Progress in Predicting Fault Zone Deformation for Fault Seal Analysis

A fundamental component of interpretation of fault behavior needed to estimate the sealing properties is the distribution of low permeability “gouge” or fault rock along the fault. Observations of well-exposed fault zones show that smearing and mixing of the stratigraphic interval into the fault zone is dependent on the mechanical properties at the time of faulting. Faults that form shallow in the section are more likely to develop smears of both sands and shales. The interpretation is that the rocks are more ductile at the time of deformation. Siliciclastic lithologies buried more deeply at a higher mean effective stress are more likely to develop a mixing by granular processes as the rocks become more brittle. The behavior may include a mix of brittle and ductile behavior that is also a function of the rock anisotropy. Outcrop examples demonstrate the differences between these distributions of fault gouge. These models have important implications to flow across and along the fault as the shearing of stratigraphy into the fault zone will depend on the local stratigraphy of the sheared layers. In a mixing model the flow properties of the faults will depend on the amount of clay within the fault zone. The smearing of the more ductile layers occurs by folding into the fault zone and then subsequently a development of a fault cutting the section preserving the sheared section. In more brittle rocks, faults link across layers to form complex lenses bound by fault rocks. Although the mechanical properties of the lithologies are difficult to predict at the time of faulting they are important indicators for the styles of deformation and the subsequent flow properties across the faults.