Liquid and gaseous hydrocarbons are intimately associated in nature, and it has become customary to shorten the expression "petroleum and natural gas" to "petroleum" when referring to both. Oil, gas, and coal originate from plant debris and are subject to the same geological processes of bacterial action, burial, compaction, and geothermal heating. Most petroleum source beds were deposited in marine environments and most coals were formed under nonmarine conditions. Accumulations of petroleum are usually found in relatively coarsegrained, porous, and permeable sedimentary reservoir rocks that contain very little, if any, insoluble organic matter. The petroleum traps are primarily associated with basement-controlled, regional, central arches, with stratigraphic traps occurring around the margins. Petroleum traps are usually associated with large arches or block uplifts. Extracontinental downwarp basins represent only 18 percent of the world's basin area but are extremely prolific petroleum producers.