Natural Maghemite Nanoparticles in Crude Oil Seen by Electron Paramagnetic Resonance

The characterization of inorganic particulate matter suspended in crude oil is crucial for understanding reservoir geochemistry, fluid provenance, and potential issues related to production infrastructure. In this regard, this work represents the first methodology that applies a physics-based model to systematically characterize natural magnetic nanoparticles (MNPs) in a diverse set of crude oils, establishing electron paramagnetic resonance (EPR) as a robust tool for magnetic speciation in complex organic fluids with a low concentration of MNPs. More specifically, it presents a comprehensive investigation into the magnetic properties of naturally occurring MNPs found in five distinct crude oil samples (A-E) from the Esprito Santo sedimentary basin. Despite the inherently low concentration of these magnetic phases, which renders them undetectable by conventional X-ray diffraction, we successfully isolated and characterized them by using a high-gradient magnetic concentration protocol, followed by EPR spectroscopy at 9.4 GHz. The EPR spectra exhibited broad, asymmetric resonance signals characteristic of randomly oriented nanoparticles with cubic-type anisotropy. Applying the Tsay model for an ensemble of Fe-oxide nanoparticles, we extracted fundamental magnetic parameters from the MNP ensemble: the peak-to-peak line width (W), the effective g-factor (g ≈ 2.08-2.15), and the anisotropy field (H ₁ ≈ -350 to -480Oe). The results revealed that the magnetic phase is predominantly maghemite (γ-Fe₂O₃). A correlation analysis between the magnetic parameters, iron concentration, and total acid number (TAN) suggests basically two distinct sources for the existence of MNPs, which are represented by Sample A, characterized by high acidity (1.81 mg of KOH·g^-1) and iron content (8.3 μg g^-1), exhibiting magnetic properties consistent with corrosion-derived particles, whereas Sample D, with low acidity (0.10 mg of KOH·g^-1) and iron content (0.9 μg g^-1), showing a signature indicative of intrinsic geogenic nanoparticles.