VPS has seen an increase in the levels of shale oil components in marine fuels bunkered recently in Singapore. Although we have received no reports of problems, from vessels using these fuels, we do recommend caution as detailed below.

Over the course of February and March 2026, VPS testing identified 90,000 mt of delivered VLSFO bunkers containing compounds consistent with an Estonian shale oil-derived stream. Estonian shale oil has and is, often used as a blend component in bunker fuel, but has also been known to de-stabilise fuel, producing sludges and blocking ships filters.

Estonian shale oil contains high-boiling phenolic compounds such as, resorcinol and alkyl benzenediols, which can give rise to high acid numbers, often above the ISO8217 specification limit of 2.5 KOH/g.

The recent VLSFO samples bunkered in Singapore, contained resorcinol and alkyl benzenediol concentrations between 4,000ppm-13,200ppm and elevated acid numbers ranging from 1.98KOH/g-2.34 KOH/g.

Despite these findings, no operational issues have been reported so far from vessels that are consuming these fuels.

Yet, while shale oil derived components may be incorporated into marine fuel blends permitted under ISO 8217, when present in marine fuel blends, it can still present operational challenges depending upon blend stability and composition. The presence of phenols, resorcinols, fatty acids, and related oxygenated compounds is not typical of conventionally refined marine fuels. Any observed elevated acid numbers may indicate, unusual blend components resulting in potential instability risks.

Based on our experience and industry data, resorcinol and alkyl benzenediols present within marine fuels may initially show such fuels as compliant to the ISO8217 specifications. However, over time there can be a risk of delayed operational issues, including, sludge formation, filter and purifier fouling/blockages, plus possible impacts on fuel injection equipment cleanliness and performance. Therefore, regarding the latest VLSFO fuels delivered in Singapore, the absence of issues at this stage does not preclude delayed effects, particularly with, storage stability, long-term use, or system cleanliness.

Consequently, VPS would recommend the following precautionary measures when handling such fuels:

• To undertake enhanced monitoring of the fuel treatment plant (purifiers and filters), differential pressures and sludge discharge rates and injection system performance.
• Regarding fuel management, avoid unnecessary co-mingling with other fuels, unless compatibility is confirmed. Maintain optimal settling and purification practices, such as regular draining of settling tanks, while the correct purifier inlet temperatures and minimum feed rates are maintained.
• Record-keeping should continue to maintain detailed engine logbook entries, including tank sounding records with details of all fuel transfers, fuel consumption periods, plus any abnormalities in system performance.
• Sampling and Testing should include the retention of additional samples from, before-purifiers and engine-inlets, plus checking the quality of the fuel entering the engine by monitoring, sediment, stability, and acid number trends.
• The vessel should seek clarification from the suppliers regarding the blend components used, plus the source of any elevated acid number and any use of alternative feedstocks (e.g. shale-derived streams).

Following these recent cases of elevated acid numbers and the presence of phenolic and oxygenated compounds, VPS has contacted each of the vessels who received these fuels. All have confirmed they have not experienced any operational issues to date, when burning the fuel.

However, based on both VPS experience and industry findings, these fuels require close operational monitoring and cautious handling, particularly with respect to fuel stability and cleanliness of the fuel treatment system.

VPS will continue to monitor the situation and provide updates as further operational feedback becomes available.

By Stanley George, VPS Group Technical & Science Manager, and Steve Bee, VPS Group Marketing & Strategic Projects Director.