From what I've seen it's the old fashion method. Los Angeles use to have a shit ton of oil fields. There are still some but you wouldn't notice the oil wells because they cover them up with a pretend building to match the city. Bakersfield going to Sequoia Park, there are a ton of oil fields there as well. The sad thing though is I've noticed that they've been putting a lot of solar power farms there.

Edit: Here is a quick 5 min vid talking about LA's Oil https://youtu.be/5XimGagIVHc?si=vrR_aUoslz24RMa4