Fuel analysis
That gap is exactly what an insurer has just put on record.
Marine insurer Skuld issued an advisory on 27 April 2026 after seeing several vessels receive bunkered fuel – mostly from Southeast Asia, particularly Singapore, Hong Kong and Malaysia – that met standard ISO 8217 parameters on delivery but showed significant contamination once advanced testing was applied.
Gas Chromatography-Mass Spectrometry (GC-MS) on the affected fuels revealed a high presence of hydrocarbon compounds, phenolic compounds and alkylresorcinol derivatives.
Concentrations of hydrocarbon compounds such as dihydro-dicyclopentadiene and indene ran between 5,000 and 14,000 ppm, with alkylresorcinol levels between 4,700 and 6,000 ppm.
The finding lines up with a Singapore-specific alert Maritec-Naias had already issued on 13 March, covering fuel oil bunker samples taken over the preceding months.
Seven cases showed alkylresorcinols at 3,000 to 17,000 ppm and six showed phenolic compounds at 2,000 to 3,000 ppm.
In the same testing round, fifteen VLSFO samples returned elevated Acid Numbers of 2.0 to 2.5 mg KOH/g – within ISO 8217 specification, but at what Maritec-Naias described as the higher side of the acceptable range.
That last point is the one worth sitting with. An Acid Number inside spec is not, on its own, evidence of a problem; the concern is what tends to sit behind an elevated reading.
Maritec-Naias noted that higher Acid Numbers can stem from contamination with acidic compounds such as phenolic compounds and alkylresorcinols, often associated with Estonian shale oil – the same chemical signature Skuld's GC-MS testing had picked up independently a few weeks later.
A fuel can therefore clear its compliance certificate while still carrying the chemistry that causes the operational problems downstream: sludge formation, fuel pump seizures and compromised injection equipment.
Some vessels bunkering in Singapore over the same period reported exactly that, including piston ring breakage and excessive sludge formation in purifiers.
Skuld's advisory placed the pattern in a commercial context: bunker prices have risen sharply amid the conflict in the Middle East, and the insurer noted that some suppliers may respond to that pressure by blending low-quality marine fuels into deliveries that still clear standard specification checks.
Skuld also pointed out this is not a new phenomenon for the region – comparable concentrations of the same compounds turned up in VLSFO from the ARA region in 2019, also linked to Estonian shale oil.
None of this means shale-oil-derived blending components are automatically a problem.
Their presence is permitted under ISO 8217 provided the resulting fuel still meets the standard's compositional requirements, and not every vessel bunkering affected fuel reports operational issues.
What both Skuld and Maritec-Naias are converging on is narrower and more actionable: an on-spec result from routine testing is not sufficient reassurance where phenolic and alkylresorcinol contamination is a known risk for the supply route, and an Acid Number sitting at the upper edge of the specification band is worth treating as a prompt for further testing rather than a pass.
For buyers and testing programmes covering Singapore-loaded VLSFO specifically, the practical response both organisations are pointing toward is the same.
Treat GC-MS screening as a genuine addition to the standard test package for this route, not an optional extra, and use an Acid Number reading toward the top of the specification range as a trigger for that additional testing rather than a result to file away as compliant.
PIN 27.3 June/July 2026
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