FORCED FOLDING OF HIGHLY POROUS SANDSTONE: STRAIN, FOLD GEOMETRY, DEFORMATION BANDS AND FLUID FLOW

Field studies and experimental work show that highly porous sandstones tend to respond to strain by the formation of deformation band populations. Moreover, progressive accumulation of strain in such sandstones leads to a gradual increase in the frequency of deformation bands until faulting initiates. This fundamental observation pertains to folding of sandstone layers, predicting a relationship between fold geometry, fold strain and deformation band density. If we know this relationship, we can predict band density from knowledge of strain, or vice versa.

Strain is related to fold geometry parameters such as fold curvature, layer dip, or fold tightness, but because this relationship depends on the folding mechanism and history, which is linked to both external conditions and mechanical (lithological) properties, going from one to the other is not necessarily straight forward. In this contribution we will consider forced folding, where a monoclinal fold evolves in the “process zone” above an upward-propagating basement-rooted fault. Examples from the Colorado Plateau show that there is a clear correlation between dip of the steep limb of the monoclinal fold and deformation band density, and therefore a simple correlation between strain and dip, as predicted by for example the trishear model. Specifically, the eolian Navajo Sandstone layer of the San Rafael Monocline in southern Utah shows a systematic evolution from (mostly) reverse bedding-parallel bands (flexural slip mechanism) to the formation of conjugate sets of deformation band networks (ladder structures) oblique to bedding. Hence, at early stages, bedding-parallel deformation bands emphasize the anisotropy represented by bedding and lamination, whereas at later stages, oblique deformation band zones impose an additional mechanical and petrophysical anisotropy that reduce bed-parallel macro-permeability and thus fluid flow. In a petroleum injection-production situation this relationship predicts fluid flow perturbation, as will be demonstrate by numerical flow simulation examples. The generality of these observations will be discussed.