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1 Formation of Oil and Gas
Chemistry of Sustainable Energy
nitrogen, and oxygen that were once a part of living biota become part of the landscape. In an aerobic environment, this organic matter is oxidized, but in an oxygenfree environment (as when trapped in a layer of sediment), oxidation is negligible.
Numerous types of organic compounds, then, are trapped in sedimentary rocks and
converted into crude petroleum and other carbon-rich materials under conditions of
heat and pressure over a geologic timescale.
Fossil fuels, then, are mixtures of hydrocarbons in a low oxidation state, formed
over a period of millions of years and trapped in sediment that makes them fairly inaccessible. What is the chemical composition of these mixtures and what is their origin? Given that ancient crude oil primarily consists of even-numbered carbon chains
and that the material is both optically active and levorotatory, it is widely accepted
that the vast majority of oil is biological in origin (Selley 1998). Decomposition of
this once-living matter into fossil fuels is a process whereby the highly oxygenated
organic matter is reduced to hydrocarbon species.
Lipids and lignins from the original plant or animal material are the primary
organic materials that decay to hydrocarbon fossil fuels. (Ultimately, of course, these
carbon atoms came from carbon dioxide in the process of photosynthesis.) Lignins,
found in all woody plant materials as a very complex polymeric mixture of phenols
and glycerol, are the source of solid fossil fuels such as peat and coal (Figure 2.1).
FIGURE 2.1 Lignin structure.
n = 10, 12, 14, 16...
A fatty acid ester
Where R, R′, R′′ = long chain alkyl group from a fatty acid
FIGURE 2.2 Representative lipid families.
Lipids are organic compounds characterized by their solubility behavior: they are of
low polarity and are thus highly hydrophobic. Terpenes, fatty acids, fatty acid esters,
and phospho- and sphingolipids (Figure 2.2) containing long (even-numbered)
hydrocarbon chains are all considered lipids.
Lipids are the primary source of crude oil. Several million years ago, a multitude
of marine phytoplankton and bacteria lived, died, and were deposited in sediment.
Through reduction by anaerobic bacteria, burial, and compression in the rock cycle,
the material decomposed with the loss of small molecules such as water, methane,
and carbon dioxide. In this process known as diagenesis, an enriched organic material is formed. Dark layers of this material are found primarily in oil shale and are
called kerogen, a very complex solid mixture containing primarily carbon and
hydrogen with lesser percentages of nitrogen, oxygen, and sulfur.
There are three basic types of kerogen, distinguished by both their origin and
their general structures. Kerogen that is algal in origin is known as “Type I” kerogen
and is heavily aliphatic (see Figure 2.3), being formed from lipids. This type of kerogen ultimately becomes oil and gas. Types II and III kerogens are considerably more
aromatic in nature and derive from marine microorganisms (Type II) and woody
plants (Type III). Kerogen from the Green River Formation oil shale deposit in the
western United States, for example, is rich in algal kerogen and has an approximate
composition of C215H330O12N5S (Cane 1976). It is the thermal decomposition of kerogen under heat and compression at depths well below 1000 m—a process known as
maturation—that leads to oil, gas, and coal. If liquid, the petroleum is expelled and
migrates to nearby deposits.
While lipid-based kerogens lead primarily to petroleum, lignin-based kerogen
(from woody plants) leads to solid fossil fuels such as peat and coal. Coal, by definition, is a biogenic sedimentary rock composed of at least 50% decomposed plant
matter. Millions of years ago, tropical flora flourished in the swamps, bogs, and
forests of the Earth. Layers of dead plant matter became incorporated into parts of
Earth’s crust. Peat, the precursor to coal, is one example. Having a high moisture
content, as the peat is compressed, small molecules (e.g., water and methane) are
Chemistry of Sustainable Energy
FIGURE 2.3 Type I kerogen.
expressed out, increasing the proportion of carbon and leading to varying grades of
coal. The overall approximate chemical composition of coal is CH0.8SxNyOz (where
x, y, and z are each <0.1). Coal is classified into various grades (Tester et al. 2005):
Anthracite is a hard, lustrous coal containing a high percentage (86–97%) of carbon and few volatile components. It typically has the highest heating value of coal
types (see Table 2.1) and is largely used for residential and commercial space heating
because of its limited availability. No new anthracite is being mined in the United
Bituminous coal is 45–86% carbon and accounts for almost one-half the coal
produced in the United States. Bituminous and subbituminous coals (see below) are
used primarily for steam-electric power generation.
Subbituminous coal consists of 35–45% carbon and is midrange between bituminous coal and lignite in terms of properties. Similarly, its appearance ranges from
the lustrous, hard appearance of the higher grades of coal to a soft brown-to-black
Lignite contains 25–35% carbon and has a high moisture content. Lignite is also
known as “brown coal.” One step removed from peat, lignite (unlike peat) is free
Types and Properties of Coal
Typical Heat Content
Source: Data from U.S. Department of Energy Information Administration, EIA-923 Monthly Time
Series File, Fuel Receipts and Cost, Schedules 2 (March 2012).
of cellulose. A 50-m layer of peat will be compressed over time to a 10-m layer of
lignite (Skinner and Porter 2000).
While the carbon and hydrogen in coal generate heat by combustion, the other
trace elements in coal make it a significant source of airborne pollution. Particulate
matter, SO2, NOx, as well as mercury (both in its elemental and oxidized form) are
released upon combustion, making coal-fired plants the largest single source of
anthropogenic mercury pollution (United Nations Environment Programme 2012).
Several approaches have been developed for mercury reduction, from injection of
activated carbon to trapping particulates by a fabric filter or electrostatic precipitation. Gasification of coal (Chapter 5) is the basis for “clean coal technology,”
wherein coal is converted into hydrogen gas with cogeneration of CO2 (which can be
sequestered) in what is known as “IGCC” (integrated gasification combined cycle)
technology (Armaroli and Balzani 2011). By integrating coal gasification with heatdriven turbines, waste heat that would otherwise be lost in the exhaust stream is captured, leading to higher efficiencies. More details on CO2 sequestration are provided
in Section 2.5.
The hydrocarbon fuels upon which most of our way of life is based are aptly
labeled as fossil fuels: it has taken unimaginably staggering volumes of marine
microorganisms with fantastic pressures and temperatures over millions of years to
yield sedimentary rock sources from which we can (with some difficulty) extract oil.
Conversion of decaying plants through the same processes forms seams of coal that
must be mined at significant financial, human, and environmental cost. “Hubbert’s
peak” describes the point at which the demand for these fossil fuels outpaces the
global production capacity—a peak that may have already been reached for oil, and
is predicted for the latter part of the twenty-first century for natural gas (Deffeyes
2005). These are in no sense sustainable fuels.
2.2 EXTRACTION OF FOSSIL FUELS
Fossil fuels, then, are finite forms of fuel that humankind has taken advantage of
over the past century to support our way of life. It is problematic enough that the formation of these fuels took eons, but getting these sources of energy out of the ground
presents additional challenges and takes a severe toll. The more scarce the fuel, the
more drastic our methods for extraction become as new nonconventional methods
for extracting petroleum resources illustrate (vide infra).
2.2.1 Conventional Petroleum
In the early years of oil exploration and discovery, petroleum geologists were able
to tap into reservoirs of light, free-flowing oil trapped underground. As can be seen
in Figure 2.4, a structural oil trap stratifies water, oil, and gas on the basis of different densities. The oil reserves in the Middle East are a good example of oil-rich
structural oil traps. Relatively straightforward technology is used to vertically drill
through the impervious rock trapping the oil, releasing and then capturing the oil
and gas. The petroleum industry has been producing oil and gas in this manner—
the “conventional” manner—since the late nineteenth century. As readily accessible
Chemistry of Sustainable Energy
FIGURE 2.4 Types of structural oil traps: (a) anticline and (b) fault. (Skinner, B.J. and S.C.
Porter: The Dynamic Earth. An Introduction to Physical Geology. 4th ed. 2000. Copyright
Wiley-VCH Verlag GmbH & Co. KGaA. Reprinted with permission.)
onshore supplies have dwindled, exploration and recovery have moved offshore, with
concomitant increase in risk and cost. The BP Deepwater Horizon explosion and
oil spill of April 2010 has the distinction of being the largest unintentional oil spill
in our planet’s history. Having killed 11 oil rig workers and spewed an estimated
4.9 million barrels of crude oil into the Gulf of Mexico, the costs of this event are
2.2.2 Nonconventional Sources
As supplies of conventional oil and gas have declined, new methods for accessing
the more elusive hydrocarbons trapped onshore have evolved. Once deemed nonproducible, these nonconventional sources of oil and gas (also called tight oil or gas)
include shale gas, oil shale, tar sands, methane coalbeds, and methane clathrates.
In other words, as the relatively accessible supplies dry up, the petroleum industry
must go to further and further lengths to find and extract petroleum from the planet,
using breakthrough technology in physics, chemistry, and engineering to meet our
220.127.116.11 Shale Oil and Gas
Shale is a fine-grained sedimentary rock that can trap natural gas in natural fractures and pores, or adsorb it onto organic matter and minerals in the shale (Gregory