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1 Formation of Oil and Gas

1 Formation of Oil and Gas

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20



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).

CH2OH



O

HO



CH2OH



OCH3

O



CH3O

O

HO



HO



CH3O



HO



O



CH2OH



OH

O



OH



CH3O

O



CH3O



O



CH3O



OH



CH2OH



HOCH2

O



O



O



OCH3



OH

H3CO

HOCH2



O

HOCH2



O



OCH3

H



CH3O

O



O



HO

OCH3

OCH3



CH3O

O



FIGURE 2.1  Lignin structure.



OCH3



21



Fossil Fuels

O



O

O



O



n



OH



n = 10, 12, 14, 16...

A saturated

fatty acid



O



O



R



O



R′



O



R′

O



O

O



O



(CH2)12CH3

R



R′′



O



P



OH



O

A fatty acid ester



A phospholipid



OH

H

N

O



O

R′

O

P



OCH2CH2N(CH3)3



O

A sphingolipid



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



22



Chemistry of Sustainable Energy

(CH2)2CH3



CH3CH2



O



HO



CH3



N

H

(CH2)10CH3



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

States.

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

form.

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



TABLE 2.1

Types and Properties of Coal

Coal Type

Bituminous

Subbituminous

Lignite

Anthracite



Typical Heat Content

Range (mmBtu/ton)

22–26

16–18

12–13

22–28



Typical Sulfur

Content (%)

≈2

≈0.3

≈0.9



Average Ash

Content (%)

11

13.8

5.3



Source: Data from U.S. Department of Energy Information Administration, EIA-923 Monthly Time

Series File, Fuel Receipts and Cost, Schedules 2 (March 2012).



Fossil Fuels



23



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



24



Chemistry of Sustainable Energy

(a)



Gas



ock

of r

o

R

ock

ir r

o

v

er

Res



Oil



Structural traps



Water



(b)



ock

of r

k

Ro

roc

oir

v

r

e

Oil

Res



Gas



Ro



o

of r



ck



Water



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

incalculable.



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

demands.

2.2.2.1  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



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