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From The Primer of Oilwell Drilling, 6th edition Copyright © 2001 Petroleum Extension Service (PETEX®) of The University of Texas at Austin. All rights reserved


Many people mistakenly believe that crude oil flows in underground rivers or pools in subterranean lakes. In fact, crude oil and natural gas, or hydrocarbons, are found in rock formations that geologists call reservoirs, which are usually located thousands of feet underground. But not all rocks can hold oil and gas. To serve as an oil and gas reservoir, rocks have to meet several criteria.



Rocks are not always as solid as they may appear. For example, a microscope will reveal that a piece of sandstone or limestone has tiny openings or voids that geologists call pores. A rock with pores is "porous," and a porous rock has "porosity." Reservoir rocks must be porous because hydrocarbons can occur only in pores. A reservoir rock is also permeable, which means that its pores are connected. Oil and gas have to be able to move from pore to pore in order to be extracted. Unless hydrocarbons can move, they stay locked in place, unable to flow into a well. So a suitable reservoir rock must be porous, permeable and hold enough hydrocarbons to make it economically feasible for the operating company to drill, extract and produce them.


Many scientists believe oil and gas originated in the vast number of living organisms that once populated the earth’s ancient seas. Some were large fishes and other marine creatures, but most were microscopic life forms. Countless numbers of these microorganisms lived and died daily, and their remains settled to the bottom for millions of years. Over time, enormous quantities of this organic sediment accumulated in thick deposits on the sea floor, mixing with mud and sand. The layers of sediments grew hundreds or thousands of feet thick.

The tremendous weight of the overlying sediments created great pressure and heat on the deeper layers, transforming them into rock. At the same time, heat, pressure, and other forces changed the decaying organic material into crude oil and natural gas. Meanwhile, millions of years of geological action from tectonic plate movement and volcanoes thrust massive blocks of land, up, down and to the side. Wind and water eroded formations, earthquakes buried them and new layers of sediment settled on them. In some cases, land surrounded vast areas of ocean, creating inland seas that evaporated over time. These geological forces slowly and constantly altered the shape of the earth and layers of rock formations. Under the right circumstances, these alterations could trap and store hydrocarbons.

All along, overlying rocks continued to weigh down on underlying rocks, forcing hydrocarbons out of their source rocks. Seeping through subsurface cracks and fissures, oozing through small connections between rock grains, hydrocarbons moved upward until a subsurface barrier stopped them or until they reached the earth's surface. Most, however, became trapped and stored in a layer of subsurface rock. Today, the oil industry explores for petroleum formed and trapped millions of years ago.


A hydrocarbon reservoir has a distinctive shape, or configuration, that prevents hydrocarbons from escaping. Geologists classify reservoir shapes, or traps, into two types: structural and stratigraphic. A deformation in the rock layer that contains the hydrocarbons forms a structural trap, which can be either a fault trap or an anticlinal trap.

A fault trap is a break in the layers of rock. A fault trap occurs when formations on either side of a fault move and come to rest in a configuration that traps oil that migrates into it. Often, an impermeable formation on one side of the fault moves opposite a porous and permeable formation on the other side. The petroleum migrates into the porous and permeable formation and gets trapped by the impervious layer at the fault line.

An anticline trap is one located in an anticline, an upward fold in the layers of rock that looks much like a domed arch in a building. Oil and gas migrate into the folded porous and permeable layer and rise to the top. They cannot escape because of an overlying bed of impermeable rock.


Stratigraphic traps form when other beds seal a reservoir bed or when the permeability changes within the reservoir bed itself. In one type of stratigraphic trap, a horizontal, impermeable rock layer cuts off an inclined layer of petroleum-bearing rock. Sometimes an impervious layer cuts off, or pinches, a petroleum-bearing formation. Other stratigraphic traps are lens-shaped. Impervious layers surround the hydrocarbon-bearing rock. Yet another type occurs when the porosity and permeability change within the reservoir itself. The upper reaches of the reservoir are nonporous and impermeable, while the lower part is porous and permeable and contains hydrocarbons.


In a combination trap, more than one kind of trap forms a reservoir. A faulted anticline, for example, occurs when several faults cut across the anticline. In some places, the faults trap oil and gas. Another type of trap is a piercement dome. In this case, molten salt pierced surrounding rock beds. The moving salt deformed the horizontal beds. Later, the salt cooled and solidified and some of the deformed beds trapped oil and gas. A piercement dome formed Texas’ famous Spindletop deposit, the first to show that buried layers of rock could contain tremendous amounts of oil. Spindletop, which produced more than 80,000 barrels of oil per day when the first well was drilled in 1901, marked the beginning of the modern petroleum industry.