I have read the Factorio Wikipedia but I can't understand what it is telling me. My Oil Refiniery does not have an option for petroleum, only "Basic oil processing."
The Basic Oil Processing recipe produces Petroleum, Light Oil, and Heavy Oil. That is how you get Petroleum, and you will also have to do something with the Light and Heavy Oil. Once you unlock Advanced Oil Processing, you can use "cracking" recipes to reprocess Light Oil into Petroleum and Heavy Oil into Light Oil.
The wiki is a bit unclear. It says petroleum 'consumes' crude oil, and that's true in a way.
- First off, you need to have Oil processing researched.
- Pumpjacks produce crude oil (when placed on an oil resource)
- Oil Refineries transform crude oil into Heavy Oil, Light Oil and Petroleum, 'consuming' the crude oil in the process.
- You need a 'recipe' in the Oil Refinery to specify mode of operation. At first this will only be 'basic processing', with different options coming with later research.
- The different oils are then processed further in Chemical plants to get sulphur, plastic etc.
Here's one of the many tutorial video's you can find on the subject.