The Petroleum Industry Is Oligopolistic Meaning There Are Few Producers Relative

The petroleum industry is oligopolistic, meaning there are few producers relatively speaking, and there are high barriers to entry.  Moreover, there are long-run economic profits for those firms due to their market power.  With that said, why do you suppose I filled up my gas tank in 1999 for $.99 per gallon, only to pay $4.15 a little over six years later and 14 years later still around $4.00?  What do you suppose happened during that time in which prices rose 300% or more and never looked back?  Is it all due to global demand rising?