History of Oil

         Crude Oil: Key Historical Events

          Fundamentals of Oil
               Finding Oil & Gas
               Securing Leases
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               Evaluating/Well Logging/Coring
               Completing the Well
               Artificial Lifts/Injection Wells
               Oil Production/Secondary Recovery
               Waterflooding in the Illinois Basin
               Operation/Sale of Oil

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Finding Oil & Gas

Hydrocarbons - crude oil and natural gas - are found in certain layers of rock that are usually buried deep beneath the surface of the earth. In order for a rock layer to qualify as a good source of hydrocarbons, it must meet several criteria.

Characteristics of Reservoir Rock
Reservoir Rock For one thing, good reservoir rocks (a reservoir is a formation that contains hydrocarbons) have porosity. Porosity is a measure of the openings in a rock, openings in which petroleum can exist. Even though a reservoir rock looks solid to the naked eye, a microscopic examination reveals the existence of tiny openings in the rock. These openings are called pores. Thus a rock with pores is said to be porous and is said to have porosity.

Another characteristic of reservoir rock is that it must be permeable. That is, the pores of the rock must be connected together so that hydrocarbons can move from one pore to another. Unless hydrocarbons can move and flow from pore to pore, the hydrocarbons remain locked in place and cannot flow into a well.

In addition to porosity and permeability reservoir rocks must also exist in a very special way. To understand how, it is necessary to cross the time barrier and take an imaginary trip back into the very ancient past.

Imagine standing on the shore of an ancient sea, millions of years ago. A small distance from the shore, perhaps a dinosaur crashes through a jungle of leafy tree ferns, while in the air, flying reptiles dive and soar after giant dragonflies. In contrast to the hustle and bustle on land and in the air, the surface of the sea appears very quiet. Yet, the quiet surface condition is deceptive. A look below the surface reveals that life and death occur constantly in the blue depths of the sea. Countless millions of tiny microscopic organisms eat, are eaten and die. As they die, their small remains fall as a constant rain of organic matter that accumulates in enormous quantities on the seafloor. There, the remains are mixed in with the ooze and sand that form the ocean bottom.

As the countless millennia march inexorably by, layer upon layer of sediments build up. Those buried the deepest undergo a transition; they are transformed into rock. Also, another transition occurs: changed by heat, by the tremendous weight and pressure of the overlying sediments, and by forces that even today are not fully understood, the organic material in the rock becomes petroleum. But the story is not over.

For, while petroleum was being formed, cataclysmic events were occurring elsewhere. Great earthquakes opened huge cracks, or faults, in the earth’s crust. Layers of rock were folded upward and downward. Molten rock thrust its way upward, displacing surrounding solid beds into a variety of shapes. Vast blocks of earth were shoved upward, dropped downward or moved laterally. Some formations were exposed to wind and water erosion and then once again buried. Gulfs and inlets were surrounded by land, and the resulting inland seas were left to evaporate in the relentless sun. Earth’s very shape had been changed.

Meanwhile, the newly born hydrocarbons lay cradled in their source rocks. But as the great weight of the overlying rocks and sediments pushed downward, the petroleum was forced out of its birthplace. It began to migrate. Seeping through cracks and fissures, oozing through minute connections between the rock grains, petroleum began a journey upward. Indeed, some of it eventually reached the surface where it collected in large pools of tar, there to lie in wait for unsuspecting beasts to stumble into its sticky trap. However, some petroleum did not reach the surface. Instead, its upward migration was stopped by an impervious or impermeable layer of rock. It lay trapped far beneath the surface. It is this petroleum that today’s oilmen seek.

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Types of Petroleum Traps
Fault Trap Geologists have classified petroleum traps into two basic types: structural traps and stratigraphic traps.

Structural traps are traps that are formed because of a deformation in the rock layer that contains the hydrocarbons. Two common examples of structural traps are fault traps and anticlines.

A fault trap occurs when the formations on either side of the fault have been moved into a position that prevents further migration of petroleum. For example, an impermeable formation on one side of the fault may have moved opposite the petroleum-bearing formation on the other side of the fault. Further migration of petroleum is prevented by the impermeable layer.

Anticline MapAn anticline is an upward fold in the layers of rock, much like an arch in a building. Petroleum migrates into the highest part of the fold, and its escape is prevented by an overlying bed of impermeable rock.

Stratigraphical MapStratigraphic traps are traps that result when the reservoir bed is sealed by other beds or by a change in porosity or permeability within the reservoir bed itself. There are many different kinds of stratigraphic traps. In one type, a tilted or inclined layer of petroleum-bearing rock is cutoff or truncated by an essentially horizontal, impermeable rock layer. Or sometimes a petroleum-bearing formation pinches out; that is, the formation is gradually cut off by an overlying layer. Another stratigraphic trap occurs when a porous and permeable reservoir bed is surrounded by impermeable rock. Still another type occurs when there is a change in porosity and permeability in the reservoir itself. The upper reaches of the reservoir may be impermeable and nonporous, while the lower part is permeable and porous and contains hydrocarbons.

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