Thermochemistry of the Formation of Fossil Fuels

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Fluid-Mineral Interactions: A Tribute to H. P. Eugster © The Geochemical Society, Special Publication No.2, 1990 Editors: R. J. Spencer and I-Ming Chou

Thermochemistry of the formation of fossil fuels

MOTOAKI SATO U.S. Geological Survey, MS 959, Reston, Virginia 22092, U.S.A.

Abstract-Photosynthesis liberatesO2 from a mixture of CO2 and H20 in forming biologicalsubstances. Formation of fossil fuels is also associated with progressive depletion of oxygen from biological substances. The two processes are thus sequential in terms of deoxygenation. To evaluate thermochemical relations of the deoxygenation processes in a simple manner, the concept of deoxygenation free energy Gdeox and deoxygenation enthalpy Hdeox (defined as Gibbs energy and enthalpy, respectively,

of reaction to release unit mole of O2 in forming a compound CxHyNzOw from CO2, H20, and N2) are computed for over 150 compounds. If compound A changes to compound B by dehydration and/or decarboxylation, and A has a higher Gdeox value than B, energy is released and the transformation

can occur spontaneously; likewise heat is released if A has a higher Hdeox' The computed results indicate that energy input is required only for the initial stage of photosynthesis, and progressive deoxygenation and aromatic condensation during diagenesis should decrease free energy and release heat. Catagenesis and metagenesis are also most probably spontaneous and exothermic because both

low C-number paraffinic hydrocarbons and polycondensed aromatic kerogens have the lowest Gdeox and Hdeox values among various homologous groups even at room temperature. The ultimate products of the spontaneous evolution of fossilfuelsare methane and graphite. The role of increased temperature associated with deeper burial is not to change the position of equilibrium, but simply to increase the reaction rate exponentially so that the extremely slow reactions would proceed in geologic time.

INTRODUCTION BACKGROUND

Photosynthesis may be viewed as a natural pro- SOLAR ENERGYIS chemically stored, via photosyn- cess in which photosynthetic organisms liberate ox- thesis, as an unstable coexistence of free oxygen ygen from a mixture of mainly H 0 and CO , and and organic matter near the Earth's surface. When 2 2 some Nand S compounds, to form biological sub- organisms die, most of the biological matter is aer- stances. Transformation of biological remains to obically degraded to gases and water-soluble com- fossil fuels may also be viewed as a process in which pounds and is recycled to the atmosphere and hy- oxygen is removed from biological substances. drosphere. Only about 0.1 % escapes chemical and Graphic presentations by OWEN (1948), VAN microbial oxidation within the first few centimeters KREVELEN (1961), and others show at a glance that of burial and undergoes anaerobic transformation o as well as Nand S are depleted progressively from to fossil fuels in sediments. Because of accumulation biological substances (e.g., carbohydrates, proteins, over geologic periods of time, however, the total lipids, and lignin) during their transformation to amount of fossilized organic matter is estimated to fossil fuels (e.g., kerogen, petroleum hydrocarbons, be orders of magnitude (about 2000 times according natural gas, and coal). In this sense, photosynthesis to WELTE, 1970) greater than the live biomass, and fossil fuel formation are natural sequential warranting a thorough understanding of the nature processes in which the compounds of the C-H-O- of the transformation process. (N-S) system are deoxygenated. Much biogeochemical work was devoted to the As caloric values of organic compounds generally molecular identification and quantification of the increase with increasing degree of deoxygenation, organic substances in relation to geologic factors some earlier investigators searched for the sources (basin structure, burial depth, age, temperature, etc.) of the energy thought to be necessary for the trans- to deduce the rate and chemical mechanisms ofthe formation, analogous to the solar energy input in degradation of biological matter and transformation photosynthesis. Bacteriological processes, radioac- to fossil fuels (BREGER, 1963; TISSOT and WELTE, tivity, heat, and pressure have been studied as the 1978; HUNT, 1979). Consequently, recent investi- possible sources of the transformation energy gators have arrived at the deduction that the mat- (BREGER, 1963). Except perhaps for the effect of uration process occurs spontaneously in stages when heat in thermal metamorphism of coal to anthracite, the temperature increases due to burial along a nor- however, the search for these energy sources had mal geothermal gradient. The present work is an remained uncertain. attempt to provide an explicit thermochemical basis More recent studies in the field and laboratory for this deduction. (summarized by TISSOT and WELTE, 1978; HUNT, 271 272 M. Sato

1979), have indicated that most biological com- For example, the formation of ethanol (C2H60) pounds undergo rapid and extensive anaerobic from glucose (C6H1206) at 25°C and 1 atm: chemical changes within several hundred meters of C6H1206 = 2 C2H60 + 2 CO2, burial, where the temperature remains within the biologically active range «50°C). Diagenetic pro- so, = -226.44 kJ (1) cesses, both chemical and microbial, transform re- (where !:;'Gis the Gibbs free energy of reaction, and active biological substances to more inert, insoluble r kJ is kilojoule) may be regarded as being composed polymers such as humic acids and kerogen. Further of two reactions, each of which releases 1 mole of burial with accompanying temperature rise (to O , in opposite directions: about 150-200°C) causes thermal degradation of 2 kerogen to petroleum-range hydrocarbons, a process CO2 + H20 = (1/6) C6H1206 + O2, called catagenesis. Burial deeper than several kilo- meters (>200°C) results in metagenesis and meta- !:;'Gr= 479.82 kJ (2) morphism, in which methane and other gaseous (2/3) CO2 + H20 = (1/3) C2H60 + O2, hydrocarbons and carbonized residues are formed. In the above scheme of fossilfuel evolution, tem- !:;'Gr= 442.08 kJ. (3) perature seems to playa controlling role. Because It is easy to see that (1/6) !:;.Gr(I) = !:;'Gr(3) - !:;'Gr of the inevitable association of the depth of burial (2). Heat is known to be generated during the fer- with increased time, temperature, and pressure, it mentation of carbohydrates. Reaction (1) can occur is difficult to clearly determine if energy input as spontaneously as indicated by the negative free en- heat is indeed necessary, or if the late stages of ergy of reaction, although the presence of enzymes transformation are merely hindered by high acti- in malt increases the reaction rates multifold. The vation energies (mechanism) and require higher spontaneity of the conversion of glucose to ethanol temperatures to proceed even given geologicperiods is also indicated by the fact that !:;'Gr(3) < !:;'Gr(2). oftime. The ambiguities are especially strong in the As seen above, whether one organic compound discussions of thermal degradation, and "cracking" should spontaneously be converted to another, of geopolymers. upon losing H20, CO2, and/or N2, could easily be surveyed by calculating the Gibbs free energy nec-

THERMOCHEMICAL CONSIDERATIONS essary to remove 1 mole of O2 in forming each

compound from CO2, H20, and N2 (if needed), and Evaluation of free energy changes for reactions by comparing the computed free-energy values. involving natural organic substances is handicapped Such a free energy of reaction may be defined as by the fact that too many complex and sometimes deoxygenation free energy, Gdeox>and can be ob- ill-defined compounds are involved, and also many tained from the Gibbs free energy of reaction for reaction paths are still insufficiently understood. the general reaction: The approach described below, however, bypasses the complexities arising from uncertainties in reaction paths and outlines energy relations among many organic compounds. The approach is based on the assumption that the deoxygenation during maturation of fossil fuels proceeds via loss of H20 and CO2, a model well documented for coalification (VAN KREVELEN, 1963) and kerogen evolution as (TISSOT and WELTE, 1978, p. 149). 4!:;'Gr(4) The principle of the approach is that when one (5) Gdeox= (4x + y - 2w) organic compound can theoretically be obtained from another by releasing H20 and/or CO2 (and and N2 if nitrogen is involved; sulfur is not considered here as it is a minor component), the free energy !:;'Gr(4)= !:;'G?(CxHyNzOw) balance in the overall reaction, however complex the actual mechanism may be, is directly related to - x!:;'G?(C02) - ~ !:;'G?(H20) (6) the difference between the Gibbs free energies required to make the two compounds from a mixture where !:;'G? is the Gibbs energy of formation for of H20, CO2, and N2 upon releasing 1 mole of free each chemical species involved at 1 atm pressure oxygen (02), and at a specified temperature (25°C in this paper Thermochemistry of the formation of fossil fuels 273

unless otherwise noted). If compound A changes to and H20 (and N2) in forming a compound in the compound B via dehydration, and/or decarboxyl- C-H-O-(N) system: ation, the transformation can proceed spontaneously with a release of energy when A has a _ 4x + y- 2w Qdeox - 4x + y (10) higher Gdeox. Similarly, deoxygenation enthalpy, H may be deoXo where x, y, and ware defined in equation (4). Note defined on the basis of equation (4) as that nitrogen as N2 is not involved in determining 4!:;.Hr(4) Qdeox (it may if incorporated in the deoxygenation (7) equation as NH3 and/or N0 , but for simplicity we Hdeox = (4x + y - 2w) 2 disregard this possibility for now). The Qdeox for where the enthalpy of the reaction can be obtained glucose, for example, is 0.667, and that for ethanol from the enthalpy offormation, !:;'H?, of the chem- is 0.857, according to the above definition. ical species involved as: The Gdeox and Qdeox values have been computed for some 150 common organic compounds, for which the Gibbs free energy data are available at 25°C (STULLet al., 1969) and listed in Table 1.

Some of these values have been plotted on a Gdeox VS. Qdeox diagram (deoxygenation free energy diagram) in Fig.!. Also, the G values of some hy- The availability of Gibbs energy of formation data deox drocarbons (all Qdeox = 1) are plotted against the for complex solid and liquid organic compounds is atomic H/C ratio in Fig. 2. rather limited, and an estimation is often difficult, Figures 1 and 2 show many interesting facts re- particularly for uncharacterized geopolymers. In garding the thermochemical relations among the contrast, enthalpy of formation data or heat of common organic compounds. Notable points are combustion (Hc) data are available for a large num- listed below. ber of organic substances in fair to good accuracy. In fact, combustion calorimetry constitutes the basis (a) Among the oxygenous homologous series,

for obtaining thermochemical data for the majority fatty acids have the lowest Gdeox for a given Qdeoxo of organic substances. It may be noted here that followed by amino acids, proteins, alcohols, car- reaction (4) is exactly reverse of the combustion bohydrates, aldehydes, and finally by ethers, in as- reaction, and that H; = !:;.Hr (4). Hence, cending order. (b) Within each oxygenous homologous series, 4Hc the Gdeox decreases regularly with increasing C deox (9) H = (4x + y - 2w) number, and the slope of the decrease against Qdeox also decreases regularly, approaching zero at Qdeox

If compound A changes to compound B via de- = 1. The differences in Gdeox among various oxy- hydration and/or decarboxylation, the transfor- genous homologous series also decrease as Qdeox mation reaction is exothermic when A has a higher approaches 1.

Hdeox• In view of complexity of estimating Gdeox (c) Among hydrocarbons, the Gdeox of the olefinic values for complex compounds as discussed later, series decreases regularly (allowing some reversals the Hdeox relation may be substituted for the Gdeox within the limits of uncertainties in the thermo- relation in certain cases. chemical data) with increasing C number, whereas In summary, if compound A changes to com- that of the paraffinic series has the reverse trend. pound B by dehydration and/or decarboxylation, The cycloparaffinic series roughly follows the trend and A has a higher Gdeox value than B, energy is of the olefinic series, but the pattern is highly irreg- released and the transformation can occur spon- ular.

taneously; heat is released if A has a higher Hdeox (d) When the C number increases, the Gdeox val- than B. ues of most common nonaromatic homologous se- In order to relate the Gdeox or Hdeox values ofvar- ries converge to a single value, which is about 424.7 ious compounds to the degree of deoxygenation, kJ at 25°C (this value will be referred to as "the which is an index of maturation of fossil fuels, an- convergence value" of the Gdeox). The Gdeox values other quantity called deoxygenation quotient, Qdeoxo of aromatic rings uniquely diverge from the con- is introduced. The deoxygenation quotient indicates vergence value as the C number increases, decreas- the fraction of oxygen atoms removed from the total ing toward the graphite value (394.4 kJ) as the de- oxygen atoms present in the starting mixture of CO2 gree of condensation increases. 274 M. Sato

Table 1. A list of Hdeox (deoxygenation enthalpy) and Gdeox (deoxygenation free energy), and Qdeox (deoxygenation quotient) values of common compounds of the C-H-N-O system. For hydrocarbons, atomic H/C ratio is given instead of Qdeox (all 1.00)

SECTION A SECTION A

C2..o. G_ C2..o. G_ Name Formula State C2..o. (kJ) (kJ) Name Formula State Q...., (kJ) (kJ)

Fatty acids Furan (I) .900 462.97 456.03 Ethyl acetate (I) .833 447.67 438.72 Formic acid CH,O, (I) .333 509.17 540.24 Acetic acid C,H.O, (I) .667 437.30 436.89 Butyric acid C,H,O, (I) .833 436.70 429.72 Ethers Valeric acid CSHIO02 (I) .867 436.57 428.57 Methyl ether C,H,O (g) .857 486.82 462.47 Palmitic acid CI6H3202 (s) .958 433.78 425.61 Ethyl methyl ether C,H,O (g) .900 468.32 447.61 Ethyl ether C,HIOO (I) .923 453.95 440.09 Polybasic and hydroxy acids Isopropyl ether C,H1,O (I) .947 445.63 433.43

Oxalic acid C2H20. (s) .200 485.82 649.61 Lactic acid C,H,O, (s) .667 447.98 457.27 Carbohydrates Succinic acid C,H,O, (s) .636 425.90 440.48 Glucose, D C6HI206 (s) .667 466.94 479.82 Pyruvic acid C,H,O, (I) .625 467.07 477.66 Alpha-Galactose, D C6HI206 (s) .667 465.12 478.20 Fumaric acid, trans. C"H..O" (s) .600 444.88 466.08 Sorbose, L C6HI206 (s) .667 467.58 480.03 Maleic acid C,H,O, (s) .600 451.70 473.57 Beta-Lactose C12H220'1 (s) .686 469.13 481.23 Citric acid- H20 C6HIOOS (s) .529 471.63 499.84 Sucrose C12H220,1 (s) .686 470.35 483.09 Alpha-Lactose- H,O C12H24012 (s) .667 474.40 487.45 Fatty alcohols Beta-Maltose- H,O C12H2"O'1 (s) .667 474.40 487.45 Galactitol C6H1406 (s) .684 463.86 473.24 Methanol CH,O (I) .750 484.41 468.36 Manitol,D C6H'406 (s) .684 465.33 474.52 Ethanol C,H.O (I) .857 455.85 442.07 Xylose, D CsHIOOs (s) .667 469.47 484.35 Propanol C,H,O (I) .900 448.74 435.83 Butanol C"HIOO (I) .923 446.23 433.73 Peptides-amino acids, proteins, etc. Pentanol CsHI20 (I) .938 443.28 43 l.l 8

Hexanol C.HI4O (I) .947 442.50 430.56 Glycine C,H,NO, .692 428.61 446.25 Heptanol C7HI60 (I) .955 442.14 430.11 00 Alanine, D, L C,H,NO, .789 431.30 437.67 Hexadecanol C1,H,.0 (s) .980 436.29 426.79 00 Valine, L C,HIlNO, 00 .871 432.84 432.22 Leucine, L C,Hi3NO, 00 .892 432.99 430.42 Polyhydric and other alcohols Isoleucine, L C,Hi3NO, 00 .892 434.05 431.62 Phenylalanine, L C,HIlNO, 00 .915 432.25 431.86 Ethylene glycol C,H,O, (I) .714 475.84 470.78 Serine, L C,H,NO, 00 .684 447.57 462.81 Glycerol C,H,O, (I) .700 472.93 472.82 Tyrosine, L C,HIlNO, 00 .872 433.38 435.93 Erythritol C04HIOO. (s) .692 465.06 472.67 Tryptophan, L CIIHI2N202 00 .929 432.96 434.00 Pentaerythritol CSHI60. (s) .778 476.43 465.08 Arginine, D CtiH1 .. N,,02 00 .895 439.80 445.45 Fufuryl alcohol C,H,O, (I) .846 463.40 459.87 Creatinine C,H,N,O 00 .913 444.94 453.06 Pentamethylene Aspertic acid, L C,H,NO, 00 .652 426.96 447.33 glycerol CSHI202 (I) .875 463.26 451.82 Glutamic acid, L C,H,NO, 00 .724 427.45 439.62 Cyciopentanol CsHIOO (I) .933 442.37 432.86 Hippuric acid C,H,NO, 00 .867 432.71 435.61 Cyclohexanol c.HuO (I) .944 438.57 430.13 Alantoin C,H,N,O, 00 .727 428.49 460.72 Aspargine, L C"HsN203 00 .750 428.41 443.57 Aldehydes G1ycylglycine C"H,N203 00 .750 430.25 452.38 Alanylglycine, L CSHION20) 00 .800 428.38 437.54 Formaldehyde CH,O (g) .667 563.45 521.66 l.eucylglycine, D, L C8HI6N203 00 .875 435.76 436.51 Acetaldehyde C,H,O (g) .833 476.93 451.94 Hippury1glycine C11H12N20. 00 .857 434.22 438.85 Propionaldehyde C,H,O (g) .889 461.50 441.07 Butylaldehyde C,H,O (I) .917 450.67 437.65 Others Heptanal C7HI40 (I) .952 444.37 432.04 Furfural C,H,O, (I) .833 468.85 466.77 Graphite C (s) 1.00 393.51 394.38 Carbon monoxide CO (g) .500 565.93 514.21 Ketones, esters Carbon dioxide CO, (g) 0.00 o Hydrogen H, (g) 1.00 571.68 474.38 Aceton C,H,O (I) .889 447.48 434.83 Water H,O (I) 0.00 o o 2-Butanone C,H,O (I) .917 444.38 431.80 Hydrocyanic acid HCN (g) 1.00 533.57 506.48 2-Pentanone C,HIOO (I) .933 442.39 430.06 Ammonia NH, (g) 1.00 510.76 452.85 2-Octanone C,H1,0 (I) .958 439.15 427.15 Hydrazine N,H. (g) 1.00 666.86 632.91 Ketene C,H,O (g) .800 505.88 482.83 Urea CH,N,O (s) .750 421.34 447.74 Thermochemistry of the formation of fossil fuels 275

Table 1. (Continued)

SECTIONB SECTION B H_. G_ H"". G_. Name Formula State H/C (kJ) (kJ) Name Formula State H/C (kJ) (kJ)

Paraffins Ethylcyclohexane CSHI6 (I) 2.00 435.21 423.47

PropylcycIohexane C.Hj, (I) 2.00 435.24 423~9 Methane CH, (g) 4.00 445.17 408.97 Butylcyclohexane CIOH2Q (I) 2.00 435.35 423.73 Ethane C,H. (g) 3.00 445.67 419.26 Cyciohexylheptane C13H26 (I) 2.00 434.76 423.17 Propane C,H, (g) 2.67 444.00 421.69 Cyciopentyldecane CISH30 (I) 2.00 436.62 424.73 Butane C,HIO (g) 2.50 439.32 422.84 Cyclohexyldecane CI6H12 (I) 2.00 435.46 423.99 Pentane C,H12 (I) 2.40 438.67 423.20 Hexane C,H" (I) 2.33 438.22 423.40 Heptane C,H" (I) 2.29 437.90 423.57 Diolefins and acetylides

Octane C,H18 (I) 2.25 437.65 423.70 Allene (g) I~ 486.08 464.98 Nonane C.H20 (I) 2.22 437.46 423.79 1,3-Butadiene (I) I~ 457.62 443.40 Decane CIOH" (I) 2.20 437.31 423.88 Isoprene (I) I~O 451.46 437.99 Dodecane C12H" (I) 2.17 437.10 424.04 1,4-Pentadiene (I) I~O 455.21 441.13 Hexadecane C"H" (I) 2.13 436.69 424.10 Acetylene (g) 519.83 494.06 Tetracosane C"H., (s) 2.08 434.98 424.70 1m Propyne (g) I~ 484.41 462.99 Dotriacontane C"H.. (s) 2.06 434.12 423.98 I-Bytyne (I) I~O 467.81 453.26 2-Butyne (I) I~O 463.89 449.89 Olefins

Ethylene C,H. (g) 2.00 470.33 443.76 Aromatic hydrocarbons Propene C,H, (g) 2.00 457.43 434.99 I-Butene C,H, (g) 2.00 452.87 432.93 Benzene CoH. ro 1.00 435.68 426.96 l-Pentene CsHlo (I) 2.00 446.67 431.48 Naphthalene CIOHs 00 0.80 429.71 424.47 l-Hexene CoH12 (I) 2.00 444.85 430.32 Anthracene CI4HIO 00 0.71 428.33 423.82 l-Heptene C7HI4 (I) 2.00 443.57 429.51 Phenanthrene CI4HIO 00 0.71 427.54 422.95 I-Octene CSHI6 (I) 2.00 442.60 428.89 Fluoranthene C'6H)O 00 0.63 427.84 423.73 l-Decene CIOH20 (I) 2.00 441.26 428.05 Pyrene C'6HIO 00 0.63 423.78 419.76 I-Hexadecene CJ6H32 (I) 2.00 439.41 426.97 Naphthacene C1sHI2 00 0.67 426.53 422.90 Perylene C20HI2 00 0.60 424.69 420.96 Cycloparaffins and other saturated alicyclic hydrocarbons Toluene C,H, ro 1.14 434.43 424.80 Ethylbenzene CsHIO ro 1.25 434.74 424.83 Cyclopropane C,H. (g) 2.00 464.74 444.25 Propylbenzene C.Hj, ro 1.33 434.85 424.74 Cyciobutane C,H, (g) 2.00 453.41 439.62 m-Xylene CsHIO ro 1.25 433.51 423.68 Cyclopentane CsHIO (I) 2.00 438.78 425.90 Mesitylenebenzene C.Hj, ro 1.33 432.76 423.03 Cyclohexane C6H12 (I) 2.00 435.54 424.01 Tetramethylbenzene CIOHI4 00 1.40 430.18 421.63 Cycioheptane C7HI4 (I) 2.00 437.84 426.20 Pentamethylbenzene CIIHI6 00 1.45 432.00 422.87 Cyciooctane CSHI6 (I) 2.00 438.80 427.54 Hexamethylbenzene CI2HI8 00 1.50 432.16 423.32 Methycyclopentane C6HI2 (I) 2.00 437.52 424.55 Biphenyl C12HIO 00 0.83 431.16 425.71 Ethylcyciopentane C7HI4 (I) 2.00 437.32 424.60 Diphenylethane C'4H14 ro 1.00 431.94 424.39 Propylcyciopentane CSHI6 (I) 2.00 437.13 424.65 Bibenzyl CI4HI4 ro 1.00 432.08 425.65 Bytylcyciopentane C.Hj, (I) 2.00 435.35 423.73 Styrene CsHs ro 1.00 439.53 430.62 Methylcyclohexane C7HI4 (I) 2.00 434.78 422.99 Stilbene, trans. CI4H12 00 0.86 432.89 427.18

(e) Among unpolymerized species, carbohy- less water-soluble and inherently slow to react, and drates have exceptionally high Gdeox for the number thus have a better chance of being preserved in sed- of carbon atoms. They are exceeded only by a iments upon resisting microbial and chemical oxi- handful of compounds such as oxalic, formic, citric dation. In fact they are the dominant precursors of acids and formaldehyde. fossil fuels. Biopolymers such as cellulose, lignin, resins,and proteins are used as the structural material Calculations now in progress for Gdeox values at to support free-standingweightsby land plants. Upon 127°C (400 K) and 227°C (SOOK) also indicate burial these polymers undergo extensive aromatic that the above relations observed for 2SoC remain condensation to kerogen, which makes up more than unchanged. Except for condensed aromatic com- 90% of the total organic matter in the Earth's crust pounds, the G values converge to 424.3 kJ with deox (HUNT,1979).For thermochemical consideration of increasing carbon number at 12rc. fossil fuel formation, therefore, it is important to es- timate the deoxygenation parameters for these poly- ESTIMATION OF DEOXYGENATION mers, although only rough approximations are pos- PARAMETERS FOR NATURAL POLYMERS sible because (1) the entropy change associated with Certain organic substances have the ability to form polymerization is generally difficultto accurately as- macromolecules upon polymerization. Compared to sess, and (2) most of these polymers are composi- the respective monomers, these polymers are much tionally variable within a range. 276 M. Sato

500

450

400 c J 1.0 .5 ,6 .7 .8 .9

FIG. 1. Plots of Gdeox (deoxygenation free energy) versus Qdeox (deoxygenation quotient) for some compounds of the C-H-N-O system. Homologous series are identified by labelled connecting lines. Arabic numerals indicate the number of carbon atoms in these homologues. Some other groups are also identified with labels, where possible. Inorganic compounds are individually labelled. Except for labelling, hydrocarbons are excluded in this diagram. The star indicates the convergence value of the homologous series.

Many methods have been proposed for estimat- monomer or major components of a complex poly- ing thermochemical properties of organic com- mer, and particularly if the monomer is gas or liq- pounds (reviewed by JANZ, 1967) including the uid, there may be a significant change in entropy method of VANKREVELENand CHERMIN(1951) associated with polymerization. The entropy of po- for Gibbs energy offormation based on group con- lymerization may be ignored if the transformation tributions. However, most of these methods are de- is from an amorphous solid to a crystalline state. rived for the ideal gas state, and, hence, are appli- For example, the transformation of amorphous Se cable only to gaseous compounds and condensed to linear macromolecule trigonal Se(a simple model compounds with known heat of vaporization and case of polymerization) decreases entropy by 6.77 measurable vapor pressures. Biopolymers and geo- J/mol· K at 25°C according to GAUERet al. (1981), polymers have negligible vapor pressures at room which corresponds to a polymerization energy of temperature and decompose if heated. So in some about 2 kJ per mole of the monomer compound. cases it is preferrable to use the heat of combustion Cellulose data and rely on the similarity of Hdeox relation to the Gdeox relation as the first approximation. Cellulose is the basic structural material for higher If heat of combustion is available only for the plants. It is a highly polymerized polysaccharide Thermochemistry of the formation of fossil fuels 277

consisting of several thousands glucose (cellbiose) KJ units in chains. Upon hydration it produces glucose.

The chemical formula, (C6HIOOS)n, gives Qdeox of

0.706. The heat of combustion of cellulose at 25°C 440 is given as 2812.4 k.I/rnole (COLBERT et al., 1981),

which gives Hdeox as 468.7 kJ according to Equation (9). The H; value gives !:;.H? of cellulose as -977 .88 kf /mole. The SO for cellulose is estimated at 205 J/ mole- K by subtracting the entropy of macrochain polymerization (6.77 J/mole· K for Se) from the entropy of glucose (212.13 J/mole· K, STULL et al., 1969, p. 680). The entropy change in the substi- tution of 2(-OH) by (-0-) linkage in a solid polyhydroxy compound may be neglected (JANZ, 1967, Table 4.1). This gives !:;'G~ for cellulose as -681.5

k.l/rnole and Gdeox as 478.5 kJ. The Gdeox value is 0.7 kJ lower than that of glucose. The reversal in

the order of Hdeox and that of Gdeox between the two

compounds is due to the entropy of liquid H20 liberated in the dehydration-polymerization.

Lignin

Lignin is a collective term for amorphous aro- Hie matic macromolecular substances, which cement cellulose in land plants, and is an important pre- FIG. 2. Plots of Gdeox versus H/C atomic ratio for common hydrocarbons. Arabic numerals indicate the number cursor to coal. It is basically constructed from of carbon atoms. For paraffinic and olefinic series, only phenyl-propane derivatives, such as coniferyl al- those of straight chains are plotted. Graphite is considered cohol, sinapyl alcohol, and p-coumaryl alcohol, that to be the product of infinite aromatic condensation, and are biosynthesized from carbohydrates (SARKANEN included in the aromatic series. The star indicates the convergence value of the oxygenous homologous series shown and LUDWIG, 1971; KIRK et al., 1980). The poly- in Fig. 1. merization of these monomer units are associated with replacement of two end hydroxyls by an ether linkage. The atomic composition ranges typically al., 1969), which is 168.6 kl/mole (510.7 kJ in terms from (C H 0 )n to (C H O)n, depending on the ll I2 3 9 s of H; between the two), and applying equation (7) average number of the methoxyl (-OCH3) groups to the chemical formula of the lignin monomers. attached to the phenol ring, giving a Qdeox range of Ignoring the small differences in the entropy term, 0.893 to 0.955. A compound that is closest to these the approximate Hdeox value for the lignin group monomers in structure, and for which the heat of ranges from 455 kJ (for dimethoxy sinapyl unit, combustion or enthalpy of formation is known, is Qdeox 0.893) to 445 kJ (for p-coumaryl unit, Qdeox isoeugenol CH3CH:CH . C6H3 (OCH3)OH. To make 0.955). Other more complex and random config- a chain polymer of coniferyllignin from isoeugenol, urations would probably average out to this range. it is required that one each of the (C-H) and (C- There is no simple reliable way of estimating the OH) bonds are replaced by one (C-O-C) bond. Using Gdeox oflignins because of their complex structures. the bond contributions for heats of combustion of liquids (applicable to amorphous solids) given by Humic matter LAIDLER (1956), and 5345.5 kJ for the H; ofisoeu-

genol, the H; and Hdeox for the coniferyl lignin is This is an operational term applied to polymer- computed as 5170 kl/mole and 450 kJ, respectively. ized, water-insoluble brown to black substances

The variations in Hdeox by addition or subtraction formed, under restricted oxidative conditions in soil of a methoxyl group is estimated at about 5 kJ per and also in young sediments in swamps and other mole of the lignin unit. This is made on the basis subaqueous environments, from the residue ofbio- of the difference in !:;.H? between 4-hydroxy-m-an- logical matter unused by microorganisms. Humic isolaldehyde (vanillin) C6H3(OH)(OCH3)CHO and substances are soluble in dilute alkali solutions, but p-hydroxybenzaldehyde C6H4(OH)CHO (STULL et only partially so in dilute acid solutions. The acid 278 M. Sato

soluble fraction is called "fulvic acids" and have a A Quinic Acid Unit Prct oce techu t c Ac; c unt t range of composition of C: 30-40%, H: 6-8%, 0: OH 45-55%, N: 4.5-5.5% in weight. The acid-insoluble 1 H OH H C= 0 fraction is called "humic acids" and have the com- I I I I positional range ofC: 50-55%, H: 5.5-6.5%, 0: 30- o C-C 0 C-C 35%, N: 1-4% (GALIMOV,1980), and atomic ratio -CII:/~-O-C ~\II;jO~C-C - C C-H range of H/C: 0.5-1.5, and O/C: 0.2-0.5 (TISSOT : \~ ~/ ketonic '\ / and WELTE,1978). Fulvic acids tend to disappear ester C- C bond C__ C bond I I I I early during the diagenesis. On their way to becom- OH H OH .. ·0 .. · ether bond ing protokerogen and eventually to kerogen, humic I acids are' increasingly depleted of oxygen, and become more aromatic. Molecular characterization of humic acids is very incomplete. A general consensus is that the biological precursors are dominantly lignins and tannins B Quin;c Acid Unit Protocatechuic Acid Unit which are widespread in plants. Probably proteins OH and perhaps carbohydrates are also involved. Lab- I oratory studies indicate that both aromatic and ali- O=C ether I phatic/alicylic substances are incorporated. Func- .bond /C'-2H tional groups detected are carboxyls, carbonyls, C-O-C-H H-C-H ' , • C-C 0 phenolic and enolic hydroxyls, and amides in the : I I II : H-C-H H H-C-O-C /;0\ C-C-O-C order of abundance. Methoxyls are present only in low concentrations. For example, a typical humic ,,~/ ether '\ / : , bond C-C acid from peat described by CHRISTMANand OG- ester OH I I bond LESBY(1971) contains C: 58.35%, H: 4.97%, 0: OH H 32.2%, N: 2.62% in weight, and the following func- FIG. 3. Suggested models of humic acids that could be tional groups in millimoles/gram: 0.2 methoxyl, 2.9 derived from lignin and cellulose, and versatile in the ali- phenolic hydroxyl, 2.2 enolic hydroxyl, 5.5 car- phatic vs. aromatic ratio; (A) a model in which the aliphatic bonyl, and 8.6 carboxyl. The functional groups in and aromatic groups are bridged by a ketonic bond, and (B) that bridged by an ether bond. total take up 63.4% of the dried, ash-free humic acid, the balance presumably being hydrocarbon chains and rings. None of the proposed structural models (reviewed by SWAIN,1963;CHRISTMANand In view of the uncertainties of the chemical

OGLESBY,1971; Huc, 1980; GALIMOV,1980) ap- structure of humic acids, direct estimation of Gdeox pears to accommodate the abundant carboxyls and values of humic acids is not attempted. Instead, a carbonyls, and, at the same time, conform with an list of chemicals that have been identified upon approximate C:H:O ratio of 2:2:1 indicated by the degradation of humic matter, or proposed to be above weight composition. The alicyclic quinones likely components on theoretical grounds, are listed, could account for the carbonyls but have too high together with their Hdeox (and Gdeox in rare cases O/C ratios. Ketonic bridges between H-rich struc- where Gibbs energy of formation is given in the tures may resolve this problem. Such a unit may literature) in Table 2. Nitrogenous compounds are be constructed, for example, from one molecule of excluded for simplicity. Compounds with meth- quinic acid (OH)4' C6H7. COOH and one molecule oxyls are eliminated in view of the low abundance of protocatechuic acid C6H3(OH)2COOH, bridged of this group. Needless to say, compounds without by a ketonic or ether bond (Fig. 3a or 3b). A ke- heat of combustion or enthalpy of formation data tonic bond of this type is found in benzoin are excluded. Sample calculations show that Gdeox C6H5. CHOH . CO· C6H5. Quinic acid is an ali- or Hdeox values are very similar among isomers when cyclic polyhydroxy acid derived from carbohydrate they have more than several C atoms. Only the most and, upon dehydration, changes to aromatic pro- stable isomer is listed when Hdeox values vary by tocatechuic acid (CONNANT,1936, p. 458). Proto- less than 1 kJ among the isomers. The compound catechuic acid is also formed by oxidation of the p- name is given by a structural name in such a case. coumaryl alcohol unit oflignin. Such units may be Alicyclic quinoids are grouped together with aro- linked together by ether, ketonic, or ester matic polyhydroxy compounds because of the close (SCHNITZERand NEYROND,1975) bonds upon re- relationship existing between the two groups. leasing H20. The Qdeox values for humic acids inferred from Thermochemistryof the formationof fossilfuels 279

the Van Krevelen (H/C vs. O/C) diagrams (Fig. II. WELTE,1978, p. 210). Also the formation of poly- 2.9 in TISSOTand WELTE,1978; Fig. 7-6 in HUNT, cyclic aromatic rings is suggested (several to tens of 1979) are in the range of 0.778 to 0.927, and a typ- rings). The main functional groups are ketonic and ical value is 0.88. The range is richer in oxygen than carboxyl groups, but ester group is missing (ROBIN the above model lignins. et al., 1977). Similar to humic acids, Gibbs energy data are available only for a small number of solid Kerogen or liquid compounds which tend to be low in C- number. Hdeox can be estimated from polycyclic ar- This term was originally applied to the organic omatic compounds with carboxyl and carbonyl matter in oil shales that produced waxy oils (keros groups in Table 2. = wax in Greek) upon heating, but the term has been broadened in recent years to include all the DISCUSSIONS disseminated organic matter of sedimentary rocks The significanceof the G versus Qdeox relations insoluble in nonoxidizing acids, bases, and organic deox shown in Fig. I regarding the transformation of solvents regardless of capacity for oil distillation. plant matter to fossil fuels becomes apparent when On the basis of microscopic characterization, one notes that typical photosynthetic products have kerogens are classified into sapropelic and humic. higher Gdeox and lower Qdeox values compared with Sapropelic kerogens originate in decomposition and fossil fuel substances. A higher G means, by def- polymerization products oflipid-rich biological re- deox inition, that more energy is chemically stored in mains such as spores and algae and deposited in liberating a unit quantity of free oxygen from a subaquatic muds, usually under oxygen-poor con- mixture of H20, CO2, and N2, and also that a sub- ditions, and have relatively high H/C ratios (1.3- stance could be deoxygenated spontaneously by loss 1.7, HUNT, 1979). Oil shales and boghead coals of CO2 and/or H20 to produce another substance mature from organic-rich sapropelic deposits and of a lower Gdeox. This fact clearly has an important correspond to Type I of the three kerogen evolution bearing on the flow of energy in the biosphere as types recognized by TISSOTet al. (1974). The mo- discussed below. lecular sizes are dominantly in the range of CI5 to C and have average chemical composition ranging 40 Photosynthesis from C40H6805(Qdeox 0.956) to C40H5401(Qdeox

0.991) on a C40basis. Qdeox increases as the depth The higher Gdeox values for primary photosyn- of burial increases. This kind ofH-rich composition thetic products indicate that photosynthetic organ- requires predominance of saturated hydrocarbons isms chemically capture as much solar energy as of aliphatic/alicyclic groups and lesser amount (in possible at the initial stage of photosynthesis by uti- terms of carbon numbers) of aromatic hydrocar- lizing the thermochemical nature of the C-H-(N)- bons. The oxygenous functional groups are scarce, o system with regard to deoxygenation. When and consist of ester and ether linkages, hydroxyls, starting out from CO2, H20, and N2 of ultimate carboxyls, and carbonyls in varying proportions volcanic origin, the most effective chemical storage (VITOROVIC,1980). The Gdeox values are probably of solar energy by deoxygenation can be achieved somewhere between those of palmitic acid (Qdeox by forming compounds with highest Gdeox values, 0.958, Gdeox 425.6 kJ, Table 1)and the convergence which turn out to be compounds oflow Qdeox values. value (Qdeox 1.00, Gdeox 424.7 kJ) discussed earlier. In terms of GdeoXo oxalic acid (Qdeox 0.200, Gdeox Humic kerogens correspond to Type III ofTISSOT 649.6 kJ), formic acid (0.333, 554.7 kJ), formal- et al. (1974), and include humic coals. Atomic ratios dehyde (0.667, 521.7 kJ), and citric acid (0.529, on C40basis vary approximately from C40H32012 499.8 kJ) lead other compounds. The formation of (Qdeox 0.875) to C40H2804 (Qdeox 0.957) as they formaldehyde, followed by its polymerization to evolve during diagenesis. This type has the lowest monosaccharides, has been suggested as the initial H content among the three types. The composi- stage of photosynthesis by many investigators tional boundary between humic kerogen and humic (CONANT,1936, p. 561). This path is energetically acids is not well defined and the two overlap sub- workable. Carbohydrates (Gdeox 464 to 487 kJ) can stantially in the Qdeox range. In contrast to the ali- form from formaldehyde spontaneously with release phatic-rich Type I, this type is characterized by a of energy as indicated by the higher Gdeox of the high proportion of aromatic carbons. Chemical and latter. The organic acids mentioned are also found physicochemical evidence suggest that aromatic in plants. These acids will change to saccharides carbon atoms increase from about 70% in hard spontaneously if catalyzed by suitable enzymes, as brown coals to over 90% in anthracites (TISSOTand exemplified by the ripening of citric fruits. 280 M. Sato

Table 2. Deoxygenation parameters of compounds relevant to humic matter

Name Formula State Qdeox Hdeox Gdeox

Biopolymers

Cellulose (C6HlOOS)n (s) .706 468.7 478.5 Lignin (conyferyl) (ClOHlOO2)n (s) .920 450.0

Aromatic acids, hydroxyacids, anhydrides

Benzoic acid C7H602 (s) .882 430.26 430.27 p-Hydroxylbenzoic acid C7H603 (s) .824 432.51 436.52 Salicylic acid C7H603 (s) .824 432.39 436.31 Phthalic acid CgH604 (s) .789 429.78 436.68 Phthalic anhydride CgH403 (s) .833 434.54 439.79 Protocatechuic acid C7H604 (s) .765 435.78 Gallic acid C7H60s (s) .706 441.97 2,4-Cresotic acid CgHg03 (s) .850 432.41 Phenylacetic acid CgHg02 (s) .900 432.80 Phenoxyacetic acid CgRg03 (s) .850 444.72 p-Toluic acid CgHg02 (s) .900 429.13 p-Anisic acid CgHg03 (s) .850 440.68 Piperonylic acid CgH604 (s) .789 448.55 Atropic acid C9Hg02 (s) .909 437.32 Cinnamic acid, trans C9Hg02 (s) .909 434.77 Hydrocinnamic acid C9HlO02 (s) .913 432.68 p-Cumaric acid C9Hg03 (s) .864 437.39 Uvitic acid C9Hg04 (s) .818 431.84 Trimesic acid C9H606 (s) .714 428.16 Phenylpropiol acid C9H602 (s) .905 449.76 p-Isopropylbenzoic acid C lOH1202 (s) .923 432.03

Aromatic hydroxyl derivatives, aldehydes

Phenol C6H60 (s) .933 436.22 432.49 Benzyl alcohol C7HgO (I) .944 439.63 433.17 o-Cresol C7HgO (s) .944 434.53 o-Hydroxybenzyl alcohol C7Hg02 (s) .889 442.15 p-Ethylphenol CgHlOO (s) .952 435.25 Thymol ClOH'40 (s) .963 434.79 Isoeugenol ClOH'202 (I) .923 445.84 Pyrocatecohol C6H602 (s) .867 441.16 Hydroquinone C6H602 (s) .867 438.83 p-Benzoquinone C6H402 (s) .857 457.65 459.17 Phloroglucinol C6H603 (s) .800 430.78 2-Naphthol CIOHgO (s) .958 430.76 1A-Naphthaquinone ClOH602 (s) .913 438.94 Benzaldehyde C7R60 (I) .941 440.37 o-Anisaldehyde CgHgO (I) .900 447.72 Cinnamaldehyde C9HgO (I) .955 443.63 Phenylpropiol aldehyde C9H60 (I) .952 452.37

Aromatic ethers, ketones

Anisole C7HgO (I) .944 444.50 Phenetol CgHlOO (1) .952 441.57 p-Dimethoxybenzene CgHlO02 (s) .905 447.24 m-Methylanisole CgHlOO (I) .952 441.99 p-Allylanisole ClOR'20 (I) .962 447.12 Methylisoeugenol CllH'402 (I) .931 448.95 Diphenyl ether C12HlOO (s) .966 437.11 433.05 Acetophenone CgHgO (I) .950 436.73 430.19 Banzophenone C13HlOO (I) .968 435.12 Benzoion C'4H'202 (s) .941 435.99 Thermochemistry of the formation of fossil fuels 281

Table 2. (Continued)

Name Formula State Qdeox Hdeox Gdeox

Others

Cumen C9H12 (I) 1.00 434.61 424.74 p-Cymen ClOH14 (I) 1.00 433.90 424.00 Decahydronaphthalene, trans ClOH1S (I) 1.00 432.89 423.19 2-Methylnaphthalene, trans CllHIO (s) 1.00 429.82 423.46 Inositol C6H1206 (s) .667 461.98 p-Cymen-2,5-diol ClOH1402 (s) .926 437.85 Borneol ClOH1SO (s) .966 438.26 Terpinol ClOH1SO (s) .966 439.25 Campholic acid ClOH1602 (s) .929 439.59 Cyclohexane carboxylic acid C7H1202 (s) .900 434.23 Diphenyl carbonate C13HlO03 (s) .903 438.81 438.36 Eicosanoic acid C2oH4002 (s) .967 433.59 Phenylurea C7HsN2O (s) .944 433.27 Quinolin C9H7N (I) 1.00 436.40 432.03

The monosaccharides and glycerol (0.700, 472.8 the absence of definitive chemical characterization kJ), once synthesized, become the primary ingre- of humic acids and entropy data for lignins, this dients for a variety of biological substances of higher point is very uncertain. The portion of humic acid Qdeox values. As long as the biosynthesis proceeds derived from cellulose and other carbohydrates have

via loss of H20 and/or CO2, the process is most definitely lower Gdeox values than the precursor probably spontaneous, because both CO2 and H20 substances. The formation of quinic acid from car- are far more stable than the parental substance. bohydrates and its subsequent transformation to Structural material (e.g., cellulose, lignin, waxes, protocatechuic acid by dehydration and oxidation resins) and energy storage material (e.g., starch, fats, mentioned earlier may be cited as an example. oils) are probably produced from the primary in- Type III humic kerogens and coals, ranging in gredients without net addition of energy. In the Qdeox from 0.875 to 0.957, probably cluster around presence of NH3 (Gdeox 452.8 kJ), which is found 430 kJ in Gdeox as discussed earlier. With the de- in volcanic condensates as NH4Cl (WHITE and crease of functional groups as burial depth increases,

WARING, 1963), proteins may also be synthesized it is probable that Gdeox falls below 430 kJ in bitu- from these ingredients. minous coal, and below 420 kJ in anthracitic coal

judging from that ofperylene C20H12 (421 kJ). The Transformation tofossil fuels aromatic polycondensation ultimately results in the formation of graphite (394.4 kJ). Fossil fuel substances, such as natural gas (meth- In the overall deoxygenation energy relation of ane), petroleum (paraffins, cycloparaffins, aromatic fossil fuels, a significant point is that the Gdeox values hydrocarbons), coal (polycyclic aromatic com- of fossil fuel substances are very close to or lower pounds), and oil shale (Type I kerogen), all have than the convergence Gdeox value (424.7 kJ) for non- higher Qdeox and lower Gdeox values than do primary aromatic compounds mentioned earlier. This fact photosynthetic products. Methane has the lowest appears to indicate that no net input of energy has

Gdeox (409.0 kJ) among organic substances. Most taken place during the fossilization of biological re- hydrocarbons found in petroleum in nature have mains and the evolutionary history of fossil fuels.

Gdeox lower than 427.2 kJ. The only known excep- If energy had been added, in net, during the pro- tion appears to be cyclobutane (439.2 kJ), which cesses, some fossil-fuel substances would show sub- was reportedly found in a trace quantity (0.001 %) stantially higher Gdeox values than the convergence in a Venezuelan oil (WHITEHEAD and BREGER, Gdeox value. It is well known that thermal cracking 1963, p. 252). Kerogens in oil shale and boghead of petroleum produces olefins (443.5 to 426.8 kJ). coal (Type I) probably have Gdeox values between Hydrogen (H2, 474.4 kJ) is scarce in natural gas. It 424.7 kJ and 425.6 kJ as discussed earlier. may be argued that H2 escapes readily because of There may be slight reversals in Qdeox and Gdeox its high buoyancy and mobility, but the accumu- when humic acids form from lignin because of ad- lation of helium, which is equally mobile, in natural dition of oxygen during oxidative degradation. In gas reservoirs does not support this argument. In 282 M. Sato

the presence of unsaturated organic compounds, at room temperature, the role of increasing tem- H2, if ever produced, is expected to be consumed perature associated with increasing depth of burial readily for hydrogenation of the unsaturated com- is to increase, by orders of magnitude, the rates of pounds. It is more probable, however, that so-called reactions which would otherwise not proceed de- hydrogenation reactions are caused by intra- or in- tectibly even in geologic time scale. The activation ter-molecular proton transfer and that H2 partial energy for the polycyclic aromatic condensation to pressure never reaches a significant level during the cause hydrogen disproportionation must be large, transformation and maturation. requiring catagenic temperatures (100 to 150°C) Chemical changes of organic sedimentary de- for the process to proceed in geologic time. This, posits are rapid at the beginning of burial and soon however, does not necessarily imply that a heat in- reach a plateau stage when the carbon content put is essential for advanced stages of fossil fuel reaches about 85 weight percent (Qdeox about 0.96) evolution. A large mass of organic sediment could (BREGER, 1963). This is where the mode of energy create its own thermal environment, if heat is release changes from deoxygenation to hydrogen trapped by impervious overlying sediments, as the disproportionation, and a sharp bend in the kerogen transformation reactions are generally exothermic. evolution lines occurs in the Van Krevelen diagram. Also, adiabatic compression of fluids may elevate The H/C ratio begins to decrease rapidly at this the temperature in a large sedimentary basin such point. With most of the oxygen depleted (the atomic as the Gulf of Mexico (JONES, 1970). O/C < 0.1), the dominant form of the energy release A totally confined environment, on the other shifts to simultaneous polycondensation of aromatic hand, may not favor evolution of fossil fuels. The rings and production of methane and, in case of mass action law indicates that, at equilibrium, the aliphatic Type I kerogens, other light paraffinic hy- removal of the reaction products facilitates for a drocarbons. As polycondensation continues, H at- reaction to proceed. The removal is particularly oms are liberated from condensing alicyclic, ke- important when the free energy of reaction is small, tonic, and benzene rings and spontaneously parti- as in the late stages of fossil fuel evolution. Geo-

tion to form saturated light (C, or smaller) logical settings in which fluids (H20, CO2, CH4, hydrocarbons. These compounds all have Gdeox and other light hydrocarbons) can escape through

values lower than the convergence Gdeox' This stage porous media or fractures, or be absorbed by re- is catagenesis. actions with lithic material (such as unhydrated The ultimate state of energy release without ox- volcanic ash), would accelerate the evolutionary idation is the graphite (393.4 kJ) and methane process. (409.0 kJ) combination. This is the extreme result In conclusion, I would like to emphasize that of the spontaneous hydrogen partioning, a process this work is a preliminary attempt to obtain a broad that operates in metagenesis. Methane escapes easily picture of the significance of the thermochemical so that graphite becomes the ultimate in situ product relationship of organic substances to the process of of spontaneous evolution of organic matter in an fossil-fuel formation. Thermochemical data for im- anaerobic environment. portant natural substances such as lignin, humic

The large positive Gdeox values for all organic acids, and kerogens are incomplete and only a very substances of the C-H-N-O system indicate that broad framework can be constructed. Also, com- isolation from free oxygen is essential for fossil-fuel putations at elevated temperatures and pressures formation. The Gibbs free energies of reaction are yet to be completed. Hopefully a better frame- evolved in the direct oxidation by free oxygen are work will be built in future. at least an order of magnitude greater than those in the deoxygenation and hydrogen disproportion- Acknowledgement-This work was seeded in my mind ation reactions. An access to free oxygen would when the late Wilmot H. Bradley recruited me to study change the course of reactions, ultimately oxidizing the algal sediment at Mud Lake, Florida, in 1967 and the whole organic matter back to the pre-photosyn- asked me if algal sediment could be converted to oil shale under normal burial conditions. It seems to me that the thetic material, i.e., H20, CO2, and N2. present paper is appropriate to this memorial volume, be- Higher temperatures (up to a few hundred "C) cause the late Hans Eugster was interested in the ther- do not appear to change the energy relations, but mochemistry of the C-H-O system and he had a close they surely would increase the reaction rates, which friendship with W. H. Bradley.I deeply cherish the memory increase exponentially following the Arrhenius of my friendship with both of them, two scientists of ex- traordinary caliber and visions. I thank the late Irving A. equation, k = Ae-Ea/RT, where Ea is the activation Breger for challenging discussions in the early days of this energy and k the rate constant. Because the spon- work. I also thank Richard A. Robie for his help and com- taneity of evolution of fossil fuels is indicated even ments. Rama K. Kotra, William H. Orem, and Everett L. Thermochemistry of the formation of fossil fuels 283

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