Permeability scaling properties of fault damage zones in siliclastic rocks

Abstract

Natural fault damage zones are composed of clusters of sub-seismic-scale faults surrounding larger faults. In siliclastic rocks these faults often form partial barriers to flow and significantly influence fluid flow. A three-dimensional statistical model of fault damage zone architecture, incorporating fault size, orientation and spatial characteristics observed in natural examples, is used with a two-dimensional discrete fault flow model to investigate fluid flow and up-scaling of permeability in fault damage zones with a permeability contrast between rock matrix and fault rock of four orders of magnitude. Models that incorporate realistic fault orientation distributions show mean bulk permeabilities that are up to two orders of magnitude lower than the rock matrix. Incorporating spatial clustering of smaller-scale faults around large faults results in a higher variance of bulk permeability. The degree of ‘efficiency’ of 50 by 50 m faulted regions (spanning the fault damage zone) is characterized by comparing their bulk permeability with that of the same sized two-dimensional region with a single spanning fault of constant thickness such that the two areas contain the same proportion of fault rock. Regions of size 50 by 50 m are found to be around 50% efficient in the direction perpendicular to the main fault and between 1 and 10% efficient parallel to the main fault. The efficiency of the fault network, thus defined, was found to be insensitive to the exponent of the power law length distribution. This concept of fault damage zone efficiency and the modelling results provide a method of estimating bulk rock permeability from measurements of the fault rock proportion from core or bore-hole logs. Bulk fault zone permeability consists of contributions from the fault damage zone and the fault slip zone on which the majority of the displacement takes place. Estimates of the contribution of the slip zone suggest that the fault damage zone contributes significantly to the bulk permeability of the entire fault zone when slip zone fault rocks have permeabilities no lower than around one order of magnitude less than that of deformation bands. However, even when the slip zone dominates the fault zone bulk permeability, the fault damage zone is likely to have an important influence on flow processes, such as fault seal.