There are a number of theories. Theories ate what you get in a Simulation (see Gravity) while trying to figure it out. I like this one: if one accepts that Earth is a sentient being, then her bodily makeup would correspond in many ways to ours. Our forms consist of Earth material; we contain salt water, and host a myriad and complex assortment of smaller life forms.

I conjecture that what we call petroleum is something like a lymph fluid produced for a function beyond our understanding of.

Questions abound. Answers fly around like buzzards huffing gasoline.