[decimon]

Does this mean we can inject carbon or CO2 into oil fields to make MORE oil?

You can inject what chemical compounds it would take but you wouldn't have the required heat, pressure and time.

[thackney]

You need to remember that heat and pressure BREAK DOWN the complex hydrocarbons found in oil into shorter, simpler molecules. We do this in refineries every day.

If you have a basic understanding of enthalpy and entropy, this will help explain:
http://www.chemcases.com/fuels/fuels-b.htm