The Evolution of Paleo-Porosity in Basalts: Reversing Pore-Filling Mechanisms Using X-Ray Computed Tomography

Abstract

Often carrying a high-volume fraction of vesicles, basaltic rocks can be an important reservoir horizon in petroleum systems, and are considered an excellent candidate for CO2 storage by in situ mineral trapping. The frequency of amygdaloidal basalts in many sequences highlights the prevalence of mineralisation, but when the vesicle network has been filled, the basalts can act as impermeable seals and traps. Characterising the spatial and temporal evolution of the porosity and permeability is critical to understanding the petro-physical properties and CO2 storage potential of basalts. We exploit X-ray computed tomography (XCT) to investigate the precipitation history of an amygdaloidal basalt containing a pore-connecting micro fracture network now partially filled by calcite as an analogue for CO2 mineral trapping in a vesicular basalt. The fracture network likely represents a preferential pathway for CO2-rich fluids during mineralisation. We investigate and quantify the evolution of basalt porosity and permeability during pore-filling calcite precipitation by applying novel numerical erosion techniques to “back-strip” the calcite from the amygdales and fracture networks. We provide a semi-quantitative technique for defining reservoir potential and quality through time and understanding sub-surface flow and storage. We found that permeability evolution is dependent on the precipitation mechanism and rates, as well as on the presence of micro fracture networks, and that once the precipitation is sufficient to close off all pores, permeability reaches values that are controlled by the micro fracture network. These results prompt further studies to determine CO2 mineral trapping mechanisms in amygdaloidal basalts as analogues for CO2 injections in basalt formations.

A Scaling for the Permeability of Loose Magma Mush Validated Using X‐Ray Computed Tomography of Packed Confectionary in 3D and Estimation Methods From 2D Crystal Shapes

“…LBflow has been validated or applied using sphere packs in cubic lattice arrangements (Llewellin, 2010b; Wadsworth et al., 2017), random overlapping sphere packs (Vasseur & Wadsworth, 2017; Vasseur et al., 2020), hard sphere packs (Vasseur et al., 2021, 2022), hexagonal arrays (Vasseur & Wadsworth, 2019), fibrous porous media (Nabovati et al., 2009), sintering systems (Wadsworth et al., 2017, 2021), and basalt pore spaces (Macente et al., 2022) among others. We calculate the specific surface area s of the particle packs using a marching cubes algorithm (Lorensen & Cline, 1987) from Python's scikit‐image toolbox (following Lewiner et al., 2003).…”

3D Quantification of Pore Networks and Anthropogenic Carbon Mineralization in Stacked Basalt Reservoirs

“…X-rays are attenuated while passing through a material following Lambert and Beer’s law, and this attenuation coefficient depends upon the atomic electron density and the bulk material density. X-ray tomographic imaging is a convenient tool for exploring energy dissipation and quantifying the microstructural properties of mafic and ultramafic rocks. ,,− To provide a robust analysis of the postinjection basalt cores, we used XMT imaging to reconstruct 3D volumetric distribution of porosity, carbonate nodules, and other mineral phases.…”