Canadian MineralogistVol. 29, pp.707-739 (1991)

ORGANIC/INORGANIC REACTIONSIN METAMORPHIG PROCESSES

HAROLD C. HELGESONDepartment of Geologt and Geophysics, Ltniversity of California, Berkeley, California 94720, U,S.A.

FoREwoRD

This paper is dedicated to Hugh Greenwood in appreciation for all he has so elegantly done to enhance ourquantidti;e understanding of metasomatic processes. It was Hugh Greenwood who first explained how T-X1co,ldiagra*s can be used to advantage in representing and interpreting complex phase relations in metamorphic systems(Grienwood 1962). I hope he will find Figure 15 in the present communication gratifying in this regard, because tomy knowledge it is the first T-Xtco,t diigram to be published in which one of the curves represents metastableequilibrium imong minerals, a Cb21h2O fluid, and hydrocarbon species in petroleum at high temperatures andpiessures. I would like to take this opportunity to publicly express my appreciation to Hugh for all he has done forme over the years, I panicularly appreciate his thoughtful, incisive, and perceptive contributions to all of the manystimulating discussions we have had. Hugh is always up for just about anything, and we've tested together manyorganic/inorganic fluids over the years in a determined effort to better understand our mutual perceptions of theworld of metimorphic petrology. Through ir all, Hugh has been a staunch supportive friend whom I shall miss greatlyas an academic colleague, but look forward to seeing often on the ski slopes and sailboats of the future. Thank you'Hugh, for sharing with us over the past 30 years or so your unselfish friendship, challenging intellect, superbcraftsmanship, and unparalleled scholarship.

ABSTRACT

Although the ultimate fate of carbon derived fromburied biomass during prograde metamorphism of crustalrocks is carbon dioxide, methane, or graphite, geologicalobservations and experimental data reported in theliterature suggest that organic compounds in intermediateoxidation states may persist at high temperatures andpressures in hydrocarbon liquids and aqueous solutionsover substantial periods of geological time. Ther-modynamic calculations indicate that under these cir-cumstances, metastable equilibrium may be achieved inmetasomatic processes among certain hydrocarbonspecies in petroleum and oxidized aqueous species suchas CH3COOH and CO2 in hydrothermal solutionscoexisting with minerals in the system KAIO2-SiO2-CaO-FeO-H2S-O-CO2-H2O. The calculations reveal thatprogressive burial of sediments containing hydrocarbonreservoirs or source rocks in which hydrocarbon speciesin petroleum are in metastable equilibrium with mineralsand oxidized carbon-bearing aqueous species favorsprecipitation of K-feldspar, calcite, and pyrrhotite at theexpense of the annite component of biotite, pyrite, siderite(or other Fe-carbonates), and hydrocarbon species incrude oil. The process leads to enrichment of the aqueousphase in CO2, which reaches a maximum mole fractionof - 0,75 at 320oC. Metastable persistence of crude oil athigher temperatures is probably limited to heavyaromatic-rich crude oils and asphaltenes coexisting withCO2-rich aqueous fluids in rocks that are devoid of phaseassemblages that are capable of buffering the fugacity ofoxygen. The calculations also indicate that tectonic release

70'7

of CO"-rich fluids from depths in excess of - l0 km mayresult in precipitation of hydrocarbon liquids as the fluidflows upward toward the surface.

Keywords: organic/inorganic reactions, petroleum,metamorphism, phase relations in the system KAIO2-SiO2-CaO-FeO-H2S-O-CO2-H2O, hydrocarbons,carbon dioxide, oxygen fugacilY.

Sotrlvatng

Le carbone d6riv6 de matdriaux biog6niques enfouis,suite au mdtamorphisme prograde de roches crustales,apparait 6ventuellement sous forme de gaz carbonique,m6thane ou graphite; cependant, les observations 96olo-giques et les donn€es exp6rimentales cit€es dans lalittdrature font penser que les compos6s organiques desniveaux d'oxydation interm€diaires pourraient persisterjusqu'i une temp6rature et une pression dlevdes dans deshydrocarbures liquides et des solution aqueuses, et ce, surune trbs longue 6chelle de temps. Les calculs thermody-namiques indiquent que dans de telles circonstances, un6quilibre m6tastable pourrait €tre atteint au cours desprocessus m6tasomatiques impliquant les esp0ces hydro-carburdes dans le p6trole et les espCces aqueuses oxyd6estelles que CH3COOH et CO2, dans les solutionshydrothermales qui coexistent avec les min6raux dusysteme KAIO2-SiO2-CaO-FeO-H2S-O-CO2-H2O. D'a-prbs les calculs, l'enfouissement progressif des s6dimentscontenant des r6servoirs d'hydrocarbures ou des roches-mbres dans lesquelles des esp€ces hydrocarburdes dup6trole sont en 6quilibre mdtastable avec des min6raux et

708 THE CANADIAN MINERALOGIST

des espdces contenant carbone oxyd6 favorise la pr6cipitation de feldspath potassique, calcite et pyrrhotite auxddpens du pdle annite de la biotite, pyrite, sid6rite (ouautre carbonate ferrifbre), et des especes hydrocarbur6esdans le mazout. Ce processus mbne i un enrichissementde la phase aqueuse en CO2, qui atteint une fractionmolaire maximum d'environ 0.75 e 320'C. La persistencem6tastable du mazout ?r des temp6ratures plus 6lev6es esttout probablement limit6e aux mzvouts riches encompos6s lourds aromatiques et aux asphaltbnes quicoexistent avec un fluide aqueux riche en CO2 dans desroches sans phases aptes i tamponner la fugacit€ deI'oxygine. Les calculs indiquenr aussi que la lib6rationtectonique des fluides riches en CO2 ir une profondeurd6passant 10 km pourrait causer la pr6cipitation d'hydro-carbures liquides i mesure que la phase fluide migre versla surface.

(Traduit par la Rddaction)

Mots-clds: r6actions organique-inorganique, pdtrole,m6tamorphisme, relations de phase, systbme KAIOr-SiO2-CaO-FeO-H2S-O-CO2-H2O, hydrocarburi,gaz carbonique, fugacit6 de I'oxygbne.

INTRoDUCTIoN

Organic molecules in kerogen, petroleum, andaqueous solutions in the Earth's crust are common-ly assumed to be unstable at high temperatures.According to the conventional view of catagenesis(Tissot & Welte 1984, Hunt 1979, North 1985),thermal degradation of kerogen with increasingburial leads to the formation of petroleum attemperatures from - 60o to 120o or, in some cases,at temperatures to l50oC. With further burialand increasing temperature to -200oC, organicmetamorphism (metagenesis) is thought to result inconversion of all organic matter that remains in thesediment to graphite and natural gas consistingprimarily of methane. Although artractive frommany points of view, this overall scenario has comeinto question in recent years, primarily in responseto mounting evidence that kerogen may retainhydrocarbon-generating potential at much highertemperatures and that under cefiain circumstances,liquid hydrocarbons may persist metastably attemperatures in excess of 400'C (Simoneit. er a/.1 98 l, Reverdatto et ol. I 984, Price er a/. 1986, Pricer99l ) .

Both catagenesis and metagenesis are essentiallyprocesses of progressive deoxygenation of organicmaterial. Thermodynamic calculations indicate thatboth the Gibbs free energies and enthalpies of thedeoxygenation reactions are negative (Sato 1990).Hence, the reactions are favored thermodynamical-Iy, but to a decreasing extent with increasingtemperature! Nevertheless, the dramatic decreasein oxygen fugacity with increasing depth of burialin the upper few kilometers of a sedimentary basin

(Helgeson 1992) results in large chemical affinitiesfor the deoxygenation reactions. The chemicalaffinity (A) of a reaction is given by A = RT ln(K/Q), where K and Q stand for the equilibriumconstant and activity quotient for the reaction,respectively. Because the second law of ther-modynamics requires A dt/dt > 0, where df,/dtrepresents the reaction rate (Prigogine & Defay1954), it follows that the deoxygenation reactionsare favored during catagenesis to an increasingdegree with increasing depth of burial to -3 km (seebelow), despite their exothermic nature. However,if the oxygen fugacity of the system is buffered atdepths in excess of - 3 km by mineral assemblages,the oxygen fugacity will increase with further burialand increasing temperature above - l00oC (Ilelgeson1992), which thermodynamically favors preserva-tion of oxidized organic species and inhibition ofthe deoxygenation reactions at greater depths. Asemphasized by Sato (1990), the role of increasingtemperature with depth of burial is restricted toincreasing exponentially the rates of the deoxygena-tion reactions. Although the extent to which eachof the reaction rates is affected by increasingtemperature remains to be determined, recenlisotopic data (Peter et al. l99l), as well as hydrouspyrolysis experiments and organic geochemistrystudies of rocks from ultradeep wells (Price 1983,l99l), indicate that the overall process is rapid inthe context of geological time. It thus seems highlyunlikely from a thermodynamic point of view thatall of the myriad of organic compounds insedimentary basins would fail to survive burialtemperatures in excess of - 150o-200oC.

If stable equilibrium is achieved in metamorphicprocesses at crustal pressures and temperatures,thermodynamic calculations indicate that all of thecarbon in the system C-H-O would be distributedamong CHa, CO2, and graphite (see below).Hydrothermal precipitation of graphite inmetamorphic rocks has received considerable atten-tion in recent years (Dobner et al. 1978, Weis e/al. 1981, Rumble et al. 1986, 1989, Rumble &Hoering 1986), but so far CHa and CO2 are theonly carbon-bearing species considered ingeochemical models that have been invoked toaccount for the precipitation process. Nevertheless,recent kinetic studies of the decarboxylation ofaqueous acetic acid to form CHa and CO2(Drummond & Palmer 1986, Palmer & Drummond1986) suggest that aqueous species with inter-mediate oxidation states between CH4and CO2 maypersist at high temperatures ove, Iong periods ofgeological time. This observatidn is consistent withthe hieh concentrationf,.of carboxylic acids inoil-field brines (Willey et al. 1975, Kharaka et al.1977, 1986, Carothers & Kharaka 1978, Workman

ORGANIC/INORGANIC REACTIONS IN METAMORPHIC PROCESSES 709

& Hanor 1985, Hanor & Workman 1986, Crosseyet ol. 1986, Fisher 1987, Means & Hubbard 1987,MacGowan & Surdam 1987, 1988, 1990a, b, Barthl99l). Analytical data and thermodynamic calcula-tions also indicate that oil-field brines may serve asore-forming solutions (Sverjensky 1984) and thatCOz and aqueous carboxylic acids are inhomogeneous equilibrium in these brines, but thatmethane is not (Shock & Helgeson 1986, Shock1987, 1988, Helgeson & Shock 1988, Helgeson1991, Helgeson et al. l99l).

The temperature limits at which various organicaqueous species become labile has yet to beestablished, but several lines of evidence suggestthat in many instances, these limits are much higherthan those considered "reasonable" by organicchemists. For example, a recent thermodynamicanalysis (Shock 1990a) of high-temperature ex-perimental data reported by Miller & Bada (1988)indicates that metastable equilibrium amongoxidized organic aqueous species may occur at hightemperatures. In this case, the species consideredwere amino acids at 250oC and 260 atmospheres.The high-temperature metastability of organicspecies is consistent with fluid-inclusion data(Levine et al. l99L), as well as the presence of suchspecies in submarine hydrothermal vent waters(Simoneit 1990, 1991, Simoneit et al. 1990) andhigh-temperature springs and fumaroles at Yel-lowstone National Park (Des Marais & Truesdell1991). Thermodynamic calculations of the relativestabilities of aqueous peptides, amino acids, andother biochemical molecules (Shock 1988, 1990a,b, c, 1991, Amend & Helgeson 1991) indicate thatincreasing temperature not only favors the meta-stable persistence of organic species in oil-fieldbrines, but also the presence of bacteria inhydrocarbon reservoirs.

Liquid hydrocarbons have been found as in-clusions in metamorphic minerals formed at -6 kbarand 300oC (Goff6 & Villey 1984) and observed inrock samples retrieved from well depths at whichthe present-day temperatures range from 225o to300oC (Price et al. 1979,1981, Price 1982). In hisreview of evidence for the thermal stability oforganic compounds, Shock (1990c) concluded thatvarious organic compounds are preserved in theEarth at temperatures and pressures extending toat least 500oC and 10 kbar, depending on thecompound.

The role of petroleum, oil-field brines, andorganic/inorganic reactions among minerals andfluids in metamorphic processes has yet to beinvestigated from a thermodynamic point of view.A preliminary step in this direction is representedby the present communication, the purpose ofwhich is to explore with the aid of thermodynamic

calculations to what extent organic species inpetroleum may be in metastable equilibrium withminerals and oxidized aqueous species duringprograde metamorphism of hydrocarbon reservoirsand source rocks in sedimentary basins.

CnlculartoN oF EQUILIBRIUM CoNSTANTSeT HICH TTVPENNTUNES AND PRESSURES

FOR REACTIONS INVOLVING MINERALS, cor GAS'

OncaNtc AND INoRGANIC AQUEous Spnctrs,

AND ORGANIC MOITCUIES IN PETROLEUM

AND NATURAL GAS

The apparent standard partial molal Gibbs freeenergy of formation (AG") of a given species at aspecified pressure (P) and temperature (T) isdefined by

ac'=d-ci+(eir-ei,,r,) (r)

where 46ol refers to the standard partial molalGibbs free 6nergy of formation of the species fromthe elements in their stable form at a referencepressure (P,) andremperature (T,) of I bar ard298.15 K, and @,t-Q,r, stands for the dif-ference in the standard partial molal Gibbs freeenergy of the species at the pressure and tempera-ture of interest, and that at P" and T". The standardstate adopted in the present study for minerals,liquid H2O, and organic species in petroleum is oneof unit activity of the thermodynamic componentsof stoichiometric minerals and pure liquids at anypressure and temperature. The standard state foraqueous species other than H2O calls for unitactivity of the species in a hypothetical one molalsolution referenced to infinite dilution at anypressure and temperature. For the thermodynamiccomponents of gases, the standard state is specifiedas unit fugacity of the pure hypothetical ideal gasat I bar and any temperature.

The last term on the right side of Eqn. (l) canbe expressed as a function of temperature andpressure by writing

6g,1 - Gi,,r, = - g,r, 1r- r,1 + Iei,ar

r P- \zt-riei-anr+ IviaP

T- P,

where S$,1", e;, and V;, denote the standardpartial modal entropy, heat capacity, and volumeof the species at the subscripted pressure andtemperature. Equations representing Cf, and Vfin Eqn. (2) are summarized below for minerals,CO2 gas, aqueous species, and hydrocarbon liquidsand gases.

710 THE CANADIAN MINERALOGIST

(4)

where Q, Y, and X denote the partial derivativesof the reciprocal dielectric constant of HrO (e) givenbv

and

C"o ana Vo refer to the conventional standardpartial molal heat capacity and volume of theaqueous species, o1, e2; a\t e4s cp a,frd c2 representtemperature- and pressure-independent coefficientscharacteristic of the species, O and V stand forsolvent parameters equal to 228 K and 2600 bars,respectively, c,r designates the conventional Borncoefficient of the species, which can be expressedfor the lh such species as

/ " '

, ' r - l . \a = nlz"1 l+ ;,v,.r, * lz iB)' - U (3.os2 + e)-' J t r r I

where r"r,...1, refers to the efflective electrostaticradius of the species at the reference pressure andtemperature, which for monatomic ions is given by(Helgeson & Kirkham 1976)

l - t .re,i,v,,r,= rx; +lzilk2 02)

and for other charged aqueous species by (Shocket al. 1989')

Minerols and CO2 gas

The standard partial molal heat capacities ofminerals and CO2 gas can be calculated as afunction of temperature at I bar from powerfunctions used to regress experimental calorimetricdata such as those adopted by Maier & Kelley(1932) arrd Kelley (1960), Haas & Fisher (t976\,Berman & Brown (1985), Fei & Saxena (1987),Holland & Powell (1990), and others. The Maier-Kelley power function is sufficiently accurate foruse in the present study. It can be expressed as

e S = c + D T + 4

where a, b, and c stand for temperature-pressure-independent coefficients characteristic of themineral. Because the standard partial molalexpansibilities t(AV"/aT)pl and compressibilities[(AV'lAP)r] of minerals in the crust of the Earthare small and have an opposing effect on thestandard partial molal volumes of minerals withincreasing temperature and pressure, Vi for mostsuch minerals in Eqn. (2) can be expresied as

.= [9],=*(!**),

"= [9],=,(@,),

(8)

(e)

(3)

without introducing undue uncertainty in calcu-lated values of G,r - G,r, at pressures < l0kbar. Equations (3) and (4)'permit Eqn. (2) to bewritten as

Gi,r - Gi,,r. = - -$,..r (r-T.)+ 4r-" -tr f+ll[("*ar]r)1r-l"y'.1 --."

',- " . (5)

- t---;4t-----:,1

+ -vP"r' (P - P')

which can be substituted in Eqn. (l) to generatevalues of AGo for minerals. In the case of dO2 gas,the standard state for gases adopted in the presentstudy (see above) requires Vi and V;,r,r, in Eqns.(2) and (5) to be equal to zero.

Aqueous species

The standard partial molal heat capacities andvolumes of charged aqueous species as functionsof temperature and pressure can be computed fromthe revised HKF (Helgeson et ol. l98l) equationsof state (Tanger & Helgeson 1988), which can bewritten as

e 3 = c , + - - 9 - - ( , , o / v ' o \ \

G-er t (r-o)rl4(P-P'l.at'lffiJJ ..+corx+zrvfPt -tii-,Y4]

" (6)- - - \ar7r ' \e ' / [ar 'zJo

and

i. = a, + --L a _--4-* =.-*-3- - .o - 11 - tYP)' Y+P r-e (Y+PXr-a) ' - - \e

-AaPir (7)

*=(#),=i[(#).-3(#),](10)

vi =vi.,x

( l 3 )

where Zi stands for the charge on the lh aqueousspecies, r,, denotes the^ crystal radius of the species,n = 1.66027 x 105 A gal mol-r, k, designares aconstant equal to 0.94 A for cationi and zero Afor anions, g represents a solvent function oftemperature and density given by Shock ef a/.(1992), Srfr,.r, refers to the standard partial molalentropy of the 7th aqueous species at the sub-scripted pressure and temperature, and o2 isdefined by

as=tlslz1l (r4)

Differentiating Eqn.(11) leads to

ORGANIC/INORGANIC REACTIONS IN METAMORPHIC PROCESSES 7 t l

and

[4e) =(ar.)'*, *ft.g'] *,[ ] ."Jr- lar.Jr^'- l .attro^t (17)

wheret, ",, , , ,-2 "\x, = -n[lzjl(',,i,0,.r, +lz js)

- -21Q.o82+ d-" ) (t 8)and

/ " ' , lo\a -z,B.oaz+r)-3) (19)

x2=2nlz j l r . ; ,p , . r ,+12, ,_, _. )

Evaluating the integrals in Eqn. (2) with the aid ofEqns. (6)-(19) leads to

Gi,r - c-i,,r, = - -sp.a(r-r,) - ", [t' (+)-'.

" ]/ - ' L ' l+ ar(e _v,) + a,

" l-** ., 7( ( r r t ( r ' \ ' \ r e - r r r . ( r . r r - e t ' \ )-al[r-6j-[r,-erl s ]-e'Irn;qy

r t t ( . /v+p' \ ' ' l (20)+lr*Jla,(n-n).*'lffiJJ/ r \ ( r l+ ol:- tJ - or,.1

ffi - t.1*.r.' Yp,.',(t- r,)

which can be combined wirh Eqn. (l) to calculatevalues of AGo for ionic aqueous species at hightemperatures and pressures.

The analog of Eqn. (20) for neutral aqueousspecies can be written as (Shock el a/. 1989, Shock& Helgeson 1990)

/ / r \ )^6'= - $.,r (r-t) - "r lr n

1;.1- r.r,.1

+a ' (p -n , )+c2 r (# )

((r r t ( r ))re-rr r. l ' rrr-et ' \)-", [r-oJ-l r,-srl e .J-F' l . iF;A))

r t t t . /v+p ) ' l Ql). lt*J 1",{n-n'). ".'' lffiJJ*.,f"*r1r-1,;.l- =r- I

l . ' ' ' e eP, 'T,

)

where c,r" denotes the effective Born coefficient ofthe species, which is independent of pressure andtemperature and consistent with

r,l" = d+6 Si,,r ez)

where d and 6stand for correlation parameters forvarious groups of neutral aqueous species given byShock el al. (1989), Shock & Helgeson (1990), andD.A. Sverjensky, E.L. Shock, and H.C. Helgeson(in prep.). Equation (21) can be used together withEqn. (l) to compute apparent standard partialmolal Gibbs free energies of formation of bothinorganic and organic neutral aqueous species athigh temperatures and pressures.

Hydrocarbon liquids

The standard partial molal heat capacities of alarge number of organic liquids, gases, and solidscan be computed as a function of temperature atone bar from equations given by H.C. Helgeson &A.M. Knox (in prep.), who demonstrated that Eqn.(3) represents closely all of the available heatcapacity data for hydrocarbon liquids. However,the c coefficient in Eqn. (3) is positive forhydrocarbon liquids, which is generally not the casefor minerals.

The standard partial molal volumes of hydrocar-bon liquids at high temperatures and pressures werecomputed in the present study from the ParametersFrom Group Contributions (PFGC) equation ofstate (Cunningham 1974, Majeed & Wagner 1986),which can be written for the standard partial molalcompressibility factor (Zo) of a given stoichiometrichydrocarbon as

f=Pr"=1_sv 'hrv : , b )_ ,Rr D \ v " /

[ '-4*,',, I ""+ 12* >Yil ----l------r I'

[v"-t'-?',,,J,Jwhere R refers to the gas constant, and V, and i{stand for the mole fractions of the atoms in the tthand lh polyatomic groups (such as CH3- and-CHz-) in the hydrocarbon of interest, which aredefined for stoichiometric hydrocarbons by

v,=JL' Lvini Q4)i

andV: l l r

Y:=---JJ-' )u jnji (2s)

where z, and v, denote the stoichiometric numberof molei of thei ith and 7th groups, respectively, inone mole of the hydrocarbon species, and ri andn; designate the stoichiometric number of moles ofaioms in one mole of the rth and lh groups. Theparameters D and s in Eqn. (23) are defined by

fa<o) _ [a3) -I aP ,/,

- [ aP Jr^t

(#),=(#),"'(15)

( l6 )

a=!v1-vii

and

s=!v;4s; e7)t

where V| and si represent the standard partial molalvolume and degree of freedom coefficient of the

(26)

7t2 THE CANADIAN MINERALOGIST

ith group, respectively, and \,7 in Eqn. (23) standsfor a group-interaction term'for the ith and lhgroups defined by

.E' 'ILTL i i= l r j i=e

u t ! t ' ' (28)

where k refers to Boltzmann's constant and/ F . . + F . \

Eu=aii l=-l" " \ 2 ) Q 9 )

where ai; represents the interaction coefficient forthe ith and 7th groups and

E,, = e(9) + e(l)[soP'2. t

- \. Rr -l.J (30)

where Efo) and E l) denote interaction parametersfor the subscripted groups. E;; in Eqn. (29) is givenby the analog of Eqn. (30) ii which the subscriprii is replaced bry /.

Values of b, s, o;1, E 0), E !), E!!1, and Efll in Eqns.(23), (29), and its'analog for-E,, (30) "have beenretrieved for a large number of hydrocarbons andother orhanic molecules by regression of pressure-volume-temperature data at pressures and tempera-tures to -350 bars and -24A"C (Cunningham 1974,Majeed & Wagner 1986). Average uncertainties incalculated values of Vt for the z-alkanes in thispressure-temperature range appear to be of theorder of a few percent or less.

The PFGC equation of state is based onempirical modification of Eyring's (1936) latticetheory of liquids (Barker 1963, Wilson l9Z2).Because its theoretical foundations are independentof temperature and pressure, it was used as a firslapproximation in the present study to predict valuesof Vo for n-alkanes at pressures and temperaturesbeyond those for which expgrimental data areavailable. The integrals of Vi in Eqn. (2) forhydrocarbon liquids were evaluated with the aid ofa numerical integration computer routine adoptedfrom Arden & Astill (1970). As indicated above,the standard partial molal heat capacity integralsin Eqn. (2) were evaluated for hydrocarbon liquidswith the aid ofEqn. (3), but those for hydrocarbonswith boiling temperatures at I bar that are lowerthan the temperature of interest were evaluated forthe liquid standard state at high temperatures andpressures by taking into account the standardpartial molal heat capacities of both the liquid andgas phases along the integration path.

Hydrocorbon gases

The standard partial molal heat capacity ofmethane gas as a function of temperature at onebar can be computed from Eqn. (3), but those ofother hydrocarbon gases at I bar are quadraticfunctions of temperature from 298.15 to 800 K

(H.C. Helgeson & A.M. Knox, in prep.), consistentwith

er,: d + 6T + erz,

where d, 6, and i stand for constants characteristicof hydrocarbon gases other than methane. As aconsequence of the standard state adopted for gasesin the present study (see above),

V" =0 (32)

for gases. Taking account of Eqns. (31) and (32),Eqn. (2) can thus be written for the standard statefor gases as

/ / r \ \6[,1 -GF,,r, = - -sr,,t, (r - r,) + a I r- !, - rhl r | | .:l

\ \ r r , ,U ' -

- 2 ft -r-\2 - 9 (r3 - nh +zr1\z ' , " 6 \ ' ' '

Equilibrium constants

The equations summarized above permit calcula-tion of equilibrium constants for reactions amongminerals and aqueous, liquid, and gas species from

/oor= - AGi- 2.303RT

where K stands for the equilibrium constant, Ragain refers to the gas constant, and AGi denotesthe standard partial molal Gibbs free energy ofreaction, which is given by

AG; = >tt,rA6i (35)i

where i?,., designates the stoichiometric reaction-coefficient of the rth solid, aqueous, liquid, or gasspecies in the rth reaction, which is positive forproducts and negative for reactants, and AGistands for the apparent standard partial molalGibbs free energy of formation of the rth species,which is given by an appropriate statement of Eqn.( l ) .

The equations of state parameters and ther-modynamic data at 25'C and I bar used in thepresent study to compute values of log K fromEqns. (l)-(35) were taken from Helgeson e/ a/.(1978), Majeed & Wagner (1986), Shock &Helgeson (1988, 1990), Shock et a/. (1989), andH.C. Helgeson & A.M. Knox (in prep.). Thecalculations were carried out with the aid ofprogram SUPCRT92 (Johnson et al. 1992) vsingequations taken from Johnson & Norton (1991) tocompute values of the density and dielectricconstant of H2O and their partial derivatives. The

(31)

(34)

ORCANIC/INORGANIC REACTIONS IN METAMORPHIC PROCESSES 7t3

g function and its partial derivatives were generaledfrom equations given by Shock et sl. (1992).

OXIDATIoN/REDUCTION EQUILIBRIA

AMoNG OncaNtc MoLECULES,MINERALS, AND AQUEOUS SPECIES

Although hydrocarbons are covalently bonded,nominal oxidation states can be assigned to thecarbon atoms in the molecule so that any changein oxidation state is equivalent to electron transferin oxidation/reduction reactions. In contrast toreactions among ionic species of different charge,oxidation/reduction reactions involving hydrocar-bons or other organic molecules involve incompletetransfer of shared electrons in the covalent carbonbonds. Nevertheless, nominal oxidation states canbe assigned to the carbon atoms in organicmolecules. As in the case of minerals, it makes nodifference if the nominal oxidation states assignedto the carbon atoms in organic species correspondto physical reality because electrons are alwaysconserved in oxidation/reduction reactions. Theonly requirement for assigning nominal oxidationstates to the covalently bonded carbon atoms inorganic molecules is that any reaction of themolecule with HrO does not change the sum of thenominal oxidation states of the carbon atoms(Hendrickson et al. 1970). This requirement is metby assigning to a given carbon atom a nominalcharge of -l for each carbon-hydrogen bond, zerofor each carbon-carbon bond, and + I for eachcarbon-oxygen, carbon-sulfur, or carbon-nitrogenbond. For example, the middle two carbon atomsin n-butane, which can be written as

H I I H H

l l l lH - c - c - c - c - H ( 3 6 )

t l l lH H H H

are assigned nominal oxidation states of -2, butthe outer two carbon atoms have a nominal chargeof -3. The overuge nominal oxidation state (Z) ofthe carbon atoms in butane is then equal to [2(-2)+ 2C3)l/4 = -2.5. Although the law of definiteproportions requires the carbon number (n) oforganic molecules to be an integer, Z" and otherdependent variables are described below as con-tinuous functions of n, with the implicit under-standing that the functions are valid only at integervalues of the independent variable. For example,Z"for n-alkanes is plotted in Figure I as a functionof carbon number for n : 1,2,...,50, where thecarbon number t? represents the stoichiometricnumber of moles of carbon atoms in one mole ofthe n-alkane chain. It can be deduced from this

N-CARBOXYLIC ACIDS

- 2(n-21LC=-T

N.ALKANES

7 - -2(n+ l )-cn

2

o

J' _l

-2

-3

o lo 2a 30 40 50n

Frc. l. Average nominal oxidation states of the carbonatoms (2") in r-carboxylic acids and r-alkanes (seetext).

figure that 2.. is an asymptotic function of n forwhich

hy,Z"=4 e7)n-t@

and

limz^ =4 t 'o\il1-c (381

Z" can be regarded as a measure of the relativeoxidation states of organic molecules. Althoughn-alkanes become more oxidized with increasingcarbon number, alkylbenzenes and oxygen-bearingspecies such as the carboxylic acids become morereduced. For example, the nominal oxidation stateof the carbon atom in formic acid (HCOOH),which corresponds to

o//

H - c\

o - His 2 because the carbon atom is assigned a chargeof I for each bond to an oxygen atom. However,the average nominal oxidation state of the carbonatoms in acetic acid (CH3COOH), which can beidealized as

(3e)

7t4 THE CANADIAN MINERALOCIST

(40)

is zero. Z,for n-carboxylic acids is plotted in FigureI as a function of n for n : 1,2,...,50. It can beseen in this figure that Zc for the carboxylic acidsalso is an asymptotic function of carbon number,which approaches the same limit as that of ther-alkanes (-2) as n * o. However, in contrast tothe curve for the n-alkanes in Figure l, Z, for ther-carboxylic acids approaches 2 as z * 1. Notethat in the case of both curves in Figure l, 90% ofthe change in the average nominal oxidation stateof the carbon atoms occurs from rz : I to n :10. At z greater than or approximately equal to 20,the absolute difference in the average nominaloxidation states of the carbon atoms in n-alkanesand r-carboxylic acids is less than or approximatelyequal to 0.3.

Acetic acid and its dissociated counterpart (theacetate anion) are apparently the most abundanrorganic aqueous species in oil-field brines, which

are known to contain as much as 10,000 ppm totalacetate (MacGowan & Surdam 1988, 1990b). Aceticacid is apparently in metastable equilibrium withaqueous CO2 and other carboxylic acids in thesebrines (Shock & Helgeson 1986, Shock 1987, 1988,Helgeson & Shock 1988, Helgeson 1991, Helgesonet al. l99l), which is consistent with

2CO44+2H2O e CH3COOHGq)+2Oz(8) (41)

where the subscripts (qq) and (g) stand for aqueousand gas states, respectively. The logarithmic analogof the law of mass action for reaction (41) can bewritten for anr6 = 1 as

lo8 acrrcoon,*, = logK+2 log koO., -2log fs4r1 @2)

where K refers to the equilibrium constant forreaction (41), arr6' acH3.cooH1on1, and a.or,on, denoteIne acllvlues oI tne subscnptecl aqueous specles,and log/or,r, designates the fugacity of oxygen inthe system. The assumption of unit activity of HrO

H Ol / /

H - C - C

t \H O - H

roo r50 250 300 360 400 450 500 550TEMPERATURE, "C

Ftc' 2. Logarithm of the fugacity of oxygen Eas Uo21n1) buffered by the equilibrium mineral assemblages depicted inthe triangular phase-diagrams as a function of iHrirperature along the fluid pressure profile shown in Figure +.The reactions represented by the univariant curves and invariant points (filled circlesjare listed in Table l. andthe mineral abbreviations are identified in Figure 13.

ORGANIC/INORCANIC REACTIONS IN METAMORPHIC PROCESSES 7t5

C'g - l l

-oI -r3

to-

ot -ls(9

9 -)7

-?5-67 -65 €3 € l -59 -57 -55

LoG to.,o,-53 -51 -49 -47

Frc. 3. Logarithm of the activity of acetic acid in an aqueous phase (a6s,sq6u,oo) coexisting with quartz, calcite,and the mineral assemblages depicted in the triangular phase-diagrams for tlid system KAIO2-SiO2-Ca-Fe-S--O-CO2-H2O as a function of the logarithm of the equilibrium fugacity_of_oxygen Vo2u1) at 100'C and 400 bars(see teit).' Equilibrium values of log oCo2tuatare represented by the dashed curves.-The mineral abbreviationsare identi f ied in Figure 13.

a.'MT

7 t6 THE CANADIAN MINERALOGIST

TABLE I. SUMMARY OF REACTIONS REPRESENTED BY TI{E TJMVARTANT CI.JRVESAND INVARTANT POINTS SHOWN IN FIGT]RE 2

UnivariantCurveando

InvariantPoint

Reaction

CB 2KFe3(AlSi3)Oro(OH)z(mitc)

+ 6 FeS2(ry"e)

c 12 FeS + 2KAlSi3Os + 3O4g+2112O1q(pynhotite) (K-feldsp()

DBH FqO4 + 3 FeS2(marctite) (pynte)

c 6 FeS + 2oqs1(pyrhotite)

EAI 6Fe2O3 ;2 4 FqOa + Qk)(lremarile) (rnanerire)

FA 4KAlSi3Os + 6Fe2O3(K-feldspo) (herrarire)

+ 4H2O14 ;2 4KFq(AlSi3)Or0(OH)2 + 3O4d1(qnile)

AB 2 KAlSi3Os + 2FqOa(K-feldspu) (magnerite)

+ 2H2O6 ? 2KF%(AlSi3)Oro(OH)z + O2@)(annite)

A r(Fe3(ANi3)oro(oH)z(annir€)

+ 3 Fe2O3(h€rnarite)

e KAlSi3Os + 3Fe3O4 + H2O14(K-feldspar) (rnagretite)

B and BG 4KFe3(ArSi3)Oro(OH)z(oudre)

+ 3 FeS2(py"a)

d 6FeS +4KAlSi3Os + 3Fe3O4 +4IJ2O1(pyrrbotite) (K-feldryd) (magnerite)

in the present study is based on the observationthat NaCl is the predominant electrolyte in mostoil-field brines and hydrothermal solutions (Hel-geson 1970). Calculation and extrapolation ofosmotic coefficients for NaCl solutions usingequations and parameters taken from Helgeson etol. (1981) and Pitzer et al. (1984\ indicateihar theactivity of HrO in these solutions falls in rhe range0.9 s asr6 S I for ionic strengths up to -3 at thepressures and temperatures considered in thepresent communication.

Equation (42) constitutes a bridge betweenoxidation,/reduction mineral equilibria and meta-stable equilibria among oxidized carbon-bearingspecies in aqueous solutions and reduced hvdrocar-bon species in petroleum. For example, logarithmicanalogs of the law of mass action foi mineralequilibria that bufferlo, like those represented bythe curves shown in Figure 2 can be combined withEqn. (42) to generate diagrams like that shown inFigure 3, which can be regarded as an isothermal_isobaric petrogenetic grid for minerals coexistingwith quartz and an aqueous phase in the systemKAIO2-SiO2-Ca-Fe-S-O-CO2-H2O at 100.C and300 bars. Many of the minerals shown in Figure 3are commonly found in hydrocarbon reservoirs(Biederman 1986).

Univariant equilibria are depicted in Figures 2and 3 as solid curves, which meet in isothermal-isobaric invariant points at A and B in Figure 2and B, C, D, E, and F in Figure 3. The reactionscorresponding to the curves and invariant points inFigure 2 are listed in Table l, where they are labeledwith the same letter designations as those shown inthe figure. Stable mineral assemblages are denotedin Figures 2 and 3 by the tie lines in the triangulardiagrams depicted in the divariant stabilitv fields.Values of log acoztoot are represented in Figure 3by the dashed isopldths. Thermodynamic calcula-tions using analytical data reported by Kharaka etal. (1977) indicate that the intersection of anisopleth for log eCo2tqa\ = -2,6 with the coordinatesrepresented by log_a6-11r66,6 Hton,? -

-4 an^d log /or*,= -59.2 in Figure 3 is typical of the oil-field brines

at - 100'C in the Houston-Galveston and CorpusChristi areas of the Texas Gulf Coast (Helgeson elal. 1991, H.C. Helgeson, A.M. Knox & E.L. Shock,in prep.). It can be deduced from these values andthe phase relations shown in Figure 3 thar thepredicted equilibrium assemblage of mineralscoexisting with the brines at 100.C includes quartz,K-feldspar, biotite lrepresented by theKFe3(AlSir)Oro(OH)z (annite) componentl, siderite(or ferroan calcite or dolomite), pyrite, and calcite,

ORGANIC/INORCANIC REACTIONS IN METAMORPHIC PROCESSES

o 50 loo 150 200 250 300 350 400 450TEMPERATURE, OC

Frc. 4. Geostatic and fluid pressures as a function of depth and temperature in the Mississippi Gulf Coast. Thegeostatic pressure is consiitent with a bulk density of 2.26 gcm-3, and the profile of fluid pressure is for a fluiddensity equivalent to that of a brine containing 80,000 ppm NaCl (Stuart 1970).

7t7

E.(DY

lrJE,fCNalrJE,(L

all of which occur at depth in the Mississippi GulfCoast (Biederman 1986, Sassen e/ al. 1989, IJ.Hanor, pers. comm., 1989). Although detritalmagnetite also is present in these sediments,thermodynamic calculations indicate that thismineral is not in equilibrium with the interstitialbrines, nor apparently are detrital pyrrhotite andK-feldspar, which should react with siderite to formpyrite and the annite component of biotite at thetemperatures (80"-150"C) and fluid pressure ( -400bars) prevailing in the hydrocarbon reservoirs ofthe Houston-Calveston and Corpus Christi areasof the Texas Gulf Coast (Helgeson et al. l99l,H.C.Helgeson, A.M. Knox & E.L. Shock, in prep.).Similarly, detrital magnetite should react with theinterstitial brines to produce siderite or pyrite (orboth). Although siderite is rarely reported to bepresent in hydrocarbon reservoirs, it may be easilymisidentified petrographically as calcite if the

observer is not looking for siderite (J.R. Boles,pers. comm.).

The phase relations described above provide aframework for investigating the consequences ofprogressive metamorphism of hydrocarbon reser-voirs and source rocks with increasing depth andtemperature. To illustrate the approach, equi-librium constraints on phase relations like thoseshown in Figure 3 can be evaluated along a"tvoical" seothermal profile in a subsiding basin.The profilE selected f<ir this purpose in the presentstudy is represented by the fluid pressure curveshown in Figure 4, which was generated from fluidpressure and temperature measurements at variousdepths in the Mississippi Gulf Coast reported bystuart (1970) and Hower et al. (1976). The fluidpressure profile in Figure 4 was chosen instead ofthe geostatic profile because thermodynamic equi-librium among minerals and aqueous solutions in

DEPTH, KM

2.5

20

r.5

l .o

350 400 450

- GENERATED FROIII DOWNHOLE /gF0f HTF,gSF-B''fo?H^to#X't"oer./- - - EXTRAPOLATED ASSUMING ^//

Pf " Pg AT DEPTHS \ loKM //'

GEOSTATICTESSURE (Ps)

FLUTD PRESSURE (Pf )

7 1 8 THE CANADIAN MINERALOCIST

nonhydrostatically stressed systems such assedimentary basins depends at a given temperatureand bulk composition solely on fluid pressure, noton overburden pressure (Bruton & Helgeson 1983,Holdaway & Goodge 1990).

-to-2.3 -2.1 -1.9 -17 -1.5 -t.3

LOG o^^vv2(oq)

Ftc. 5. Logarithms of the activities of n-alkanes in crude oil (ac,rrzrn+ rr. rJ as a function of the logarithm of the activityof. aqueous Coz (acoztost) in metastable equilibrium wi*r iillfili]r'ditfnd orher hydrocarbonsln perroleum and thePtl:j3,ur-:.Iblages dbpicted in the triangular phase diagrams at 200'C along the fluid pressure profile shown inrlgure 4. tne numbers on the curves denole the carbon numbers of the r-alkahes (see text). Lines-a and b refer totwo particular crude oils, which are similarly annotated in Figures 7 and 8. The mineral abbreviations are identifiedin Figure 13.

Metastable equilibrium among r-alkane speciesin petroleum and coexisting oil-field waters alongthe fluid pressure profile shown in Figure 4 can bedescribed in terms of the general reaction

LOG f.o.1.3

a

=+E

$tT

g

C)o

6

4

2

o

(9oJ -4

ANN

FeMT PO PY

HMT PO

t2 b20

+ PETROLEUM

ORCANIC/INORGANIC REACTIONS IN METAMORPHIC PROCESSES'n9

cnHz(,,+0,(r).($)our, e@3)

nCOr61+Q+\H2o

where the subscript (/) denotes the liquid state, andn again stands for the carbon number of the organicspecies, which corresponds to the stoichiometricnumber of moles of carbon atoms in one mole ofthe organic molecule. The logarithmic analog ofthe law of mass action for reaction (43) can bewritten for asr6 = I (see above) as

lo8 cc,Hr,,rr,u, = logKn + nlo$acour,

(+) bsroo,y(44)

where K, stands for the equilibrium constant forthe zth statement of reaction (43). Equation (44) isplotted in Figure 5 for n = 4,8, 12,20, and 33 \npetroleum coexisting with an aqueous solution andK-feldspar, annite, pyrite, and pyrrhotite aI 200"Calong the fluid pressure curve shown in Figure 4.The oxygen fugacity characteristic of this as-semblage corresponds to that shown for 200oCalong curve CB in Figure 2, which represents thereaction with the same label in Table l. The lawof mass action for this reaction can be written forstoichiometric K-feldspar, annite, pyrite, and pyr-rhotite coexisting with an aqueous solution forwhich a"ro= I (see above) as

loeKtoEloz=-i- (45)- 5

where K refers to the equilibrium constant forreaction cB.

Equilibrium values of log ./coz,*, are shown atthe top of the diagram depicted inTigure 5. Thesevalues were computed from the law of mass actionfor

co4t) r2 coz{e)

which can be written in its logarithmic form as

log/cor(r) =logK+lo8acoO*, @j)

where K stands for the equilibrium constant forreaction (46). Equation (47) is plotted in Figure 6for 100o, 200o, 300o, and 400'C along the fluidpressure curve in Figure 4. It can be deduced fromthe isotherms in this figure that log .fco21", for agiven log eCo2r,a\ is insensitive to temperature andpressure along the fluid pressure profile shown in

LOG o,.,.""2(oq)

Frc. 6. Logarithm of the fugacity of CO2 gas (/co2,n1 ) asa function of the logarithm of the equilibrium ac-iivityof aqueous co2 (ags11,") at constant temperature(labeled in oC) along thdlluid pressure profile shownin Figure 4.

Figure 4, but it maximizes slightly with increasingtemperature from l00o to 400"C.

The equilibrium state depicted in Figure 5 canbe described in terms of the logarithmic analog ofthe law of mass action for

(3n + 1) I(AlSi3Os+ 6(32 + l) FeS(K-feldspar) (prtnomr)

(46)

*3ttcorc.t +2(3n +2) HzO ?(48)

(3n + l) I(FqAlSi3Ols(oH),(armite)

+ 3(3n + l) FeS2+ 3CnH21n*11.1;(pfe) (n-alkme)

which can be written for asn15;,6, = oFes =4KFe3(Alsi3)oto(o{)2 : QHzo= I as

log4c,H4rrr),{D =logK+3logkoa.., @9)

where K stands for the equilibrium constant forreaction (a8). No attempt was made in the presentstudy to take into account the equilibrium conse-quences of compositional variation in minerals onthe distribution of n-alkanes in the coexistingmetastable crude oil. Preliminary calculations

720 THE CANADIAN MINERALOCIST

jt

s_2(\l

-c

oo(, _4oJ

4030too 20n

Ftc. 7. Logarithm of the activities of n-alkanes in crude oil (as,sr," * ,, ,,,) as a func-tion of carbon number (/r) ar constant log aco)t,^\ (represehr?il bf ihe numberson the curves) at2A0"C along the fluid pressur-d-p'rofile shown in Figure 4. Theletters a and D denote particular crude oils similarly designated in Figures 5 and 8.

indicate that these effecs are negligible in thepresent context. Reaction (48) is represented inFigure 5 by the differenr rieJines in the triangularphase-diagrams on opposite sides of the bundle ofn-alkane isopleths. It can be seen that the slopes ofthese isopleths decrease dramatically with decreas-ing carbon number, and that all of the curves

intersect each other atlog ag,,y1rr,,+lr.r/r = 2.3 andloe 4co21u,,y : 1.51, where log K for reaction (49) isindependent of carbon number (see below).

Equilibrium speciation of n-alkanes in petroleum

The equilibrium distribution of ,?-alkanes in a

fo t5 20 25 30nn

Ftc. 8. Total mole fraction of r-alkanes (X5) computed from Eqn. (50) for crude oils a and b in Figures 5 and 7 asa function of carbon number. The solid curves represent homogeneous metastable equilibrium as well asheterogeneous metastable equilibrium with aqueous CO2, but the dashed curves annotated a' and b' designatenonequilibrium distributions of the light r-alkanes (see rext).

(/

ORGANIC/INORGANIC REACTIONS IN METAMORPHIC PROCESSES 721

(50)

mole fraction of

given crude oil coexisting with the mineral as-semblage shown in Figure 5 corresponds to aparticular loE ocozronr Two such crude oilsrepresenting typical aciivities of aqueous CO2 inhydrothermal solutions are designated a and b inthis figure. LoE ecuszrn*r,.,n for the two values oflog ecoz,or, represented by a and b in Figure 5 isplotted as a function of carbon number in Figure 7,together with log ecrT2lr*,,.,r, fo, crude oilsrepresented by intermediate values of log esor,uur-It can be seen in Figure 7 that the curves are lineai,which is a consequence of the fact that log K forreaction (49) and the apparent standard partialmolal Gibbs free energies of formation of n-alkanes(as well as other groups of organic molecules) arelinear functions of carbon number. The slopes ofthe curves in Figure 7 are negative, and the slopeincreases with increasing log eCo2taot and ap-proaches zero atlog ocoztoot: -1.51'i i i Figure 5.Note that the curves appi<iach loE ocruztn.n.tn :0 at carbon numbers ranging from -4 for curve ato - 13 for curve b. As a consequence, homo-geneous equilibrium among the a-alkanes in crudeoils a and b in Figure 7 cannot be achieved atcarbon numbers lower than -7 and -23, respec-tively. This can be demonstrated with the aid ofFigure 8, where the sums of the mole fractions ofthe z-alkanes in crude oils a and b are plottedagainst carbon number. The solid curves shown inthis figure were generated from

29Xr = ! xc,tt4"*11r1

where Xe stands for the total

=

+cN

Ig

c)l-

t

25 30

Frc. 9. Trace activity coefficients of r-alkanes in a model crude oil computed fromthe MOSCED equation developed by Thomas & Ecken (1984) for 100',200',and 300'C (see text).

t5 3520

liquid n-alkarrgs, Xgn112,n*,,.,n denotes the molefraction of the subscripted sliecies, and

i = 3 0 - n . ( 5 1 )

The values of Xcnttzrr*,.,n corresponding to thecurves annotated a and b in Figure 8, whichrepresent homogeneous equilibrium, were generated

from Eqn. (50) using the values of acnH2h+t\,r lcorresponding to curves a and b in Figure 7,

respectively, and the relation

y_ -_ _ aCoHr("*0,<rl (52)^c'H21"*r;,1r1 - qil"*rr,,,,

where \6,Hr,,*,,,n stands for the rational activitycoefficient iif thd'subscripted species in crude oil.The values of \c,Hzrn+rr.rr, used to generate curvesa and b in Figure 8 correspond to trace activitycoefficients 6tr,cnHz@+t),(rt) generated in the mannerdescribed below.

Although activity coefficients of hydrocarbonsin binary and, in some instances, ternary and morecomplex hydrocarbon liquids can be computedfrom equations of state such as those developed byCunningham (1974), Peng & Robinson (1976),Schreiber & Pitzer (1989), Anderko &Pitzer (1991),

300" ,/\.. .200"

722 THE CANADIAN MINERALOGIST

and others (see Prausnitz et al. 1986\, insufficientexperimental data are available for multicom-ponent hydrocarbon liquids at high pressures andtemperatures to permit retrieval by regressioncalculations of appropriate mixing parameters for

crude oil at depth along the fluid pressure profilein Figure 4. An alternative approach involvescalculation of trace activity coefficients for speciespresent in relatively low concentrations in crude oil(trrr,cnHz(r+.r).(/;), which consists predominantly of

(to

^loc ) _

o(,oJ

roo t50 t75 200 225 250 275 300

TEMPERATURE, OC

FIc. 10. Logarithm of the activity of aqueous CO2(ag6r,,,,,) as a function of temperature for stable and metastableequilibrium among quartz, calcite, oxidized species ofEiibon in the aqueous phase, r-alkanes and other hydrocar-bons in petroleum, and the mineral assemblages depicted in the triangular phase-diagrams along the fluid pressureprofile shown in Figure 4. The reactions represented by the univariant curves and invariant points (filled circles)are listed in Table 2, and the mineral abbreviations are identified in Figure 13. Points B and D denote indifferentcrossings of univariant curves ABC and GBN, and IDL and CDE, respectively.

125

[')

n FeD

slD

./

G ry

ORGANIC/INORGANIC REACTIONS IN METAMORPHIC PROCESSES 723

light paraffins with carbon numbers less than orapproximately equal to l5 (Helgeson l99l). Takingaccount of compositional data reported by Mair(19@) and Bestougeff (1967), a speciation modelof crude oil was generated to represent the"solvent" for the various hydrocarbon speciesconsidered in the present study. Calculation oftrace activity coefficients for n-alkanes in thismodel crude oil at temperatures from 100o to300oC using the modified separation of cohesiveenergy density (MOSCED) equation developed byThomas & Eckert (1984) resulted in the curvesshown in Figure 9, where it can be seen that\,r,crH2(u+l).(/) for z-alkanes differs only slightlyfrom uhity for r less than or approximately equalto 12, but it then decreases with increasing carbonnumber from 0.98 at n = 12 to -0.45 at n = 35.Values of \,"crn2rr*r.,.,,, computed in this mannerwere used to generatii'ihe values of X" shown inFigure 8 from those of acnrr@+r),(r, along curvesa and b in Figure 7.

Thermodynamic consideration of the relativevolume percents of z-alkanes and other hydrocar-bons reported to be in crude oil samples subjectedto analytical procedures in the laboratory indicatethat the hydrocarbons in the API Research Project6 samples for which analyses are reported by Mair(1964) may be in homogeneous metastable equi-librium with one another at temperatures below- 150oC, but only at fugacities of oxygen greaterthan those corresponding to the hematite-mag-netite/o2 buffer shown in Figure 2 (Helgeson l99l).It thus appears that hydrocarbon speciation inpetroleum detected in the laboratory may not berepresentative of hydrocarbon speciation in undis-turbed petroleum in hydrocarbon reservoirs. Never-theless, extrapolation to lower carbon numbers ofMair's (1964) data for r-alkanes at n > l0 tocompensate for the loss of the light volatileparaffins prior to collection of the API ResearchProject 6 samples suggests that the total numberof moles of hydrocarbon species in crude oil is ofthe order of 7 moles liter-t (Helgeson 1991).Computed mole fractions of z-alkanes using thisvalue, together with analytical data taken fromMair (1964) and the extrapolated values of thenumber of moles liter-r of z-alkanes in the APIResearch Project 6 samples at n > l0 resulted inthe dashed curves labeled a' and b' in Figure 8.These curves represent two of many possiblenonequilibrium distributions of r-alkanes in thelighter paraffin range of crude oils a'a and b'b inFigure 8. At carbon numbers higher than -7 arld- 23, respectively, all of the n-alkanes in crude oilsq and b may be in homogeneous metastableequilibrium with each other, as well as inheterogeneous metastable equilibrium with the

mineral assemblage and coexisting aqueous solu-tions represented by the curves shown in Figure 5.However, metastable equilibrium cannot beachieved among both light and heavy n-alkanes inpetroleum (Helgeson 1991). Regardless of thetemperature and pressure, the relative charac-teristics of the curves shown in Figures 5, 7, and 8are typical of those for other mineral assemblagesin metastable equilibrium with r-alkanes in crudeoil. In all cases, the identities of the predominantn-alkanes in homogeneous metastable equilibriumwith one another in petroleum are sensitivefunctions of log ogqr,rr, but the reverse is not true(see below).

Stasls AND METASTABLE PHASE RrlartoNsAMONG MINTNAIS, T.ALKANES IN PETROLEUM,

eNo Aqurous SPECIES IN THE SYsrrnaKAIOz-SiOz-CaO-FeO-H2S-O-CO2-H2O

Stable and (in the case of reactions involvinghydrocarbons) metastable phase relations amongminerals, rz-alkanes in petroleum, and oxidizedaqueous carbon species are depicted in Figure l0for acau',rt,.= I (see below) in terms of logo.grrnn,as a

-ftiiiciion of temperature along the fldid

pressure curve shown in Figure 4. The univariantcurves and invariant points shown in this figurewere generated from statements of the law of massaction for the reactions listed in Table 2. The stablemineral assemblages in the divariant fields betweenthe univariant curves include quartz and calcitecoexisting with those represented by the tie lines inthe triangular composition-diagrams shown in thefigure. The univariant curves and invariant pointsin Figure l0 represent reactions with the same letterdesignations as those in Table 2. lt can be seen inthis table that two of the three invariant reactionsinvolve n-alkanes that are in metastable equilibriumwith mineral assemblages and aqueous speciesalong curves HG, FG, ABC, and GCDE in Figure10. Because the positions of these curves arerelatively insensitive to hydrocarbon speciation,petroleum was represented in the calculations byn-octane, which was assigned a reference activityof unity. The consequences of departures ofecsurs,rtt from this reference value are discussedbelow.

Reactions HG, FG, ABC, and GCDE in Table 2represent four of nine possible metastable equilibriainvolving n-alkanes in petroleum coexisting withaqueous fluids and quartz bgether with otherminerals in the system KAIO2-SiO2-CaO-FeO-H2S-O-CO2-H2O. The other equilibria are un-stable. The five reactions representing these un-stable equilibria can be written as

THE CANADIAN MINERALOGIST1' ' A

.l

l-

!)(n

>n3'<e d2eFXFH16F 5 Z

xutY FZ Ede2 A62dtz t lz dr1v:3>

Ft11

F

(t)

7{

r. z s$ssFt,

^ :I H

gA'5 :F J

8 . =

+5F+

Xo

E l ^5 >= ' Far a)

N

+(,t,

^ :il

-f

= ' v- 9 x

6'vt

N

tA:

a+Fx- E . >E A

€+o

€ ; !

$+9 .eg F j

?,

+(,; ' I

= ' H

5' 6',6 X

+{:t+s\)

A Xr >ga€ %*1 --

+? u rE H

E 6 'E %5*"

+H

N

TJNd'

; 6E ! .I t o

F X

p

+c t ,1 6 uF 6 'P P

5. J/

so^ :

: H

A AF :t l +5 . =- €

+'r

^ tE+E 5g . EF Og/ rf,

5

+€it

i ;= ' v

oN

t lt 9

5G ' -g: q- ,9

+N

- F.ia s1 +5 ( ,g . v

5 x

qt

+.tr:+

H

N

A Xr >gE€ g3- --

+^ ( ,g . a

f i 'As5

+|\)

$X6'

; 6E 6 '= . v

a

u+(,XvI

?' ttx : oE - f )

a.i o

+F xB d ,P l.)

f f - -- a ,

t r ,

+El ttr 5 OH ,*.: ' 5- *

Xvs

* 6 ')_. at! N)- *

5a l l lx ' o5 - O

a i u

lJ* Ni 9 H6 6 ' .z . ob- *

5 H

Ei5= ' so* *

5

Xvs

^ :: F

E } t, t lP +t 6 . 3

v l

+t,s

E T

E. -=O H- 6 ' ,

(t)N

+(,:

e +F V5 . xo o- o

lJN

=' (,i : F

9 . 5-6_ E- o

ut+ss+

ov6A

+:+

N

(,(,F

E -= ' Hg 6 '

+:+

p

al

^ :: t s

al iF :

8ev : 1

(,(,

?. :lE +6 ;E i /F.F

5

+Q:+(,

vs

t\)

o^ SI t sI H

A,RF :D !

8e.>+t,:

^ +5 F

! l :5 Nt r vO H

+(,(,s

^ +. i 5t ' \*eg

\t(!U)p

T.t(,

- ( ,.,:: S

E+f. i-t i Hr d ,

(tt+l'.)sov6{+t\)s+

N

(,o

a !! t sI Hp.,!a

F : tE +5 . =e r a

+(,I+

X6'

el r-\H }6 it)

N

+t(,

^ :€ += ' 9

- 9 +(!(a

p

f I

(,s

^ f

l v

Sx5 >3.7

@

+o\

f t ,{ z :3.+g . e

- ( !

ut+(,:l

ov

+l'J(,I+b'J

N

.!a

Foso

CnHz(n+r),(0+{3n+l) Fe2Q d(t-alke) (lem,ti!e)

2(3a+l)Fe3oa + nco4*1 (53)

(mo4€'i!s)

+ (z+1)H2O

3CrHzla+r),it1+ 2(3n + l)KAlSi3Os(r-alkme) (K-foldspu)

+ 3(3n+l)Fe2Q + (3n-r)H2O p(hmtite)

(54)

2(3n + 1)KFe3(Alsi3)o1o(oH)2(mite)

+3nCOr1*,,

CaHz(n+r),(l) + (3n + 1)KAlSi3Og(r-alkm) (K-feld$s)

+(3ztl)Fe3oa +2nH2O {(magrtire)

(55)

(3't + l)t{Fe3 (Alsi3)o1e(oH)2(mir")

+ nCO4*1

2(3n+l)FeCO3+(n+1)H2O P(sidsite)

CrHz(r+r),(D + (3n+l) Fe2O3 (56)

(r-allme) (hmarite)

+(5n+2jCO4r1

and

(3a+l)CaCO3 + (32+1)FeS2(calcite) (pynte)

+(3n+1)H2O C

(s7)3caHz(r+r)1f + (3n+l)CaSO4

(r-alkme) (urhydrite)

+(3a+l)FeS +COqq).(pynlotite)

The phase assemblage represented by reaction (53)is unstable because the equilibrium constant for thereaction requires the activity of aqueous CO, to behigher at all temperatures than the upper stability-limit of magnetite in Figure 10. Similarly, in the

725

ca$e of the metastable equilibrium-states repre-sented by reactions (54) and (55), agsr,o., must behigher at all temperatures than the uppef stability-limit of annite and annite + magnetite, respective-ly. Hence, these equilibria also are unstable.Reaction (56) represents an unstable phase-as-semblage because its equilibrium constant requiresan activity of aqueous CO2at all temperatures thatis below the lower limits of hematite stability.Finally, the phase assemblage represented byreaction (57) is unstable because it can occur onlyat activities of CO2@q) that are far below thestability field of anhydrite.

It follows from the observations summarized inthe preceding paragraph that the presence of crudeoil in which rz-alkanes are in metastable equilibriumwith aqueous COr in hydrocarbon reservoirs andsource rocks favors the irreversible reaction ofr-alkanes and detrital hematite to form authigenicmagnetite and CO2ron, until all of the hematite isdestroyed. Similarly, CnHz(n+ rr,r0 should reactirreversibly with K-feldspar and hematite orK-feldspar and magnetite to produce iron-richbiotite and COzoqt.Also, detrital hematite shouldreact with n-alkanes and CO2ron, to form siderite,and CnH2@+t,,rn should react irreversibly withanhydrite, prrrhotite, and CO4on1 to produce pyriteand calcite until all of the anhydrite or petroleumis consumed. The latter observation is particularlyrelevant to hydrocarbon accumulations associatedwith the cap rocks of salt domes.

It can be seen in Figure l0 that-log aco2@(t)increases with increasing temperature along each oTthe univariant curves shown in the figure, all butone of which (AB) are within - 1 log unit of eachother at any given temperature. Of the reactionsinvolving rz-alkanes, those labeled HG and FG inTable 2 terminate in Figure l0 at invariant pointG, which occurs at l20oC along the fluid pressureprofile depicted in Figure 4. At higher tempera-tures, only pyrrhotite, siderite, and pyrite orK-feldspar, biotite, pyrrhotite, and pyrite cancoexist in metastable equilibrium with aqueous CO2and n-alkanes in petroleum. The latter of these twometastable equilibria is represented by curve ABCin Figure 10, which terminates at invariant pointC. Point B in this figure designates an indifferentcrossing of univariant curves ABC and GBN. Attemperatures above invariant point C, which occursat 230"C, only one mineral assemblage in thesystem KAIO2-SiO2-CaO-FeO-H2S-O-CO2-H2Ocan coexist in metastable equilibrium with aqueousCO2 and r-alkanes in petroleum. This state ofmetastable equilibrium corresponds to the revers-ible reaction of pyrrhotite with aqueous CO2 toform pyrite, siderite, and z-alkanes in the presenceof quartz and calcite along curve GCDE in Figure

ORGANIC/INORGANIC REACTIONS IN METAMORPHIC PROCESSES

726 THE CANADIAN MINERALOGIST

ttoN

o( J '

o(9oJ

TEMPERATURE, "CFtc. I l. Logarithm of the activity of aqueous CO2 (agg11,,-,) as a funcrion of temperature for stable equilibrium among

quartz, calcite, oxidized species of carbon in the aquddirs phase, and the mineral assemblages depicted in the trian-gular phase diagrams along the fluid pressure profile shown in Figure 4. The reactions represented by the univariantcurves and invariant points (filled circles) are listed in Table 3, and the mineral abbreviations are indentified in Figure13. Point B denotes an indifferent crossing of univariant curves ABC and BN. The shaded region of the diagramis enlarged in Figure 12.

10. Point D in this figure represents an indifferenrcrossing of univariant curves IDL and CDE.

The metastable phase-relations depicted in Fig-ure 10 can be compared with their stable counter-parts in Figures 11 and 12, where stability relationsare depicted in terms of log a.or,._, and temperaturefor graphite coexisting with q'rlHrtz, calcite, andother stable minerals in the system KAIO2-SiO2-CaO-FeO-HrS-O-CO2-H2O. The univariant cur-ves and invariant points shown in Figures l1 and12 were generated by taking account of the law ofmass action for the reactions shown in Table 3.These reactions are designated by letters cor-responding to those denoting the curves and

invariant points in Figures 1l and 12. All of thelatter designations for both stable and metastableequilibria shown in Table 2 and Figure 10 are thesame as those denoting the corresponding stableequilibria in Table 3 and Figures 11 and 12.

It can be deduced from comparison of Figure l0with Figures ll and 12 that metastable equilibriumamong minerals, aqueous CO2, and r-alkanes inpetroleum causes invariant points G and C to occurat much lower temperatures (by -?,40" and - 150oC,respectively) in Figure l0 than their stable counter-parts for corresponding equilibria involvinggraphite in Figures 1l and 12. Invariant point Pand curve PQ in Figures ll and 12 correspond to

KAtO,

ORGANIC/INORGANIC REACTIONS IN METAMORPHIC PROCESSES 727

325 350 375 400 4?5 450

TEMPERATURE, OCFrc. 12.Logarithmof theactivi tyof aqueousCO2(ag6r,, . ,) asafunctionof temperatureintheshadedregionof Figure

ll, depicting stable equilibrium among quartz, cali)l-b", oxidized species of carbon in the aqueous phase, and themineral assemblages depicted in the triangular phase diagrams along the fluid pressure profile shown in Figure 4(see caption of Fie. l1).

invariant point B and curve BC in Figure 2. Thesecurves and invariant points represent the upper-temperature stability limit of annite along the fluidpressure profile shown in Figure 4.

Although the phase relations involving hydrocar-bons depicted in Figure l0 are metastable relativeto those depicted in Figures ll and 12, they maynevertheless persist over long periods of geologicaltime. Ample evidenie indicates that this is the casein hydrocarbon reservoirs and source rocks, as wellas hydrocarbon-bearing rocks that have beensampled at temperatures as high as 300"C (Price1982, 1983, 1991, Pr ice et a l . 1979,1981, 1986).The extent to which metastable phase-relationsinvolving hydrocarbons like those shown in Figurel0 persist in geological systems during metamor-phism can also be assessed with the aid of isotopicstudies and fluid-inclusion analyses from metamor-phosed source-rocks and reservoirs (see below).

As indicated above, curves HG, FG, ABC, andCCDE in Figure l0 were generated for 4.a"rr.,r, :l. Although the specification of n-alkanes in meta-stable equilibrium with aqueous CO2 is highlysensitive to the magnitude of a.or,oo,, the sensitivityof log aqns.,, I lr.{i ) to lo9 ecozruot is greater forreactions involving siderite than if is for reactionsHG and ABC. Metastable equilibrium amongCOz@qtand ,?-alkane species in petroleum coexistingwith siderite-bearing mineral assemblages alongcurves FG and GCDE in Figure l0 constrains logeCo2ruut to be within + 0.05 or less of the valuesrepresented by the curves. [n contrast, metastableequilibrium in the case of curves HG and ABC maycause log eCoztqqtto vary by as much as t -0.3 logunits from the values represented by the curves,depending on the predominant t?-alkane species inthe hydrocarbon phase. Nevertheless, in all fourcases, log eCoz(oqt is sufficiently insensitive to

crgN

oC)

o l(9oJ

ffi'MT PO PY

728

hydrocarbon speciation that the states of meta-stable equilibrium represented by curves FG, HG,ABC, and GCDE in Figure 10 can be regarded aslog acoztoot buffers to within limits that are of theorder of magnitude of the uncertainties in thecomputed equilibrium-constants for the reactions

THE CANADIAN MINERALOGIST

TABLE 3. STJMMARY OF REACTIONS REPRESENTED BY TTIE UNTVARIANT CI]RVESAND INVARIANTPOINTS SHOWN INFIGURES II A$12

at high temperatures and pressures. As indicatedabove, the equilibrium and nonequilibrium statesrepresented by the curves shown in Figure 8 aretypical of the crude oils in the phase assemblagesrepresented by these univariant curves (Helgesonl99l ) .

UnivariantCurveand,/or

InvriantPoint

Reaction

SIKP KAlSi3Os + 3 FeCo3 +H2O p lce3(atSi3pro(OH)z +3@2qy(K-fel&p6) (sid€rir) (mite)

TR 4F€CO3 P 2Fe2O3 + C +3C94*1(sidsrie) (h€rnarite) (gophite)

UR 6Fe2O3+ C P4Fe3Oa+@4q)(b€lruite) (gnphite) (rnagnetite)

RIJL FeCO3+ Fe2O3 e+ Fe3O4 +CO&aq)(sid€rite) (hematite) (magn€tite)

RFG 6FeCO3 p 2Fe3Oa + C +5@2qaq1(sid€rits) (mrgnotite) (gaphite)

VHG 3FeS2+ Fe3Oa + 2C(pYnt") (nagetire) (graphire)

c 6 FeS +2@v1aqy(pyrrhotio)

WABC 2 KFe3(AlSi3)oro(oH)2 + 6 FeS2 +(arrdtr) (PY"t )

3C P 2KAlSi3O3 +(snphite) (K-feldspar)

12 FeS +3 CO21sd+2H2O(pyrrnotite)

GCE 2FeS2 +2FoCO3 + C(pydte) (sid€'rire) (gaptrite)

c 4 FeS +3&21aq1(pFhotite)

GPBN FeS2 +4FeCO3 P 2FeS + Fe3Oa +4@X@q1(pynts) (sidsrire) (pyrrtrotre) (Inaglerite)

Q, QP 4KAlSi3Os + 6FeS(K-rehspr) (pyrrmrite)

+ 4WO e 4 I(Fe3(AlSt:)Oro(OH)z + 3 FeS2 + 3 Fe3Oa(amire) (pyna) (magnerite)

R 5 Fe2O3 +(hemadrc)

C p 3Fe3Oa +FeCO3(srpm") (magnetio) (sidsfio)

I KAISi3OE + 3 Fe3Oa(K-feldrpa) (nasnetite)

+ HzO a2 KFe3(AlSi3)Oro(OH)z + 3 Fe2o3(amite) (h€rnatite)

G l2FeCO3 + 30FeS(sidsrit€) (pyrrhotite)

P 9 Fe3Oa + 15 FeS2 + lzc(marrtte) (Pvnto) (gqtite)

c Kre3(AlSi3)oro(oH)z + 2Fes2 + c e KAsi3os + 4FeS +FeCo3 +H2o(!tmits) (pyr) (graphie) (K-felttspa) (pyrrhorite) (sid€rire)

The states of metastable equilibrium representedby curves HG, FG, ABC, and GCDE in Figure l0require equilibrium fugacities of methane, ethane,propane, and other light hydrocarbons that are toolarge to be achieved in the Earth's crust (Helgeson1991). For example, metastable equilibrium withmethane along all four of these curves wouldrequire .fcuo to be > 105 bars. In the case of thestable equilibria portrayed in Figure ll and 12,equilibrium with methane can be achieved in theEarth's crust along all of the curves except VH andWA below -75o and -175"C, respectively. The factthat the fugacities of the light paraffins requiredfor metastable equilibrium among aqueous COr,

729

n-alkanes in petroleum, and the mineral as-semblages represented by curves HG, FG, ABC,and GCDE in Figure l0 cannot be achieved in theEarth's crust precludes metastable equilibriumamong the light and heavy r-alkanes in crude oi.,which provides a driving force for hydrocarbonmaturation (Helgeson 1991). It also precludesmetastable equilibrium among the light paraffinsand carboxylic acids, CO2, and other oxidizedspecies of carbon in oil-field brines, which areapparently in equilibrium with calcite. However,the light hydrocarbons in these brines maynevertheless be in metastable equilibrium with theircounterparts in crude oil (Helgeson 1991).

ORGANIC/INORGANIC REACTIONS IN METAMORPHIC PROCESSES

I5DEPTH, KM

67 to t l

T.o-

T

N I(o+NoO

g

ooJ

TEMPERATURE, OC

Frc. 13. Logarithm of the activity of the calcium ion over the square of the activity of the hydrogen ion 0og a6z- / &11")in aqueous solutions coexisting in stable equilibrium with quartz, calcile, and other minerals in the systemKAIO2-SiO2-CaO-FeO-H2S-O-CO2-H2O and (in the case of univariant curves ABC, HG, PG, and GCDE)metastable equilibrium with n-alkanes and other hydrocarbons in petroleum as a function of temperature alongthe fluid pressure profile shown in Figure 4. Points B and D denote indifferent crossings of univariant curvesABC and GBN, and IDL and CDE, respectively.

5.5

4.O

75 roo 125 r50 t75 200 % 250 275

ANN = ANNITEHM = HEMATITEKSP '' K-FELDSPARMT 'MAGNETITEPET = PETROLEUMPO = PYRRHOTITEPY . PYRITECAL = CALCITEANH = ANHYDRITE

730 THE CANADIAN MINERALOGIST

Mgresovarrc CoNsreurNCES oF MgresrnsltEQUILIBRIUM AMONG SII-IcaTgs, SULFIDES,

OxtDES, r-ALKANES tN PErnoLsuM, CO2@q),AND COEXISTING CALCITE IN THE SysTgvKAIO2-SiO2-FeO-CaO-H2S-O-COr-HrO

Univariant and invariant stable and (for reac-tions involving rz-alkanes in petroleum) metastableequilibria in the system KAIO2-SiO2-CaO-FeO-H2S-O-CO2-H2O are depicted in Figure 13 interms of temperature and log (as",./dH.) in theaqueous phase along the fluid pressuri: profileshown in Figure 4. The.labels on the curves andinvariant points in Figure 13 are the same as thosefor the reactions listed in Table 2 and the

corresponding curves and invariant points in Figure10. The curves in Figure 13 were generated fromthose in Figure l0 by taking account of the law ofmass action for

CaCO3+2H+ p Ca2++CO4 ad+Hzo (58)(caicile)

which can be written in its logarithmic form fordcaco: : oH2o = I (see above) as

tog(a*2. lQ,)=bgr+b1acorrr, (59)

where K stands for the equilibrium constant forreaction (58). Equation (59), which is representedby the curves depicted in Figure 14, can be

t5

l3

+N:E n(o

+N

C ) i lo

x toIJJ

6- 7 - 6 - 5 - 4 - 3 - 2 - l O

LOG o' c o z ( o q )

Frc. 14. Logarithm of the activity of the calcium ion over the square of the activityof the hydrogen ion (log as^zr/&rt-) in an aqueous solution coexisting in stableequilibrium with calcite as a function of the logarithm of the activity of aqueousCOV.!o.9r1on) at constant temperature (abeled in oC) along the fluid pressureprofile shoVn in Figure 4.

-8

ORGANIC/INORCANIC REACTIONS IN METAMORPHIC PROCESSES 731

combined with statements of the law of mass actionfor the reactions listed in Table 2 to generate allbut one of the curves (XY) shown in Figure 13.The reaction represented by curve XY in Figure 13can be written as

l5CaCO3+ FeS2 +7Fe2O3 +26H+ e.(calcile) (pynte) (hsurile)

2CaSo4 + lSFeCOr + l3ca2+ + l3H2o (60)(mhydite) (siderire)

The pH scale shown on the right side of thisfigure is consistent with log eg^2+ = -1.4, whichwas computed from the concentration of Ca2* in"average" oil-field brine (3000 ppm), generated byKinghorn (1983) using a representative value of 0.5for the individual ion activity coefficient of Ca2-in the brine. The latter value was computed fromthe geochemical approximation developed by Hel-geson el al. (1981).

It can be seen in Figure 13 that the equilibriumpH shown in the figure ranges from -6 for aqueoussolutions in which CO, is in metastable equilibriumat 100'C with a-alkanes in petroleum coexistingwith pyrite, pyrrhotite, and magnetite, or magnetiteand siderite, fo -3.6 at 300oC for metastableequilibrium among r-alkanes in petroleum, Ca2*,H*, and carbonate species in the aqueous phase,and pyrrhotite, pyrite, siderite, quarr.z, and calcite.If the Ca2+ concentration in the aqueous solutionwere taken to be an order of magnitude higher orlower than 3000 ppm, the solution pH at a giventemperature shown in Figure 13 would be half alog unit lower or higher, respectively. Changing theactivity coefficient of Ca2+ by a few tenths has onlya slight effect on the calculated values of solutionpH shown in Figure 13.

It can be shown that the calculated values of pHfor aqueous solutions coexisting with the variousphase-assemblages represented by the univariantcurves and invariant points in Figures l0 and 13are in general agreement with independentlypredicted values of pH generated from equilibriumand mass-transfer calculations for a wide varietyof hydrothermal systems (Helgeson 1970). In-variant point C in these figures is of particularinterest in this regard because it representsequilibrium among the annite component of biotiteand K-feldspar, quartz, calcite, pyrrhotite, pyrite,and siderite, all of which are in metastableequilibrium with z-alkanes in petroleum andoxidized species of carbon in an aqueous phase witha pH of - 4.3 at230'C and a depth of - 8 km alongthe fluid pressure curve shown in Figure 4. Ifinvariant point C is reached as a result ofprogressive burial of a hydrocarbon reservoir orsource rock along curve ABC in Figure 13,K-feldspar, pyrrhotite, and calcite shouldprecipitate at the expense of annite, pyrite, and

z-alkanes, which is accompanied by an increasinglypositive change in volume, resulting in decreasingporosity. If invariant point C is reached byprogressive burial of a hydrocarbon source rock orreservoir along curve GC in Figure 13, pyrrhotiteand calcite replace pyrite and siderite in the process,with a concomitant decrease in porosity andconversion of n-alkanes in petroleum to COr,or, andother oxidized aqueous species of carbon. In bothcases, the reactions cause 4qu2+ and the solutionpH to decrease, and eCo2toot to increase, whichfavors replacement of detiital plagioclase bycalcite. With continued burial and further reactionalong curve CDE in Figures l0 and 13, the solutionpH continues to decrease, and the aqueous solutionbecomes increasingly CO2-rich, reaching a con-centration of - 6 m (264,000 ppm) at 300oC, wherethe solution pH is of the order of 4 or less. Thereactions accompanying progressive burial of ahydrocarbon reservoir or source rock along curvesABC or GCDE in Figures l0 and 13 also generateaqueous acetic acid in equilibrium with CO2,on,, butto a decreasing degree with increasing temperature(Helgeson l99l). Calculation of the activity ofbenzene in the hydrocarbon liquid represented bycurve CDE in Figures l0 and 13 indicates thatd6uH..,,./ increases from .*0,01 at C to -0.1 at E.At high temperature, petroleum speciation may bedominated by aromatic hydrocarbons.

The decrease in porosity with increasing depthof burial along curves ABC and GCDE in Figure13 favors progressive isolation of the fluids in thehydrocarbon reservoir or source rock, whichpromotes increasing fluid pressure toward itsgeostatic equivalent. Essentially all of the CO2 inthe CO2-rich aqueous fluid at point E in Figure 13is derived from liquid hydrocarbons during theburial process. With further burial above 300oC,all of the crude oil may be converted to CO2, HrO,and other species in the aqueous phase as the twofluids become completely miscible (see below). Thereaction represented by curve GCDE in Figure 13consumes H2O. Hence, the process leads progres-sively to complete conversion of hydrocarbons incrude oil to carbonate in calcite and CO2 and otheroxidized species of carbon in the aqueous phase.

TeN{psRArunE-Xcoz Dtacnnras DEPICTINGSrest-e nNo MetesrABLE EeurLlBRrA

IN THE SYSTEM

KAIO2-SiO2-CaO-FeO-H2O-O-CO2-H2O

A temperature-Xgo, diagram depicting stableand (in the case of curve DEE'E') metastableequilibria in this system is depicted in Figure 15 fortemperatures and depths corresponding to the fluidpressure profile shown in Figure 4. Equilibrium

732 THE CANADIAN MINERALOGIST

C)o

350

250o.2 o.3

among pyrite, pyrrhotite, and siderite coexistingwith a CO2-H2O fluid in metastable equilibriumwith r-alkanes for r > 10, as well as benzene,toluene, and other hydrocarbons in crude oil, isrepresented in this figure by curve DEE'E", whichis an extension in T-Xc.6, space of curve GCDEin Figure 10. As indicated above, this is the onlymineral assemblage in the system that can coexistin metastable equilibrium with hydrocarbon speciesin crude oil, and thereby buffer the oxygen fugacityand mole fraction of CO2 in the system attemperatures in excess of 230oC during burial alongthe fluid pressure curve shown in Figure 4. Thestable analog of Figure 15 is depicted in Figure 16,where stable equilibrium phase-relations are shownfor graphite coexisting with a COr-H2O fluid and

o.8 o.9 t.o

other minerals in the system KAIOz-SiOz-CaO-FeO-H2S-O-CO2-H,O. Where possible, the uni-variant curves and invariant points shown inFigures 15 and 16 are labeled with the same letterdesignations as those of the corresponding curvesin Figures 10-13. The curves in Figures 15 and 16were generated for X.6, * Xszo = I using thefugacity coefficients of H2O and CO2 shown inFigures 17 and 18, which were obtained fortemperatures and pressures along the fluid pressurecurve shown in Figure 4 by extrapolating fugacitycoefficients computed from the modified Redlich-Kwong equation of state by Holloway (1977) andBowers & Helgeson (1983). The curve representingthe miscibility gap in the fluid in Figures 15 and16 (curve ZZ') is consistent with the analysis by

trJE

P ozsE.U.J(L

H soo

o.70.6o.5o.4^coz

Frc. 15. Temperature-X66, diagram representing stable and (in the case of the reaction involving n-alkanes in petro-leum merastable equilibiium among a CO2-H2O fluid and quartz, calcite, and the mineral assemblages depictedin the triangular phase-diagrams for the system KAIO2-SiO2-CaO-FeO-H2S-O-CO2-H2O along the fluid pressureprofile shown in Figure 4. Curve DEE'8" represents metastable equilibrium among pyrrhotite, pyrite, siderite, ir-alkanes in petroleum, and a CO2-H2O fluid. The letters denoting the univariant curves and invariant points (filledcircles) correspond to those designating the corresponding curves and points in Figure 10 and 13. Point D representsan indifferent crossing of univariant curves IDL and CDE. The mineral abbreviations are identified in Figure 13.

KAIO,KSP -

s

PO\ S,/

6t-'\{\r\ YJ

ML

hSID -PY,/

E t '

+K.FELDSPAR

H2O r Cp.

oo

L;E,:fF

E,lrJ(L=ldF

ORCANIC/INORGANIC REACTIONS IN METAMORPHIC PROCESSES t 5 5

Xco.Frc. 16. Temperarure-X6..o, diagram representing stable equilibrium among a CO2-H2O fluid and quartz, calcile, and

the mineral assemblag-ei depicted in the triangular phase diagrams for the system KAIO2-SiO2-CaO-FeO-H2S-O-CO2-H2O along the fluid pressure profile shgwn in Figure 4. Except lor Z, Z' , and Y' , the letters denoting theunivariant curves and invariant points (filled circles) refer to those designating reactions in Table 3 and the correspon-ding curves and points in Figures I I and 12. The mineral abbreviations are identified in Figure 13. Point B representsan indifferent crossing of univariant curves ABC and CBN.

05

Bowers & Helgeson (1983) of phase relations in thesystem CO2-H2O.

It can be seen in Figure 15 that curve DEE'E"reaches a maximum at -320"C, which represents thehighest temperature at which hydrocarbon speciesin crude oil can be in metastable equilibrium withpyrite, pyrrhotite, and siderite along the fluidpressure curve shown in Figure 4. However, it

should be emphasized in this regard that 320oC isnot necessarily the highest temperature at whichcrude oil may exist in the Earth's crust. Forexample, if the fugacity of oxygen in the system isnot buffered by mineral assemblages coexisting inmetastable equilibrium with COr,on, and hydrocar-bon species in crude oil, but instead is controlledsolely by metastable equilibrium among aqueous

325

P

r4_oi

TCALCITE

YI

slD fr' '

CoOCAL

+CALCITE ANH

#sFLUID +GRAPTIITE

HaO + CO,

o(\J

Its

o.8X.o,

Ftc. 17. Fugacity coefficient of H2O (1se2) in CO2-H2Ofluids as a funclion of Xs6, at constant temperature(labeled in 'C) along the flujd pressure profile shownin Figure 4 (see text).

CO2, CH3COOH, and hydrocarbon species inpetroleum, the hydrocarbon liquid may coexist witha COr-rich aqueous fluid at much higher tempera-tures than 320"C. Candidates for survival at thesehigh temperatures include heavy aromatic-richcrude oils and asphaltenes.

Progressive burial of hydrocarbon reservoirs orsource rocks along curve DEE' in Figure 15 isaccompanied by formation of pyrrhotite at theexpense of siderite and pyrite as the CO2-H2O fluidbecomes increasingly enriched in CO, derived fromhydrocarbon species in crude oil. At point E, inFigure 15, the mole fraction of CO, in the aqueousphase reaches -0.75. With further burial andincreasing temperature above -320oC, all of thepyrite and siderite must be replaced by pyrrhotite,or all of the crude oil must be converted to CO,.In any case, if tectonic events cause release of "a

CO2-rich fluid at depths in excess of - 12 km, wherethe temperature exceeds -320"C, metastable crudeoil may be precipitated from the fluid as it movesin fractures toward shallower depths and lowertemperatures, fluid pressures, and oxygenfugacities. As the immiscible petroleum phaseprecipitates, hydrocarbon species of successivelyhigher molecular weight should form in metastableequilibrium with one another, which is the opposite

FIc. 18. Fugacity coefficient of CO2 (166r) in CO2-H2Ofluids as a function of Xgo, at consta-nt temperature(labeled in oC) along the fluid pressure profile shownin Figure 4 (see texr).

734 THE CANADIAN MINERALOCIST

o.6

o.5

o.4

o.3

o.2

o. l

oo o.2 0.6OA t.o

order of their stoichiometric solubilities. Theprocess favors two-phase flow, dissolution ofcalcite, and increasing porosity and permeabilityalong the flow path.

CoNclunrNc Rruenrs

It can be deduced from the calculations describedabove that progressive burial of petroleum reser-voirs and source rocks may lead to preservation ofliquid hydrocarbons in metastable equilibrium withtheir mineralogical environment and COzu.n,CH3COOH(',), and other oxidized aqueous species.The presence of both pyrite and pyrrhotite in themineral assemblage is required for this state ofmetastable equilibrium to persist at temperaturesabove - 120'C. The other minerals in the assemblagemay be biotite and K-feldspar, or siderite, ankerite,ferroan dolomite, or ferroan calcite. Under theseconditions, petroleum with hydrocarbons in meta-stable equilibrium with this mineral assemblagemay persist at temperatures as high as 230'C. Athigher temperatures, from 230o to -320oC, only themineral assemblage pyrite + pyrrhotite + Fe-bear-ing carbonate in metastable equilibrium withhydrocarbons in petroleum and a CO2-HrO fluidcan buffer both the fugacities of 02 and CO2. Itfollows that assemblages of pyrrhotite + pyrite +Fe-bearing carbonate in low-grade metamorphic

rocks may be associated with metastable preserva-tion of petroleum during the metamorphism ofhydrocarbon reservoirs and source rocks attemperatures and pressures of the order of 320oCand *2.1 kbar or less. Such states of metastableequilibrium that might have been present duringmetamorphism should be manifested by carbonisotopes and the compositions of fluid inclusionsin metasomatic minerals. The inclusions shouldcontain aqueous carboxylic acids and otheroxidized species of carbon, high concentrations ofCO2 and, in the absence of graphite, both liquidand gas hydrocarbons. The fluid inclusions alsomay contain abundant sulfate and possibly sulfidedaughter minerals. Fluid-inclusion studies shouldalso serve to corroborate the thermodynamiccalculations summarized above, which indicate that*320oC is the maximum temperature at whichmetastable equilibrium involving COxsql, hydrocar-bon species in petroleum, and pyrite + pyrrhotite+ Fe-bearing carbonates can be achieved duringprograde metamorphism. At temperatures between230' and 320"C, the calculations suggest thatpreservation of liquid hydrocarbons in metastableequilibrium with this mineral assemblage is prob-ably limited to rocks containing ferruginouscarbonates and relatively sulfur-rich crude oils. Inthe absence of either pyrite or pyrrhotite, heavycrude oils and asphaltenes may persist in fer-ruginous-carbonate-bearing rocks at temperaturesin excess of 320"C. In any event, the metamorphicrocks would be expected to contain abundantsulfides in veins and veinlets throughout thepaleoreservoir or source rock.

The calculations summarized above indicate thatprogressive burial of hydrocarbon reservoirs leadsto CO2-rich aqueous fluids at depths in excess of- 10 km. This observation, as well as the results ofthe thermodynamic calculations described in thepreceding pages, are ripe for experimental verifica-tion. Carefully controlled experiments in which thefugacity of oxygen is buffered are needed todocument the extent to which hydrocarbon speciesin crude oil may persist in metastable equilibriumwith their mineralogical and aquatic environmentsat high temperatures and pressures. Other experi-ments are required to demonstrate that liquidhydrocarbons can indeed precipitate from CO2-richaqueous fluids at high temperatures and pressures.The fact that oxidized "organic" species may format high temperatures and pressures has already beendemonstrated by French (1964), who unintention-ally produced propanoic acid, ethyl alcohol, butyricand acetic acids, and ketones during the synthesisof siderite in experiments at 2 kbar and tempera-tures from 350o to 380'C. Many more suchexperiments are needed to even begin to clarify the

735

murky division between organic and inorganicreaetions in metamorphic processes.

-:.,:. ACKNOWLEDGEMENTS

This paper co-uld never hdve been written withoutthe superb rese'arch sppport I have in theLaboratory of Theoretital Geochemistry (otherwiseknown as Prediction Central) at the University ofCalifornia, Berkeley. I am indebted to Eric Oelkersfor expertly and cheerfully expediting the calcula-tions described above. Eric coded up the MOSCEDequation for trace activity coefficients of organicspecies and the PFGC equation of state forhydrocarbons. He also carried out countlesscomputer calculations for me, as did my researchassistants through the years, Annette Knox, MaryConnolly, Laurie Stenberg, and Christine Owens.I am grateful for all of their help in sifting throughendless mountains of papers and computer output.Thanks are also due Vitalii Pokrovskii for manyhelpful discussions and for generating Figures 11,12, and 16, Christine Owens for bringing order outof chaos by supervising the delicate balancing actthat is involved in writing organic,/inorganicreactions among minerals and hydrocarbons, Bar-bara Ransom, Jan Amend, Everett Shock, andLeigh Price for many discussions of diageneticsystems and organic reactions, Rebecca Aringtonfor drafting the figures, Joachim Hampel forphotographing them, and last but not least, JoanBossart for her incredible talent at translating myhandwriting into the word processor with flawlessprecision and unflagging dedication. Themanuscript benefited from reviews by GeorgeSkippen, Bernard Boudreau, and Robert Martin.Financial support was supplied by the NationalScience Foundation (NSF Grant EAR 8606052), theDepartment of Energy (DOE Grant DE-FGO3-85ER-13419), and The Committee on Research atthe University of California, Berkeley, all of whichis acknowledged with thanks.

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