Characteristics of 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 (Figure 1).
AFigure 1: Porositynother 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 (Figure 2). 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.
Figure 2: Permeability 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 sea floor. 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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