Petroleum, like coal, is found in sedimentary rocks, and was probably formed from long-dead living organisms. The rocks in which it is found are almost always of ocean origin and the petroleum-forming organisms must have been ocean creatures rather than trees. Instead of originating in accumulating woody matter, petroleum may be the product of the accumulating fatty matter of ocean organisms The Petroleum in which is is found in almost always of ocean origin and the petroleum-forming organisms must have been ocean creatures rather than trees. Instead of originating in accumulating woody matter, petroleum may be the product of the accumulating fatty matter of ocean organisms