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.
Naturally occurring oil seep near McKittrick, California, United States.
Petroleum seep near the Korňa in northern Slovakia.
Tar "volcano" in the Carpinteria, California, asphalt mine. Oil exudes from joint cracks in the petroliferous shale forming the floor of mine. 1906 photo, U.S. Geological Survey Bulletin 321
Tar bubble at La Brea Tar Pits, California
Bitumen is an immensely viscous constituent of petroleum. Depending on its exact composition it can be a sticky, black liquid or an apparently solid mass that behaves as a liquid over very large time scales. In American English, the material is commonly referred to as asphalt. Whether found in natural deposits or refined from petroleum, the substance is classed as a pitch. Prior to the 20th century the term asphaltum was in general use. The word derives from the ancient Greek ἄσφαλτος ásphaltos, which referred to natural bitumen or pitch. The largest natural deposit of bitumen in the world is the Pitch Lake of southwest Trinidad, which is estimated to contain 10 million tons.
Natural bitumen from the Dead Sea
Refined bitumen
The University of Queensland pitch drop experiment, demonstrating the viscosity of bitumen
Bituminous outcrop of the Puy de la Poix, Clermont-Ferrand, France