Lithofacies Controls on Deformation Band Development: Implications for Reservoir Quality

Abstract

Deformation bands are a major feature of fault zones within porous sandstones, which significantly alter petrophysical properties of hydrocarbon reservoirs. It is therefore important to understand the fundamental processes governing their distribution, geometry and character. Previous work has focused mostly on homogeneous aeolian sedimentary successions, where their relationships to major faults and impact on fluid flow is reasonably well understood.

However, little attention has been given to deformation bands within fluvial lithofacies, which commonly express a broad range of distinctly different intrinsic host rock properties such as grain size distribution, angularity and composition. Current observations and theory suggest that strain localization and deformation band development is inhibited in poorly sorted sands, in favour of mass cataclastic flow. However, preliminary evidence from outcrop and core suggest that deformation bands can and do develop in sandstones with a wide range of textures.

This study examines the effect of lithofacies on the faulting and deformation band development within a major UK onshore basin. Field, petrographic and experimental work will be utilized to investigate the controls on band development, with an aim to better understand the mechanical stratigraphy and petrophysics of a lithologically variable reservoir.

Initial field results show variations in band thickness, density and petrophysics with facies. Damage zones of faults in fluvial lithofacies are less extensive, and thus one would assume less impactive on fluid flow. However, initial petrographic evidence shows porosity reduction of between 93-98%, compared to 60% in comparable Aeolian units. Grain size distribution is identified as a major control on deformation band development and geometry, in co-operation with porosity and grain size controls, hugely affecting a unit’s ability to accommodate and distribute strain. Using a combination of textural analysis and controlled triaxial tests, field and core samples will be examined to understand the style and effects of deformation.

Identifying facies that are susceptible to deformation band development is crucial to optimise sweep and recovery of hydrocarbons. These results have huge implications for the prediction of reservoir quality and fluid migration in faulted reservoirs, as well as the deformation effects associated with depletion.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018