I have a group of ecoterrorists who want to limit the amount of CO2 the world puts out in a drastic way: by poisoning oil fields so that the oil becomes unuseable, or at least forever uneconomical to pump up.
Unfortunately I haven't yet thought of a way to do this, not even with unrealistic wealth and technical capabilities. Burning it underground or having it "eaten" by bacteria or so would just output the CO2 immediately. Injecting water into the field is actually used as a way to get more production out of it.
Injecting sulfur to make the quality of the oil really low? We get sulfur from oil in the first place, so there is apparently a way to get it out, and there will be a market for all that sulfur because there is an ecoterrorist group buying all of it...
I guess my best bet is another chemical that makes the oil too dangerous / uneconomical to use, but can't be easily extracted from the oil. That exists in sufficient quantities to poison at least a small oil field.
Another option is to make the field inaccessible somehow, make it impossible to have wells there, but I don't know how.