DIAGENETIC IRON OXIDE PRECIPITATION: A PROXY FOR SUPERCRITICAL CO2 SPATIOTEMPORAL FLOW

Understanding subsurface supercritical CO₂ flow is essential for feasibility assessment of commercial scale CO₂ injection and sequestration. In a case study of Jurassic strata of the Colorado Plateau, we evaluate lithologic heterogeneity, fractures and faults preserved in both targeted reservoirs and seals to ascertain potential spatiotemporal effects on CO₂ plume migration.

Jurassic eolian units on the Colorado Plateau (e.g., Wingate Sandstone, Navajo Sandstone and Entrada Sandstone) are commonly modeled as thick, homogenous units. However, heterogeneities (e.g., stratification, soft-sediment deformation, bounding surfaces, sabkha deposits) exist on submeter to kilometer scales. Jurassic eolian units also exhibit iron oxide precipitation patterns on bedding to regional scales that record diagenetic fluid flow through the reservoirs and commonly reflect preferential flow paths along heterogeneities. Thus, iron oxide precipitation patterns can serve as “ground truth” for supercritical CO₂ fluid flow models.

Concretion precipitation geometries also record the mode of reactant mass transfer. Geochemically self-organized spheroidal geometries are precipitated by diffusive mass transfer through isotropic rock, often where the bedding is homogenized or massive. More elongate concretion geometries record advective mass transfer, oriented in the direction of fluid flow. Faults and fractures are commonly lined with meters to 10s of meters of concretionary cement precipitation. Modes of mass transfer, directional fluid flow and preferential flow paths can provide parameters to predict spatiotemporal CO₂ plume migration.

Another important factor is that supercritical CO2 may mobilize iron, and iron oxide minerals commonly adsorb trace elements such as arsenic and lead. For example, whole rock analysis shows that iron oxide concretions in the Navajo Sandstone are enriched in arsenic relative to the host rock by over an order of magnitude. The release of heavy metals such as iron, lead, and arsenic could impact subsurface mineralogical systems and may potentially contaminate drinking water supplies.

This project is supported in part by the Southwest Regional Partnership on Carbon Sequestration, funded by the U.S. Department of Energy and managed by DOE's National Energy Technology Laboratory.