A petroleum seep is a place where natural liquid or gaseous hydrocarbons escape to the Earth's atmosphere and surface, normally under low pressure or flow. Seeps generally occur above either terrestrial or offshore petroleum accumulation structures. The hydrocarbons may escape along geological layers, or across them through fractures and fissures in the rock, or directly from an outcrop of oil-bearing rock.

The petroleum fly, Helaeomyia petrolei, is a species of fly from California, USA. The larvae feed on dead insects and other arthropods that become trapped in naturally occurring petroleum pools, making this the only known insect species that develops in crude oil, a substance which is normally highly toxic to insects.