Grain Mixing Versus Fragmentation in Sealing Fault Cores from Poorly Lithified Sands: Granulometric and Morphometric Constraints
Fault zone development significantly impacts fluid flow in poorly lithified siliciclastic reservoirs, depending on the petrophysical properties of protolith material and on the environmental conditions of deformation. Cataclasis, the progressive comminution of grains by fracturing, has been recognised in fault cores of both sub-seismic and seismic faults developed in sandy sediments at very shallow crustal levels. When fault slip exceeds bed thickness, an additional variable is given by the mixing of grains from different levels during fault movement. Although many laboratory experiments describe the process of grain fragmentation and mixing in sands, few studies concentrate on the quantitative evaluation of the competitive contribution between grain comminution and mixing. This is particularly true for field analogue studies. In this contribution, we present grain size, shape and permeability data of undeformed and faulted counterparts from a fault zone cutting three different clay-free, quartz-dominated sandy sediments of uniform grain sizes with mean diameters of about 625 μm, 363 μm and 176 μm, respectively, in order to: 1) establish a quantitative relationship between grain size reduction intensity in the fault core and the initial grain size; 2) evaluate the relative contribution of grain fragmentation and mixing within the fault core; 3) constrain the influence of grain size reduction modalities on permeability variations across the fault zone. Our data indicates that cataclasis in fault cores developed in poorly lithified sandy sediments is a size-sensitive process and that grain mixing is subordinated to fragmentation processes. This results in a two order of magnitude permeability reduction in the fault core.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009