Bunker fuel weathering and fingerprinting

Intentional and illegal discharge of bunker fuel oil and waste oils at sea from vessels is the most common type of oil spill investigated by Australian regulatory agencies.

Tracing oil spills

We have compiled oil spill statistics which highlight an increasing proportion of mystery fuel oil spills where the source of the spill is unknown. This creates a problem in meeting the obligations to properly enforce the International Convention for the Prevention of Pollution from Ships (MARPOL), and makes recovery of any response costs increasingly difficult.

When investigating and prosecuting mystery spills, authorities need to be able to determine how long the spilt oil has been at sea. Recent court cases in Australia have raised concerns about a lack of fundamental data on the rate and timing of bunker fuel oil weathering at sea.

Weathering of oil changes its chemical properties, so chromatographic profiles for weathered oils can look very different to profiles for fresh oil samples from ships. To address the identified lack of data, funding was allocated from the National Plan and a consultant engaged from Leeder Consulting in Melbourne, to carry out a research project on the effects of weathering at sea of various grades of bunker oils.

Oil weathering studies can provide physical and chemical data on how bunker oils change over time and can provide an estimate of how long they have been on the water. This data can be used as an input to improve modelling capabilities and is useful for prosecution purposes as well as determining the window of opportunity for combat techniques.

This report has been finalised by Leeder Consulting and the National Plan Environment Working Group. Below is a summary of the report's main outcomes and conclusions.

Research outcomes

A series of marine bunker fuel oils of different viscosities (180, 280 and 380 cSt) were artificially weathered in a laboratory under moderate and extreme wind and sea temperatures. The weathering process was videoed and the physical properties (viscosity and pour point) and mass balance of the remaining oil was determined after 9, 24, 48 and 96 hours of simulated summer and winter conditions. 

Samples were taken of the oil residues for chemical (biomarker) fingerprinting analysis by gas chromatography and mass spectrometry (GC/MS). Results of the GC/MS chemical analysis technique undertaken on the weathered bunker fuel oil residues demonstrated that even after extreme weathering at summer conditions over 96 hours the biomarkers were still intact. This highlights that the specific biomarker used and the ratios calculated are highly resistant to weathering and an excellent marine oil spill source identification tool.

This provides the National Plan with demonstrated proof of the effectiveness of the oil spill fingerprinting techniques used in our investigations of oil pollution incidents.

The weathering, mass balances and oil properties tested provided some interesting results:

  • Bunker fuels that were rated with equivalent oil specifications on viscosity had quite different weathering rates and residual oil properties.
  • Both the pour point and kinematic viscosity of the bunker fuel oils were found to increase with time as the oil weathered.
  • The viscosities of the weathered bunker fuels after 96 hours increased on average by two orders of magnitude.
  • All bunker fuels tested were found to have lost an average of 30 per cent by volume after 96 hours weathering under all test conditions. Even after the most extreme weathering, approximately 70 per cent of the bunker fuels remained as a solid residue on the water surface.
  • Once the pour point of oil exceeded the seawater temperature, generally after nine to twelve hours of weathering, the oil became a solid, and dispersant effectiveness was very low.
  • After only nine hours weathering of all the oils tested, even under moderate winter conditions, the pour point of the remaining oil was higher than the sea temperature and became a solid residue on the water surface.

The main conclusion reached is that even where the initial grade and specification of the bunker fuel and weathering conditions experienced at sea is known, it is not possible to predict the physical properties of weathered bunker oil residues. The weathering characteristics of unknown bunker fuels cannot be predicted with any accuracy as bunker fuels are made from a blend of heavy residual oils and lighter viscosity cutting oils from around the world that vary in physical properties and chemistry.

To determine the weathering rate of marine bunker fuel and the physical properties and behaviour of any residues it would therefore be necessary to carry out laboratory weathering of the oil in question under controlled conditions. Caution should also be taken with any oil weathering models in spills involving marine bunker fuel oils that are not fully characterised.


The results of this successful project have demonstrated an improved understanding of marine bunker fuel properties, behaviour, variability and weathering. The study has demonstrated the effectiveness of the oil spill fingerprinting techniques employed by local analytical testing laboratories for National Plan oil spill investigations.

Research report

Read the research report Bunker fuel weathering study.

Last updated: 

Tuesday 11 August 2020