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The evolution of multicomponent systems at high pressures: VI. The thermodynamic stability of the hydrogen–carbon system: The genesis of hydrocarbons and the origin of petroleum


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The spontaneous genesis of hydrocarbons that comprise natural petroleum have been analyzed by chemical thermodynamic-stability theory. The constraints imposed on chemical evolution by the second law of thermodynamics are briefly reviewed, and the effective prohibition of transformation, in the regime of temperatures and pressures characteristic of the near-surface crust of the Earth, of biological molecules into hydrocarbon molecules heavier than methane is recognized. For the theoretical analysis of this phenomenon, a general, first-principles equation of state has been developed by extending scaled particle theory and by using the technique of the factored partition function of the simplified perturbed hard-chain theory. The chemical potentials and the respective thermodynamic Affinity have been calculated for typical components of the H-C system over a range of pressures between 1 and 100 kbar (1 kbar = 100 MPa) and at temperatures consistent with those of the depths of the Earth at such pressures. The theoretical analyses establish that the normal alkanes, the homologous hydrocarbon group of lowest chemical potential, evolve only at pressures greater than approximately 30 kbar, excepting only the lightest, methane. The pressure of 30 kbar corresponds to depths of approximately 100 km. For experimental verification of the predictions of the theoretical analysis, a special high-pressure apparatus has been designed that permits investigations at pressures to 50 kbar and temperatures to 1,500 degrees C and also allows rapid cooling while maintaining high pressures. The high-pressure genesis of petroleum hydrocarbons has been demonstrated using only the reagents solid iron oxide, FeO, and marble, CaCO3, 99.9% pure and wet with triple-distilled water.
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The evolution of multicomponent systems at high
pressures: VI. The thermodynamic stability of
the hydrogen–carbon system: The genesis of
hydrocarbons and the origin of petroleum
J. F. Kenney
, Vladimir A. Kutcherov
, Nikolai A. Bendeliani
, and Vladimir A. Alekseev
Gas Resources Corporation, 11811 North Parkway, Floor 5, Houston, TX 77060;
Russian Academy of Sciences, Joint Institute of Earth Physics,
Bolshaya Gruzinskaya 10, 123810 Moscow, Russia;
Russian State University of Oil and Gas, Leninski Prospect 65, 117917 Moscow, Russia;
Russian Academy of Sciences, Institute for High Pressure Physics, 142092 Troitsk, Moscow Region, Russia
Communicated by Howard Reiss, University of California, Los Angeles, CA, June 24, 2002 (received for review April 3, 2002)
The spontaneous genesis of hydrocarbons that comprise natural
petroleum have been analyzed by chemical thermodynamic-stability
theory. The constraints imposed on chemical evolution by the second
law of thermodynamics are briefly reviewed, and the effective pro-
hibition of transformation, in the regime of temperatures and pres-
sures characteristic of the near-surface crust of the Earth, of biological
molecules into hydrocarbon molecules heavier than methane is rec-
ognized. For the theoretical analysis of this phenomenon, a general,
first-principles equation of state has been developed by extending
scaled particle theory and by using the technique of the factored
partition function of the simplified perturbed hard-chain theory. The
chemical potentials and the respective thermodynamic Affinity have
been calculated for typical components of the H–C system over a
range of pressures between 1 and 100 kbar (1 kbar 100 MPa) and
at temperatures consistent with those of the depths of the Earth at
such pressures. The theoretical analyses establish that the normal
alkanes, the homologous hydrocarbon group of lowest chemical
potential, evolve only at pressures greater than 30 kbar, excepting
only the lightest, methane. The pressure of 30 kbar corresponds to
depths of 100 km. For experimental verification of the predictions
of the theoretical analysis, a special high-pressure apparatus has been
designed that permits investigations at pressures to 50 kbar and
temperatures to 1,500°C and also allows rapid cooling while main-
taining high pressures. The high-pressure genesis of petroleum hy-
drocarbons has been demonstrated using only the reagents solid iron
oxide, FeO, and marble, CaCO
, 99.9% pure and wet with triple-
distilled water.
atural petroleum is a hydrogen–carbon (H–C) system, in
distinctly nonequilibrium states, composed of mixtures of
highly reduced hydrocarbon molecules, all of very high chemical
potential and most in the liquid phase. As such, the phenomenon
of the terrestrial existence of natural petroleum in the near-surface
crust of the Earth has presented several challenges, most of which
have remained unresolved until recently. The primary scientific
problem of petroleum has been the existence and genesis of the
individual hydrocarbon molecules themselves: how, and under what
thermodynamic conditions, can such highly reduced molecules of
high chemical potential evolve?
The scientific problem of the genesis of hydrocarbons of natural
petroleum, and consequentially of the origin of natural petroleum
deposits, regrettably has been one too much neglected by compe-
tent physicists and chemists; the subject has been obscured by
diverse, unscientific hypotheses, typically connected with the ro-
coco hypothesis (1) that highly reduced hydrocarbon molecules of
high chemical potentials might somehow evolve from highly oxi-
dized biotic molecules of low chemical potential. The scientific
problem of the spontaneous evolution of the hydrocarbon mole-
cules comprising natural petroleum is one of chemical thermody-
namic-stability theory. This problem does not involve the properties
of rocks where petroleum might be found or of microorganisms
observed in crude oil.
This paper is organized into five parts. The first section reviews
briefly the formalism of modern thermodynamic-stability theory,
the theoretical framework for the analysis of the genesis of hydro-
carbons and the H–C system, as similarly for any system.
The second section examines, applying the constraints of ther-
modynamics, the notion that hydrocarbons might evolve sponta-
neously from biological molecules. Here are described the spectra
of chemical potentials of hydrocarbon molecules, particularly the
naturally occurring ones present in petroleum. Interpretation of the
significance of the relative differences between the chemical po-
tentials of the hydrocarbon system and those of biological mole-
cules, applying the dictates of thermodynamic-stability theory,
disposes of any hypothesis of an origin for hydrocarbon molecules
from biological matter, excepting only the lightest, methane.
In the third section is described a first-principles, statistical
mechanical formalism, developed from an extended representation
of scaled particle theory (SPT) appropriate for mixtures of aspheri-
cal hard-body molecules combined with a mean-field representa-
tion of the long-range, attractive component of the intermolecular
In the fourth section, the thermodynamic Affinity developed
using this formalism establishes that the hydrocarbon molecules
peculiar to natural petroleum are high-pressure polymorphs of
the H–C system, similarly as diamond and lonsdaleite are to
graphite for the elemental carbon system, and evolve only in
thermodynamic regimes of pressures greater than 25–50 kbar (1
kbar 100 MPa).
The fifth section reports the experimental results obtained using
equipment specially designed to test the predictions of the previous
sections. Application of pressures to 50 kbar and temperatures to
1,500°C upon solid (and obviously abiotic) CaCO
and FeO wet
with triple-distilled water, all in the absence of any initial hydro-
carbon or biotic molecules, evolves the suite of petroleum fluids:
methane, ethane, propane, butane, pentane, hexane, branched
isomers of those compounds, and the lightest of the n-alkene series.
1. Thermodynamic Stability and the Evolution of
Multicomponent Systems
Central to any analysis of chemical stability is the thermody-
namic Affinity, A({
}), which determines the direction of
evolution of a system in accordance with the second law of
thermodynamics as expressed by De Donder’s inequality, dQ⬘⫽
0 (2). The Affinity of an n-component, multiphase system
of p phases involving r chemical reactions is given as
Abbreviations: STP, standard temperature and pressure; SPT, scaled particle theory; SPHCT,
simplified perturbed hard-chain theory.
To whom reprint requests should be addressed. E-mail:
August 20, 2002
vol. 99
no. 17 www.pnas.orgcgidoi10.1073pnas.172376899
p, T, n
其兲, [1]
in which
are the chemical potential and stoichio-
metric coefficients of the ith component in the
th reaction,
designates the respective phase.
The second law states that the internal production of entropy
is always positive for every spontaneous transformation. There-
fore, the thermodynamic Affinity (Eq. 1) must always be posi-
tive, and the direction of evolution of any system must always
obey the inequalities:
p, T, n
The inequalities in Eq. 2 express the irreversibility of spontaneous
transitions and state that for a spontaneous evolution of a system
from any state, A, to any other state, B, the free enthalpy of state
B must be less than that of state A, and at no point between the two
may the free enthalpy be greater than that of state A or less than
that of state B.
The sum of the products on the second line of inequality in Eq.
2, of the thermodynamic Affinities and the differential of the
variables of extent, d
, is always positive, and the circumstance for
which the change of internal entropy is zero defines equilibrium,
from which there is no spontaneous evolution. This is De Donders
The sum of products on the second line of inequality in Eq. 2
deserves particular note. In the second line of Eq. 2, F
and dX
general thermodynamic forces and flows, respectively, and subsume
Newtons rule, F
, as a special case (3, 4). The expression in the
second line of Eq. 2 states further that for any circumstance for
which the Affinity does not vanish, there exists a generalized
thermodynamic force that drives the system toward equilibrium.
The constraints of this expression assure that an apple, having
disconnected from its bough, does not fall, say, half way to the
ground and there stop (a phenomenon not prohibited by the first
law) but must continue to fall until the ground. These constraints
force a chemically reactive system to evolve always toward the state
of lowest thermodynamic Affinity.
Thus, the evolution of a chemically reactive, multicomponent
system may be determined at any temperature, pressure, or com-
position whenever the chemical potentials of its components are
known. To ascertain the thermodynamic regime of the sponta-
neous evolution of hydrocarbons, their chemical potentials must
be determined.
No consideration has been given in the foregoing discussion of
chemical thermodynamic stability to the rate of increase of the
variables of extent, d
. Such is the subject of chemical kinetics, not
stability theory. The rate at which a reaction might occur cannot
alter its direction as determined by the second law of thermody-
namics; otherwise the second law would not exist. The evolution of
a system can admit intermediate states, in which one (or more)
intermediate product might possess a chemical potential consider-
ably greater than that of any of the initial reagents. The presence
of a selected catalyst can enhance a fast reaction, and if the system
is removed rapidly from thermodynamic environment at which such
reactions proceeding to the final state occur, the intermediate
product(s) can be separated. The petrochemical industry routinely
operates such processes. However, such complex industrial pro-
cesses are not mimicked spontaneously in the natural world.
2. The Thermodynamic Energy Spectrum of the H–C System
and the Effective Prohibition of Low-Pressure Genesis
of Hydrocarbons
The thermodynamic energy spectrum of the chemical potentials
(molar Gibbs energies of formation, G
)oftheHC system at
standard temperature and pressure [(STP, 298.15 K; 1 atm
(1 atm 101.3 kPa)] is available from tables of chemical data
(5). The chemical potentials of naturally occurring members of
the of the HC system at STP are shown graphically in Fig. 1.
Examination of the energy spectrum of these chemical potentials
of the HC system establishes at once that, at STP, the chemical
potentials of the entire hydrocarbon system are remarkable for
both their characteristic increase with degree of polymerization
as well as their linear, and almost constant, magnitude of such
increase with carbon number. With increasing polymerization,
the n-alkane molecules manifest increased chemical potential of
very approximately 2.2 kcal per added carbon atom or CH
(There exist also branched isomers, the chemical potentials of
which differ from such of the normal configuration by, typically,
24%.) Such increase in chemical potential with increased
degree of polymerization contrasts strongly with the thermody-
namic spectrum of the highly oxidized biotic carbon (‘‘organic’’)
compounds of the hydrogencarbonoxygen (HCO) system,
which manifest consistently decreasing chemical potentials with
increasing polymerization. This latter property allows the
high degree of polymerization and complexity of the biotic
Examination of the HCO system of oxidized carbon com-
pounds establishes that the chemical potentials of almost all biotic
compounds lie far below that of methane, the least energetic of the
reduced hydrocarbon compounds, typically by several hundred
kcalmol. Although there exist biotic molecules of unusually high
chemical potential such as
-carotene (C
), vitamin D
O), and some of the pheromone hormones, such com-
pounds are relatively rare by abundance. They are produced by
biological systems only when the producing entity is alive (and at
formidable metabolic cost to the producing entity), and the pro-
duction ceases with the death of the entity. Such compounds are not
Fig. 1. Molar Gibbs energies of formation, G
, of the naturally occurring
hydrocarbons at STP (5).
Kenney et al. PNAS
August 20, 2002
vol. 99
no. 17
decomposition products of other biotic compounds and are labile
and themselves decompose rapidly. For these foregoing reasons,
such compounds cannot be considered relevant to the subject of the
origin of natural petroleum.
The properties of the thermodynamic energy spectrum of the
HC and HCO systems, together with the constraints of the
second law (Eq. 2) establish three crucial properties of natural
(i) The HC system that constitutes natural petroleum is a
metastable one in a very nonequilibrium state. At low pres-
sures, all heavier hydrocarbon molecules are thermodynami-
cally unstable against decomposition into methane and carbon,
as similarly is diamond into graphite.
(ii) Methane does not polymerize into heavy hydrocarbon mole-
cules at low pressures at any temperature. Contrarily, increas-
ing temperature (at low pressures) must increase the rate of
decomposition of heavier hydrocarbons into methane and
(iii) Any hydrocarbon compound generated at low pressures,
heavier than methane, would be unstable and driven to the
stable equilibrium state of methane and carbon.
These conclusions have been demonstrated amply by a century
of refinery engineering practice. The third conclusion has been
demonstrated by many unsuccessful laboratory attempts to convert
biotic molecules into hydrocarbons heavier than methane.
There are three generic chemical processes that deserve specific
consideration: the ‘‘charcoal burners,’’ ‘‘bean-eaters,’’ and ‘‘oc-
tane-enhanced bean-eaters’’ reactions. All describe limited reac-
tions by which a highly oxidized biotic molecule can react to
produce elemental carbon when ‘‘carried’’ by a more thorough
oxidation process. In both the following examples, the simple
carbohydrate, sugar C
, is used as a typical biotic reagent; the
same reasoning and results hold also for any of the highly oxidized
biotic compounds.
The charcoal burners reaction is:
3 6C 6H
O. [3]
The chemical potential of water vapor at STP is 54.636 kcalmol.
The thermodynamic Affinity for the charcoal burners reaction
(Reaction 3) to produce amorphous carbon, or graphite, is 109.10
kcal. Therefore, the genesis of coal from biological detritus in an
oxygen-poor environment is permitted by the second law. However,
the thermodynamic Affinity for the charcoal burners reaction to
produce diamond is 105.02 kcal, the quantity of which is also
positive, and therefore not immediately prohibited by the second
law as expressed solely by de Donders inequality, the first of
equations (Eq. 2). Nonetheless, no charcoal burner ever scrabbles
through his ashes hoping to find diamonds. Such reasonable
behavior demonstrates an effective appreciation of the dictates of
the second law as expressed by Eq. 2. In this case, the generalized
force is the difference in thermodynamic Affinity between the
reactions for graphite and diamond, respectively,
AT, which,
in the regime of temperatures and pressures of the near-surface
crust of the Earth, assures always the genesis of graphite, never
diamond. Similarly, for reactions involving hydrocarbons heavier
than methane, the same generalized force,
AT, always drives the
system toward the state of lowest free enthalpy, i.e., methane plus
free carbon.
The bean-eaters reaction is:
3 3CH
. [4]
The chemical potentials at STP for the simple carbohydrate
, methane, and carbon dioxide are 218.720, 12.130, and
94.260 kcalmol, respectively. The thermodynamic Affinity for
the reaction accordingly is 100.42 kcalmol and therefore permitted
by the second law. Indeed, reactions of the type in Reaction 4 are
typical of those by which methane is produced in swamps, sewers,
and the bowels of herbivores.
The octane-enhanced bean-eaters reaction is:
3 2CH
. [5]
Since the chemical potential of n-octane is 4.290 kcalmol at STP,
and that of molecular hydrogen is zero, the thermodynamic Affinity
for the octane-enhanced bean-eaters reaction is A (100.42
12.130 4.2908) 87.70 kcalmol, still positive and thereby not
prohibited outright by the constraints of De Donders inequality.
However, no biochemical investigation has ever observed a mole-
cule of any hydrocarbon heavier than methane resulting from the
decomposition of biological detritus. After a meal of, e.g., Boston
baked beans, one does experience biogenic methane, but not
biogenic octane. No such process produces heavier hydrocarbons,
for such a process would involve effectively a reaction of low-
pressure methane polymerization, similarly as the effective prohi-
bition of the evolution of diamonds by the charcoal burners
reaction. In the previous section, we described the industrial
technique by which useful intermediate products can be obtained
by controlling the reaction process. The FischerTropsch process
uses reactions essentially identical to Reaction 5 to generate liquid
petroleum fuels from the combustion of coal, wood, or other biotic
matter. However, the highly controlled industrial FischerTropsch
process does not produce, spontaneously and uncontrolled, the
commonly observed large accumulations of natural petroleum.
The foregoing properties of natural petroleum and the effective
prohibition by the second law of thermodynamics of its spontaneous
genesis from highly oxidized biological molecules of low chemical
potentials were clearly understood in the second half of the 19th
century by chemists and thermodynamicists such as Berthelot and
later confirmed by others including Sokolov, Biasson, and Men-
deleev. However, the problem of how and in what regime of
temperature and pressure hydrogen and carbon combine to form
the particular HC system manifested by natural petroleum re-
mained. The resolution of this problem had to wait a century for the
development of modern atomic and molecular theory, quantum
statistical mechanics, and many-body theory. This problem now has
been resolved theoretically by determination of the chemical po-
tentials and the thermodynamic Affinity of the HC system using
modern quantum statistical mechanics and has also now been
demonstrated experimentally with specially designed high-pressure
3. Calculation of the Thermodynamic Affinity Using SPT and
the Formalism of the Simplified Perturbed Hard-Chain
Theory (SPHCT)
To calculate the thermodynamic Affinity of a distribution of
compounds of the HC system in general regimes of temperature
and pressure, one must use a rigorous mathematical formalism
developed from first-principles statistical mechanical argument.
No approximate, or interpolated, formalism developed for the
low-pressure regime can suffice. A sufficiently rigorous formal-
ism has been developed by extending the SPT equation of state
of Pavlı´cek, Nezbeda, and Boublı´k (6, 7), for mixtures of con-
vex hard-body systems, combined with the formalism of the
SPHCT (8).
Following the procedure enunciated originally by Bogolyubov (9)
and developed further by Feynmann (10) and Yukhnovski (11), a
factored partition function is used that employs a reference system:
. The reference system used is that of the hard-body
fluid as described rigorously by SPT (12–15). The description of the
hard-body fluid by SPT is one of the few exactly solvable problems
in statistical mechanics. This property is especially valuable because
the thermodynamic evolution of a system at high pressures is
www.pnas.orgcgidoi10.1073pnas.172376899 Kenney et al.
determined almost entirely by the variable components that are
obtained from the reference system.
For mixtures of hard-body particles of different sizes and shapes,
SPT generates the following analytical expression for the contri-
bution to the pressure of the hard-core reference system:
1 2
, [6]
in which the geometric compositional variables, r, s, and
, are
defined by the SteinerKihara equations:
, q
, s
(A thorough discussion of the SteinerKihara parameters may be
found in ref. 16.) The following geometric functions are introduced:
, and
. The geometric parameter
is the
multicomponent analogue of the Boublı´k parameter of asphericity
for a single-component fluid,
), and may be inter-
preted as the systems weighted degree of asphericity. The param-
has no analogue in a single-component fluid, for which it is
always equal to unity.
might be interpreted as a parameter of
interference that measures the degree of difference in the mean
component dimensions of radii. When these definitions are used,
the Boublı´k equation (Eq. 6) can be written in a simple form as
, [8]
in which c
, c
, and c
are variables of composition that depend on
the combined geometries of the molecular components and their
fractional abundances: c
1, c
1) 2, and
Similarly, the contribution of the reference system to the free
enthalpy may be written, as
in which I 2c
, J ⫽⫺12(3c
), and K 16(3c
). When these identities are used, the contribution of the
reference system to the pressure and the free enthalpy become
simplified functions of the packing-fraction,
, and the geometric
compositional variables,
The contributions to the pressure and the chemical potentials
from the long-range van der Waals component of the intermolec-
ular potential are described using the formalism of the SPHCT
(1719). The SPHCT uses the mean-field technique (20) of the
BethePeierlsPrigogine ‘‘lattice-gas’’ model, in which has been
applied the shape-independent scattering formalism (21). As dem-
onstrated previously (22) at elevated pressures, the pressure and
chemical potential are dominated by their respective hard-core
components, and the attractive component is several orders of
magnitude smaller and of little consequence. The representation of
the attractive components of the pressure and chemical potential
used has been that developed for the SPHCT by Sandler (23),
Donohue and Prausnitz (19), and Lee and Chao (24) using the
mixing rules of Kim et al. (25). The Prigogine shape c factors used
by the SPHCT are related to the Boublı´k geometric parameters
such that c
(1 3R
) and V
). The values of V
were taken from van Pelt et al. (26). The chemical potential
of the i-th specie of a multicomponent system is given by
, in which
represents the reference value of the
chemical potential at STP.
4. The Evolution of the Normal Alkanes, Ethane, Hexane, and
Decane from Methane at High Pressures
At STP, methane possesses the lowest chemical potential and is
the only thermodynamically stable hydrocarbon. At low pres-
sures and all temperatures, all hydrocarbons are thermodynam-
ically unstable relative to methane or methane plus carbon
(either graphite or amorphous carbon). At normal temperatures
and pressures, the evolution of methane will dominate and
effectively exhaust the HC system of its elemental components.
Because methane is the sole hydrocarbon specie that is thermo-
dynamically stable at low pressures, the chemical Affinities of
each of the heavier species have been calculated in comparison
with methane. Accordingly, the chemical Affinity calculated for
the thermodynamic stability of, for example, the methane 7
(n-octane hydrogen) system is that for the reaction CH
The chemical potentials of the hydrocarbon and methane
molecules and the resulting thermodynamic Affinities of the
(methane 3 hydrocarbon hydrogen) system have been eval-
uated for the n-alkanes from methane through C
. In Fig. 2
are shown the Gibbs energies for the set of hydrocarbons
methane (CH
) and the n-alkanes, ethane (n-C
), hexane
), and decane (n-C
). These thermodynamic vari-
ables have been determined at pressures ranging from 1 to
100,000 bar and at the supercritical temperature 1,000 K, the
temperature of which corresponds conservatively to the geolog-
ical regime characterized by the respective pressures of transition.
The values of the SPHCT parameters, c,
, and Y, for the
individual compounds that have been used were taken from van
Pelt et al. (27), and the reference values of the chemical
potentials of the pure component were taken from standard
reference tables (3).
The results of the analysis are shown graphically for the temper-
ature 1,000 K in Fig. 2. These results demonstrate clearly that all
Fig. 2. Gibbs energies of methane and of the HC system [(1n)C
(n 1)nH
Kenney et al. PNAS
August 20, 2002
vol. 99
no. 17
hydrocarbon molecules are unstable chemically and thermodynam-
ically relative to methane at pressures less than 25 kbar for the
lightest, ethane, and 40 kbar for the heaviest n-alkane shown,
The results of this analysis, shown graphically in Fig. 2, establish
clearly the following:
(i) With the exception of methane, heavier hydrocarbon mole-
cules of higher chemical potentials are not generated sponta-
neously in the low-pressure regime of methane synthesis.
(ii) All hydrocarbon molecules other than methane are high-
pressure polymorphs of the HC system and evolve sponta-
neously only at high pressures, greater than at least 25 kbar
even under the most favorable circumstances.
(iii) Contrary to experience of refinery operations conducted at
low pressures, heavier alkanes are not unstable and do not
necessarily decompose at elevated temperatures. Contrarily, at
high pressures, methane transforms into the heavier alkanes,
and the transformation processes are enhanced by elevated
The theoretical analyses reported here describe the high-pressure
evolution of hydrocarbons under the most favorable chemical
conditions. Therefore, although this analysis describes the thermo-
dynamic stability of the HC system, it does not explicitly do the
same for the genesis of natural petroleum in the conditions of the
depths of the Earth. The chemical conditions of the Earth, partic-
ularly near its surface, are oxidizing, not reducing; of the gases in the
Earths atmosphere and crust, hydrogen is significantly absent, and
methane is a very minor constituent.
Although both methane and heavier hydrocarbons were present
in the carbonaceous meteorites that participated in the accretion
process of the formation of the Earth, such molecules were unlikely
to have survived in their initial composition. The heat of impact that
accompanied accretion most likely would have caused decompo-
sition of heavier hydrocarbons and the release of methane. For both
the theoretical analyses described in this section and the experi-
mental investigations described in section 5, the conservative per-
spective has been taken that hydrocarbons evolve from the solid,
abiotic carbon compounds and vestigial water present in the upper
mantle of the Earth.
5. Experimental Demonstration of Hydrocarbon Genesis Under
Thermodynamic Conditions Typical of the Depths of the Earth
Because the HC system typical of petroleum is generated at
high pressures and exists only as a metastable me´lange at
laboratory pressures, special high-pressure apparatus has been
designed that permits investigations at pressures to 50 kbar and
temperatures to 1,500°C, and which also allows rapid cooling
while maintaining high pressures (28). The importance of this
latter ability cannot be overstated; for to examine the sponta-
neous reaction products, the system must be quenched rapidly to
‘‘freeze in’’ their high-pressure, high-temperature distribution.
Such a mechanism is analogous to that which occurs during
eruptive transport processes responsible for kimberlite ejecta
and for the stability and occurrence of diamonds in the crust of
the Earth.
Experiments to demonstrate the high-pressure genesis of petro-
leum hydrocarbons have been carried out using only 99.9% pure
solid iron oxide, FeO, and marble, CaCO
, wet with triple-distilled
water. There were no biotic compounds or hydrocarbons admitted
to the reaction chamber. The use of marble instead of elemental
carbon was intentionally conservative. The initial carbon com-
pound, CaCO
, is more oxidized and of lower chemical potential,
all of which rendered the system more resistant to the reduction of
carbon to form heavy alkanes than it would be under conditions of
the mantle of the Earth. Although there has been observed igneous
(carbonatite) of mantle origin, carbon should be more
reasonably expected to exist in the mantle of the Earth as an
element in its dense phases: cubic (diamond), hexagonal (lonsda-
leite), or random-close pack (chaoite).
Pressure in the reaction cell, as described in ref. 25, of volume 0.6
was measured by a pressure gauge calibrated using data of the
phase transitions of Bi, Tl, and PbTe. The cell was heated by a
cylindrical graphite heater; its temperature was measured using a
chromel-alumel thermocouple and was regulated within the range
5°C. Both stainless steel and platinum reaction cells were used; all
were constructed to prevent contamination by air and provide
impermeability during and after each experimental run.
The reaction cell was brought from 1 bar to 50 kbar gradually at
a rate of 2 kbarmin and from room temperature to the elevated
temperatures of investigation at the rate of 100 Kmin. The cell and
reaction chamber were held for at least1hateach temperature for
which measurements were taken to allow the HC system to come
to thermodynamic equilibrium. The samples thereafter were
quenched rapidly at the rate of 700°Csec to 50°C and from 50°C
to room temperature over several minutes while maintaining the
high pressure of investigation. The pressure was then reduced
gradually to 1 bar at the rate of 1 kbarmin. The reaction cell was
then heated gently to desorb the hydrocarbons for mass spectrom-
eter analysis using an HI-120 1B mass spectrometer equipped with
an automatic system of computerized spectrum registration. A
specially designed high-temperature gas probe allowed sampling
the cell while maintaining its internal pressure.
At pressures below 10 kbar, no hydrocarbons heavier than
methane were present. Hydrocarbon molecules began to evolve
above 30 kbar. At 50 kbar and at the temperature of 1,500°C, the
system spontaneously evolved methane, ethane, n-propane,
2-methylpropane, 2,2-dimethylpropane, n-butane, 2-methylbutane,
n-pentane, 2-methylpentane, n-hexane, and n-alkanes through
, ethene, n-propene, n-butene, and n-pentene in distribu-
tions characteristic of natural petroleum. The cumulative abun-
dances of the subset of evolved hydrocarbons consisting of methane
and n-alkanes through n-C
are shown in Fig. 3 as functions of
temperature. Methane (on the right scale) is present and of
abundance 1 order of magnitude greater than any single com-
Fig. 3. Cumulative abundances of n-alkanes through n-C
on left ordinate,
methane abundance on right, as functions of temperature at the pressure of
40 kbar. (Scales are in ppm.)
www.pnas.orgcgidoi10.1073pnas.172376899 Kenney et al.
ponent of the heavier n-alkanes, although as a minor component of
the total HC system. That the extent of hydrocarbon evolution
becomes relatively stable as a function of temperature above
900°C, both for the absolute abundance of the individual hydro-
carbon species as well as for their relative abundances, argues that
the distributions observed represent thermodynamic equilibrium
for the HC system. That the evolved hydrocarbons remain stable
over a range of temperatures increasing by more than 300 K
demonstrates the third prediction of the theoretical analysis: Hy-
drocarbon molecules heavier than methane do not decompose with
increasing temperature in the high-pressure regime of their genesis.
6. Discussion and Conclusions
The pressure of 30 kbar, at which the theoretical analyses of
section 4 predicts that the HC system must evolve ethane and
heavier hydrocarbon compounds, corresponds to a depth of
more than 100 km. The results of the theoretical analysis shown
in Fig. 2 clearly establish that the evolution of the molecular
components of natural petroleum occur at depth at least as great
as those of the mantle of the Earth, as shown graphically in Fig.
4, in which are represented the thermal and pressure lapse rates
in the depths of the Earth.
As noted, the theoretical analyses reported in section 4 describe
the high-pressure evolution of hydrocarbons under the most favor-
able chemical conditions. The theoretical calculations for the evo-
lution of hydrocarbons posited the presence of methane, the genesis
of which must itself be demonstrated in the depths of the Earth
consistent with the pressures required for the evolution of heavier
hydrocarbons. Furthermore, the multicomponent system analyzed
theoretically included no oxidizing reagents that would compete
with hydrogen for both the carbon and any free hydrogen. The
theoretical analysis assumed also the possibility of at least a
metastable presence of hydrogen. Therefore, the theoretical results
must be considered as the determination of minimum boundary
conditions for the genesis of hydrocarbons. In short, the genesis of
natural petroleum must occur at depths not less than 100 km, well
into the mantle of the Earth. The experimental observations
reported in section 5 confirm theoretical predictions of section 4,
and demonstrate how, under high pressures, hydrogen combines
with available carbon to produce heavy hydrocarbon compounds in
the geochemical environment of the depths of the Earth.
Notwithstanding the generality and first-principles rigor with
which the present theoretical analysis has used, the results of the
theoretical analyses here reported are robustly independent of the
details of any reasonable mathematical model. The results of this
theoretical analysis are strongly consistent with those developed
previously by Chekaliuk and Kenney (2932) using less accurate
formal tools. The analysis of the HC system at high pressures and
temperatures has been impeded previously by the absence of
reliable equations of state that could describe a chemically
reactive, multicomponent system at densities higher than such of
its normal liquid state in ordinary laboratory conditions and at
high temperatures. The first analyses used the (plainly inade-
quate) Tait equation (33); later was used the quantum mechan-
ical Law of Corresponding States (34); more recently has been
applied the single-fluid model of the SPHCT (31, 32). None-
theless, all analyses of the chemical stability of the HC system
have shown results that are qualitatively identical and quantita-
tively very similar: all show that hydrocarbons heavier than methane
cannot evolve spontaneously at pressures below 2030 kbar.
The HC system does not spontaneously evolve heavy hydrocar-
bons at pressures less than 30 kbar, even in the most favorable
thermodynamic environment. The HC system evolves hydrocar-
bons under pressures found in the mantle of the Earth and at
temperatures consistent with that environment.
This article is dedicated to the memory of Nikolai Alexandrovich
Kudryavtsev, who enunciated what has become the modern Russian
Ukrainian theory of abyssal, abiotic petroleum origins (35), and to the
late Academician E. B. Chekaliuk (J.F.K.).
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Fig. 4. Pressure and temperature in the depths of the Earth.
Kenney et al. PNAS
August 20, 2002
vol. 99
no. 17
... Hydrogen and other reduced gases support unique ecosystems with chemolithoautotrophic metabolic pathways on the modern deep seafloor (Takai et al., 2004;Ohara et al., 2012), may provide energy for long-term survival of microbial cells (Morita, 2000) and may have contributed to the origin of life on Earth and perhaps other planets (Nisbet and Sleep, 2001;Sharma, 2005, 2007;McCollom and Seewald, 2013;Holm et al., 2015;Glein and Zolotov, 2020;McCollom et al., 2022). Molecular H 2 plays a significant role in the formation of natural hydrocarbon gases via the processes of microbial hydrogenotrophy (Wagner et al., 2018;Katz, 2011;Milkov, 2011Milkov, , 2018 and Fischer-Tropsch synthesis (Etiope and Whiticar, 2019), and may play a role in the formation of liquid petroleum fluids (Pratt, 1934;Szatmari, 1989;Levshounova, 1991;Kenney et al., 2002;Wang et al., 2019). ...
... e., hydrogenotrophic methanogenesis) is well recognized by petroleum/ organic geochemists, the role of H 2 in making oils is highly debatable. Theories of abiotic (abiogenic) origin of oil rely on the availability of molecular H 2 in the deep subsurface (e.g., Szatmari, 1989;Kenney et al., 2002;Kutcherov and Krayushkin, 2010). However, the vast majority of petroleum geoscientists and geochemists do not support theories of abiotic origin of large natural accumulations of petroleum (Peters et al., 2005;Glasby, 2006). ...
Geologic molecular hydrogen (H2) occurs in the subsurface and vents and seeps at the surface. However, this valuable natural resource is under-utilized in the economy because the distribution, abundance and origins of H2 are poorly understood. I studied a global dataset of 6246 natural gases with reported H2 concentrations from 16 different geological habitats. The average H2 concentration in all gas samples is 3.5%, but the median concentration is only 0.01%. Gases sampled in Mid-Ocean Ridges and in serpentinites have the highest average concentrations of H2 (~24% and ~21%, respectively). More than 30 different processes may produce H2 observed in natural gases. Hydrogen isotopic composition (expressed as δ²H-H2 values) may indicate crustal (<-650‰) or mantle and primordial (from -650‰ to -100‰) sources of H2, or may result from temperature-dependent equilibration of H2 with water. Much of crustal H2 may be sourced by the reactions of serpentinization, while the quantitative significance of other H2-generating processes such as radiolytic decomposition of water and hydrocarbons, fracture-induced reduction of water, petroleum cracking and coal metamorphism remain speculative. Primordial H2 perhaps vents in some volcanic settings. Provided better understanding of H2 abundance and origins in different geological settings should enable the purposeful exploration for geologic H2 and the assessment of its economic resources.
... First, our models show that CH 4 is stable under relatively cold (U)HP conditions, whereas P-T paths along ''hot" subduction geothermal gradients would intersect elevated f O 2 fields (Fig. 10a). Thermodynamic predictions by other studies similarly demonstrated that abiogenesis production of CH 4 is favorable under low-temperature conditions (Kenney et al., 2002;Connolly and Galvez, 2018). Lower temperatures favor CO 2 dissociation and carbon polymerization . ...
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Subduction is a key element in the carbon cycle between the Earth’s surface and deep interior. In addition to pressure (P) and temperature (T), oxygen fugacity (ƒO2) is an intensive variable that can change the speciation, stability, and mobility of carbon-bearing phases in subduction zones. However, tracking redox evolution and associated carbon speciation in high-pressure metamorphic rocks from subduction zones remains challenging. In this study, we identified CH4-rich fluid inclusions in ultra-high-pressure (UHP; i.e., coesite-bearing) eclogites (the UHP-CH4 eclogite) and CO2-rich fluid inclusions in high-pressure (HP; i.e., quartz-bearing) eclogites (the HP-CO2 eclogite) from the Chinese southwestern (SW) Tianshan palaeosubduction zone, respectively. Combining the garnet-clinopyroxene oxybarometry and comprehensive thermodynamic calculations, we revealed that the ƒO2 of the UHP-CH4 eclogite decreases to a minimum of ∼FMQ−3 at peak P–T conditions (2.8 GPa, 525 °C) and CH4 is expected to form in graphite-saturated C-H-O fluids. Whereas during the early-exhumation stage (2.5 GPa, 600 °C), the UHP-CH4 eclogite is predicted to buffer higher ƒO2 conditions of ∼FMQ−0.9. In contrast, the HP-CO2 eclogite, which has a higher bulk CO2 content (∼10 wt. %) and elevated Fe3+/ΣFe ratio (0.24) than the UHP-CH4 eclogite, suggesting a stronger oxidative seafloor alteration of the HP-CO2 eclogite protolith before subduction, stabilizes graphite and CO2-rich fluids at peak ƒO2 conditions between FMQ and FMQ+1 (2.6 GPa, 500 °C). These data demonstrate that the thermal structure (P–T conditions) of the subduction zone and the composition of the slab are first-order controls on the redox evolution of subducting slabs and the changing speciation of carbon in slab fluids buffered by the metamorphic rock system. Therefore, we propose that the cold subduction of a weakly altered oceanic crust (e.g., UHP-CH4 eclogites from the SW Tianshan) provides ideal conditions for the formation of ultra-deep abiotic CH4.
... Thermodynamic modelling of C-H and C-H-O systems demonstrates that high-molecular hydrocarbons become stable at high pressure [27][28][29] . This phenomenon was recently confirmed by the experiments [30][31][32] . ...
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Diamonds grown by high pressure high temperature process (HPHT) are usually characterized by yellow color and high contents of nitrogen. Introduction of Ti decreases nitrogen content in diamond. Understanding the formation of nitrogen-poor diamond is very important not for the progress of HPHT process only, but because these diamond varieties represent the rare natural stones, although their crystallization conditions have not been clarified yet. Here we studied the composition of fluid phase in synthetic diamonds. The experiments were performed using a high-pressure apparatus BARS at pressures 5.5–6.0 GPa and temperatures 1350–1400 °C. It was found that introduction of metallic Ti leads to concentration of nitrogen mainly as nitrogenated hydrocarbons. The hypothesis that elucidates the formation of low-nitrogen diamond in Fe–Ni is proposed: the presence of Ti leads to an increase of hydrogen fugacity in the metal melt which drastically reduces the nitrogen solubility. As a result, nitrogen concentrates in the form of complex hydrocarbon compounds, while diamond grows colorless and characterized by very low nitrogen content. It is suggested that the proposed mechanism acts the same way in the presence of other metals which are strong reducing agents.
... Hydrocarbons accompanying the pulsed hydrogen degassing of the Earth are synthesized in deep alkaline magma chambers under conditions of high pressures and temperatures, which is fundamentally substantiated as in theoretical ones (Kudryavtsev, 1973(Kudryavtsev, , 2013Porfir'ev, 1974;Kropotkin, 1986;Karpov et al., 1998 ;Gold, 1992;Letnikov, 2005;Marakushev, Marakushev, 2006, 2008Zubkov, 2009;Kutcherov, Krayushkin, 2010;Marakushev et al., 2014) and in experimental (Kenney et al., 2002;Mukhina et al., 2017;Kolesnikov et al., 2017;Kucherov, Serovayskiy, 2018;Tao et al., 2018;Sokol et al., 2019Sokol et al., , 2020 studies. Aquatic fluids coexist with HC fluids originating in deep magma chambers, which, reaching the crust at depths of about 50 km, transform into an oil fluid , which seeps to the surface along faults and cracks (migration channels) and forms oil and gas deposits in rocks of the most diverse lithological composition, genesis and age. ...
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On the basis of an inorganic concept of the petroleum origin, the phase relationships of crystalline kerogens of black shales and liquid oil at the physicochemical conditions of a typical geobarotherm on the Texas Gulf Coast are considered. At the conditions of the carbon dioxide (CO2) high fluid pressure, the process of oil transformation into kerogens of varying degrees of "maturity" (retrograde metamorphism) takes place with decreasing temperature and hydrogen pressure. Kerogen generation in black shale rocks occurs by the sequential transition through metastable equilibria of liquid oil and crystalline kerogens (phase "freezing" of oil). The upward migration of hydrocarbons (HC) of oil fluids, clearly recorded in the processes of oil deposit replenishment in oil fields, shifts the oil ↔ kerogen equilibrium towards the formation of kerogen. In addition, with decreasing of the hydrogen chemical potential as a result of the process of high-temperature carboxylation and low-temperature hydration of oil hydrocarbons, the "mature" and "immature" kerogens are formed, respectively. The phase relationships of crystalline black shale kerogens and liquid oil under hypothetical conditions of high fluid pressure of the HC generated in the regime of geodynamic compression of silicate shells of the Earth in the result of the deep alkaline magmatism development. It is substantiated that a falling of hydrogen pressure in rising HC fluids will lead to the transformation of fluid hydrocarbons into liquid oil, and as the HC fluids rise to the surface, the HC ↔oil ↔ kerogen equilibrium will shift towards the formation of oil and kerogen. It is round that both in the geodynamic regime of compression and in the regime of expansion of the mantle and crust, carboxylation and hydration are the main geochemical pathways for the transformation of oil hydrocarbons into kerogen and, therefore, the most powerful geological mechanism for the black shale formations.
... High-temperature and high-pressure experiments by Kenney et al. (2002) indicated that petroleum hydrocarbons have been produced using reagents: solid iron oxide (FeO), marble (99.9% pure CaCO 3 ) and water at 500-1500 • C and 1-5 GPa. In particular, hydrocarbon compounds heavier than methane (C 2 H 6 -C 6 H 14 ) spontaneously evolved at pressures greater than 3 GPa (Fig. 9a). ...
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Tectonism often plays an important role in the mineralization process, which is generally thought to be the main controlling factor in the accumulation of economic materials (e.g., gold, coal, oil and gas) through deformation. However, numerous experimental and theoretical studies have suggested that tectonic stress not only causes deformation (physical changes) in rocks and minerals but also promotes their chemical changes by acting directly on chemical bonds and causing bond scission or regeneration, called tectonic stress chemistry (TSC). In recent years, TSC actions caused by tectonic activities have provided new ideas and evidence for explaining the chemical structural evolution of coal, hydrocarbon formation, organic (coal-derived) and inorganic graphitization and hydrothermal mineralization under shear stress. These background studies have provided incentives and insights into how tectonic stress affects the chemical structures of minerals, rocks and even ore-forming fluids in the process of mineralization. In this paper, we briefly review: (1) the concept of TSC; (2) the TSC process in the formation of shear zone type gold deposits from stress concentration, brittle fracturing, sudden reduction of fluid pressure, and flash vaporization to gold precipitation; (3) mechanisms of the macromolecular structural evolution of coal and gas generation under shear stress from deformation experiments and molecular dynamic simulations; (4) coal-derived graphitization caused by preferred orientation and extension of the basic structural units (BSUs) under shear stress; and (5) some preliminary experimental explorations on inorganic graphitization in carbonate-hosted shear zones. In addition, some existing problems and possible solutions for these processes are also discussed. Finally, we propose additional potential TSC processes in extensive geological processes, e.g., the relationship between deformation and metamorphism and trigger mechanisms of slow-slip earthquakes. To further explore these processes, a combination of experiments and molecular dynamic simulations should be undertaken by researchers.
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The behavior of cementite (Fe3C) in aqueous environments was investigated in the thermobaric range of 180–950°C and 2–6 GPa. When interacting with water, cementite was transformed into wüstite and magnetite. The gaseous reaction products were represented mainly by saturated hydrocarbons with linear and branched structures up to C7. The composition of the hydrocarbon products synthesized from cementite and water at extreme thermobaric parameters varied from light mixtures similar to «dry» natural gas to complex hydrocarbon systems similar to «wet» natural gas and gas condensate. During the investigation, it was discovered that the chemical reaction between iron carbide and water begins at 220°C under extreme pressure, which is significantly lower than the temperature at which the reaction of cementite with water begins at ambient pressure.
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El ser humano es un animal energético. Este carácter ontológico humanidad-energía se distingue por un ejercicio permanente de poder en torno a la obtención de fuentes de energía en todo su ciclo: producción-distribución-consumo, todo ello a gran escala. Si bien desde la década de los setenta del siglo XX se comenzó a estudiar la urgencia de transitar a otros energéticos menos agresivos contra el ambiente, hasta la segunda década del siglo XXI el petróleo conserva su papel neurálgico en la matriz. Este hidrocarburo tiene una capacidad inherente a su naturaleza, la de ser un elemento civilizador, tal como lo son la roca, la madera y el acero.
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Molecular dynamics simulation of the pressure-density-temperature properties of supercritical methane (CH4) are made with the COMPASS II force field model in the range of 200–3000 K, 0.1–3.0 GPa, and 0.22–0.668 g·cm⁻³, where 710 states are simulated using NPT ensemble, and 212 states are simulated using NVT ensemble. These results are in good agreement with experimental data and the calculated results from highly accurate reference model of Setzmann and Wagner (J Phys Chem Ref Data 20:1061–1155, 1991) and its extrapolation in the region where the reference model can be validated. The simulation results are calibrated with the reference model. The calibrated simulations results and the reference model are used simultaneously to develop an accurate cubic equation of state for supercritical CH4 in the range of about 300–3000 K and 0–3 GPa (0–0.53 g·cm⁻³). The equation are tested against experimental and simulated data at high temperatures and pressures. Compared with the overwhelming majority of experimental results, the volume deviations are within 0.4 % to 1.1 %, with averages of about 0.1 % to 0.4 %; Compared with the molecular simulation results in literature and this work, the volume deviations are within 0.6 % to 3.7 %, with averages of about 0.1 % to 1.2 %. The equation can accurately predict the fugacity coefficients, residual enthalpies, and entropies and other thermodynamic properties.
Petroleum reserves continue to grow, despite record-breaking pace of oil and gas production worldwide. The energy scenario is remarkably improved when basement reservoirs are considered. The term “basement” in “basement reservoirs” refers to crystalline formations ranging from intrusive and extrusive magmatic bodies (especially granites) to the family of low to medium-grade metamorphic rocks. Hydrocarbons have been under production from these types of rocks around the world for many decades but since around 1990 there has been growing interest and exploration in these formations where matrix porosity is negligible, and storage and production are dominated by the fracture system. As such, developing these reservoirs is a formidable challenge. This chapter offers prospects, potential and optimum methods of developing basement reservoirs.
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The horizontal distribution and periodic accumulation of hydrocarbon in reservoirs are well understood. However, our understanding on the vertical distribution of hydrocarbon accumulation in reservoirs is not clear and remains mysterious to some extent. We proposed a concept of hydrocarbon accumulation depth limit (HADL) to characterize the hydrocarbon's vertical distribution in petroliferous basins, which is determined by statistical analyses of the variation trends of reservoir layers' essential properties, including hydrocarbon saturations (So), movable hydrocarbon ratios (Mo) and dry layer ratio (Ko), when the depth is increased. A total of 80,762 drilling results from 12,237 exploration wells in six representative petroliferous basins in China were collected and analyzed. The reservoir layers' essential properties So, Mo and Ko and their correlations with porosity, permeability, pore throat radius and thermal evolution degree were investigated. The critical values of So, Mo and Ko that define HADL were quantified to be So=0, Mo=0 and Ko=100. Our study indicates that the HADL in petroliferous basins varies from less than 3,000 m to more than 13,000 m deep, depending on the hydrocarbon composition, reservoir lithology, reservoir age, geothermal gradient, tectonic movement, etc. Two factors play an essential role in the formation and variation of HADL: 1) the depletion of hydrocarbon generation potential of source rocks which cuts off hydrocarbon contribution for reservoirs formation and 2) the termination of differential compaction which eliminates capillary pressure difference between the outer surrounding rocks and inner reservoir layers, ending the dominant driven force for hydrocarbon migration and accumulation in deep and tight reservoir layers. All proven oil reserves of 33.95 billion tons equivalent in China, as well as world's discovered 52,926 oil and gas reservoirs and their unproved potential resources, are distributed above the HADL we defined.
Scaled particle theory, last reviewed in 1965, is reviewed in an up-to-date manner. However, discussion is limited to hard sphere fluids. In particular developments which have introduced new exact conditions, and permitted the use of all exact conditions are reported. In addition, methods of advancement based on the application of the thermodynamics and statistical thermodynamics of curved boundary layers are reviewed. Recent work in which scaled particle theory is applied to the derivation of the radial distribution function is discussed. Finally, in the last section, some new results are reported.
The fundamental concepts of the statistical theory of phase transitions of the second kind are developed using the three-dimensional Ising model as an example of a physical system with phase transitions. The theory is constructed from the Hamiltonian of the system up to expressions for the thermodynamic functions. Explicit expressions are obtained for the free energy, entropy, specific heat, mean moment, susceptibility, and critical temperature. Graphs of these functions are presented, and the values of the critical indices are determined. Generalizations of the theory to n-component systems, cluster models of ferroelectrics, and binary alloys are examined.
The van der Waals model of a dense fluid is represented by a simple partition function. It is shown that this function, coupled with a choice of plausible ad hoc assumptions, leads to any one of several well-known equations of state including the empirically successful equation of Redlich and Kwong.A modern version of the van der Waals equation is obtained by a perturbation on the hard-sphere equation of state of Carnahan and Starling; the effect of attractive forces is found from a corresponding-states representation of the observed properties of argon. Finally, it is shown that when Prigogine's simplifying assumptions are made about the effect of rotational and vibrational degrees of freedom, the simple partition function can be used to derive a useful treatment of liquid polymers and other complex liquids.
The thermodynamic stability of n-octane has been investigated as a function of temperature, pressure, and degree of molecular clustering at supercritical temperatures. At low pressures, the free enthalpy is shown to be always lowest in the unassociated, gas state, and the system is, in that regime, robustly resistant to clustering. At high pressures, the free enthalpy of the unassociated, gas state exceeds that of the clustered, liquid state. At the pressure at which the values of the free enthalpies of the gas and liquid states become equal, the system becomes abruptly unstable, and will then spontaneously cluster into effective `cluster-polymers', and undergo a phase transition to a liquid state. This phenomenon is a geometric effect, and occurs even at supercritical temperatures. The gas–liquid phase transition reported here is closely related to the Alder–Wainwright gas–solid phase transition, the onset of which is applied to approximate the optimal clustering parameter. This phase transition is of the class of entropically-driven phase transitions, characterized by an increase in spatial order accompanied by an increase in entropy, and manifests an inverted latent heat of transformation, analogous to adiabatic demagnetization.