Optimizing kerogen purification and its impact on density measurements

Although kerogen purification workflows are well documented, inconsistent protocols and conclusions in the literature limit their applicability to density determinations. Accurate kerogen density (ρk) is essential to derive organic‑matter volume and porosity from petrophysical logs, particularly the density log, but measurements are often biased because standard HCl/HF demineralization leaves heavy mineral residues that inflate apparent density. We evaluated an optimized laboratory workflow on kerogens from the Agrio Formation (outcrop) and the Vaca Muerta Formation (core) to approach real ρk by minimizing impurity carryover. After the conventional HCl/HF step, we introduced a second HCl treatment at 120 °C to remove ralstonite‑type fluorinated phases, followed by pyrite elimination via either reductive dissolution with CrCl₂ or oxidative treatment with HNO₃. Structural integrity and functional groups were monitored by ss‑NMR, FTIR, and XPS, and densities were measured at 25 °C by gas pycnometry (Anton Paar Ultrapyc 5000) before and after purification. The hot‑acid step efficiently removed fluoride residues, the CrCl₂ route preserved the kerogen molecular framework and achieved >95% purity but required multiple steps, heavy‑metal reagents (Zn, Cr, Hg), and 2–3 days to reach and adequate purification with traditional methods. In contrast, the HNO₃ route induced kerogen nitration, but does not alter other functional groups or the measured ρk and is markedly faster, simpler and more reliable for pyrite removal. In both cases, purified samples showed substantial decreases in measured density relative to pre‑purification values, confirming the magnitude of mineral‑residue bias, and petrographic standards exhibited a correlation between ρk and reported %VRo. We conclude that a pragmatic sequence of HCl/HF → HCl at 120 °C (defluorination) → pyrite removal with HNO₃ for rapid, routine work or CrCl₂ when strict molecular preservation is required—balances purity, turnaround, and operational safety, yielding more representative kerogen densities for integration with petrophysical analyses and porosity models in unconventional reservoirs.