# Natural Maghemite Nanoparticles in Crude Oil Seen by Electron Paramagnetic Resonance

## TL;DR

This paper introduces a new method using EPR to detect and characterize natural magnetic nanoparticles in crude oil, revealing their sources and properties.

## Contribution

First physics-based model to characterize magnetic nanoparticles in crude oil using EPR, enabling magnetic speciation in low-concentration organic fluids.

## Key findings

- EPR successfully identified maghemite as the dominant magnetic phase in crude oil samples.
- Two distinct sources of magnetic nanoparticles were identified based on acidity and iron content.
- Magnetic parameters like g-factor and anisotropy field were extracted using the Tsay model.

## Abstract

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.