Heavy crude oil is highly viscous, with an API gravity below 22.3°API. These oils typically have low hydrogen-to-carbon ratios, so simple distillation will produce fewer of the desirable light hydrocarbon products.

Heavy crude oils also contain high levels of asphaltene, sulfur and other acidic compounds, nitrogen, and heavy metals. These contaminants require significant additional processing to yield profitable products from refining. Heavy crude oil typically produces Grades 3 and higher fuel oils and heavy marine fuel. Usable byproducts include asphalt, lubricant oils, and waxes.