Abstract: Fault Sealing Mechanisms in Sandstones

R.C.M.W. Franssen, C.J. Peach, E.J.M. Willemse

The understanding of fault seal mechanisms is crucial to the assessment of trap formation and reservoir compartmentalization. The impact of faulting on the permeability of reservoir sandstones was addressed using triaxial deformation experiments (n>40) and field studies. The experimental results show three distinct types of failure behaviour: 1) Shear localisation (faulting) with internal particulate flow (i.e., the sliding and rolling of intact grains past one another); here, failure was characterized by strong dilation and associated permeability increase; 2) Shear localisation (faulting) with internal cataclasis (i.e., comminution of grains resulting in extensive grain size reduction); this failure mode was accompanied by either compaction (permeability decrease) or dilation (pe meability increase) of the fault gouge depending on starting material and test conditions; 3) Homogeneous cataclastic flow; no shear localisation occurred and the samples compacted during deformation.

A deformation mechanism map has been constructed which shows how the failure behaviour is controlled by the initial porosity of the sandstone and effective confining pressure. The change in permeability which accompanied localised shear failure ranges over 5 orders of magnitude and depends upon initial porosity and permeability and effective pressure. The failure mechanism that operates can be determined from the conditions under which failure occurs, using the deformation mechanism map. This, coupled with a theoretical model for fault-gouge permeability, offers a means of predicting the permeability of active fault zones (i.e., fault zones that have not undergone diagenetic changes).

The hydraulic properties of natural inactive fault zones have been investigated using porosity, permeability and capillary displacement pressure data collected from outcrop (aeolian sandstones from Utah, U.S.A. and U.K.) and North Sea core samples. Permeabilities in these natural faults may be up to 6 orders of magnitude lower than the matrix permeability. Displacement pressures measured during mercury injection tests may be as high as 500 psi.

Comparison of the data from experiments vs. outcrop suggests that during faulting (i.e., active stage) the permeability in the fault zone may be significantly higher than in the matrix. However, after fault slippage stopped (i.e., inactive stage), commonly the fault zone permeability and porosity are reduced, and capillary entry pressures are increased, by processes such as mechanical compaction, pore collapse, cementation and pressure solution. Ultimately, the fault zone will become a seal or barrier to flow.

Combining field and laboratory data with theoretical models allows the hydraulic properties of active and inactive fault zones to be estimated.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994