On December 9, 2014, similar to 94 000 gallons of furnace oil spilled into the Shela River in Bangladesh, a designated World Heritage Site by the United Nations Educational, Scientific and Cultural Organization. It was the largest recorded oil spill in the Sundarbans region. Visually, furnace oil appears similar to heavy fuel oil, but little is known about its composition even though it is heavily utilized worldwide. A shift in global oil production to heavier, less well-known feeds (i.e., heavy oil and bitumen) requires molecular-level knowledge for efficient response, damage assessment, and restoration in the event of any oil spill. However, little is known about the chemical composition of furnace oil in chronic and acute releases. For the first time, we catalog the molecular-level composition of a relatively unknown furnace oil collected immediately after the 2014 Bangladesh spill and compare it to a well-characterized intermediate fuel oil (IFO) spilled in Texas City, Texas (U.S.A.) in March 2014. Through a combined technique approach, we apply comprehensive two-dimensional gas chromatography (GCxGC) analysis and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to contrast the unknown furnace oil to IFO. Combined, these techniques capture the continuum of oil components and access the less volatile, highly complex non-GC amenable compounds. GCXGC analysis provides biomarker signatures that suggest the furnace oil likely originated in the Middle East and is a refined product. We further compared the furnace oil with the Arabian light crude from Middle East origin (WP681) and revealed remarkable similarities between the two oils. Simulated distillation for the furnace oil showed that 42% of the oil mass is not volatile below 478 degrees C (equivalent to C-40; the upper limit for GC-based techniques), whereas the IFO contained 38% of the total mass >C-40. Furthermore, FT-ICR MS extends the carbon number range and unlocks the molecular composition of non-GC amenable compounds. Atmospheric pressure photoionization (APPI) and electrospray ionization (ESI) FT-ICR MS resolve and identify tens of thousands of molecular formulas in each oil and report furnace oil composition similar to whole heavy crudes. To the best of our knowledge, this is the first report of the detailed compositional characterization of any furnace oil.