Deformation Mechanism of Unconsolidated Quartz Sands as a Function of Grain Size Distribution: Insights Into Deformation Band Formation in Mixed Aeolian-Fluvial Reservoirs

Abstract

Deformation bands are a strain localisation structure of sandstones often associated with faulting. The impact on microstructural and petrophysical properties of reservoirs due to deformation band formation, whilst varied, generally is disruptive to fluid flow due to porosity loss through grain re-organization, fracturing, and cataclasis. The yield and subsequent cataclasis of sandstones are controlled mainly by porosity and mean grain size, as such strain localisation is favoured in high porosity, coarse-grained sandstones. Mixed aeolian-fluvial reservoirs exhibit a wide range of porosity and permeability distributions, with sandstones of both mature and immature grain textures. Whilst it has been reported that strain localisation is inhibited in texturally immature sandstones with a wide grain size distribution, we report the occurrence of deformation bands in sandstones with a broad range of grain size profiles, as well as interactions between sandstones of different properties where deformation band characteristics change. The effect of grain size distribution on both the inelastic yield of sandstones, and the dominant deformation mechanism by which yield occurs is examined. We present results of a range of deformation experiments, using unconsolidated quartz sands, allowing us to control and replicate textural parameters observed in the field in order to investigate yield behaviour. Hydrostatic experiments were conducted on both under- and over-consolidated samples with a range of mean grain sizes from 128-700µm, as well as constant displacement rate triaxial experiments up to 10% axial strain, to explore yield behaviour in both the brittle dilatant regime and shear-enhanced compactive regime. Experiments were then repeated with systematically varied grain size distribution whilst mean grain size maintained. The development of both localized dilation and compaction bands, as well as distributed cataclastic flow, are observed using a range of techniques including micro X-ray computed tomography and back scattered electron microscopy. The prediction of deformation bands, particularly of their occurrence in complex reservoirs, is of huge importance for the effective sweep of hydrocarbons. These results offer insight into the role of grain size distribution on the mechanical behaviour of sandstones, and ultimately the development and predictability of deformation bands within aeolian-fluvial reservoirs.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019