Reservoirs and Traps

Oil is found in reservoirs in sedimentary rock.  It is composed of compressed hydrocarbons, and was formed millions of years ago in a process that began when aquatic plant and animal remains were covered by layers of sediments (particles of rock and mineral).   As bacteria and chemicals broke down the organic plants and animal material, increasing layers of sediment settled on top.  Heat and pressure transformed the layers of sediment into sandstone, limestone and other types of sedimentary rock, and transformed the organic matter into petroleum.  Tiny pores in the rock allowed the petroleum to seep in.  These "reservoir rocks" hold the oil like a sponge, confined by other, non-porous layers that form a "trap."  (See illustration.)

The first challenge for geologists and petroleum engineers is finding these traps, and the second is assessing the characteristics of the reservoir rock and thus the producibility of the oil.   (Once extracted, the crude oil must be refined into usable products, as discussed in the chapter on oil refining.) 

Different regions have different geologic features, formed as the Earth's crust shifted, and thus some regions have more and larger traps.  Furthermore, in some reservoir rock, the oil is more concentrated in pools and thus easier to extract, while in other, more porous, rock, it is diffused throughout.  The Middle East is a region that exhibits both favorable characteristics -- the petroleum traps are numerous, large, and the reservoir rock holds the oil in pools.  Its dominance in world oil supply is the clear result (see linked graph).  Other regions, however, also have large oil deposits, even if more difficult to identify and more expensive to produce.  The United States, with its rich oil history, is such a region.  (Regional roles in oil production are discussed more fully in the Supply Chapter.)