How do asphaltenes and resins make bitumen thick and dense?

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Arabian-Iranian basin oil fields — heavy oil and tar sand Major oil fields of the Arabian-Iranian basin region.

Bitumen, often found in oil sands, is characterized by its extreme density and high viscosity. While these substances are present in lighter oils, bitumen is defined by a high concentration of large hydrocarbon molecules known as asphaltenes and resins.1

These complex molecules are the primary drivers of bitumen's physical properties. Because they are significantly larger and more complex than the hydrocarbons found in conventional petroleum, their high concentration creates a substance that is too thick to flow at ambient temperatures. In addition to these hydrocarbons, bitumen often contains high levels of metals like nickel and vanadium, as well as elements such as sulfur, nitrogen, and oxygen, all of which contribute to its dense, heavy nature.1