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Deformation sequence and fluid migration within a fault zone - analogue observations from the Little Grand Wash Fault, Utah

Session: Faults, Fractures, and Geomechanics for the Energy Transition

Presenting Author:

Elizabeth S. Petrie

Authors:

Petrie, Elizabeth S¹, Skurtveit, Elin², Høe, Markus Johan³, Evans, James P⁴

(1) Western Colorado University, Gunnison, CO, USA, (2) NGI, Oslo, Norway; University of Oslo, Oslo, Norway, (3) NGI, Oslo, Norway; University of Oslo, Oslo, Norway, (4) Utah State University, Logan, UT, USA,

Abstract:

Permeability and mechanical rock properties within a fault zone are influenced by fluid + rock interactions and associated mineralization. These factors, in combination with variations in host rock lithology, create a complex mixture of physical properties that make it challenging to assess fault zone permeability and the likelihood of fault reactivation.

Observations from outcrop, core, and thin section analyses of rocks exposed at the Little Grand Wash fault, Utah, USA, provide a clear sequence of fracture development that is related to fluid flow. This location which has long served as a natural analogue for CO₂ injection provides a comprehensive data set of rock + CO₂+ brine interactions. Here we assess the changes in deformational behaviours linked to changes in rock mechanical properties using cross-cutting relationships. Using field mapping of fracture distribution and orientation we also quantify the dilation and slip tendencies within the fault zone.

The observed cross-cutting relationships reflect development of carbonate cements, deformational features, and strengthening of fault gouge resulting from prolonged fluid-rock interactions. Cross-cutting relationships indicate that shear failure preceded the creation of open-mode fractures which post-date the development of carbonate cement in reservoir type sands. Gouge samples at the main slip surface contain folded calcite veins that cut carbonate cemented, iron-rich illite bearing foliated gouge. At this study site, elevated CO₂ flux occurs at the crest of the Green River anticline and is associated fault segments with dilation tendency ≥0.8 and in damage zones that contain a greater number of optimally oriented fractures.

All lithologies in this study provide evidence for developing fracture permeability and cycles of fracture reactivation and fluid migration. Structural diagenesis in the fault zone results in the formation of a variety of fracture types and the creation of new and reactivation of existing fracture permeability pathways. This study provides detailed observational data that can be used to understand the sequence of deformation associated with fluid-rock interactions, a method for assessing fault zone permeability and the probability for reactivation within a CO2+ brine saturated fault zone.