What happens to oil?

Question

If it is left in the ground? What does it turn into after time ?

Answer 1

Depends upon the 'oil'...

Mineral oils (eg motor oil) are hydrocarbons, which are very stable in our enviroment, so little happens. This is why oil leaks/spills are so problematic. However, some of the constituents of 'crude oil' can oxidize/polymerize to a tarry mess.

Vegetable oils are organic esters,  hydroxyl-containing materials esterified by carboxylic acids.  These will slowly hydrolyze to their respective constituents, giving R-OH and R'-COOH...they may also oxidize (turn rancid).