Steam injection to force oil out of existing wells in the area. Apparantly, there's more wells in the area than they thought. Most are capped (apparantly not too terribly well), and the oil popped out at the point of least resistance.
The LA Basin is full of old oil fields just waiting to blow crude or gas out of them. Born and raised out here. Saw the wells go, and dummies putting housing developments on top of them.
FWIW