Evolution of Porosity in Fractures in the Newberry Volcano Geothermal System, Oregon, USA: Feedback between Deformation and Alteration

Abstract

In crystalline andesitic to basaltic rocks from Newberry Volcano in Oregon, the amount of porosity generated through dilation accompanying slip varies with the stage of fault development. Detailed analysis of porosity structure from the micron to centimeter scale across six distinct deformation stages from intact rock to fractures exhibiting distinct fault rock and breccia captured in core from approximately 1300 m MD reveals that porosity varies due to the relative role of four distinct mechanisms: (1) dilation, (2) healing by mineral precipitation, (3) wear and brecciation of the protolithic wall rock, and (4) mineral replacement through metasomatic alteration or local dissolution/re-precipitation. At early stages the first two mechanisms compete. However at latter stages separation of fracture walls by a growing thickness of fault rock, especially where containing frictionally weak minerals such as phyllosilicates, inhibits dilation. Since, sustainable permeability generated during EGS stimulation depends on the ability of these fractures to dilate and then remain propped open after they are induced to slip via stimulating fluid, these results suggest a network of small, early stage fractures should be targeted for stimulation