The role of mechanical stratigraphy on CO2 migration along faults – examples from Entrada Sandstone, Humbug Flats, Utah, USA

Abstract

Bleaching patterns along a faulted succession provides new insight into fluid migration pathways controlled by the fault zone and mechanical stratigraphy. In order to investigate this effect, we logged stratigraphic layers in Humbug Flats, Utah, USA both in host rocks and faulted rocks, where a 40 m offset normal fault cuts through the Entrada Sandstone. We measured petrophysical and mechanical properties using TinyPerm II, Schmidt Hammer and laboratory core testing for selected layers. Bleaching has been identified as a proxy for fluid migration parallel to bedding, which fades out with distance from faults. A new observation is that fracturing of bed interfaces contributes to horizontal flow out from the fault zone. Longer-distance comprehensive bleaching dominates the most permeable beds (units), hosting porosity-controlled deformation band associated to fault zones. Fractures prevail in low permeability and stiffer units, where isolated fracture parallel bleaching haloes are observed within fault damage and process zones. Strong correlation is observed between permeability/porosity and stiffness/strength properties for the tested samples of different layers along the stratigraphic logs. For the studied fault, fractures provide important conduits for vertical flow across low permeability and stiff layers, whereas the more porous layers provide bedding-parallel flow paths out of the fault zone.