Oil and natural gas are products of plant and animal remains or organic material. For the most part, these plants, corals, and animals lived in seas. Over millions of years, the remains settled on the ocean floor together with sediment which washed down from exposed earth and rock. In later periods, these layers of organic material and sediment were covered by more sediment which, as a result of time and pressure, converted these layers to sedimentary rock filled with hydrocarbons. To get a clearer picture of how this process occurred we need to understand some geological history.

The earth is believed to have formed some 4.6 billion years ago from a cloud of cosmic dust and ice. As the planet pulled itself together by gravity, compressing matter ever more densely, it became molten. The heaviest components, such as nickel and iron, sank to the center to form the planet's core while the lighter materials, including silicon, aluminum, magnesium and other light elements, solidified into a thin rocky crust. Water vapor moved to the surface to form oceans.

Over time the earth's crust has become thicker and more stable. Today, the planet can be viewed as a system of plates that fit together like a puzzle which slowly move and change shape.

Pangaea

As recently as the Triassic Period 225 million years ago, the Earth's continents were still connected together in a single super continent called Pangaea. Pangaea fragmented into several pieces, each piece floating on a molten layer. These mobile plates of the Earth's outer crust, lithosphere, have positioned themselves to form the Earth as we know it today.

During this 225 million year period, the continents and oceans experienced additional changes. As the earth's plates moved against one another, the forward or leading edges either lifted up on top of the next plate or moved below it causing edges to crumple and increase in height thus producing the mountains we see today. As plates separated, magma rose from the mantle and solidified in the rift forming mid-ocean ridges. The new crust, thinner than the continents, spread between the plates. While the rate of plate movement is very slow (averaging perhaps the speed of a growing fingernail), over the time spans we are considering, the results are dramatic. The theory that explains these plate movements is called plate tectonics.

The two basic types of crust are oceanic and continental. While the oceanic crust is thin, about 5 to 7 miles thick and composed of heavy igneous rock that formed from magma flows, the continental crust is 10 to 30 miles thick. Because of these differences, the continents tend to float, rising high above sea level as in the mountainous regions. These continental mountains were gradually worn down by rain and the action of ice, and the freed particles of rock were carried to the sea where they were deposited in thick sedimentary beds, cemented together by minerals and the pressure of more sediment deposited above.

At various times (some spanning tens of millions of years) much of North America was covered by seas. This occurred in warmer periods when the polar ice caps were much smaller and consequently held less water. This phenomenon of changing sea levels helps explain why oil and gas deposits are present far inland from any existing ocean.

Generally, oil and gas are found in a geologic structure called a "trap" that prevents the oil and gas from escaping. There are two general types of reservoir traps: a) structural and b) stratigraphic. Structural traps are formed by the deformation of the reservoir formation while stratigraphic traps are the result of an updip seal of porosity and permeability.

The anticline trap is formed by the folding of rocks into a dome. These anticlinal traps contain petroleum that has migrated from a source below. Further upward, migration of hydrocarbons was prevented by an impenetrable layer of rock above the reservoir. Fault traps are formed by the shearing and offsetting of rock strata. The escape of petroleum from a fault trap is prevented by non-porous rocks that have moved into position opposite the porous petroleum bearing rock formation. Dome and plug traps are porous formations on or around great plugs of salt or serpentine rock that has pierced or lifted the overlying rock layers. Stratigraphic traps are caused either by a nonporous formation sealing off the top edge of a reservoir or by a change in the porosity and permeability of the bed itself.

Chart courtesy of Earth Science World and Exxon-Mobil