Power Plant Engineering

OPEC AND ITS MEMBERS
1. Algeria
2. Libya
3. Indonesia
4. Nigeria
5. Iran
6. Qatar
7. Iraq
8. United Arab Emirates
9. Kuwait
10. Saudi Arabia
11. Venezuela
Origins of Oil. Oil and gas are names given to a wide variety of hydrocarbons found in sedimen-
tary basins on or under the earth’s surface. Oil or petroleum is generally a complex mixture of the
heavier (non-gaseous) hydrocarbons, averaging about two atoms of hydrogen to each carbon atom. Oil
found in different reservoirs differs in composition, and many even vary within a single reservoir. Its
properties vary from a light fluid to viscous heavy oil, grading to asphalt.
The process of oil formation started with the mixing of marine organisms with sand and salt to
form sedimentary deposits, in periods ranging from tens of millions of hundreds of millions of years
ago. Continued deposits of material led to burial, with a concomitant rise in pressure and temperature,
resulting in compaction of the sediment into sedimentary rock, called the ‘source rock’, and conversion
of the organic material into hydrocarbons (oil) embedded in the source rock. Increasing pressure from
continued burial, together with the movement of water, with which rock below the water table is satu-
rated, resulted in movement of the small oil globules into the porous and permeable environment of
reservoir rocks. In some situations the oil became trapped in the reservoir rocks by a neighboring layer
of impermeable rock, and these oil bearing reservoir rocks are the sources from which oil is now ob-
tained.
Many types of geological structure can give rise to possible traps for oil. The first, called an
anticline trap is in the form of a dome, in which gas, oil and water are held within the reservoir rock
overlain by a layer of impermeable rock that prevents the oil and gas, more buoyant than the underlying
water, from escaping to the surface. The second type, is called a fault trap, and may occur where imper-
meable rock at a fault in the strata of reservoir rocks prevents upward movement of oil. In the fourth

---

42
POWER PLANT ENGINEERING
type, the reservoir rock changes in permeability so that further movement of oil through the pores of the
reservoir rock becomes possible.
To Summaries: Hydrocarbons are generated in source rocks from the remains of marine organ-
ism deposited and buried in the rocks. They are transported by surface tension, gravitational and pres-
sure forces into reservoir rocks, where, if there are suitable traps, they accumulate in the pores of the
rock and form the reservoirs of oil and gas found today. For oil to be formed within the source rocks,
they must have been buried for a million years or more at depths over 1 km, to get the pressure and
temperature high enough, but rarely more than about 4 km or the higher temperature at those depths
would usually decompose the oil, leaving methane gas and petroleum coke.