PROCESSES OF FAULT GOUGE DEVELOPMENT IN LAYERED SAND-MUDSTONE SEQUENCES

Fault zones cause dramatic discontinuous changes in mechanical and transport properties. The clay content of the fault gouge is one of the main controlling parameter in these changes.

Using field observations and analogue and numerical models, we describe the development of normal fault zones in layered sand-mudstone sequences, with special focus on the incorporation of clay into the fault gouge. We highlight the processes of lateral clay injection and telescoping on parallel strands.

Lateral clay injection into normal faults is a process by which unexpected amounts of clay can be added to the fault gouge. Our model of this process starts with the presence of a releasing bend in a clay layer and the development of a "squeezing block" pushing the clay into the fault zone. The two mechanical constraints on the process of lateral clay injection are: 1) the clay must flow under the overburden load, and 2) the sand layer must be stronger as the clay layer. A simple analytical model predicts the onset of clay injection when C=s_v*(1-sin f)/(2cos f), where C is cohesion (MPa), s_v is vertical stress (MPa) and f (°) is friction angle. The finite element models (with elasto-plastic continuum elements for the sand and clay and contact elements for the pre-existing faults) show a somewhat higher friction angle dependence.

Many faults consist of multiple parallel strands. Along these strands, wall rock material is "telescoped" inside the fault zone introducing extra complexity which has a major effect on local fault gouge width and juxtaposition. We present a model which quantifies the influence of multiple fault strands on cross-fault connectivity of reservoirs.