been used to study the role of pre-existing faults in compressive regimes. From a theoretical point of view, reactivation is mainly controlled by fault attitude, stress regime and frictional properties of fault planes. In scaled-down sandbox experiments, precut faults are introduced in the homogeneous granular media with a nylon wire which is forced through the sand cake producing a thin planar disturbed zone. Systematic experiments of thrust inversion with various dips and strikes of such planar discontinuities have been modelled. Comparison of experimental results with theoretical diagrams indicate that disturbed zones have a friction angle which is 10–20% lower than the homogeneous sand and that the compressive regime in the sandbox has a shape factor close to 0.4. The static analysis of fault reactivation is in accordance with the experimental observations except for pre-existing faults dipping at very low angle. However, numerical modelling using the Udec code shows that low-angle faults can be reactivated as a result of stress concentration in the lower part of the fault. In addition, sandbox experiments indicate that in thrust systems, reactivation of pre-existing faults is not only dependent on their attitude but also on their spacing and location relative to the thrust system.

Mechanical and Chemical Effects of Pore Fluids on Rock Properties: ABSTRACT

The disposal of wastes underground involves injection of fluids into a rock mass that through natural processes has probably attained a state of at least metastable equilibrium. Injected fluids tend to disrupt this equilibrium mechanically and chemically. The primary question concerns what effect this disruption will have on the rock body. Fracturing, faulting, or the reactivation of old faults could lead to earthquakes and to the creation of fractures that may provide permeability channels through which injected fluids could escape from the intended disposal beds. Accelerated compaction, brought about by chemically weakening the load-bearing framework of the rock or by reducing fluid pressures upon withdrawal, can cause surface subsidence. At depths of 3-4 km most rocks are brittle, and fracture and rigid-body rotation are the dominant mechanisms of deformation. The most important physical parameter in this regime is the effective confining pressure Pe, defined as the difference between the confining pressure Pc and the pore-fluid pressure Pp. Both the breaking strength and the compaction of rocks are dependent upon the magnitude of Pe regardless of the absolute values of Pc and Pp. Increases in Pp produced by injection of fluids decrease the normal stresses but do not change shear stresses across potential failure surfaces. This could lead to fracturing, faulting, or the reactivation of pre-existing faults. Decreases in Pp produced y withdrawal of fluids from a reservoir can lead to compaction and surface subsidence as the Pe is increased. The second important aspect of the problem is the role of the pore-fluid chemistry. Significant reductions in rock strength have been shown to result from the lowering of the surface energy of solids that occurs from adsorption of pore fluids and associated modification of bonding. Triaxial compression tests on sandstone show that the coefficient of internal friction is not altered by pore solutions of 0.002 to 2 ppm of FeCl3. The lowering of the breaking strength is due rather to the reduction of the intragranular cohesive strength as a function of the concentration of the electrolyte solution. The frictional characteristics of already broken rocks may be significantly altered by the introduction of surface-active fluids. In light of man-made earthquakes, such as those near Denver, Colorado, the influence of pore-fluid pressure and chemistry suggests that more sophisticated tests of rock properties should be made if problems caused by unexpected rock failure are to be eliminated. End_of_Article - Last_Page 2090------------