In some parts of the world, this substance is found in pure state and in different forms. So far three types of natural bitumen (asphalt) have been identified:
1- Sedimentary Rock Asphalt: which is produced from natural penetration of bitumen into Calcic Rocks or Sand Stones. This type of bitumen is found as solid substance in the surface and vertical layers of the ground, like bitumen mines in Kermanshah Province, Ilam Province, Lorestan Province and Khoozestan Province, Iran.
2- Asphalt Lakes: Which is found on the surface of the ground and it is not solid, and as shown by its name, it is seen in the form of lakes, like Behbahan bitumen lake in Iran and Trinidad bitumen lake in USA.
3- Natural asphaltites: In some regions, bitumen is found with very high grade of purity including 98% of bitumen and 2% of minerals, known as the purest type of natural bitumen with relatively high softening point. This substance is found in some mines of Kermanshah Province and Ilam Province as well as in the USA. In USA, this product is known as “Gilsonite”.