Experimental Deformation of Unconsolidated Quartz Sands, Examining the
Effects of Grain Sorting on Deformation Band Formation Within Mixed
Aeolian-fluvial Reservoirs
Abstract
Deformation bands are the main structural element of fault damage zones
within sandstone reservoirs. The prediction of band occurrence and their
petrophysical impacts is based largely on the understanding that the
yield and deformation mechanism of sandstones is primarily controlled by
porosity and mean grain size. Whilst this is supported by field
observations within aeolian successions, where bands are predictably
favoured within coarse-grained, high-porosity sandstones, the prediction
of deformation bands within texturally complex mixed aeolian-fluvial
reservoirs on the basis of porosity and grain size alone, may be
unreliable. The effect of grain sorting on the mechanical behaviour of
sandstones is not well understood, although it is generally regarded
that deformation band formation is inhibited in texturally immature
sandstones with a poor level of sorting. We examine the effect of
sorting on both the inelastic yield of sandstones, and the dominant
deformation mechanism by which yield occurs, using a series of triaxial
experiments on unconsolidated quartz sands, which allow us to precisely
control textural parameters. Hydrostatic experiments were conducted on
over-consolidated samples of very well- and moderately-sorted sands with
a range of mean grain sizes from 128-700µm. We report much lower P*
values than those estimated from porosity and grain size relationships,
with P* further reduced with decreased sorting. Constant displacement
rate triaxial experiments are performed at up to 10% axial strain, to
explore yield behaviour in both the brittle dilatant regime and
shear-enhanced compactive regime. Experiments were repeated with
systematically varied grain sorting whilst mean grain size and porosity
was maintained. The textural and petrophysical changes are observed and
quantified using pore volumometry, back scattered electron microscopy,
digital image analysis and point counting. Results show that deformation
band formation is exclusive to well sorted sands, with deformation
occurring by distributed Hertzian fracture within poorly sorted sands.
We hypothesise that band formation within poorly sorted sandstones is
promoted by the formation and propagation of bands in adjacent well
sorted sandstones where band formation is favoured.