Oil was once organic plant life. Just like coal. Once it is gone, it is gone.
At current consumption, viable oil reserves will be exhausted in less than 50 years. It is not a sustainable resource.
From what I've read it appears there are several ways oil is created, but the most common large deposits we've been using are mostly old life. They tend to provide the best quality oil. It's possible to create synthetic oil in the lab and do so even cheaply but, again, its just not the same quality.
I truly feel no one knows the answer to this question.