UNDERSTANDING LONG-TERM CO2-DRIVEN ALTERATION OF BASALT PORE STRUCTURES THROUGH NEWBERRY VOLCANO, OREGON, USA

The Newberry Volcano, located in the Cascade Mountain Range in central Oregon, is a 400,000 years old composite volcano that serves as a natural laboratory for studying long-term CO₂ transport and mineralization in basalts within the Cascades region and beyond. This work characterizes the interplay between pore size distribution and alteration mineralogy and morphology in a series of porous (altered) and seal (unaltered) Newberry drill core samples. We resampled core collected from USGS N2 well, specifically focusing on zones of hydrothermally altered and unaltered (fresh) basalts alternating from bottom hole (932 meters) to 700 meters below surface and unaltered and altered basaltic siltstones found 300 meters below surface. Previous analysis of the core (1982) emphasized mineralogical alteration from CO₂-rich, low-ionic strength hydrothermal fluids that were transported laterally by permeable lava flows, with upward fluid migration through microfractures in the basalt matrix. In accordance, hot springs, sourced from underground hydrothermal fluids, have a CO₂ composition between 77-97% mol CO₂ at nearby Paulina and East Lakes. However, little characterization has been conducted on the unaltered and altered Newberry core samples in the context of rock flow properties and geologic CO₂ sequestration. We integrate sample porosity and pore-size distribution analysis from BET, NMR, and MICP, SEM/EDS, thin section, and He pycnometry measurements for a variety of basalt sample depths at Newberry. Results are cast in terms of dominant pore type signals and the interplay between permeability, clay and carbonate alteration, and sealing capacity. The nanoporous (tight) basalt samples have exceptionally low matrix permeability and tend to maintain their "fresh" state over geologic time. The more porous vesicular samples have undergone significant alteration by CO₂-rich fluids, transforming primary minerals like plagioclase into clays, quartz polymorphs, and carbonates, thereby filling the pore systems and creating a secondary nanoporosity network. Remarkably, all the hydrothermally altered basalts examined in this work share a similar pore size distribution, with clay minerals playing a prominent role in the petrophysical/core measurements and, possibly, carbon storage and mineralization capacity.