Fragility

If the Niger Delta Avengers are proving anything to the world, it is how fragile oil infrastructure is. A small group of people are able to severely damage the production of a country with minimal amounts of effort.
Huge amounts of oil moves around the world in above ground pipelines. It only takes a small amount of explosives to completely destroy one. And with the vast distances that these pipes travel, they are impossible to protect. A small dedicated group like the NDA easily roams around, and can damage pipes faster than they could ever be repaired. The same could be done to the wells themselves. A lot of those are unmanned or barely staffed. If a group was inclined it would not be hard to create a repeat of the Kuwait fires of the Gulf War. ISIS seems to be turning its attention to Saudi Arabia, and there is no better way to hurt them than to go after their oil. Damaging production would piss off the Sauds, something major like a refinery fire would hurt them.
If a terrorist group wanted to make their point, knocking a few million barrels out of production would get the world listening. Because unfortunately, affecting the stock market gets more global attention then the slaughter of innocents.