Geology Reference
In-Depth Information
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Figure 6.28
Ancient Sedimentary Rocks and Their Interpretation
Muav Limestone
Bright Angel Shale
Tapeats Sandstone
Image not available due to copyright restrictions
a
The Jurassic-aged Navajo Sandstone in Zion National Park in
Utah is a wind-blown dune deposit. Vertical fractures intersect cross-
beds, hence the name Checkerboard Mesa for this rock exposure.
Historically, most coal mined in the United States has
been bituminous coal from the Appalachian region that
formed in coastal swamps during the Pennsylvanian Period
(299-318 million years ago) (see Chapter 20). Huge lignite
and subbituminous coal deposits in the western United
States are becoming increasingly important. During 2005,
more than a billion tons of coal were mined in this country,
more than 60% of it from mines in Wyoming, West Virginia,
and Kentucky.
Anthracite coal (see Chapter 7) is especially desir-
able because it burns more efficiently than other types of
coal. Unfortunately, it is the least common variety, so most
coal used for heating buildings and generating electricity
is bituminous (Figure 6.21d).
Coke
, a hard, gray substance
consisting of the fused ash of bituminous coal, is used in
blast furnaces where steel is produced. Synthetic oil and gas
and a number of other products are also made from bitumi-
nous coal and lignite.
rock must have an overlying, nearly impervious
cap rock;
otherwise, the hydrocarbons would eventually reach the
surface and escape (
Figure 6.29a, b). Effective reser-
voir rocks must have appreciable pore space and good
permeability, the capacity to transmit fluids; otherwise,
hydrocarbons cannot be extracted from them in reason-
able quantities.
Many hydrocarbon reservoirs consist of nearshore
marine sandstones with nearby fine-grained, organic-rich
source rocks. Such oil and gas traps are called
stratigraphic
traps
because they owe their existence to variations in the
strata (Figure 6.29a). Indeed, some of the oil in the Persian
Gulf region and Michigan is trapped in ancient reefs that
are also good stratigraphic traps.
Structural traps
result when
rocks are deformed by folding, fracturing, or both. In sedi-
mentary rocks that have been deformed into a series of folds,
hydrocarbons migrate to the high parts of these structures
(Figure 6.29b). Displacement of rocks along faults (fractures
along which movement has occurred) also yields traps for
hydrocarbons (Figure 6.29b).
Other sources of petroleum that will probably become
increasingly important in the future include
oil shales
and
tar
sands.
The United States has about two-thirds of all known
oil shales, although large deposits are known in South Amer-
ica, and all continents have some oil shale. The richest de-
posits in the United States are in the Green River Formation
of Colorado, Utah, and Wyoming. When the appropriate ex-
traction processes are used, liquid oil and combustible gases
can be produced from an organic substance called
kerogen
,
which is found in oil shale (Figure 6.29c). Oil shale in the
Green River Formation yields between 10 and 140 gallons
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Petroleum and natural gas are
hydrocarbons
, meaning
that they are composed of hydrogen and carbon. The re-
mains of microscopic organisms settle to the seafloor, or
lakefloor in some cases, where little oxygen is present to
decompose them. If buried beneath layers of sediment,
they are heated and transformed into petroleum and nat-
ural gas. The rock in which hydrocarbons form is known
as
source rock
, but for them to accumulate in economic
quantities, they must migrate from the source rock into
some kind of
reservoir rock.
And finally, the reservoir
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