The petroleum occurrences in India are usually associated with anticlines and fault trap in rock formations of the tertiary age. In regions of folding, anticlines or domes, it occurs where oil is trapped in the crest of the up fold. The oil bearing layer is a porous limestone or sandstone through which oil may flow. The oil is prevented from rising or sinking by interviewing non-porous layers. Petroleum is also found in fault traps between porous and non-porous rocks. Gas,being lighter, usually occurs above the oil.