Mineralogy and geochemistry of basalt reservoirs for CO2 Storage in Helguvík, Iceland

It then calculates the global grain density ( \(\rho _{g}\) ) using the mathematical formula: \(\rho _{g}=\sum _{i=1}^{n}(V_{i}\times \rho _{i})\) Where \(V_{i}\) is the volumetric fraction of mineral \(i\), and \(\rho _{i}\) is the theoretical density of that mineral. 2. Porosity QEMSCAN determines porosity volumetrically by identifying and quantifying the empty, unmineralized, or epoxy-filled spaces within the SEM backscattered electron (BSE) images.

Image Segmentation: The software separates the image into distinct phases: mineral grains and pore spaces (voids). Calculation: It computes the areal percentage of pores across the scanned surface area. Because the surface of a polished 2D section statistically represents the 3D volume, this 2D areal porosity is converted into a 3D volumetric porosity ( \(\phi \) ): \(\phi =\frac{\text{Volume\ of\ Pores}}{\text{Total\ Volume}}\) 3.

Bulk Density Bulk density (the mass of the entire rock or sample divided by its total volume) is determined by combining the mineral-derived grain density with the QEMSCAN porosity measurement Calculation: The system calculates the dry bulk density ( \(\rho _{b}\) ) using the relationship between the solid matrix density and the measured void space: \(\rho _{b}=\rho _{g}\times (1-\phi )\) Where \(\rho _{g}\) is the grain density and \(\phi \) is the fractional porosity.

CHEMICAL COMPOSITION The chemical composition reported here refers to a method based with X-ray fluorescence. X-ray Fluorescence (XRF) determines element percentages by measuring the characteristic X-rays emitted from a sample when it is excited by a primary X-ray source. How XRF Calculates Element Percentages XRF spectrometers use a multi-step process to convert raw light signals into precise chemical percentages: • Excitation: Primary X-rays hit the rock sample. • Emission: Atoms eject inner-shell electrons. • Fluorescence: Outer electrons drop down, emitting secondary X-rays. • Detection: Sensors measure the energy and intensity of these X-rays. • Energy Identification: X-ray energy identifies which elements are present. • Intensity Measurement: Peak brightness relates directly to element abundance. • Calibration: Software compares intensities against certified reference materials. • Matrix Correction: Algorithms adjust for absorption and enhancement effects.

Why the Sum Can Exceed 100% In theory, the sum of all components in a rock should equal exactly 100%. In practical XRF analysis, totals can finish above 100% (often 101% to 103%) due to several analytical factors: • Oxide State Assumptions: Most geological XRF software calculates elements as standard oxides (e.g., \(SiO_{2}\), \(Fe_{2}O_{3}\)). If the iron in the rock is actually a pure metal or a lower-mass oxide (\(FeO\)), the software artificially adds excess oxygen mass to the final calculation. • Mineralogical Heterogeneity: Uneven mineral distribution causes disproportionate signal bouncing. • Surface Roughness: Microscopic bumps alter the expected X-ray takeoff angle. • Matrix Effect Overcorrection: Mathematical models can overcompensate for element interactions. • Calibration Drift: Instrumental variations can cause standard lines to shift.

Laser Ablation The chemical composition from laser ablation is also presented. Chemical composition by laser ablation offers significantly higher sensitivity; it can reliably detect trace and ultra-trace elements. Laser ablation (specifically LA-ICP-MS) measures elements in a rock by focusing a high-energy laser onto the sample to vaporize a microscopic pit.

The resulting aerosol is carried into a plasma torch where it is atomized and ionized. A mass spectrometer then separates and measures the ions based on their mass-to-charge ratio. Major elements like Aluminum (\(Al\)) and Calcium (\(Ca\)) are traditionally reported as oxides (e.g., \(Al_{2}O_{3}\) and \(CaO\)) rather than pure elements or raw cations.