The commonality between all compounds in crude oil is that they are hydrocarbons. This means they are primarily composed of hydrogen and carbon atoms.

However, it's important to note that:

* They vary greatly in their specific chemical structure. This results in a wide range of physical and chemical properties, leading to the different fractions that are extracted from crude oil.

* They can contain small amounts of other elements. These include sulfur, nitrogen, oxygen, and metals.

So, while all compounds in crude oil are hydrocarbons, they are not identical. They differ in their chain length, branching, and presence of other elements, which ultimately dictates their properties and uses.