Ancient reef deposits often contain large amounts of oil and natural gas. In order to contain petroleum, reefs must be buried beneath the surface and covered by a "cap rock" that does not allow the passage of gas and fluids. If the cap rock is eroded, the oil and gas seeps upward and is lost at the ground surface. In this example from southeastern Michigan, a successful oil well has penetrated a buried Silurian reef. The well to the left, known as a "dry hole," missed the reef and did not encounter oil and gas Many Silurian reefs in the Milwaukee and Chicago areas once contaied petroluem, but removal of the cap rock and exposure of the reefs at the ground surface allowed the oil and gas to escape. This is indicated by residues of natural asphalt in the reef rocks. In this specimen from Thornton Quarry, black asphalt can be seen in cavities within fossil corals.