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    Oil and Gas: Characteristics and Occurrence

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    Oil and Gas: Characteristics and Occurrence

    مُساهمة من طرف Admin في الخميس نوفمبر 05, 2009 5:24 pm













    Oil and Gas: Characteristics and Occurrence
    <table border="0" cellpadding="2" cellspacing="2"><tr> <td>OIL AND GAS RESERVOIRS

    Hydrocarbons and
    their associated impurities occur in rock formations that are usually
    buried thousands of feet or metres below the surface. Scientists and
    engineers often call rock formations that hold hydrocarbons
    "reservoirs."
    Oil does not flow in underground rivers or pool up in subterranean
    lakes, contrary to what some people think. And, as you've learned,
    gasoline and other refined hydrocarbons do not naturally occur in
    pockets under the ground, just waiting to be drilled for. Instead,
    crude oil and natural gas occur in buried rocks and, once produced from
    a well, companies have to refine the crude oil and process the natural
    gas into useful products. Further, not every rock can hold
    hydrocarbons. To serve as an oil and gas reservoir, rocks have to meet
    several criteria.
    </td>
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    <td>Characteristics of Reservoir Rocks

    Nothing looks more
    solid than a rock. Yet, choose the right rock-say, a piece of sandstone
    or limestone-and look at it under a microscope. You see many tiny
    openings or voids. Geologists call these tiny openings "pores" (fig- 5
    i). 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-that is, its pores are connected (fig- 5 2). If
    hydrocarbons are in the pores of a rock, they must be able to move out
    of them. Unless hydrocarbons can move from pore to pore, they remain
    locked in place, unable to flow into a well. A suitable reservoir rock
    must therefore be porous, permeable, and contain enough hydrocarbons to
    make it economically feasible for the operating company to drill for
    and produce them.
    </td>
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    </table>

    Origin and Accumulation of Oil and Gas


    To understand how hydrocarbons get into buried rocks,
    visualize an ancient sea teeming with vast numbers of living organisms.
    Some are fishes and other large swimming beasts; others, however, are
    so small that you cannot see them without a strong magnifying glass or
    a microscope. Although they are small, they are very abundant. Millions
    and millions of them live and die daily. It is these tiny and plentiful
    organisms that many scientists believe gave rise to oil and gas.


    When these tiny
    organisms died millions of years ago, their remains settled to the
    bottom. Though very small, as thousands of years went by, enormous
    quantities of this organic sediment accumulated in thick deposits on
    the seafloor. The organic material mixed with the mud and sand on the
    bottom. Ultimately, many layers of sediments built up until they became
    hundreds or thousands of feet (metres) thick.

    The tremendous weight of the overlying sediments created great pressure
    and heat on the deep layers. The heat and pressure changed the deep
    layers into rock. At the same time, heat, pressure, and other forces
    changed the dead organic material in the layers into hydrocarbons:
    crude oil and natural gas.


    Meanwhile,
    geological action created cracks, or faults, in the earth's crust.
    Earth movement folded layers of rock upward and downward. Molten rock
    thrusted upward, altering the shape of the surrounding beds.
    Disturbances in the earth shoved great blocks of land upward, dropped
    them downward, and moved them sideways. Wind and water eroded
    formations, earthquakes buried them, and new sediments fell onto them.
    Land blocked a bay's access to open water, and the resulting inland sea
    evaporated. Great rivers carried tons of sediment; then dried up and
    became buried by other rocks. In short, geological forces slowly but
    constantly altered the very shape of the earth. These alterations in
    the layers of rock are important because, under the right
    circumstances, they can trap and store hydrocarbons.

    Even while the earth
    changed, the weight of overlying rocks continued to push downward,
    forcing hydrocarbons out of their source rocks. Seeping through
    subsurface cracks and fissures, oozing through small connections
    between rock grains, the hydrocarbons moved upward. They moved until a
    subsurface barrier stopped them or until they reached the earth's
    surface, as they did at Oil Creek. Most of the hydrocarbons, however,
    did not reach the surface. Instead, they became trapped and stored in a
    layer of subsurface rock. Today, the oil industry seeks petroleum that
    was formed and trapped millions of years ago.


    Petroleum Traps

    A hydrocarbon reservoir has a distinctive shape, or
    configuration, that prevents the escape of hydrocarbons that migrate
    into it. Geologists classify reservoir shapes, or traps, into two
    types: structural traps and stratigraphic traps.

    Structural Traps

    Structural traps form because of a deformation in the
    rock layer that contains the hydrocarbons. Two examples of structural
    traps are fault traps and anticlinal traps (fig. 5 3)




    <table border="0" cellpadding="2" cellspacing="2"><tr> <td>Fault Traps

    A fault is a break in the layers of rock. A fault trap
    occurs when the formations on either side of the fault move. The
    formations then come to rest in such a way that, when petroleum
    migrates into one of the formations, it becomes trapped there. 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. Once there, it cannot
    get out because the impervious layer at the fault line traps it.
    Anticlinal Traps

    An anticline is an upward fold in the layers of rock,
    much like a domed arch in a building. The 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.</td>

    <td></td>

    </tr>
    </table>

    Stratigraphic Traps

    Stratigraphic traps form when other beds seal a
    reservoir bed or when the permeability changes within the reservoir bed
    itself In one stratigraphic trap, a horizontal, impermeable rock layer
    cuts off, or truncates, an inclined layer of petroleum-bearing rock
    (fig- 54A). Sometimes a petroleum-bearing formation pinches out-that
    is, an impervious layer cuts it off (fig- 54B). Other stratigraphic
    traps are lens-shaped. Impervious layers surround the
    hydrocarbon-bearing rock (fig. 54C) - Still another occurs when the
    porosity and permeability change within the reservoir itself The upper
    reaches of the reservoir are nonporous and impermeable; the lower part
    is porous and permeable and contains hydrocarbons (fig- 54D).





    Other Traps
    Many other traps occur. In a combination trap, for
    example, more than one kind of trap forms a reservoir. A faulted
    anticline is an example. Several faults cut across the anticline. In
    some places, the faults trap oil and gas (fig- 55). Another trap is a
    piercement dome. In this case, a molten substance-salt is a common
    one-pierced surrounding rock beds. While molten, the moving salt
    deformed the horizontal beds. Later, the salt cooled and solidified and
    some of the deformed beds trapped oil and gas (fig- 56). Spindletop was
    formed by a piercement dome.



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