Canadian Mineralogist
Vol. 26, pp. 827-840(1988)
PETROLEUMGENERATIONIN SUBMARINEHYDROTHERMALSYSTEMS:AN UPDATE
BERND R.T. SIMONEIT
OregonStateUniversity,Comallis,Oregon97331,U.S.A.
of
Oceanography,
PetroleumResearchCroup, College
ABSTRACT
The conversion of organic matter to petroleum by
hydrothermal activity is a processwhich occurs in many
types of natural environments. Geologically immature
organic matter in marine sedimentsis being altered by this
processin the Guaymas Basin, EscanabaTrough, and
Atlantis II Deep.In GuaymasBasinthe sedimentaryorganic
matter is derived primarily from algal and microbial detritus, with a minor influx of terrestrial higher plant components. lt is easily cracked to petroleum and migratesin
hydrothermal fluids. Emanationsat the seabedresult in dispersal of the more volatile components(Cr-Cro hydrocarbons) and solidification of the heavier (C16C4e+)
petroleum within the mineral matrix of the hydrothermal
mounds. The petroleum compositions vary from condensates(C1-C15)to naphthenic (probably extensivelybiodegradedresidues,the most common surface samples)to
waxy, all with sigrrificant amounts of asphaltenesand toxic
polynuclear aromatic hydrocarbons@AII). Contemporary
organic detritus and living microorganisms are also converted in part to petrolzum-like products when they become
entrained by turbulent mixing into the vent waters, resulting in "instantaneous"hydrous pyrolysis. The latter process
occurs in all hydrothermal vents, but is most clearly evi
dent in vents which emanate from sediment-free oceanic
ridges,as for exampleon the East Pacific Riseat l3o and
2l"N and on the Mid-Atlantic Ridge at 26'N. The
hydrocarbon products generatedin the various areas are
comparedin terms of composition, organic matter sources
and analogy to reservoir petroleum.
Keywords: hydrothermal petroleum, Guaymas Basin,
EscanabaTrough, East Pacific Rise, Atlantis II Deep,
Mid-Atlantic Ridge, r-alkanes, polynuclear aromatic
hydrocarbons,petroleum migation.
Solravetnr
La transformation de la matidreorganiqueen p€trole par
activit€ hydrothermale est un processusqui seproduit dans
divers milieux naturels. La matidre organique gdologiquement immature des s6dimentsmarins est altdree par cette
voie dans le Bassinde Guaymas,la Fossed'Escanaba,et
Atlantis II Deep.La matidreorganiquesddimentairedu Bassin de Guaymasprovient essentiellementde d€tritus microbiens et algaires avec un faible apport de compos6sissus
desv€g6tauxsupdrieursterrestres.Cette matiire organique
forme facilement par craquagedesproduits p€troliers qui
migrent avecle fluide hydrothermal. Les 6manationsde fluides chauds au fond de I'oc6an entralnent une dispersion
des composdsplus volatiles (hydrocarburesC1-C1s)et une
solidification desp6trolesplus lourds (Cro-Cao+)dansla
matrice mindrale desmonticules hydrothermaux. La composition des p6troles varie des condensats(C1-C15)aux
naphtbnes(probablementdesresidusconsid6rablementbiod6graddsrencontr6sdansla plupart des€chantillonsde sur-
face)et cires,avecdesquantit6simportantesd'asphaltepolycycliques
(HAP).
aromatiques
neset d'hydrocarbures
et les microorgaLes dEtritusorganiquescontemporains
nismesvivantssont aussiconvertispartiellementen proinstanduitsprochesdu pEtrolepar unepyrolyseacqueuse
dansles
tandelorsqu'ilssontentrain€spar lesturbulences
apparaitdansles
eauxdes6vents.Cedernierph€nombne
pasunecouverchampshydrothermauxqui nepossddent
teldesridesoc€aniques
tures&imentaireissuedirectement
lesquela rideduPacifiqueEsti 13et2loN etla rideMddioproduits
hydrocarbon€s
Atlantiquei 26"N.Lescompos6s
danscessitesont eGcompardsentermesdecomposition,
de sourcesde matrbreorganiqueet d'analogleau pdtrole
de rdservoir.
Fosse
BassindeGuaymas,
Mots<i&: fttsolehydrothermal,
Ridedu PacifiqueEst,AtlantisII Deep,
d'Escanaba,
hydrocarbures
aroz-alcanes,
RideM€dio-Atlantique,
matiquespolycycliques,migrationdu p6trole.
INTRODUCTION
Organic matter of marine sedimentary basins is
derived from the syngenetic residues of biogenic
debris that originates from both autochthonous
(marine) and allochthonous (continental) sources
(e.g., Simoneit 1978, 1982a),The preservationof
organic matter in sedimentsdependson the initial
diageneticprocesses,which involve microbial degradation and chemicalconversions,togetherwith the
acidity and redox potential of the environment (e.g.'
Didyk e/ ol. 1978,Demaison& Moore 1980).Subsequentsedimentmaturation and lithification causes
metamorphism of the organic matter' ultimately
yielding petroleum products through the effects of
temperature,pressureand petrology (Hunt 1979,Tissot & Welte 1984).However, the action of hydrothermal processeson such sedimentaryorganic matter
was found to generate petroleumlike products in
GuaymasBasin almost "instantaneously" in geological time (Simoneit& Lonsdale 1982).The present
paper reviewsthis processusing examplesfrom the
extensivelystudied Guaymas Basin and other areas
of hydrothermal activity at both sedimented and
bare-rock spreadingcenters.
The analytical techniques of organic geochemistry havebeenusedextensivelyto examinethe character of organicmatter in the geologicrecord in tenns
of its structural and compositional makeup (e.8.,
Simoneit 1978, van de Meent el a/. 1980, Johns
1986).The sources,diageneticand catagenetichistories, and migration mechanisms of this organic
matter can be evaluatedfrom such data. In the fol-
827
THE CANADIAN MINERALOGIST
828
TABLE 1. ORGAI.{ICMATIBR OF SEDIMENTSAND PETROLEI,IM
SEDIMENTS
Recent(generallybiqgeniccomponentgapprorl agerangef0-104 yr):
Lipids
Gai
HumicandFulvic
subshnces
CII4, CO2,H2S
Cs-C46+
minoamount
of otaf Cr,
mingamunt
(mx.-IM)
C-arbonaceous
detritus
(humin"pseudokerogpn)
masonoteculr
tvtW. -1Or to >lff
variableamrmt
nacronolecrrla
>humarcs
majmamount
yr):
Ancient(generallygcogeniccmponeotst approx.agerange104-104
Cas
Biomen
Asphaltenes
(sorehttmat€s)
Kerosen
CHa-Ca,CO2,II2S
CeCae+
minoamount
of oul C-,
minoramount
(mx--l0%)
masmlecular,
l,Lw. -104 to>lff
variableamount
macromoleculr,
>asphaltcnes
majoamount
PETROLEIJM
!06
inm4ure 66g6ig matgin
hydrclhermal sy*ems):
frgs(ad$soline
range)
Binrmen
Asphaltenes
Kerogen
CH4-Cs+,CO2,
H2S, N2
minmamou$
of total C*
CgCao+
masomlecltar,
lv!.W.-1Ol to>lff
majoramdnt
absent
m4i6amunt
spentkePgen
tpmarnsln
sourcerocks
lowing drscussion,organic matter is classifiedas gas, macromolecularfraction consisting of fulvic and
lipids (bitumen),humic substances(with fulvic sub- humic acidsand particulatedetritus (e.g., biopolymer
stances,or asphaltenes)and kerogen (with "pseu- fragments, cell membranesand miscelaneous cardokerogen"). The characteristicsof this organicmat- bonaceousmatter; Table 1). The lipids and macroter are outlined in Table 1, which summarizesthe molecular material undergo alteration and diageneequivalent orgaric fractions in recent and ancient sis (including microbiological) according to the
sediments.
environmentalconditions during transport and in the
Various basic organic geochemicalfractionation depositional sinks, i.e., oxidative degradation in
procedureshave been routinely applied, with only high-energy,oxygenatedenvironmentsand reductive
minor modifications, prior to instrumental analyses. changes in low-energy anaerobic environments
They are describedin detail elsewhere(e.9., Reed (Didyk et al. 1978, Demaison & Moore 1980'
1977,Boon et ol. 1977,Simoneit et ql. 1978,1979, Simoneit 1983).
1980,1981,Simoneit1978,1981,1982a,b,Stuermer
Following completion of early diagenesis,matuet al. 1978, Philp e/ al. 1978, van de Meent et ol.
ration of organic matter commenceswith increas1980).
ing burial and concomitant rise in the geothermal
gradient. This produces from kerogen (macromolecular organic matter) some low-temperature
Nature of organic motter in maturing bosins
cracking products, such as natural gas (CHa-Cs+),
Immature organic matter in recent sedimentsis and bitumen Cs-C46+,including a large envelopeof
comprisedof minor amounts (basedon total organic an unresolved complex mixture of compounds or
cilbon content) of biogenic gas (CHo and CO2, UCM), which are addedto the endogenousbiogenic
sometimesHzS), significant lipid residuesof ter- gas and lipid residues(Table 1). Maturation of pseurigenousor marine origins (or both), and a major
dokerogenoccursvia molecular rearrangementsand
PETROLEUM GENERATION
addition of geomonomersby copolymerization. Further heating during catagenesisgeneratesadditional
bitumen and gas, which far exceedthe original concentrationsof endogenouslipids and gas,thus erasing their compositional signatures and resulting in
the characteristic distributions of petroleum compounds(Simoneit1983).The spentkerogenremains
as amorphouscarbon. Late stagesofcatagenesisand
the subsequenthigh-temperaturephasecalled metagenesis(very deepburial) primarily generatemethane
from both bitumen and kerogen; HrS can also be
formed, especially in carbonate sequences.
Nature of organic matter in hydrothermal systems
Hydrothermal systemscan also act on sedimentary organicmatter and on enlrainedambientorganic
detritus in water; these processesresult in "instantaneous" diagenesisand catagenesis,and thus
producepetroleumproductsanalogousto thosefrom
slower acting geothermalchanges(Simoneit & Lonsdale 1982, Simoneit 1983, 1984a,b, 1985). Gas
(CH4-Cs+, CO2 and H2S) and bitumen (Cs-Ca4*,
with a largeUCM) are crackedfrom the pseudokerogen and biopolymers, and are added to the
endogenousgas and lipids. The bitumen additionally containsproducts characteristicof elevatedthermal processes(e.9., olefins, polynuclear aromatic
hydrocarbonsor PAH, stabilizedmolecularmarkers,
etc.). T:he spent kerogen remains as amorphous
"actiYated" carbon.
The effects of pressure,temperature and time on
the chemistry of the organic matter are all interrelated. Temperature and time, which primarily
effect petroleum generation,control mineralogical
interactionsand the elementalcompositionof kerogens. Migration processesare understoodleast. In
sedimentarybasins, migration of petroleum seems
to occur by diffusion and in solution
(H2O/ CH4/ CO2solvent), whereasin hydrothermal
areasmigration proceedsby thermally driven diffusion, in solution, and by advectionand masstransport as oil or emulsions(or both). Despite these
differences,the overall result may be the samefor
both regimes.Pressurein both casesaids mainly the
solubility of the petroleum in t}lreH2O/CHa/CO2
fluids.
SsaTloon SPREADING
CENTERS
AND
HYDRoTTmRMAL
PETRoLEUMGENERATIoN
829
nomic interest becauseof the lack of structural traps
and generallypoor source-rockpotential (i.e.,low
organic carbon content). Thesesystemsdo, however,
provide a "natural laboratory" for studying the
processes
ofpetroleum generationand the behavior
of petroleum in high-temperaturefluids. The following data illustrate the diverse suite of hydrothermal
samplesfrom different environments, and the wide
ranges of organic-carbon source material.
Guaymos Bosin, Gulf of California
Guaymas Basin is an actively spreading oceanic
basin which is part of the systemof spreadingaxes
and transform faults that extends from the East
Pacific Riseto the SanAndreas fauk (Cvray et ul.
1982,Lonsdale1985).Ocean-plateaccretionoccurs
through dyke and sill intrusionsinto the unconsolidatedsediments,leadingto high conductiveheat flow
@inseleet ol. 1980,Curray el ql. 1982,Einsele1985,
Lonsdale & Becker 1985). Sedimentsaccumulate
rapidly (>2m/1000 yr) and have coveredthe rift
floors to a depth of -400 m (Curray et al. 1982).
The organicmatter of theserecantsedimentsis derived primarily from diatomaceous and microbial
detritus, and averages about 2t/o organic carbon.
Influx of terrigenou$organic matter is low because
desertsborder the Gulf. Thermal stresscausesrapid
maturation with concomitant petroleum generation;
the "oil window" seemsto migrate upward as the
magmatic heat front risesin the sedimentarycolumn
(Simoneit 1984a,Simoneit et al. 1984).Petroleum
products havebeencharacterizedin samplesobtained
by shallow gravity coring in both rifts (Simoneit el
al. 1979),piston coring in the North Rift (Simoneit
1983)and deep coring by Leg 64 of the Deep Sea
Drilling Project (DSDP) (Curray et al. 1982).
Petroleum-bearingsampleshavealso beenrecovered
from the seafloorby dredgingoperations(Simoneit
& Lonsdale 1982)and submersiblesamplingwith the
ALVIN in 1982and 1985(Fig. l; Simoneit 1984a,b,
1985,Kawka & Simoneit 1987,Simoneit & Kawka
198n.
Lee & of the DSDP encountered sills and
hydrothermal alteration at depth in all holes drilled
in the basin (Fig. l). Thermogenichydrocarbon gas,
HrS, and CO2 were identified at all sites. Lipids
(bitumen) werethermally altered closeto and below
the sills, especially at Site 477 in the South Rift
(Simoneit& Philp 1982,Simoneitet al. 1984).This
alterationis indicatedby: i) lossofthe odd-carbonnumber predominance of the r-alkanes (CPI
approaches1.0*); ii) appe:uanceof a broad hump
Although this review focuseson the occurrenceof
petroleum products in six geographiclocations of
hydrothermal activity, it is the author's opinion that
petroleum generation and migration is a ubiquitous
process associated with hydrothermalism and * CPI-Carbon PreferenceIndex: for hydrocarbonsit is
metallic-mineral formation in ocean-ridgesystems.
expressedas a summation of the odd carbon number
It should also be noted that the quantities of
homologs over a range, divided by a sumrnation of the
petroleum associated directly with submarine
even carbon number homologs over the s:rme range
(Cooper & Bray 1963,Simoneit 1978).
hydrothermal systemsare negligible in terms of eco-
830
THE CANADIAN MINERALOGIST
a)
Flc. 1. (a) Location map of GuaymasBasin showingthe
North and Soutl Rifts, and positions of the DSDP and
gravity core (30G)sites;index map showsthe location
of Guaymas Basin in the Gulf of California. (b) Map
of the South Rift shorvingthe ALVIN dive areasby dive
number (also dredge 7D and DSDP Site 477); the
hachuredareas,ue hills and high areasabove the rift
floor.
of UCM (naphthenes);iii) isomerizationof various
biomarkers;iv) presenceof large amountsof terminal olefins (alk-1-enes),isoprenoid hydrocarbons,
and elementalsulfur; and v) appearancaof PAH.
The petroleum products havemigrated away from
heat sources(e.g., sills or magmaat depth) by advection, diffusion, distillation, and especiallyby cosolution in hydrothermal fluids (Simoneit& Philp 1982'
Simoneit 1982b,Simoneit et al. 1984).Cosolution
in this contextrefers to the solution in any proportion of petroleum (bitumen), itself a solution of a
large number of compounds,in hydrothermal water,
i.e., the stagebefore emulsion. The kerogens(i.e.,
insoluble detrital organic matter in sediments)of the
shallow DSDP core samplesare typical of unaltered
marine organic matter (Simoneit & Philp 1982,
Simoneitetal. 1984).In deepersections(> - 180m
depth at Sites477 and 478), the kerogensreflect completeexpulsionof pyrolysate;in thesezonesof high
thermal stress(entering greenschistfacies)the kerogen residuesresembleactivated amorphous carbon
(Simoneit 1982b).
Numerous hydrothermal mounds rise to 20-30 m
above the South Rift floor (water depth about 2000
m) and most are actively dischargingvent fluids wilh
water temperaturesof 315"C at -2@ bars (Lonsdale 1985,Lonsdale& Becker 1985,Merewetheret
al. 1985).Typical samplesfrom these mounds are
stainedand cementedwith petroleum; the samples
have a strong odor reminiscent of diesel fuel
(Simoneit & Lonsdale 1982). Sampleshave very
diversepetroleum contents and hydrocarbon distributions @igs.2, 3; Simoneit 1984a,b,1985,Kawka
& Simoneit 1987,Simoneit& Kawka 1987),but are
analogousto thosedescribedfor bitumensat depth
in the DSDP holes (Simoneit 1983, 1984b). The
r-alkanes range from methane to greater tharr nCaa,with usual maxima in the mid r-C2s region and
no carbon-numberpredominance(CPI = 1.0, Table
2). These data and the kinetic parameters of the
biomarkers iddicate that the petroleumswere generated by rapid and intenseheating.
An exampleof a gas chromatographic (GC) trace
of an aromatic/naphthenic fraction (F2) of a sample is shownin Figure 3b. The major resolvedpeaks
are PAH, a group of compounds uncommon in
petroleums but ubiquitous in higher temperature
pyrolysis residues(Geismannet al. L967, Blumer
1975, Hunt 1979). The dominant analogsare the
pericondensedaromatic series,for example,phenanthrene, pyrene, benzopyrenes,perylene, benzoperylene and coronene. A pyrolytic origin is also supported by the presenceof five-membered alicyclic
rings (e.9., acenaphthene,methylenephenanthlene,
fluorene, fluoranthene,elc). Theseare found in all
pJrolysatesfrom organic matter; once formed, they
do not easily revert to pericondensedaromatic
hydrocarbons@lumer 1975,1976,Scott 1982).These
fractions also contain significant amounts of toxic
PAH, e.g., the benzopyrenes.In addition, perylene
is present; it is the predominant PAH of unaltered
lipids in sedimentsdepositedunder oxygen-minimum
environmentsin the Gulf (Simoneit& Philp 1982,
831
PETROLEUM GENERATION
Simoneit 198?a,,b).
Thus, the chemicalcomposition
of the aromatic fractions indicatesderivation through
high-temperaturepyrolysis and rapid quenching,
presumably by hydrothermal fluids.
Hydrothermal petroleum migration in Guaymas
Basin seemsto occur as bulk-phase(pure petroleum),
cosolute fluid and aqueous solution (highlemperature solution of predominantly z-alkanes oq
only) upward to the seabed,where the petroleum co
condenses(solidifies)and collectsin the condui* and (l)o
vugs of hydrothermal mineral mounds in response E,
to ambient temperatures.PAH and sulfur accumu- .9
E
late in the hot vents;waxescrystallizein intermediate- c,
temperature regions (20-80"C), and volatile
petroleum partly collects in cold areas (3'C) and o
emanatesinto the seawaterwith plume discharges E
(Simoneit 1984a,b,1985,Merewetheret al. 1985). o
Both the extensivematuration of organic matter to
o
bitumen in the DSDP holes, and the significant ('
accumulations of petroliferous exudate at the rift
floor, confirm the importance of hydrothermal
metamorphism (pyrolysis) as a feasible mechanism
for the formation of petroleum.
Escanaba Trough, northeastern Pacific
The Escanaba Trough represents the southern
extension of the Gorda Ridge, an active oceanic
spreadingcenterabout 300 km long, boundedon the
north and south by the Blanco and Mendocino fracture zones,respectively(McManuset al. 1970),The
Trough is filled with up to 500 m of Quaternary turbidite sediments(McManus et ol. L97O),
Petroleum cements the sediments and sulfide
deposits that blanket the ridge axis and is derived
from hydrothermal alteration of sedimentaryorganic
matter (Kvenvolden et al. 1986),Typical GC traces
ofthe saturatedand aromatic hydrocarbon fractions
are shown in Figure 4. The organic source material
for thesepetroleumsis terrigenous,on the basisof
CPI, carbon-numberrange (Table 2), biomarker
composition, and sedimentologicalconsiderations
(Kvenvolden& Simoneit 1987).
In the aliphatic hydrocarbon fraction, the zalkanes range from Cl4 to C40, with a carbonnumber maximum at n-Cy.,.A predominanceof
odd carbon numbers )n-Czs(CPI = 1.25,Fig. 4a)
is typical of a terrestrial, higher plant origin. Homologs of a marine origin (<r-Crr) are less concentrated. This petroleum was probably generatedby
intense heating of short duration, as indicated by
kinetic parametersof the biomarkers and by the high
concentrationsof unsubstitutedPAH (Fig. 4b; Kvenvolden & Simoneit 1987).
East Pocific Rise, l3"N and 2loN
Hydrothermal activity and associatedmassivesulfide depositsare found on the unsedimentedaxis of
the East Pacific Rise (EPR) in the region of 13oN;
il
il
Time(relotive).-*
gas chromatograms
of total oils
Frc.2. Representative
recovered
by DSVALVIN in
from samples
extracted
theSouthRift: (a) I lTGl interiorof largechimneybase;
(b) l177-3oilycruston rock.Numbers
on all GCtraces
Pr : pristane,
arecarbonchainlengthof n-alkanes.
conditionsgiven
Ph : phytane;gaschromatographic
in Simoneit(1984a).
abundant faunal communitiesare also associated
with this activity (H6kinian et ql. 1983). Aliphatic
hydrocarbons have been analyzedin hydrothermal
plumesand in metalliferoussedimentsnear the active
vents and at the baseof an inactive chimney (Brault
et al. 1985, 1988).Hydrocarbons from metalliferous sedimentshave characteristicsof immature
organic matter, which was recently biosynthesized
and microbiologicallydegraded,asindicatedby the
abundance of low-molecular-weight (>Cz) nalkanesand phytane; a contribution of continental
higher plant material is shown by the presenceof
high-molecular-weight z-alkanes with an oddcarbon-numberpredominance(Fig. 5a, Table 2). The
immature characterof the organicmatter is also indicated by the presenceof biomarker hydrocarbons
derived from steroidsand triterpenoids, which are
the result of low temperatures,as might be expected
in the talus of an extinct vent system. The contents
of a sediment trap deployed in the area within
- ?-0m of the vents are characterizedby biologically
derivedmaterialand also by biomarker compounds
affected by thermal alteration (Brault et al. 1985),
832
THE CANADIAN MINERALOGIST
U
z
I
U
E
i
I
E.
I
p
a
E
I
lo
20
30
40
+
T|ME(min)
50
t0
60
20
30
40
TIME(MlN)+
50
60
Ftc. 3. Gas chromatograms of the (a) aliphatic and (b) aromatic hydrocarbons from dredgesample7D-2B in Guaymas
Basin (Simoneit & Lonsdale 1982). The structures of pristane and phytane are representedby the inset in (a).
Ftc. 4. Gas chromatogramsof the (a) aliphatic and (b) aromatic hydrocarbonsin a petroleum samplefrom the Escanaba
Trough (Kvenvoldenet al.1986).
TABLE2 ST'MMARYOFTIIE CHARACTERISTICS
PETROLEUMS
FROMSEVERALSEAFIOOR
OFFTYDROTHERMAL
SPREADING CENTERS.
Lacation
Toel
PetroleumFractions(wt7o)
organic carbon
Aliphatic
Aomatic
NSO opds
ofseds(averageTd hydocarbons hydrocarbons &aspha!
GuaynasBasin, 2
Gulfof Califomia
3
65
23
15
g-Allane
rangp
Cnl
13-35 1.03
r-40 r.02
74
2r
RscanabaTrough,0.4
GordaRidge
l44a
histate:
Phytane
Referenc€s
1.1
0.3-2.5
Simoneit&Lonsdale
1982,Simoncit& Kawka
pn
L25
1.7
Kvenvoldenggl.1986
AdantistrDeep, 0.f4
RedSea
n.d.
(230ad02
n.d.
n.d.
15-40 1.1
0.8
SinoneitetiL19&/a
EasrPacificRise,
13'N
(t.opde)2
n.d.
n.d.
n.d.
15-34 1.1
1.2
Bnultqgl. 1985
EastPactficR.ise,n.d.
21'N
n.d.
@2.{aglg1z
n.d.
n.d.
Mid-AdardcRidgan.d.
TAGArea26.t.[
n.d.
n.d.
n.d.
0.4
14-40+ 0.9-1.03 0.5-1.0
10-25
1.01
0.9-1.3
&atiltqgl. (unpublished
data)
Brault&Simneit1988
I CPI- carbonpreference
inder,calcularcd
herewer therangeC2 o Q3a.
2Valuesin parenthes€s
arototalyieldp€rgran dry sampla
n.d.= notdeteimined
This indicates that higher temperature degradation
of entrained organic detritus is an important process
near hydrothermal discharge sites. Thermally
matured compounds (Fig. 5b) ile also present at
trace levelsin waters collected within - 1 km above
the hydrothermal vents. The hydrocarbon pattern of
thesewatersis indicativein many casesof pyrolysis
of bacterial matter in entrained oceanwater during
833
PETROLEUM GENERATION
(l)
o
o) ou,
b)
E
CL
(l)
E
.9
CL
0
ttl
o
('
E
o
E
()
o
o
(t
.-*
(relotive)
Time (relotive)
Time
Time(relotive)
Frc. 5. Gas chromatograms of aliphatic hydrocarbons from extracts of: (a) hydrothermal metalliferous sediment and
O) surrounding water above the vents from the EPR at l3'N (Brault et al. 1985). SI = internal standard (n-C)l
Pr : pristane; Ph = phytane.
UJ
o
z
o
!J
t
i
t
Y
a
E
3f ee
a,
3eaoer
u,
TIMEFrc. 6. Gas chromatogram of the total hydrocarbon fraction from an extract of a pyritized tube worm in sulfide matril
from the EPR at 21"N (Pr pristane; Ph phytane: Py pyrene; a, b cholestenes;c diploptene) (Brault et al. submitted).
cooling of discharging fluids (Brault et al. 1988).
Extensivehydrothermal activity and associatedsulfide depositshave also been describedat 2loN on
the EPR wherethe crust is unsedimented(Spiessel
ol. 198O,Ballard et al. l98l). The hydrocarbon contents of massive sulfides from vent chimneys are
extremely low but definitely thermogenic (Table 2).
A GC trace for total hydrocarbonsis shownin Figure
6 (Brault e/ a/. submitted). The r-alkanes in massive sulfides range from Cl4 to greater than C4u,
with no carbon-number predominance; a pyritized
tube worm from a chimney has a slight odd-carbonnumber predominance. All samplescontain PAH,
providing evidencefor hydrothermal activity. Coupled with the carbon-number maxima at n-Cy, or
higher, this indicates that the hydrocarbons were
entrapped/condensedin a high-temperalure regime
suchas an active chimney. The samplewith pyritized
tube worm residues(Fig. 6) also contains hydrothermally altered derivatives of biomarkers (e.9.'
cholestenes,hopenes)from the vent biota, i.e., probably mainly tube worms and bacteria.
Atlsntis II DeeP, Red Seq
The Atlantis II Deep contains stratified brine
layers, the deepestof which is at a temperatureof
62'C (Hartmann 1980,1985).Bulk organic matter
and hydrocarbons have been analyzed in two sediment cores(No. 84 and12'6,CHAIN 6l cruise)from
the Deep (simoneit et al. 1987).Although the brine
overlying the coring areasis reported to be sterile,
sedimentary organic material derived from
autochthonous marine planktonic and microbial
inputs and minor terrestrialsourcesis present.The
834
THE CANADIAN MINERALOGIST
o
o
c
o
o
o
o)
g
,=
cl
o!
o
E
o
@
(9
Time (relotive)+
Ftc. ?. Cas chromatogram of the total hydrocarbon fraction extracted from the
Atlantis II Deep core sample 84, 443-453 cm (Simoneit et al. 1987).
I
o
o
o
.:o
6
t
Timo*
Low-temperature maturation in the sediments
results in petroleum generation, even from low
amounts of organic matter (average0.0590).Both
steroid and triterpenoid hydrocarbons (biomarkers)
show that extensive acid-catalyzed reactions are
occurring in the sediments.In comparisonwith other
hydrothermal systemsdriven by intrusions (e.9.,
Guaymas Basin; Cape Verde Rise, Simoneit et a/.
1981),sedimentsin the Atlantis II Deep exhibit a
lower degree of thermal maturation, as is clearly
shown by the elementalcomposition of the kerogens
and the absenceof pyrolytic PAH in the bitumen.
The lack of carbon-numberpreferenceamong the
n-alkanes(CPI : 1.0)suggests,especiallyin the case
of the long-chainhomologs (e.g., Fig. 7, Table 2),
that the organic matter has been affected by catagenesis.However, the yields of hydrocarbonswith
respectto sedimentweight are much lower than those
observedin other hydrothermal areas.The low temperature and low organic-carboncontent ofthe sediments in the Atlantic II Deep seemto be responsible for this difference.
Frc. 8. Gaschromatoetrams
of (a) aliphaticand (b) aromatic hydrocarbonsfrom an extract of a massive
sphaleritesamplefrom theMid-AtlanticRidge(Brault Mid-Atlantic Ridge, TAG areo 26"N
& Simoneit1988).
The Trans-Atlantic GeotraverseCIAG) hydrothermal field on the Mid-Atlantic Ridge crest at 26oN
is one of two active vent systemsknown from slowspreadingoceanicridges(Ronaet ol. 1984,Thomporganic input derived from the water column above sonet al. 1988).Hydrothermal depositslying directly
the brine is further metabolizedby microorganisms, on oceanic crust have been dredged from the area
and the reworked compounds with organic detritus (TAG 1985-1).Three sulfide-rich samples,consistare apparently then incorporated into the sediments ing mainly of anhydrite, sphalerite,and chalcopyrite'
under the brine by sinking as material adsorbedor respectively,containedminor amountsof the more
attachedto particlesof metallic oxide precipitates. volatile (Cro-Cd hydrothermal petroleums (Brault
PETROLEUMGENERATION
835
COMPAREDTO
TABLE 3. FEATIJRESOF IIYDROTIIERMAL PE'TROLEI,'M
RESERVOIRPETROLET,JM
1. Nanral gasandgasoline-range
$rdrocarbons
(CPI = 1.G12)
2. Full rangeof g-alkanes,no carbon-number-predminance
(ndorhury of UOvf)
3. Naphtbenicco'mponents
4. fropenoid hydmcartons(nclding signmcantpdstaneandphytane)
5. Biomaten (e.g.lrusre l7o(Hlhopades andst€ranes)
6. Allrylsmadc hydrocsbonsandasphaltenes
tlydm'therml Perolsm mly:
1. Polynucler aromaticby<kocarbos(PAII) > alkyl armaic hydrocarbons
hopenes'
2. Residualimnaane biomarkenandinternediares(ag. 178(tl)-hopanes'
gerenes)
3. Significatrtamatic sulfirrheteroconrpounds
4. Itrghsulfincontcnt
5. Alkenecontentnear"sourcerock"
& Simoneit 1988). The saturated and aromatic
hydrocarbon fractions separatd from the extract of
the sphalerite sample are shown in Figure 8. The
z-alkanesrange from Cttto C2twith a CPI = 1.0;
pristaneand phytaneare present,and the UCM maxThis patimizes at the GC retention time for r?-C17.
tern is analogousto that observedfor samplesfrom
the EPR at l3oN and from the Atlantis II Deep.The
aromatic fraction, which contains naphthalene,
phenanthrene,their alkyl homologs and sulfur aromatic compounds,supportsa hydrothermalorigin.
DIscussIoN
Petroleum generation
The principal zone of petroleum formation in
under normal geothermalgrasedimentarysequences
dients extendsfrom about I km to 3 km depth (e.9.,
Hunt 1979,Tissot & Welte 1984).This depth correspondsto a temperaturerangeof 50"-120'C. The
effect of pressureon this processis significant,
although it has not been quantified (Tissot & Welte
1984).The cracking of organic matter to natural gas
is thought to takJ place at high temperatures of
150o-250oC(e.g., Kartsev et al. 1971,Vassoevich
et al. 1974,Hunt 1979).Theseproposedtemperature regimesfor the oil and gas"windows" may need
some adjustment in light of more recent data on
hydrothermal systemsand ultra-deep wells.
The "instantansous" petroleum generation in
hydrothermal systemsis a facile processwhich occurs
at temperaturesapproaching a maximum of 400"C.
At such high temperatures, organic matter is only
partly destroyed,probably becausethe thermogenic
products are rapidly removed from the hot zone.
Formation of hydrothermal petroleumseemsto com-
mencein low-temperatureareas,generatingproducts
from weaker bonds (e.g., ether, sulfide, carbonyl,
tertiary carbon linkages).As the temperatureregime
rises, products are derived from more refractory
organic mattgr and gven"resynthesized" (e.9., PAH
compounds). The major similarities and differences
between hydrothermal petroleums and reservoir
petroleums are summarized in Table 3. Most
hydrocarbon products occur in both types of
petroleum; the major differenceis the enhancedcontent of PAH and sulfur in the hydrothermal
products.
Organic matter associatedwith deeperhydrothermal systems(e.g., epithermal ores in volcanic terranes)is typically more asphalticand hasa high PAH
content. Such organic matter is widely distributed
(e.g., California mercury deposits:Geissmanel a/.
1967, Blumer 1975; other hydrothermal sulfides:
Germanov & Bannikova 1972). Deep well drilling
(>7000 m) hasintersectedCretaceousshaleswhich
were at in situ temperatruesof about 260'-300oC
(e.g., Price et sl. 1981,Price 1982).Thesesamples
were rich in bitumen componentsand their kerogens
still had significant hydrocarbon-generation potential. This indicates that in sila petroleum is stable
at much higher temperatures and pressuresthan
those discussedabove, and over long geologictime
periods.
During mineral diagenesis and metamorphism
under non-oxidizingconditions,the organic-matter
composition changesprogressivelyto more aromatic
and asphaltic residues by the expulsion of volatile
components(e.g., CO2,CH4, H2O, etc.). The carbonaceousresiduesare often associatedwith heavymetal enrichments,as for exampleuranium (e.g.,
Schidlowski 1981)or Carlin-type gold and silver ores
836
THE CANADIAN MINERALOGIST
(e.9., Radtke & Scheiner 1970). Heavy aromatic
hydrocarbons(PAH), presentin all the hydrothermal sites describedhere, are a product of hightemperaturealteration and thus may be good indicators of such alteration peripheral to sulfide orebodies(e.9., Germanov& Bannikova 1972,Blumer
r975).
alteration during diagenetic and early catagenetic
processes;this changesthe hydrocarbon signature
.(Fig. 9). During oil generation, however, large
amounts of additional hydrocarbonsare superimposed on the syngeneticlipids, thus obscuringthe
earlier signature(e,9,, lossor reduction of the oddcarbon-number predominooce )C25; cf. Fig.9
versasFigs. 3, 4). Syngeneticlipids of sedimentscan
Organic-motter type
usually be utilized to elucidate the various sources
The constitution of the initial organicmatter deter- of the total organic matter (e.g., Simoneit 1978,
mines the types of petroleum products that form in 1981, 1982a).For example,lipid hydrocarbonsof
basins with a normal geothermal gradient, and in normal sediment in Guayrnas Basin (Fig. 9a) conhydrothermal systems. The major source of tain r-alkanes mostly <C21, and a minor seriesof
petroleum compoundsis kerogen,the sedimentary homologs >C1 with a strong odd-carbon-number
macromolecular organic detritus which generally predominance. This composition is typical of a
constitutesthe bulk of the total organiccarboncon- predominantly marine planktonic and microbial oritent (Tissot& Welte 1984,Table 1). In general,ter- gin, and a minor influx of terrigenousvascularplant
restrial detritus from mainly vascular plants yields wax (>C25). By contrast, lipid hydrocarbons of
an aromatic kerogen(e.9., coal) which has a natural- sediment in the EscanabaTrough (Fie. 9b) contain
gas potential, whereasmarine/lacustrine organic n-alkanes,mostly >C, with a strong odd-carbonmatter from primarily microbial and planktonic number predominance.This signature is derived
residuesyields an aliphatic kerogen(e.9., sapropel) mainly from terrigenous vascular plant wax, with a
which has a paraffinic petroleum potential (Hunt minor microbial component (Kvenvolden et al.
1979,Tissot & Welte 1984).Kerogensin sedimen- 1986).In both the Guaymasand Escanabasamples,
tary basinsare alwaysmixtures of theseinferred end the compositions of the kerogens support the
members.
interpretations based on the nature of the lipid
Syngenetic sedimentary lipid matter undergoes hydrocarbons(Simoneit1978,Simoneitet ol. 1979).
Migration processes
The aqueoussolubility of petroleumsand various
hydrocarbon fractions has been determined
experimentally (Price 1976). Petroleum solubility
increasesexponentiallyfrom l@ to 180'C and solubilities are high enoughto account for the formation of petroleum reservoirsby migration of molecular or co-solutions.Increasedsalinitiesof 1507*NaCl
U
q
z
causedrastic exsolution of lhe petroleum and at
c
3500/oo
alrnosttotal "salt-out " occurs (Price 1976).
ztzcru26"r28
a
ffi
U
E
This finding is consistent with the requirement for
the separationof petroleum from migrating solutions
-o
in the salty waters of reservoir sands. It has been
demonstratedthat methanein the presenceof water
P
&
is an even better carrier for petroleum than water
E
or methanealone @rice et al. 1983).Both increases
o
in pressure(to about 1800bar, 1.8 x ld Pa) and
a
temperature (to 250'C) raised the solubility of
petroleum.Cosolubilitywas found at rather moderate conditions (e.g., 100"C at ld Pa, 200'C at 0.5
x lS Pa). The addition of other gasessuch as CO,
and ethaneto this mixture also increasesthe solubility of petroleum. Under theseexperimentalconditions, CH4, C2H2and CO, are all supercritical
and HrO approachesits near-critical state; thus, the
to
20
30
40
50
60
(min)+
TIME
gases and HrO are all mutually soluble by the
reducedhydrogenbonding in water, and are a good
Frc.9. Gas chromatograms of total hydrocarbons in
extracts from surfacesedimentsof (a) GuaymasBasin, cosolvent for petroleum. Therefore, primary migraSite 30G (Simoneit et al. 1979), and (b) Escanaba tion seemsto proceedas gaslfluid and aqueoussoluTrough (Kvenvolden et al, 1986),
tion (seealso Hunt 1979).
837
PETROLEUM GENERATION
CoNcrustoNs
Recentimmature sedimentsreceivebiogenic detritus which, upon deposition, undergoesdiagenetic
and additional microbial alteration. Increasingburial
in sedimentarybasinsresultsin the onset of organicmatter maturation, which generatessome volatile
products from the kerogen (easily crackedmoieties)
that becomeaddedto the endogenouslipid residues.
This is the beginningof petroleum formation. As the
depth of burial (i.e., temperature)increases,catagenesiscommencesand major petroleum generation
takes place. At still greater depths of burial the
metagenetic stage is envisaged, where extensive
cracking, disproportionation,and reforming of the
organic matter (both petroleum and kerogen
residues)occur to yield primarily gasesand residual
amorphous carbon.
In the case of hydrothermal systems, these
processesare compressedinto an "instantaneous"
geologicaltime frame. At seafloor spreadingaxes,
hydrothermal systemsoperating below a sediment
blanket (e.9., GuaymasBasin and EscanabaTrough)
generatepetroleum from generallyimmature organic
matter in the sediments. This petroleum then
migratesupward, leaving behind a spent carbonaceousresidue.The Guaymaspetroleums€uecomprised of: l) gasoline-range
hydrocarbons(C1-C1);
2) a broad distribution of n-alkanes(C12-Ca6+)
with
no carbon-numberpredominance;3) a naphthenic
hump, UCM; 4) pristaneand phytaneat significant
concentrations; 5) mature biomarkers (e.9. ahopanes);and 6) significant concentrationsof PAH
and sulfur. Exterior exposed petroleum, and
petroleumsin unconsolidatedsurfacesediments,are
microbially deeradedand leached, whereasinterior
samples are chiefly unaltered.
Hydrothermal systemsoperating in unsedimented
rift areas(e.9.,EPR at 13'N and 2loN, Mid-Atlantic
Ridge at 26'N) generatetrace amounts of petroleum
and emit methane.Low amounts of petroleum are
generatedby pyrolysis of suspendedand dissolved
biogenic organic detritus (including bacteria and
algae)entrained during the turbulent cooling of the
vents both for sedimentedand bare-rock systems.
However, this type of petroleumis swampedin the
former caseby the large quantify of petroleumgenerated from the sedimentary organic matter. In addition, low-level maturation is observedin the surrounding areaat vent sites,probably due to warming
of ambient detritus. Pyrolysis of organic matter and
petroleum migration also appear to have occurred
in geothermal regions characterized by epithermal
ore deposits.
Gases,bitumen (lipids) and kerogencomplement
each other in providing information about the
sourcesand thermal history of sedimentaryorganic
matter. Kerogenis a sensitivein situ rndtca;torof temperature, and petroleum (bitumen including asphalt
and gas) representsthe product mixture of hightemperature stress. These products may have
migrated or remained in situ.
ACKNOwLEDGEMENTS
I thank the Deep SeaDrilling Project for access
to DSDP samples,and the National ScienceFoundation for funding participation on the D.S.V.
ALVIN cruises.Samples,data and assistancewere
provided by P. Lonsdale, R.P. Philp, P. Jenden,
M.A. Mazurek, E. Ruth, O.E. Kawka, M. Brault,
J. Baross, K.A. Kvenvolden, P.A. Rona and A.
Lorre. Funding from the Division of OceanSciences,
National ScienceFoundation(GrantsOCE81-18897,
OCE-8312036,
OCE-8512832
is
and OCE-8601316)
gratefully acknowledged.I thank two anonymous
reviewersfor commentsand especiallyT. Barrett for
excellenteditorial assistanceto make this paperintelligible for geologists.
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