Chapter 14 - Energy Resources
Many of our energy resources originate from geologic processes. Oil, gas, coal,
and the fuel for atomic power plants all comes from rocks. Geothermal energy
originates in the heat generated by internal Earth processes. Wind and water
power involve the movement of wind and water involves cycles in the Earth’s
system.
There are five fundamental sources of energy in the Earth system: (1) solar
energy in the form of electromagnetic radiation, (2) gravitational energy, (3)
nuclear fission reactions, (4) stored energy in the Earth’s interior, (5) energy
in the form of chemical bonds in compounds
Usable energy resources:
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solar energy
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gravitational energy that drives the tides
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energy involving both solar energy and gravity. Wind, which powers
wind turbines, is a result of convective air movement as radiation heats the air
causing it to rise and gravity causes cooler air to sink. Solar energy evaporates
water which then falls as rain and restores surface and subsurface water
reservoirs. Moving water can power hydroelectric power plants.
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geothermal energy comes from the Earth’s internal energy sources
including radioactive decay
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energy from nuclear fission - during fission, a tiny amount of mass
transforms into a large amount of energy
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photosynthetic energy - green plants absorb solar energy which powers
the photosynthetic process and the manufacture of food energy
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energy from chemical reactions - some inorganic chemical compounds
can burn to produce light and energy
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fossil fuel energy - oil, gas and coal comes from the stored solar energy
captured by organisms that lived long ago
The increase in energy usage
for the past 150 yrs. Wood
was the main energy source in
1850. Hydrocarbons are the
main energy source now.
Oil and gas are organic chemical hydrocarbons composed of chain-like molecules of carbon
and hydrogen. The viscosity and volatility of a hydrocarbon depends on its molecular size.
Short-chain molecules tend to be less viscous and more volatile (ex. natural gas).
Hydrocarbons, like wood, can burn to release carbon dioxide, water and heat. The primary
sources of the organic chemicals in oil and gas come from plankton and algae. When algae
and plankton die, they sink to the bottom of a lake or sea and mix with clay particles to create
a muddy ooze. Eventually, layer after layer of sediment gets deposited on the lake bed and
bcomes lithified to black organic shale. This shale is referred to as a source rock.When the
buried shale reaches 100oC, chemical reactions occurring over millions of years transform it
into kerogen > then into a mixture of oil, tar, and gas.
Hydrocarbon reserves
only exist within the
top 15-20% of the
crust. Shale containing
kerogen is called oil
shale. At temperatures
> 160oC, the oil breaks
down to form natural
gas and at
temperatures > 250oC,
the remaining organic
matter transforms into
graphite. Oil itself
only forms within a
narrow temperature
range called the oil
window.
An oil field is a region that contains a significant amount of oil. The known
supply of oil in an area is known as an oil reserve. The development of an oil
reserve requires a source rock, a reservoir rock, and a trap.The largest known
oil reserves are in the region surrounding the Persian Gulf.
To be a reservoir rock, a
rock must have high
porosity and permeability.
Porosity refers to the
amount of open space or
pores in a rock. Shale has
a porosity of about 10%,
sandstone one of 35%.
Permeability refers to the way pore spaces connect to one another. In an oil well,
oil flows from the permeable reservoir rock into the well and then up to the ground
surface, usually assisted by pumping.
Oil resides in the source rock.
Oil migrates upward to
be trapped by the seal
rock.
Traps and seals
Oil and gas must be trapped underground to be available for pumping. An oil
trap is a geologic configuration that allows oil and gas to be trapped. A seal rock
is a relatively impermeable rock such as shale, salt, or unfractured limestone. The
seal rock and the reservoir rock must be arranged in a geometric configuration
that allows the oil to be collected in a restricted area. There are different types of
oil-trap geometries.
An anticline trap
An anticline is a fold with an
arched shape. If the folded
layers include source,
reservoir, and seal rocks, than
an oil reserve exists.
A salt dome trap
A salt dome traps oil
because it is impermeable
A fault trap
A fault trap is a fracture
where the slip on the fault
created an impermeable
layer.The oil collects in
tilted strata adjacent to the
fault.
A stratigraphic trap
A tilted reservoir rock bed
pinches out between two
impermeable layers. Oil
collects at the pinch-out.
Natural gas is a volatile, short-chain hydrocarbon and includes methane, ethane,
propane, and butane. Gas occurs above the oil. If temperatures are very high,
gas-only fields develop.Gas burns more cleanly than oil (producing CO2 and
H2O). Burning oil produces CO2, H2O and complex organic pollutants.
Coal is a sedimentary rock that consists of C, organic chemicals, quartz, and
clay. Coal forms from plant material that previously grew in vast coal swamps.
American’s use about 1 billion tons of coal/year (~23% of the energy supply).
Of course, significant coal supplies could not form until vascular land plants
appeared during the late Silurian period, about 420 mya. The most extensive
deposits of coal were formed during the Carboniferous period (290-360
mya).The extensive coal deposits during this period reflect the environmental
conditions of the time when North America, Asia and Europe straddled the
equator and sea levels rose.
Plate tectonics influenced
the climate of ancient coal
swamps, allowing the
accumulation of enormous
masses of plant material
that decayed and were
buried to form the large
coal deposits of today.
In order for coal to form, plant material has to be buried anaerobically (without
oxygen) and deeply. Transgression and regressions of the sea level allow the
accumulation of many sedimentary layers.The deeply buried layers heat up, driving
off volatiles. When C is > 70% of the residue, coal is formed. Organic material
buried shallowly in an anaerobic environment becomes peat. Peat contains 50% of
the world’s C.
Coal formation proceeds from peat > lignite coal > bituminous coal > anthracite.
The more volatiles are driven off by compaction and heat, the higher the C
content and the higher the coal ranking of the coal formed. Coal can’t exist in
metamorphic rocks. The high heat and pressures transform it into graphite.
The distribution of coal in North America.
The
distribution
of coal by
region.
Coal occurs in seams or beds because it is a sedimentary material. To be economic
to mine, coal has to be >1 m thick. Strip mining, where soil and rock layers are
scraped off to get to the coal seam, is the most economic way to mine coal. It can
also be the most environmentally destructive method unless steps are taken to
revegetate and fill in the damaged land.
Deep coal is mined by underground mining where a shaft to the depth of the
coal seam is dug and the coal is mined by machine. Underground coal mining
is dangerous because of the weaknesses of the sedimentary layers above the
coal seam and the accumulation of explosive gases. Miners can also contract
black-lung disease from inhaling coal dust.
Nuclear power
Nuclear power comes from a different process than the burning of fossil fuels.
One gram of nuclear reactor fuel = 2.7 barrels of oil. Nuclear power comes
from fission, the breaking of the nuclear bonds that hold protons and neutrons
together in the nucleus.Some elements, such as uranium, undergo fission
naturally. This process is called radioactive decay. In a nucler power plant,
uranium 235 is packed into fuel rods. Chain reactions occur when neutrons
released during fission strike other atoms, triggering more fission, etc. If
enough radioactive atoms are in a small enough space, a critical mass is
formed. Problems with nuclear power plants include the possibility of a
meltdown where the fission rate proceeds uncontrolled and melts the fuel rods
and possibly melts the containment structures to escape into the
environment.Chernobyl in 1985 was a near-meltdown when a steam explosion
blew the roof off the reactor and 10% of the radioactive material escaped into
the atmosphere. Nuclear waste disposal remains a contentious issue.
A chain reaction of
uranium. Fission of
atoms produces lots
of heat energy. When
a uranium atom
undergoes fission, it
produces 3 neutrons +
barium-141, krypton92, and energy.
How a reactor is
configured to
produce
electricity. The
control rods are
of boron-steel
and can be
raised or
lowered to
control the
reaction rate.
Geothermal energy
The heat produced by the Earth’s internal processes can be used as an
energy resource. This is common in countries such as Iceland where magma is
close to the surface because Iceland is in a rift zone
A few hydroelectric
power plants use the sea
and the tides to generate
electricity.
Oil reserves are predicted to run
out by 2050, only 47 years from
now!
Other fuel alternatives
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tar sand
solar energy
wind energy
Problems with extracting and/or implemention of conventional sources of fuel
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nuclear accidents and nuclear waste disposal
oil spills
acid mine runoff
acid rain
CO2 and global warming
End of Chapter 14