Adam Vonk explains that a hydrocarbon is a molecule made up of hydrogen and carbon. Hydrocarbons can be very simple structures of one carbon atom and four hydrogen atoms (CH4 – methane) or they can be much longer, complex chains. As the hydrocarbon molecules get longer, they move from existing in a gas state to a liquid state. The liquids become progressively more waxy until, eventually, they have the consistency of tar. This means that the longer and more complex the hydrocarbon chain is, the more difficult it is to get it out of the ground, as it cannot flow freely.
A hydrocarbon is… is a molecule made of hydrogen and carbon. They can be very simple hydrocarbon molecules, in which case you have one called methane, which is a gas – its chemical formula is CH4. As the hydrocarbon molecule gets longer and more complex, then eventually you go from a gas into a liquid, and then the liquid becomes more waxy. So the longer the hydrocarbon chain, the more waxy the oil becomes, until eventually you get a very sticky tar sort of bitumen type substance, which has a very long and complex chemical formula. So, say if you had a sticky bitumen type long chain hydrocarbon to get out of the ground, it would be very difficult because it doesn't flow as easy.
Lloyd Homer – GNS Science