   #copyright

Oil reservoir

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   An oil reservoir, petroleum system or petroleum reservoir is often
   thought of as being an underground " lake" of oil, but it is actually
   composed of hydrocarbons contained in porous rock formations.

Formation

   The crude oil found in oil reservoirs forms in the Earth's crust from
   the remains of living things. Crude oil is properly know as petroleum,
   and is a kind of fossil fuel. Scientific evidence indicates that
   millions of years of heat and pressure changed the remains of
   microscopic plant and animal remains into crude oil and natural gas.

   Roy Nurmi, an interpretation adviser for Schlumberger described the
   process as follows: "Something in the order of 500 million years ago
   there was only simple life in the seas, and these shallow seas would be
   rich with organic, living organisms. Plankton and algae, proteins and
   the life that's floating in the sea, as it dies, falls to the bottom,
   and these organisms are going to be the source of our oil and gas. When
   they're buried with the accumulating sediment and reach an adequate
   temperature, something above 50 to 70°C they start to cook. This
   transformation, this change, changes them into the liquid hydrocarbons
   that move and migrate, will become our oil and gas reservoir."^

   In addition to the water environment mentioned, which is usually a sea
   but might also be a river, lake, coral reef or algal mat, the formation
   of an oil or gas reservoir also requires a sedimentary basin that
   passes through four steps: burial under miles of sand and mud, pressure
   cooking, hydrocarbon migration from the source to porous rock, and
   trapping by impermeable rock. Timing is also an important
   consideration; it is suggested that the Ohio River valley could have
   had as much oil as the Middle East at one time, but that it escaped due
   to a lack of traps.^ The North Sea, on the other hand, endured millions
   of years of sea level changes that successfully resulted in the
   formation of more than 150 oilfields.^

   Although the process is generally the same, various environmental
   factors lead to the creation of a wide variety of reservoirs.
   Reservoirs exist anywhere from 1,000 to 30,000 ft below the surface and
   are a variety of shapes, sizes and ages.^

Traps

   The traps required in the last step of the reservoir formation process
   have been classified by petroleum geologists into two types: structural
   and stratigraphic. A reservoir can be formed by one kind of trap or a
   combination of both.

Structural traps

   Structural traps are formed by a deformation in the rock layer that
   contains the hydrocarbons (e.g., fault traps and anticlinal traps).

Stratigraphic traps

   Stratigraphic traps are formed when other beds seal a reservoir bed or
   when the permeability changes ( facies change) within the reservoir bed
   itself.

   An example of this kind of trap starts when salt deposited by shallow
   seas. Later, a sinking seafloor deposits organic-rich shale over the
   salt, which is in turn covered with sandstone. As the Earth's pressure
   pushes the salt up, the shale is "cooked," producing oil that seeps up
   into the sandstone above. In some places, the salt breaks through the
   shale and sandstone layers into a salt dome that effectively traps the
   hydrocarbons beneath it.^

Production

   To obtain the contents of the oil reservoir, it is usually necessary to
   drill into the Earth's crust, although surface oil seeps exist in some
   parts of the world.

Location

   Active areas of surface oil reservoirs

     * Texas

   Active areas of existing sub-sea oil reservoirs

     * North Sea
     * West Africa
     * South America
     * Gulf of Mexico

   Retrieved from " http://en.wikipedia.org/wiki/Oil_reservoir"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
