The amount of economically recoverable coal is upward of 1 trillion tons.The
United States holds substantial reserves of coal (Fig. 112), which remain prac-
tically untouched. Since coal is the cheapest and most abundant fossil fuel, it
will be a favorable alternate source of energy to replace petroleum when
reserves run low. However, because coal produces more pollution than other
fossil fuels, new technology will be required to clean up coal-fired plants.
Paleozoic sediments hold a large portion of the world’s oil reserves, indi-
cating a high degree of marine organic productivity during this time. The for-
mation of oil and gas requires special geologic conditions. These include a
sedimentary source of organic material, a porous rock to serve as a reservoir, and
a confining structure to act as a trap. The source material is organic carbon in
fine-grained, carbon-rich sediments. Porous and permeable sedimentary rocks
such as sandstones and limestones serve as reservoirs. Geologic structures created
by folding or faulting of sedimentary layers trap or pool the oil and gas.
Most organic material that produces petroleum originates from micro-
scopic organisms that originated primarily in the surface waters of the ocean
and were concentrated in fine particulate matter on the seafloor. For organic
material to become petroleum, either the rate of accumulation must be high
Figure 111Fossilized
plants of the upper
Potsville series,
Washington County,
Arkansas.
(Photo by E. B. Hardin,
courtesy USGS)CARBONIFEROUS AMPHIBIANS