Applied Geochemistry. Vol, 5, pp. 65-69, 1990 0883-2927/90 $3.00 + .00 Printed in Great Britain Pergamon Press plc

Carbon isotope composition of hydrothermal petroleums from Guaymas Basin, Gulf of California

M . SCHOELL a n d R . J . HWANG

Chevron Oil Field Research Company, La Habra, CA 90631, U.S.A.

a n d

B. R. T. SIMONEIT Petroleum Research Group, College of Oceanography, Oregon State University, Corvallis, OR 97331,

U.S.A.

Abstract--Petroleum hydrocarbons are a major emanation from the active hydrothermal vent fields of Guaymas Basin. The samples we describe here occur in hydrothermal mound crusts which have been collected in situ with a submersible. The relative concentrations of the compound classes in the hydrothermal petroleum are highly variable with the following ranges: saturate hydrocarbons 1-60%; aromatic hydrocarbons 18-38%; NSO-compounds 10-27%; and asphaltenes 0.2-25%. The sulfur concentrations range between 1.2 and 3.4%. The S-rich hydrothermal petroleums tend to be low in saturated hydrocarbons and high in NSO and asphaltene components and vice versa. Stable C isotope

13 concentrations in whole hydrothermal petroleum vary little between 6 C = - 21.2 to -22.2%o vs. PDB. Carbon isotope patterns for the compound classes are similar to typical crude oils with the saturated hydrocarbons being depleted in 13C ( - 22.9 to - 21.7%o) and asphaltenes being relatively enriched ( - 20.5 to -20.8%0). The variability decreases from saturates to aromatics, NSO and asphaltenes. This indicates that the hydrothermal petroleums originate from one source. The higher variability in the non-asphaltic compound classes is attributed to distillation and/or biodegradation effects. Organic matter in recent unaltered sediments in the immediate vicinity of the hydrothermai vents has an average 613C of approximately -20.5%0, in a range consistent with their being the source for the oils. This similarity supports the contention that the hydrothermal petroleums are pyrolysis products of the immature sediments which are permeated by the hydrothermal fluids.

INTRODUCTION

THE HYDROTHERMAL vents in the Guaymas Basin (Fig. 1) are of special interest to pe t ro leum geo- chemists because they represent a natural laboratory where the formation of pe t ro leum occurs at a hot spot in the earth 's crust. Hot fluids, hea ted by intrusive and magmatic basalts, conver t organic mat ter in the young unconsolidated sediments to pet roleum-l ike products which occur in the vent systems, in the mineral mounds and some few cent imeters below the surface in surrounding sediments (StMONEIT and LONSDALL 1982; SIMONEtT, 1983a, 1985a; KAWKA and SIMONEIT, 1987). Al though it is generally agreed that these oils are derived f rom convers ion of organic mat ter in these rift sediments (SIMONEIT, 1983a,b, 1985a), no conclusive geochemical correlat ion has yet been conducted that unambiguously establishes the origin of the oils from the sedimentary organic matter. Here we present results of C isotope analyses on the hydrothermai vent pe t ro leums and compare those to isotope data for sediments in the vicinity.

SAMPLING AND METHODS

Samples were collected during dives 1170, 1172 and 1177 of the Deep Submergence Vehicle (D.S.V.) Alvin (Fig. 1).

Various chimneys from hydrothermal mounds were sampled with the manipulators and sediments near mounds were cored. Tar-like, highly odorous petroleum products were evident in the samples, which were stored in methyl- ene chloride or chloroform with methanol to dissolve the hydrothermal petroleums and to prevent biodegradation, A dredge sample (7D-SA), recovered from earlier dredging of a mound on the sea bottom is included in this study.

Laboratory procedures consisted of standard solvent ex- traction and HPLC separation of compound classes. The hydrothermal petroleums were deasphaited by addition of hexane prior to fractionation. The polar fraction was removed by running the deasphalted sample through a Waters silica SepPak, using hexane as eluent. The remain- ing hydrocarbons were fractionated into saturates and aro- matics, using a Waters HPLC equipped with a Whatman silica column (50 cm x 9.4 mm i.d.). The saturates and aromatics were eluted with hexane at a flow rate of 2 ml/min and were monitored with RI and UV detectors, respect- ively. Stable C isotope analyses were performed on an automatic oxidation and trapping system attached online tO a Finnigan Delta E mass spectrometer. Longterm analytical reproducibility of the NBS 22 standard is 6J"C = - 29.75 + 0.04%0 relative to Peedee belemnite. The S analyses were performed by the normal microcombustion method.

RESULTS AND DISCUSSION

Composition of the hydrothermal petroleums The composit ions of the hydrotherrnal pe t ro leums

vary considerably (Fig. 2 and Table 1). The samples

65

66 M. Schoell et al.

111 * 26'W 24'W 22'W

0,. / v2_0 ~¢" Y').t W~--- "/./Locations

@ 7D 2 7 ° ~ ~ Mound I~ ~.~ s.,to. 00'N

~ . ' \ '~ r rougt l , l klTl, / " ' * Hydrothermal Patch ~ , Y~'I :~ Faul' Trace

30"N . Oelfln Basin

Location ' ~";" ~ J - ' ~ ' f

Ft6.1. Location of hydrothermal petroleum samples in the southern troughs of Guaymas Basin. Gulf of California. Sample locations and DSDP holes are indicated.

can be ordered by decreasing relative concentrations of saturates (a--d in Fig. 2). Parallel to the decrease of the saturates, both the n-alkane distributions and their total concentration change. The sample highest in saturates has n-alkanes with an almost normal distribution around n-C23. Sample 1172-1A, which is lower in saturates, has an alkane maximum around n-C15. Samples 1177-2C and 7D-5A (the dredge sample) show a clear decrease in n-alkanes most likely due to biodegradation (SIMoNErr, 1985b). The S concentrations of the oils range from 1.2 to 3.4% (Table 1). These values are not atypical compared to normal crude oils. The S-rich hydrothermal pet- roleums (e.g. 7D-5A) are generally low in saturates and higher in NSO and asphaltene components, and vice versa.

Carbon isotopic composition

The C isotopic compositions of the compound classes (Table 2 and Fig. 3) show a pattern similar to that typical for crude oils. The saturated hydro- carbons tend to be the most t3C depleted fraction and the ]3C concentration systematically increases in the aromatic hydrocarbons, NSO-compounds and asphaltenes. Some details are of interest. First, the isotopic variability is greatest in the whole oil and is

lower in the saturates, aromatics, NSO compounds and asphaltenes. Second, the NSO compounds and asphaltenes are isotopically very similar to each other and the asphaltenes show a variability almost within analytical error, with a mean isotopic composition of -20.7%o, The isotopic composition of the whole oil is controlled primarily by the compound class compo- sition. This is particularly evident for oil 7D-5A (Figs 2d and 3d) which is composed predominantly of aromatics, NSO compounds and asphaltenes. Conse- quently, the whole oil is isotopically very close to the mean isotopic composition of the aromatics, NSO compounds and asphaltenes. Oil 1172-1A is almost identical in its bulk isotopic composition to the satu- rates. The isotope mass balance suggests that the proportion of the saturates in the oil is much higher than indicated by the compound class composition (Table 1). This is likely due to evaporative loss of saturates during sample preparation which could lead to an increase in the relative amount of the polar compounds. Oil 1172-1A is rich in volatile com- pounds (<n-C17) which are easily affected by losses during sample handling. Oil 1177-2C is more de- pleted in 13C than any of its compound classes (Table 2). This cannot be explained by sample handling effects. At present, it is not clear why the 6 t3 C of this oil is so different from its separated compound classes.

C isotopes in hydrothermal petroleum, Guaymas Basin 67

0 1170-1At"

1172-1A

1172-2C

20 40 60 80

Ph

C17

C17 ~j,- Cla

Pr

Pr

Ph C17~ ~r~C~e

Ii. , - . ~ \ "

10100 20100 30.00 40100 Time (rain)

FIG. 2. Compound class composition and capillary gas chromatograms of the hydrothermal petroleum samples. Note the shift to n-alkanes with lower chain length in sample b compared to sample a and the

lower concentrations of n-alkanes in samples c and d (Pr = pristane, Ph = phytane).

Carbon isotope variations in source kerogens

For comparison with the hydrothermal petroleum samples from Guaymas Basin, we use published stable C isotope data from Deep Sea Drilling Project (DSDP) Holes 477 and 477A (JEt, DES et al., 1982; SXMONErr et al., 1984). These kerogens were isolated from samples retrieved from 50 to 200 m sediment depths. Both holes penetrated a basalt sill which had intruded the sediments and resulted in contact meta- morphism and pyrolysis of the organic matter (Fig. 4). Anomalously high temperatures were recorded in borehole 477A with a bottom temperature of > 135°C at 267 m depth. The kerogens show a large variability in atomic H/C ratios and C isotopic compositions (Fig. 4). Organic matter in the shallow sections of the holes, which was unaffected by thermal alteration, shows consistent high H/C ratios and C isotopic compositions between - 2 0 and -21%o. In the inter- mediate vicinity of the basalt sill, and in particular in

the lower section of Hole 477A, a systematic trend to more negative isotope values and lower H/C ratios are observed (Fig. 4). Isotopically, the kerogens in the unaltered sediments ( - 2 0 to -21%0) are close to the composition of the hydrothermal oils ( - 2 1 to -23%0).

Data interpretation

Secondary alteration of the hydrothermal pet- roleums in the sediments likely accounts for their compositional variability. Specifically, the removal of n-aikanes is typically an effect of biodegradation (SIMoNEIT, 1985b). Samples 1172-2C and 7D-5A are affected most by microbial action (Fig. 2). Samples 1170-1A and 1172-1A seem to be unaffected, or only mildly affected, by biodegradation (Fig. 2). The shift of the n-alkane envelope of sample 1172-1A to a maximum at Ct5 could have resulted from exposure

Table 1. Chemical composition of the hydrothermal petroleums from Guaymas Basin

Chemical composition

Saturates Aromatics NSO compounds Asphaltenes Per cent Sulfur Sample (%) (%) (%) (%) recovered (%)

ll70-1A 61.3 17.8 10.4 0.2 89.7 2.44 l172-1A 42.3 22.9 13.7 1.6 80.5 1.23 1177-2C 24.5 26.9 21.9 19.8 93.1 1.63 7D-5A 1.1 33.4 27.7 25.4 87.6 3.42

68 M. Schoell et al.

®

Whole Oil

Saturates

Aromatic HC

NSO-Compounds

Asphaltenes

- 2 3

:)

i ~ v

I I - 2 2 - 2 1 - 20

~;3 C (00,)

(~) • 1170-1A ] Hydrothermal (~) • 11~/2-1A I Mound ~) • 1177-2C Crusts (~) * 7D-5A Dredge Sample

FIG. 3. Carbon isotope variations in compound classes of the hydrothermal petroleums and in the whole oils of Guaymas

Basin.

to higher temperature or from processes similar to distillation.

Isotopically, the oils are very similar to each other suggesting that they are derived from one source. The most reasonable source of the hydrothermal petroleums is the surrounding sediments of the vent area. A comparison of the isotope variations in the hydrothermal petroleums with those of the sediment kerogens (Fig. 4) indeed shows the same range of isotope values. The low atomic H / C ratios of the kerogens from Guaymas Basin sediments (Fig. 4, samples 4--6 and 8-10) reported by JENDEN et al. (1982) were obviously caused by the effect of heat. The considerable decrease of the H/C ratios suggests that some of the kerogen in the samples has already been converted to bitumen or oil.

The altered sediment samples with low H/C ratios tend also to be depleted in 13C. Heating and forma- tion of oil should, however, have the opposite effect

and should lead to an enrichment of 13C in the kerogens as labile pyrolysis products tend to be depleted in t3C relative to kerogen (LEWAN, 1983). The lower I3C concentrations in the altered kerogens could be due to removal of 13C-enriched humic acids or alternatively to the presence of minor refractory terrestrial components, which also might be isotopi- cally more depleted in 13C than marine organic mat- ter. It should be noted that humic acids were not removed from the DSDP sediments analyzed by JENDEN et al. (1982) and they could comprise up to ~20-30% of the total organic C in the surface sedi- ments of Guaymas Basin (SIMONEIT et a l . , 1979). Conversion (and expulsion as petroleum) of pre- dominantly H-rich, 13C-rich marine kerogen would leave a relatively 13C-depleted residual altered kero- gen. For purposes of correlating the hydrothermal petroleums in the vent mounds, it is, therefore, more reasonable to compare only the unaltered sediments with high H/C ratios with these oils (Fig. 4, samples 1-3 and 7).

A comparison of the unaltered kerogens with the hydrothermal oils shows an almost perfect isotopic fit between the polar compounds of the oils and the bulk isotopic composition of the kerogens. We consider this to be a clear indication of a genetic link between the predominantly marine component of the organic matter in the young unaltered sediments of Guaymas Basin and the petroleum-like products in the hydro- thermal emanations. The oils in the Guaymas Basin hydrothermal system are therefore the product of a natural pyrolysis process in the subsurface induced by hot percolating waters. This natural pyrolysis pro- ceeds rapidly at the observed temperatures of the waters (up to ~320°C). Complete conversion of ker- ogen at these temperatures is attained in the labora- tory under hydrous pyrolysis conditions over a time period of about 72 h (JENDEN et al . , 1982; LEWAN, 1983; SIMONEIT et al . , 1984). This observation, together with the positive correlation between the Hole 477 sediments and the hydrothermal oils is, in our opinion, clear evidence that these oils are derived from hydrothermal pyrolysis of young marine sedi- ments.

Table 2. Carbon isotopic composition of the hydrothermal petroleum and their compound classes

613CpDs (%o)

Sample Whole oil Saturates Aromatics NSO compounds Asphaltenes

II70-1A -22.1 -22.3 -21.3 -21.1 -22.2 -21.3

1172-1A -22.8 -22.9 -22.1 -21.4 -20.5 1177-2C -22.2 -21.9 -20.1 -21.5 -20.7

-22.3 -21.5 -22.3 -21.3

7D-5A -21.2 -21.7 -21.8 -21.4 -20.8

mean -22.2 -22.2 -21.3 -21.4 -2(I.7

C isotopes in hydrothermal petroleum, Guaymas Basin 69

~13 C (%.)

-24 -23 -22 -21 -20

~T

Nso

,o ° 2ff2',2!22_

H,C 0.8 !~/L~/ 1 Altere

0.6

0.4

- 2 4 - 2 3 - 2 2 -21 - 2 0

;~13 C Kerogen I%*)

FIG. 4. Carbon isotopic oil-to-source correlation of hydro- thermal petroleum fractions with sedimentary organic mat- ter from the vicinity. Data for the sedimentary kerogens are from JENDEN et al. (1982) (the histograms represent the data

in Table 2).

C O N C L U S I O N S

Stable J3C concentrat ions in petroleum-l ike ema- nations f rom hydrothermal vents of the Guaymas Basin are almost identical to those of kerogens in unal tered marine sediments in the immediate vicinity of the vents. This is considered to be clear evidence for a genetic link be tween hydrothermal oils and sediments , i.e. that the oils are formed by hydrous pyrolysis in the subsurface. The Guaymas Basin hydrothermal system is a natural pyrolysis laboratory where oil formation can be studied, geologically speaking, in "zero time.'"

Acknowledgements--We thank Chevron Oil Field Re- search Company for financial support and permission to publish these results. We thank M. A. Beeunas and J. Ortis for analytical support, and P. D. Jenden and K. E. Peters for reviewing the manuscript.

REFERENCES

JENDEN P. D., SIMONEIT B. R. T. and PHILP R. P. (1982) Hydrothermal effects on protokerogen of unconsolidated sediments from Guaymas Basin, Gulf of California: Elemental compositions, stable carbon isotope ratios, and electron-spin resonance spectra. In Initial Reports of the DSDP (eds J. CURRAY, D. MOORE et al.), Vol. 64, Part 2, pp. 905-912. U.S. Government Printing Office, Wash- ington, D. C.

'KAwKA O. E. and SXMONEIT B. R. T. (1987) Survey of hydrothermally-generated petroleums from the Guay- maR Basin Spreading Center. Org. Geochem. 11, 311- 328.

LEWAN M. D. (1983) Effects of thermal maturation on stable organic carbon isotopes as determined by hydrous pyrolysis of Woodford Shale. Geochim. cosmochim. A cta 47, 1471-1480.

SIMONEIT B. R. T. (1983a) Effects of hydrothermal activity on sedimentary organic matter: Guaymas Basin, Gulf of California--petroleum genesis and protokerogen degra- dation. In Hydrothermal Processes at Seafloor Spreading Centers (eds P. A. RONA, K. BOSTROM, L. LAUBIER and K. L. SMITH, JR.), pp. 451-471. Plenum Press.

SIMONEIT B. R. T. (1983b) Organic matter maturation and petroleum genesis: Geothermal versus hydrothermal. In Proc. Symp., The Role of Heat in the Development of Energy and Mineral Resources in the Northern Basin and Range Province, pp. 215-241. Geotherm. Res. Council, Special Report No. 13, Davis, California.

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SIMONEIT B. R. T. (1985b)Hydrothermal petroleum: Com- position and utility as a biogenic carbon source. In Hydro- thermal Vents of the Eastern Pacific: An Overview, (ed. M. L. JONES). Bull. Biol. Soc. Wash. 6, 49-56.

SIMONEIT B. R. T. and LONSDALE P. F. (1982) Hydrothermal petroleum in mineralized mounds at the seabed of Guay- mas Basin. Nature 295, 198-202.

SIMONEIT B. R. T., MAZUREK M. A., BRENNER S., CRISP P. T. and KAPLAN I. R. (1979) Organic geochemistry of recent sediments from Guaymas Basin, Gulf of Califor- nia. Deep-Sea Res. 26A, 879-891.

SIMONEIT B. R. T., PHILP R. P., JENDEN P. D. and GALIMOV E. M. (1984) Organic geochemistry of Deep Sea Drilling Project sediments from the Gulf of California-- hydrothermal effects on unconsolidated diatom ooze. Org. Geochem. 7, 173--205.