So Crude oil, is actually many different things all lumped with the same name, it is widely used to describe any oil that is drilled out of the ground, whether under land or sea. However, it’s contents can be very different depending on where in the world it is extracted, varying the different amounts of hydrocarbons present, normally consisting of alkanes (of varying lengths), naphthalenes, and aromatics. There can also be a large proportion of complicated large molecules that become resins or asphalt.
The actual original of crude oil is debated slightly, whether it is the fossilised remains of trees or sea creatures. Essentially large amounts or ‘organic matter’ collected on the sea floor and was covered by sedimentary layers before it could be decomposed. Here, with the correct combination of temperature and pressure, and without oxygen present, the matter decomposes into hydrocarbons and into crude oil.