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Canadian Mineralogist
Vol. 26, pp. 827-840
(1988)
PETROLEUM GENERATION
IN SUBMARINE
HYDROTHERMAL
SYSTEMS:
AN UPDATE
BERND R.T. SIMONEIT
Petroleum
Research Croup, College
of Oceanography,
Oregon State
University,
Comallis,
Oregon
97331, U.S.A.
ABSTRACT
The conversion of organic matter to petroleum by
hydrothermal activity is a process
which occurs in many
types of natural environments. Geologically immature
organic matter in marine sediments
is being altered by this
process
in the Guaymas
Basin, Escanaba Trough, and
Atlantis II Deep. In Guaymas Basin the sedimentary
organic
matter is derived primarily from algal and microbial detri-
tus, with a minor influx of terrestrial higher plant compo-
nents. lt is easily cracked
to petroleum
and migrates
in
hydrothermal fluids. Emanations at the seabed
result in dis-
persal of the more volatile components
(Cr-Cro hydrocar-
bons) and solidification of the heavier (C16C4e+)
petroleum within the mineral matrix of the hydrothermal
mounds. The petroleum compositions vary from conden-
sates
(C1-C15)
to naphthenic
(probably extensively
bio-
degraded residues,
the most common surface samples)
to
waxy, all with sigrrificant
amounts of asphaltenes
and toxic
polynuclear
aromatic hydrocarbons
@AII). Contemporary
organic detritus and living microorganisms are also con-
verted in part to petrolzum-like
products
when they become
entrained by turbulent mixing into the vent waters, result-
ing 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 example on the East Pacific Rise at l3o and
2l"N and on the Mid-Atlantic Ridge at 26'N. The
hydrocarbon products generated
in the various areas are
compared
in terms of composition, organic matter sources
and analogy to reservoir petroleum.
Keywords: hydrothermal petroleum, Guaymas Basin,
Escanaba
Trough, East Pacific Rise, Atlantis II Deep,
Mid-Atlantic Ridge, r-alkanes, polynuclear aromatic
hydrocarbons,
petroleum
migation.
Solravetnr
La transformation de
la matidre organique en p€trole
par
activit€ hydrothermale
est un processus
qui se
produit dans
divers milieux naturels. La matidre organique gdologique-
ment immature des s6diments
marins est altdree
par cette
voie dans
le Bassin de Guaymas,
la Fosse d'Escanaba,
et
Atlantis II Deep. La matidre organique
sddimentaire
du Bas-
sin de Guaymas
provient essentiellement
de d€tritus micro-
biens et algaires avec un faible apport de compos6s
issus
des v€g6taux supdrieurs
terrestres.
Cette matiire organique
forme facilement par craquage
des
produits p€troliers qui
migrent avec
le fluide hydrothermal. Les 6manations
de flui-
des chauds au fond de I'oc6an entralnent une dispersion
des composds
plus volatiles (hydrocarbures
C1-C1s)
et une
solidification des
p6troles
plus lourds (Cro-Cao+)
dans
la
matrice mindrale des
monticules hydrothermaux. La com-
position des
p6troles
varie des condensats
(C1-C15)
aux
naphtbnes
(probablement
des residus consid6rablement
bio-
d6gradds rencontr6s
dans la plupart des €chantillons
de sur-
827
face) et cires,
avec des
quantit6s
importantes d'asphalte-
nes et d'hydrocarbures
aromatiques
polycycliques (HAP).
Les dEtritus organiques
contemporains
et les microorga-
nismes vivants sont aussi
convertis
partiellement
en
pro-
duits
proches
du pEtrole par
une
pyrolyse
acqueuse
instan-
tande
lorsqu'ils sont entrain€s
par les turbulences
dans les
eaux
des 6vents. Ce dernier
ph€nombne
apparait
dans les
champs
hydrothermaux
qui ne
possddent pas
une couver-
ture s&imentaire
issue directement des
rides oc€aniques tel-
les
que
la ride du Pacifique
Est i 13 et 2loN et
la ride Mddio-
Atlantique i 26"N. Les compos6s
hydrocarbon€s
produits
dans
ces sites
ont eG compards
en termes de composition,
de sources de matrbre
organique et d'analogle au pdtrole
de
rdservoir.
Mots<i&: fttsolehydrothermal,
Bassin de Guaymas,
Fosse
d'Escanaba,
Ride du Pacifique
Est, Atlantis II Deep,
Ride M€dio-Atlantique,
z-alcanes,
hydrocarbures aro-
matiques
polycycliques,
migration du 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 preservation
of
organic matter in sediments depends
on the initial
diagenetic
processes,
which involve microbial degra-
dation and chemical conversions,
together
with the
acidity and redox potential of the environment (e.g.
'
Didyk e/ ol. 1978,
Demaison & Moore 1980). Sub-
sequent sediment
maturation and lithification causes
metamorphism of the organic matter' ultimately
yielding petroleum products
through the effects
of
temperature,
pressure
and petrology (Hunt 1979,
Tis-
sot & Welte 1984). However,
the action of hydrother-
mal processes
on such
sedimentary
organic
matter
was found to generate petroleumlike products in
Guaymas Basin almost "instantaneously" in geolog-
ical time (Simoneit
& Lonsdale
1982). The present
paper
reviews this process
using
examples
from the
extensively
studied Guaymas
Basin and other areas
of hydrothermal activity at both sedimented and
bare-rock
spreading
centers.
The analytical techniques
of organic geochemis-
try have been used
extensively
to examine
the charac-
ter of organic
matter in the
geologic
record
in tenns
of its structural and compositional
makeup (e.8.,
Simoneit 1978,
van de Meent el a/. 1980, Johns
1986). The sources,
diagenetic
and catagenetic
his-
tories, and migration mechanisms
of this organic
matter can be evaluated
from such data. In the fol-
828 THE CANADIAN MINERALOGIST
TABLE 1. ORGAI.{IC
MATIBR OF SEDIMENTS AND PETROLEI,IM
SEDIMENTS
Recent
(generally
biqgenic componentg approrl age
range f0-104 yr):
Gai Lipids HumicandFulvic
subshnces C-arbonaceous
detritus
CII4, CO2,
H2S Cs-C46+
minoamount mingamunt
of otaf Cr, (mx.-IM)
Ancient
(generally
gcogenic
cmponeotst
Cas Biomen
CHa-Ca, CO2, II2S CeCae+
minoamount minoramount
of oul C-, (mx--l0%)
(humin"
pseudokerogpn)
masonoteculr nacronolecrrla
tvtW. -1Or to >lff >humarcs
variableamrmt majmamount
approx. age
range 104-104
yr):
Asphaltenes
(sorehttmat€s)
masmlecular,
l,Lw. -104
to>lff
variable amount
Kerosen
macromoleculr,
>asphaltcnes
majoamount
PETROLEIJM
!06 inm4ure 66g6ig matgin hydrclhermal sy*ems):
frgs(ad$soline Binrmen
range)
CH4-Cs+, CO2, CgCao+
H2S, N2
minmamou$ m4i6amunt
of total C*
Asphaltenes Kerogen
masomlecltar,
lv!.W.
-1Ol to>lff
majoramdnt
absent
spentkePgen
tpmarns ln
source
rocks
lowing drscussion, organic matter is classified
as
gas,
lipids (bitumen),
humic substances
(with fulvic sub-
stances, or asphaltenes) and kerogen
(with "pseu-
dokerogen"). The characteristics
of this organic mat-
ter are outlined in Table 1, which summarizes the
equivalent orgaric fractions in recent and ancient
sediments.
Various basic organic
geochemical
fractionation
procedures
have been routinely applied, with only
minor modifications, prior to instrumental analyses.
They are described in detail elsewhere
(e.9., Reed
1977, Boon et ol. 1977, Simoneit
et ql. 1978, 1979,
1980,
1981, Simoneit
1978, 1981, 1982a,b,
Stuermer
et al. 1978, Philp e/ al. 1978, van de Meent et ol.
1980).
Nature of organic motter in maturing bosins
Immature organic matter in recent
sediments
is
comprised
of minor amounts
(based
on total organic
cilbon content) of biogenic gas (CHo and CO2,
sometimes HzS), significant lipid residues
of ter-
rigenous
or marine origins (or both), and a major
macromolecular
fraction consisting
of fulvic and
humic acids
and particulate
detritus (e.g.,
biopolymer
fragments, cell membranes
and miscelaneous car-
bonaceous
matter; Table 1). The lipids and macro-
molecular material undergo alteration and diagene-
sis (including microbiological) according to the
environmental
conditions
during transport and in the
depositional sinks, i.e., oxidative degradation
in
high-energy,
oxygenated
environments
and reductive
changes in low-energy anaerobic environments
(Didyk et al. 1978, Demaison & Moore 1980'
Simoneit 1983).
Following completion
of early diagenesis,
matu-
ration of organic matter commences
with increas-
ing burial and concomitant rise in the geothermal
gradient. This produces from kerogen (macro-
molecular organic matter) some low-temperature
cracking
products,
such
as
natural gas
(CHa-Cs+),
and bitumen Cs-C46+,
including a large envelope
of
an unresolved complex mixture of compounds or
UCM), which are added
to the endogenous
biogenic
gas
and lipid residues
(Table 1). Maturation of pseu-
dokerogen
occurs
via molecular
rearrangements
and
PETROLEUM GENERATION 829
addition of geomonomers
by copolymerization.
Fur-
ther heating during catagenesis
generates
additional
bitumen and gas, which far exceed the original con-
centrations
of endogenous
lipids and
gas,
thus
eras-
ing their compositional signatures and resulting in
the characteristic distributions of petroleum com-
pounds (Simoneit
1983). The spent kerogen
remains
as amorphous
carbon. Late stages ofcatagenesis and
the subsequent
high-temperature
phase
called meta-
genesis (very deep burial) primarily generate
methane
from both bitumen and kerogen; HrS can also be
formed, especially in carbonate sequences.
Nature of organic matter in hydrothermal systems
Hydrothermal systems can also act on sedimen-
tary organicmatter and on enlrained ambient
organic
detritus in water; these
processes
result in "instan-
taneous" diagenesis and catagenesis, and thus
produce petroleum products
analogous
to those from
slower acting geothermal
changes
(Simoneit
& Lons-
dale 1982, Simoneit 1983, 1984a,b, 1985). Gas
(CH4-Cs+, CO2 and H2S) and bitumen (Cs-Ca4*,
with a large UCM) are cracked from the pseudokero-
gen and biopolymers, and are added to the
endogenous
gas and lipids. The bitumen addition-
ally contains
products characteristic
of elevated
ther-
mal processes (e.9., olefins, polynuclear
aromatic
hydrocarbons or PAH, stabilized
molecular markers,
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 inter-
related. Temperature and time, which primarily
effect petroleum generation,
control mineralogical
interactions and
the elemental composition
of kero-
gens.
Migration processes
are understood
least. In
sedimentary basins,
migration of petroleum
seems
to occur by diffusion and in solution
(H2O
/ CH4/ CO2
solvent), whereas in hydrothermal
areas
migration proceeds
by thermally driven diffu-
sion, in solution, and by advection
and mass trans-
port as oil or emulsions
(or both). Despite
these
differences, the overall result may be the same for
both regimes. Pressure in both cases aids mainly the
solubility of the petroleum
in t}lre
H2O/CHa/CO2
fluids.
SsaTloon SPREADING CENTERS AND
HYDRoTTmRMAL PETRoLEUM
GENERATIoN
Although this review focuses on the occurrence of
petroleum products in six geographic
locations
of
hydrothermal activity, it is the author's opinion that
petroleum generation and migration is a ubiquitous
process associated with hydrothermalism and
metallic-mineral formation in ocean-ridge systems.
It should also be noted that the quantities of
petroleum associated directly with submarine
hydrothermal systems are negligible in terms of eco-
nomic interest because
of the lack of structural traps
and generally poor source-rock
potential (i.e.,low
organic carbon content).
These systems
do, however,
provide a "natural laboratory" for studying the
processes
ofpetroleum generation
and the behavior
of petroleum
in high-temperature
fluids. The follow-
ing data illustrate the diverse
suite of hydrothermal
samples from 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 system of spreading
axes
and transform faults that extends from the East
Pacific Rise to the San
Andreas fauk (Cvray et ul.
1982, Lonsdale
1985).
Ocean-plate accretion
occurs
through dyke and sill intrusions
into the unconsoli-
dated sediments,
leading to high conductive
heat
flow
@insele
et ol. 1980, Curray el ql. 1982, Einsele
1985,
Lonsdale & Becker 1985). Sediments
accumulate
rapidly (>2m/1000 yr) and have covered
the rift
floors to a depth of -400 m (Curray et al. 1982).
The organic
matter of these
recant sediments
is der-
ived primarily from diatomaceous and microbial
detritus, and averages about 2t/o organic carbon.
Influx of terrigenou$ organic
matter is low because
deserts
border the Gulf. Thermal stress causes
rapid
maturation with concomitant petroleum generation;
the "oil window" seems
to migrate
upward as the
magmatic
heat front rises
in the sedimentary column
(Simoneit 1984a, Simoneit
et al. 1984). Petroleum
products
have been characterized
in samples
obtained
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-bearing
samples
have also been
recovered
from the seafloor
by dredging
operations
(Simoneit
& Lonsdale 1982) and submersible
sampling
with the
ALVIN in 1982 and 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). Thermogenic
hydrocarbon gas,
HrS, and CO2 were identified at all sites. Lipids
(bitumen) were thermally altered close to and below
the sills, especially at Site 477 in the South Rift
(Simoneit
& Philp 1982, Simoneit
et al. 1984).This
alteration
is indicated
by: i) loss ofthe odd-carbon-
number predominance of the r-alkanes (CPI
approaches
1.0*); ii) appe:uance
of a broad hump
* CPI-Carbon Preference
Index: for hydrocarbons it is
expressed as a summation
of the odd carbon number
homologs over a range, divided by a sumrnation of the
even carbon number homologs over the s:rme range
(Cooper
& Bray 1963, Simoneit 1978).
830
a)
THE CANADIAN MINERALOGIST
Flc. 1. (a) Location map of Guaymas Basin
showing
the
North and Soutl Rifts, and positions of the DSDP and
gravity core
(30G)
sites;
index map shows the location
of Guaymas Basin in the Gulf of California. (b) Map
of the South Rift shorving the ALVIN dive areas by dive
number (also dredge 7D and DSDP Site 477); the
hachured
areas ,ue hills and high areas above the rift
floor.
of UCM (naphthenes);
iii) isomerization
of various
biomarkers; iv) presence
of large amounts of termi-
nal olefins (alk-1-enes),
isoprenoid hydrocarbons,
and elemental sulfur; and v) appearanca of PAH.
The petroleum products
have migrated away from
heat sources
(e.g., sills or magma at depth)
by advec-
tion, diffusion, distillation, and especially
by coso-
lution in hydrothermal
fluids (Simoneit
& Philp 1982'
Simoneit 1982b,
Simoneit
et al. 1984).
Cosolution
in this context
refers
to the solution in any propor-
tion of petroleum
(bitumen),
itself a solution of a
large number of compounds,
in hydrothermal water,
i.e., the stage before emulsion.
The kerogens
(i.e.,
insoluble detrital organic matter in sediments)
of the
shallow DSDP core samples
are typical of unaltered
marine organic matter (Simoneit & Philp 1982,
Simoneit
etal. 1984).
In deeper
sections
(> - 180
m
depth at Sites
477 and 478), the kerogens
reflect com-
plete
expulsion
of pyrolysate;
in these
zones of high
thermal stress
(entering greenschist
facies)
the kero-
gen residues
resemble activated 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 discharging
vent fluids wilh
water temperatures
of 315"C at -2@ bars (Lons-
dale 1985,
Lonsdale
& Becker
1985,
Merewether
et
al. 1985).
Typical samples
from these mounds are
stained
and cemented
with petroleum;
the samples
have a strong odor reminiscent of diesel fuel
(Simoneit & Lonsdale 1982). Samples
have very
diverse
petroleum contents
and hydrocarbon distri-
butions
@igs.
2, 3; Simoneit
1984a,b,
1985,
Kawka
& Simoneit
1987, Simoneit
& Kawka 1987), but are
analogous
to those
described
for bitumens
at depth
in the DSDP holes (Simoneit 1983, 1984b). The
r-alkanes range from methane to greater tharr n-
Caa, with usual maxima in the mid r-C2s region and
no carbon-number
predominance
(CPI = 1.0, Table
2). These data and the kinetic parameters of the
biomarkers iddicate that the petroleums
were
gener-
ated by rapid and intense
heating.
An example
of a gas
chromatographic (GC) trace
of an aromatic/naphthenic fraction (F2) of a sam-
ple is shown
in Figure 3b.
The major resolved
peaks
are PAH, a group of compounds
uncommon in
petroleums
but ubiquitous in higher temperature
pyrolysis residues
(Geismann
et al. L967, Blumer
1975, Hunt 1979).
The dominant analogs
are the
pericondensed
aromatic series,
for example,
phenan-
threne, pyrene, benzopyrenes,
perylene,
benzopery-
lene and coronene. A pyrolytic origin is also sup-
ported by the presence
of five-membered alicyclic
rings (e.9.,
acenaphthene,
methylenephenanthlene,
fluorene,
fluoranthene, elc). These are
found in all
pJrolysates
from organic matter; once formed, they
do not easily revert to pericondensed
aromatic
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 sediments deposited
under oxygen-minimum
environments
in the Gulf (Simoneit
& Philp 1982,
PETROLEUM GENERATION 831
Simoneit 198?a,,b).
Thus,
the chemical
composition
of the aromatic fractions indicates
derivation
through
high-temperature
pyrolysis and rapid quenching,
presumably by hydrothermal fluids.
Hydrothermal petroleum migration in Guaymas
Basin
seems
to occur as bulk-phase
(pure petroleum),
cosolute fluid and aqueous solution (high-
lemperature solution of predominantly z-alkanes
only) upward to the seabed, where
the petroleum
condenses
(solidifies)
and collects in the condui* and
vugs of hydrothermal
mineral mounds in response
to ambient temperatures. PAH and sulfur accumu-
late in the hot vents; waxes
crystallize in intermediate-
temperature regions (20-80"C), and volatile
petroleum partly collects in cold areas
(3'C) and
emanates into the seawater
with plume discharges
(Simoneit 1984a,b,1985,
Merewether
et al. 1985).
Both the extensive maturation of organic matter to
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
spreading
center about 300 km long, bounded on the
north and south by the Blanco and Mendocino frac-
ture zones,
respectively
(McManus
et al. 1970),The
Trough is filled with up to 500 m of Quaternary
tur-
bidite sediments
(McManus et ol. L97O),
Petroleum cements the sediments and sulfide
deposits that blanket the ridge axis and is derived
from hydrothermal
alteration of sedimentary
organic
matter (Kvenvolden et al. 1986), Typical GC traces
ofthe saturated and aromatic hydrocarbon fractions
are shown in Figure 4. The organic source material
for these
petroleums
is terrigenous,
on the basis
of
CPI, carbon-number
range (Table 2), biomarker
composition, and sedimentological
considerations
(Kvenvolden
& Simoneit 1987).
In the aliphatic hydrocarbon fraction, the z-
alkanes range from Cl4 to C40,
with a carbon-
number maximum at n-Cy.,. A predominance
of
odd carbon numbers
)n-Czs(CPI = 1.25,
Fig. 4a)
is typical of a terrestrial, higher plant origin. Homo-
logs of a marine origin (<r-Crr) are less
concen-
trated. This petroleum
was
probably generated
by
intense heating of short duration, as indicated by
kinetic parameters
of the biomarkers
and by the high
concentrations
of unsubstituted
PAH (Fig. 4b; Kven-
volden & Simoneit 1987).
East Pocific Rise, l3"N and 2loN
Hydrothermal activity and associated
massive
sul-
fide deposits
are found on the unsedimented
axis of
the East Pacific Rise
(EPR) in the region of 13oN;
Time
(relotive).-*
Frc. 2. Representative
gas
chromatograms
of total oils
extracted
from samples
recovered by DSV ALVIN in
the South
Rift: (a)
I lTGl interior of large chimney
base;
(b)
l177-3 oily crust
on
rock. Numbers
on all
GC
traces
are
carbon chain
length of n-alkanes.
Pr : pristane,
Ph : phytane;
gas
chromatographic
conditions
given
in Simoneit
(1984a).
abundant faunal communities are also associated
with this activity (H6kinian et ql. 1983). Aliphatic
hydrocarbons have been analyzed in hydrothermal
plumes
and in metalliferous sediments
near the active
vents and at the base of an inactive chimney (Brault
et al. 1985,
1988). Hydrocarbons from metallifer-
ous sediments
have characteristics of immature
organic matter, which was recently biosynthesized
and
microbiologically degraded, as
indicated by the
abundance of low-molecular-weight (>Cz) n-
alkanes and phytane;
a contribution of continental
higher plant material is shown by the presence
of
high-molecular-weight z-alkanes with an odd-
carbon-number
predominance (Fig. 5a, Table 2). The
immature character of the organic matter is also
indi-
cated by the presence
of biomarker hydrocarbons
derived from steroids and 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
- ?-0 m of the vents are characterized
by biologically
derived material and also by biomarker
compounds
affected by thermal alteration (Brault et al. 1985),
il
il
o
q
c
o
o
(l)
E,
.9
E
c,
o
E
o
o
('
832 THE CANADIAN MINERALOGIST
lo 20 30 40 50 60 t0 20 30 40 50 60
T|ME
(min)
+ TIME
(MlN)+
Ftc. 3. Gas chromatograms
of the (a) aliphatic and (b) aromatic hydrocarbons from dredge sample
7D-2B in Guaymas
Basin (Simoneit & Lonsdale 1982). The structures of pristane and phytane are represented by the inset in (a).
Ftc. 4. Gas chromatograms
of the (a) aliphatic and (b) aromatic hydrocarbons
in a petroleum sample from the Escanaba
Trough (Kvenvolden
et al.1986).
TABLE
2 ST'MMARY
OF TIIE CHARACTERISTICS OF
FTYDROTHERMAL PETROLEUMS FROM SEVERAL
SEAFIOOR
SPREADING CENTERS.
U
z
I
U
E
i
I
E.
I
p
a
E
I
Lacation histate:
Cnl Phytane Referenc€s
Toel PetroleumFractions(wt7o)
organic carbon Aliphatic Aomatic NSO opds g-Allane
ofseds(averageTd hydocarbons hydrocarbons &aspha! rangp
Guaynas Basin, 2
Gulf of Califomia
RscanabaTrough,
0.4
Gorda
Ridge
AdantistrDeep, 0.f4
Red
Sea
Easr
Pacific Rise, 0.4
13'N
EastPactficR.ise,
n.d.
21'N
Mid-AdardcRidga
n.d.
TAGArea26.t.[
n.d. n.d.
(230ad02
n.d. n.d.
(t.o
pde)2
n.d. n.d.
@2.{aglg1z
n.d. n.d.
13-35 1.03 1.1
r-40 r.02 0.3-2.5
l44a L25 1.7
15-40 1.1 0.8
15-34 1.1 1.2
14-40+ 0.9-1.03 0.5-1.0
10-25 1.01 0.9-1.3
Simoneit
&Lonsdale
1982,
Simoncit & Kawka
pn
Kvenvoldenggl.
1986
SinoneitetiL 19&/a
Bnultqgl. 1985
&atiltqgl. (unpublished
data)
Brault&Simneit 1988
23 74
15 2r
3
65
n.d.
n.d.
n.d.
n.d.
I CPI
- carbon
preference
inder, calcularcd here wer the
range C2 o Q3a.
2
Values in parenthes€s
aro total
yield p€r gran
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 levels in waters collected
within - 1 km above
the hydrothermal vents. The hydrocarbon pattern of
these
waters is indicative in many cases of pyrolysis
of bacterial matter in entrained ocean water during
PETROLEUM GENERATION 833
o) b)
Time
(relotive)
- 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.
TIME-
Frc. 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).
(l)
o
E
o
CL
u,
(l)
E
.9
CL
0
ttl
o
('
E
o
E
()
o
o
(t
UJ
o
z
o
!J
t
i
t
Y
a
E
a,
u,
cooling of discharging fluids (Brault et al. 1988).
Extensive hydrothermal
activity and associated
sul-
fide deposits
have also been described
at 2loN on
the EPR where the crust
is unsedimented
(Spiess
el
ol. 198O, Ballard et al. l98l). The hydrocarbon con-
tents of massive sulfides from vent chimneys are
extremely
low but definitely thermogenic (Table 2).
A GC trace
for total hydrocarbons
is shown
in Figure
6 (Brault e/ a/. submitted). The r-alkanes in mas-
sive sulfides range from Cl4 to greater
than C4u,
with no carbon-number predominance; a pyritized
tube worm from a chimney
has a slight odd-carbon-
number predominance. All samples
contain PAH,
providing evidence
for hydrothermal activity. Cou-
pled with the carbon-number maxima at n-Cy, or
higher, this indicates that the hydrocarbons were
entrapped/condensed
in a high-temperalure regime
such
as an active
chimney.
The sample
with pyritized
tube worm residues
(Fig. 6) also contains
hydrother-
mally altered derivatives of biomarkers (e.9.'
cholestenes,
hopenes)
from the vent biota, i.e., prob-
ably mainly tube worms and bacteria.
Atlsntis II DeeP, Red Seq
The Atlantis II Deep contains stratified brine
layers, the deepest
of which is at a temperature
of
62'C (Hartmann 1980,
1985).
Bulk organic
matter
and hydrocarbons have been analyzed in two sedi-
ment cores
(No. 84 and12'6,
CHAIN 6l cruise)
from
the Deep (simoneit et al. 1987).
Although the brine
overlying
the coring areas
is reported
to be sterile,
sedimentary organic material derived from
autochthonous marine planktonic and microbial
inputs and minor terrestrial
sources
is present.
The
Time
(relotive)
.-*
3f ee
3e
ao
er
834 THE CANADIAN MINERALOGIST
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).
o
o
c
o
o
o
o)
g
,=
cl
o!
o
E
o
@
(9
I
o
o
o
o
.:
6
t
Timo*
Frc. 8. Gas
chromatoetrams
of (a) aliphatic and (b) aro-
matic hydrocarbons
from an extract of a massive
sphalerite sample from the
Mid-Atlantic Ridge
(Brault
& Simoneit 1988).
organic input derived from the water column above
the brine is further metabolized by microorganisms,
and the reworked compounds with organic detritus
are apparently then incorporated into the sediments
under the brine by sinking as
material adsorbed or
attached to particles
of metallic oxide
precipitates.
Low-temperature maturation in the sediments
results in petroleum generation, even from low
amounts
of organic
matter (average
0.0590).
Both
steroid and triterpenoid hydrocarbons (biomarkers)
show that extensive acid-catalyzed reactions are
occurring in the sediments.
In comparison
with other
hydrothermal systems driven by intrusions (e.9.,
Guaymas Basin; Cape Verde Rise, Simoneit et a/.
1981), sediments
in the Atlantis II Deep exhibit a
lower degree of thermal maturation, as is clearly
shown by the elemental
composition of the kerogens
and the absence
of pyrolytic PAH in the bitumen.
The lack of carbon-number
preference
among the
n-alkanes
(CPI : 1.0) suggests,
especially
in the case
of the long-chain
homologs
(e.g., Fig. 7
, Table 2),
that the organic matter has been affected by cata-
genesis.
However, the yields
of hydrocarbons
with
respect to sediment
weight are
much lower than those
observed
in other hydrothermal areas.
The low tem-
perature
and low organic-carbon
content ofthe sedi-
ments
in the Atlantic II Deep seem to be responsi-
ble for this difference.
Mid-Atlantic Ridge, TAG areo 26"N
The Trans-Atlantic Geotraverse
CIAG) hydrother-
mal field on the Mid-Atlantic Ridge
crest at 26oN
is one of two active vent systems
known from slow-
spreading
oceanic
ridges
(Ronaet ol. 1984, Thomp-
son et al. 1988).
Hydrothermal deposits
lying directly
on oceanic crust have been dredged from the area
(TAG 1985-1).
Three sulfide-rich samples,
consist-
ing mainly of anhydrite, sphalerite,
and chalcopyrite'
respectively, contained
minor amounts
of the more
volatile (Cro-Cd hydrothermal petroleums (Brault
PETROLEUM
GENERATION
TABLE 3. FEATIJRES OF IIYDROTIIERMAL PE'TROLEI,'M
COMPARED
TO
RESERVOIRPETROLET,JM
1. Nanral gas
and
gasoline-range
$rdrocarbons
2. Full range of g-alkanes,
no
carbon-number-predminance
(CPI
= 1.G12)
3. Naphtbenic co'mponents
(ndorhury of UOvf)
4. fropenoid hydmcartons
(nclding signmcant
pdstane
and
phytane)
5. Biomaten (e.g.
lrusre l7o(Hlhopades and st€ranes)
6. Allrylsmadc hydrocsbons and
asphaltenes
tlydm'therml Perolsm mly:
1. Polynucler aromatic by<kocarbos
(PAII) > alkyl armaic hydrocarbons
2. Residual imnaane biomarken and
internediares
(ag. 178(tl)-hopanes'
hopenes'
gerenes)
3. Significatrt
amatic sulfirr
hetero conrpounds
4. Itrghsulfincontcnt
5. Alkene content near
"sourcerock"
835
& Simoneit 1988). The saturated and aromatic
hydrocarbon fractions separatd from the extract of
the sphalerite sample are shown in Figure 8. The
z-alkanes
range
from Cttto C2twith a CPI = 1.0;
pristane
and phytane
are
present,
and the UCM max-
imizes at the GC retention time for r?-C17. This pat-
tern is analogous
to that observed
for samples
from
the
EPR at l3oN and
from the
Atlantis II Deep. The
aromatic fraction, which contains naphthalene,
phenanthrene,
their alkyl homologs and sulfur aro-
matic compounds,
supports
a hydrothermal
origin.
DIscussIoN
Petroleum generation
The principal zone of petroleum formation in
sedimentary
sequences
under normal geothermal
gra-
dients extends
from about I km to 3 km depth (e.9.,
Hunt 1979, Tissot & Welte 1984).
This depth cor-
responds to a temperature
range of 50"-120'C. The
effect of pressure
on this process
is 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).
These
proposed
tempera-
ture regimes
for the oil and
gas
"windows" may need
some adjustment in light of more recent data on
hydrothermal systems
and ultra-deep wells.
The "instantansous" petroleum generation in
hydrothermal systems
is a facile process
which occurs
at temperatures
approaching a maximum of 400"C.
At such high temperatures, organic matter is only
partly destroyed,
probably because
the thermogenic
products are rapidly removed from the hot zone.
Formation of hydrothermal
petroleum
seems
to com-
mence
in low-temperature
areas,
generating
products
from weaker
bonds (e.g., ether, sulfide, carbonyl,
tertiary carbon linkages).
As the temperature
regime
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 difference
is the enhanced
con-
tent of PAH and sulfur in the hydrothermal
products.
Organic matter associated
with deeper
hydrother-
mal systems
(e.g., epithermal
ores
in volcanic ter-
ranes)
is typically more asphaltic
and has a high PAH
content. Such organic matter is widely distributed
(e.g., California mercury
deposits:
Geissman
el a/.
1967, Blumer 1975; other hydrothermal sulfides:
Germanov & Bannikova 1972). Deep well drilling
(>7000 m) has intersected
Cretaceous
shales
which
were at in situ temperatrues
of about 260'-300oC
(e.g., Price et sl. 1981,
Price 1982).
These samples
were
rich in bitumen components
and their kerogens
still had significant hydrocarbon-generation poten-
tial. This indicates that in sila petroleum is stable
at much higher temperatures and pressures
than
those
discussed
above,
and over long geologic
time
periods.
During mineral diagenesis and metamorphism
under
non-oxidizing
conditions,
the organic-matter
composition changes
progressively
to more aromatic
and asphaltic residues by the expulsion of volatile
components
(e.g., CO2,
CH4, H2O, etc.). The car-
bonaceous
residues
are often associated
with heavy-
metal enrichments,
as for example
uranium (e.g.,
Schidlowski 1981)
or Carlin-type gold and silver ores
836 THE CANADIAN MINERALOGIST
(e.9., Radtke & Scheiner 1970).
Heavy aromatic
hydrocarbons
(PAH), present
in all the hydrother-
mal sites described here, are a product of high-
temperature
alteration and thus may be
good indi-
cators of such alteration peripheral
to sulfide ore-
bodies
(e.9., Germanov
& Bannikova 1972,
Blumer
r975).
Organic-motter type
The constitution of the initial organic matter deter-
mines the types of petroleum products that form in
basins with a normal geothermal gradient, and in
hydrothermal systems. The major source of
petroleum
compounds is kerogen,
the sedimentary
macromolecular organic detritus which generally
constitutes
the bulk of the total organic carbon con-
tent (Tissot
& Welte 1984, Table 1). In general,
ter-
restrial detritus from mainly vascular plants yields
an aromatic kerogen
(e.9., coal) which has
a natural-
gas potential, whereas
marine/lacustrine organic
matter from primarily microbial and planktonic
residues
yields
an aliphatic kerogen
(e.9.,
sapropel)
which has a paraffinic petroleum potential (Hunt
1979,
Tissot & Welte 1984). Kerogens in sedimen-
tary basins are always mixtures of these inferred end
members.
Syngenetic sedimentary lipid matter undergoes
to 20 30 40 50 60
TIME
(min)+
Frc.9. Gas chromatograms of total hydrocarbons in
extracts from surface sediments of (a) Guaymas Basin,
Site 30G (Simoneit et al. 1979), and (b) Escanaba
Trough (Kvenvolden et al, 1986),
alteration during diagenetic and early catagenetic
processes;
this changes
the hydrocarbon signature
.(Fig. 9). During oil generation, however, large
amounts
of additional hydrocarbons are superim-
posed
on the syngenetic
lipids, thus obscuring the
earlier
signature
(e,9,,
loss or reduction of the odd-
carbon-number predominooce )C25; cf. Fig.9
versas Figs. 3, 4). Syngenetic
lipids of sediments
can
usually be utilized to elucidate the various sources
of the total organic matter (e.g., Simoneit 1978,
1981, 1982a).
For example,
lipid hydrocarbons of
normal sediment in Guayrnas Basin (Fig. 9a) con-
tain r-alkanes
mostly <C21,
and a minor series of
homologs >C1 with a strong odd-carbon-number
predominance.
This composition is typical of a
predominantly marine planktonic and microbial ori-
gin, and a minor influx of terrigenous
vascular
plant
wax (>C25). By contrast, lipid hydrocarbons
of
sediment
in the Escanaba Trough (Fie. 9b) contain
n-alkanes,
mostly >C, with a strong
odd-carbon-
number predominance.
This signature
is derived
mainly from terrigenous vascular
plant wax, with a
minor microbial component (Kvenvolden et al.
1986). In both the Guaymas and
Escanaba samples,
the compositions of the kerogens support the
interpretations based on the nature of the lipid
hydrocarbons
(Simoneit
1978, Simoneit
et ol. 1979).
Migration processes
The aqueous solubility
of petroleums
and
various
hydrocarbon fractions has been determined
experimentally
(Price 1976). Petroleum solubility
increases exponentially
from l@ to 180'C and solu-
bilities are high enough
to account
for the forma-
tion of petroleum
reservoirs by migration of molecu-
lar or co-solutions.
Increased
salinities
of 1507* NaCl
cause
drastic exsolution of lhe petroleum and at
3500/oo alrnost total "salt-out " occurs
(Price 1976).
This finding is consistent
with the requirement for
the separation
of petroleum
from migrating solutions
in the salty waters of reservoir sands.
It has been
demonstrated
that methane
in the presence
of water
is an even better carrier for petroleum
than water
or methane alone @rice et al. 1983).
Both increases
in pressure (to about 1800 bar, 1.8 x ld Pa) and
temperature (to 250'C) raised the solubility of
petroleum.
Cosolubility
was found at rather
moder-
ate conditions
(e.g., 100"C at ld Pa, 200'C at 0.5
x lS Pa). The addition of other gases
such as CO,
and ethane
to this mixture also increases the solu-
bility of petroleum.
Under these experimental
con-
ditions, CH4, C2H2
and CO, are all supercritical
and HrO approaches
its near-critical state; thus, the
gases and HrO are all mutually soluble by the
reduced hydrogen bonding
in water, and are a good
cosolvent for petroleum. Therefore, primary migra-
tion seems to proceed
as
gaslfluid and aqueous
solu-
tion (see
also Hunt 1979).
U
q
z
c
a
U
E
-
o
P
&
E
o
a
ffi ztzcru26"r28
PETROLEUM GENERATION 837
CoNcrustoNs
Recent immature sediments
receive
biogenic detri-
tus which, upon deposition, undergoes diagenetic
and additional microbial alteration. Increasing
burial
in sedimentary basins results in the onset of organic-
matter maturation, which generates
some volatile
products from the kerogen
(easily
cracked moieties)
that become added to the endogenous
lipid residues.
This is the beginning of petroleum
formation. As the
depth of burial (i.e., temperature) increases,
cata-
genesis
commences and 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 gases
and residual
amorphous carbon.
In the case of hydrothermal systems, these
processes
are compressed into an "instantaneous"
geological
time frame. At seafloor spreading axes,
hydrothermal systems
operating below a sediment
blanket (e.9., Guaymas Basin
and Escanaba
Trough)
generate petroleum
from generally
immature organic
matter in the sediments. This petroleum then
migrates upward, leaving behind a spent carbona-
ceous
residue. The Guaymas
petroleums
€ue
com-
prised
of: l) gasoline-range
hydrocarbons
(C1-C1);
2) a broad distribution of n-alkanes
(C12-Ca6+)
with
no carbon-number
predominance;
3) a naphthenic
hump, UCM; 4) pristane
and
phytane
at significant
concentrations; 5) mature biomarkers (e.9. a-
hopanes); and 6) significant concentrations of PAH
and sulfur. Exterior exposed petroleum, and
petroleums
in unconsolidated surface sediments, are
microbially deeraded and leached, whereas interior
samples are chiefly unaltered.
Hydrothermal systems operating in unsedimented
rift areas
(e.9.,
EPR at 13'N and
2loN, Mid-Atlantic
Ridge at 26'N) generate
trace amounts of petroleum
and emit methane.
Low amounts of petroleum
are
generated
by pyrolysis
of suspended and dissolved
biogenic organic detritus (including bacteria and
algae) entrained during the turbulent cooling of the
vents
both for sedimented and bare-rock systems.
However, this type of petroleum
is swamped in the
former case by the large
quantify of petroleum gener-
ated from the sedimentary
organic matter. In addi-
tion, low-level maturation is observed in the sur-
rounding area at 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 kerogen complement
each other in providing information about the
sources and thermal history of sedimentary organic
matter. Kerogen
is a sensitive in situ rndtca;tor
of tem-
perature, and petroleum (bitumen including asphalt
and gas) represents the product mixture of high-
temperature stress. These products may have
migrated or remained in situ.
ACKNOwLEDGEMENTS
I thank the Deep Sea
Drilling Project for access
to DSDP samples, and the National Science
Foun-
dation for funding participation on the D.S.V.
ALVIN cruises. Samples, data and assistance were
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 Ocean Sciences,
National
Science
Foundation
(Grants
OCE81-18897,
OCE-8312036, OCE-8512832
and
OCE-8601316) is
gratefully acknowledged. I thank two anonymous
reviewers for comments and especially T. Barrett for
excellent editorial assistance
to make
this paper
intel-
ligible for geologists.
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