Carbon isotope systematics of individual hydrocarbons in hydrothermal petroleum from Escanaba Trough, northeastern Pacific Ocean

Pergamon PII: S014643f30(97)00042-9

Org. Geochem. Vol. 26, No. 718. pp. 5 I I-515. 1997 ~0 1997 Elsevier Science Ltd. All rights reserved Printed in &eat Britain 0146-6380197 $17.00 + 0.00

NOTE Carbon isotope systematics of individual hydrocarbons in hydrothermal petroleum from Escanaba Trough, Northeastern Pacific Ocean BERND

R. T. SIMONEIT’*,

MARTIN

SCHOELL’ and KEITH

A. KVENVOLDEN’

‘Petroleum and Environmental Geochemistry Group, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, U.S.A., *Chevron Petroleum Technology Company, 1300 Beach Blvd. La Habra, CA 90631, U.S.A. and ‘U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, U.S.A. (Received April 1997; accepted 28 May 1997) Abstract-We submitted individual aliphatic and polycyclic aromatic hydrocarbons in samples of hydrothermal petroleum from Escanaba Trough to compound specific isotope analysis to trace their origins. The carbon isotope compositions of the alkanes and polycyclic aromatic hydrocarbons (means -27.5 and -24.7%, respectively) reflect a primarily terrestrial organic matter source. 0 1997 Elsevier Science Ltd Kq ~ords~ompound Trough

specific isotope analysis, alkanes, PAH, hydrothermal

petroleum, Escanaba

(Kvenvolden et al., 1987, 1994; Magenheim Gieskes, 1994).

INTRODUCTION

Hydrothermal vent systems at sediment covered ridges are of interest to petroleum geochemists because they are natural laboratories where active petroleum generation (hydrous pyrolysis of immature organic matter), expulsion and migration can be studied (Simoneit and Lonsdale, 1982; Simoneit, 1993; Kvenvolden et al., 1986, 1987). The prove-

and

Extraction

The frozen samples were thawed and crushed or broken into smaller pieces prior to extraction with a methylene chloride (or chloroform) and methanol mixture (3:l). Exhaustive extraction was carried out by repeated ultrasonic agitation of solvent aliquots over a bulk sample. Two samples were extracted and separated for a prior study by Kvenvolden et al. (1986, 1987). The extracts were concentrated and asphaltenes precipitated by addition of excess hexane. Aromatic and saturated hydrocarbons were separated from the deasphaltened samples by column chromatography on neutral alumina over silica gel (3.8 g of each resulting in a column length/width ratio of 15/ 1). Separation was by elution with heptane (30 ml as Fl), toluene (35 ml as F2), and then methanol (35 ml as F3) into the respective fractions.

nance of the source organic matter may be primarily marine (e.g., Guaymas Basin, Simoneit and Schoell, 1995) or terrigenous (e.g., Escanaba Trough, Kvenvolden and Simoneit, 1990). The carbon isotopic compositions of the bulk organic matter or those of individual compounds can further define the origins of the source organic matter. Here we present results of compound specific isotope analyses (CSIA) on the hydrocarbons of four samples of hydrothermal petroleum from Escanaba Trough, northeastern Pacific Ocean (Fig. 1).

Instrumental analysis EXPERIMENTAL

Samples

Hydrothermal petroleum was derived from samples collected by dredging and by minicoring on dives by the Deep Submergence Vehicles (D.S.V.) Alvin and Sea Cliff at the locations shown in Fig. 1 *Author to whom correspondence should be addressed.

Compound Specific Isotope Analyses (CSIA) of carbon by online gas chromatography-isotope ratio mass spectrometry were conducted on the aliphatic and aromatic hydrocarbon fractions (Fl and F2, respectively). The CSIA were performed on a modified version of a Finnigan GC-C-MS system (Hayes et al., 1988, 1990; Schoell et al., 1992). The data system was ISODAT version 4.0. The gas flow and the combustion interface were completely

511

512

Note

41°11

127"40

127'30

127"2O'W

66%Rl 2040~~_’ 89

41~00'1

14D-6H

40"5C

\

SESCA 32D-2.

.DSDP35

40"40

km

40"30

:

3

sndocino

,

contour

CALIF.

I

Fracrure

Zone

I

I

Interval

500 m

I

Fig. I. Location and index map for the sample sites in Escanaba Trough (adapted from Kvenvolden rl (Il., 1994). rebuilt for better resolution of complex mixtures with closely eluting peaks. A VALCO four way valve was installed at the end of the GC-column, but within the GC oven, for easier diversion of gas flows. The oven is a quartz capillary (0.1 mm i.d.) with a cupric oxide wire insert and uses a combustion temperature of 850°C. The GC conditions were as follows: Column, Hewlett-Packard Ultra 1 (50 m x 0.32 mm i.d., 0.5 pm stationary phase, cross linked methyl silicone); carrier gas, helium (1.5 ml/ min); temperature program, 3”/min from 60” to

320°C and 20 min hold at 320°C. A representative chromatogram as monitored at mass 44 by the mass spectrometer was shown in a previous report (Simoneit and Schoell, 1995). The standard was COz injected during analysis through the changeover valve of the mass spectrometer with or without sample background. The reproducibility of isotope values from different background corrections performed on one run alone is +0.5%0 (lo standard deviation). The carbon isotopic compositions of individual compounds were calculated relative to

513

Note Table I. Stable carbon isotope data for individual

hydrocarbons

in hydrothermal

petroleums

Samples 6°C Compound

Composition

Whole oil n-Alkanes Pentadecane Hexadecane Heptadecane Octadccane Nonadecane Eicosane Heneicosane Docosane Tricosane Tetracosane Pentacosane Hexacosane Heptacosane Octacosane Nonacosanr Triacontane Hentriacontane Dotrlacontane Tritriacontane Tetratriacontane Pentatrlacontane Hexatriacontane Heptatriacontane Mean (G-C&

659-R

I

Trough

14D-6H

2040-AC2

-24.8

-27.0 -27.2 * 0.8 -26.2 f 0.4 -27.0 + 0.3 -26.4 f 0.7 -27.2 f 0. I -27.4 _+0.6 -27.2 f 0.4 -27.2 + 0.2 -27.6 f 0.5 -28.0 f 0.6 -28.6 f 0.5 -29.0 f 0.9 -29.3 f 0.4 -29.3 _+0.9

Isoprenoids Norprlstane Pristane Phytane Mean PAH Phenanthrene Fluoranthene Pyrene MethylRuoranthene/pyrene Benzofluorene Dimethylfluorantheneipyrene Benzanthracene Chrysene Kethylchrysene Benrofluoranthenes Benzo(e)pyrene Benzo(a)pyrene Indeno( 1,2,3-cd)pyrene Benzo(ghi)perylene Methylbenzoperylene Coronene Mean *Standard

32D-2

from Escanaba

(%, vs. PDB)*

deviation

-25.4 + -25.3 -24.8 -24.9 -25.9 + -25.9 + -26.5 f -26.8 f -28.6 f -28.2 + -28.2 -28.2 t -28.8 f -28.5 f -30.0 + -29. I f -29.9 f -28.9 + -29.2 +

0.0

-27.7 f 0.9

-21.5 -+ 1.6

-21.4 -26.9 k 0.4 -26.9 + I .2 -27.3 + 0.3 -27.7 + 0.1 -27.4 _+0.9 -28. I + 0.8 -27.8 + 0.5 -27.8 + I.1 -28.3 + 0.4 -29.2 + 0.6 -29.6 f 0.6 -29.9 + 0.4 -29.5 + 0.6 -30. I f 0.8 -30.4 rt_0.9 -3o.o* I.1 -28.6 i 1.4 -28.8 f I .o -28.1_+ 0.7 -30. I * 0.4 -29. I -27.4 f 1.4

-26.3 + 0.2 -25.5 -25.9 f 0.8

-24.9 + I .3 -25.0 + 0.6 -25.0 _t 0.1

-24.5 + I. I -26.2k 1.1 -26.9 + 0.8 -25.9 f 1.3

-23.6 + 0.3 -23.3 + 0.5 -24.5 + 0.1 -23.8 _+ I .2 -24.6 f 0.7 -24.2 f 0.9 -24.0 -24.9 -25.2 -24.6 -24.9

f 0.3 + 0.7 + 0.8 f 0.0 _+0.9

-25.6 + I .O -24.4 + 0.6

0.2 0.4 I.0 0. I 0.9 0. I 0. I 0.0 0.4 0.3 2.1 0.2 1.6 2.6

-24.9 -24.2 -26.3 -24.6 -24.7

+ + + + +

1.7 0. I 2.9 0.3 0.4

--25.2 -24.4 k 0.4 --23. I -24.9 &- 1.4 --25. I -24.6 f 0.5 -25.2 + 0.6 --28.4 -25.0 f 0.8

is for n = 2-4.

the COZ standard and are reported in the usual delta notation vs. the PDB standard. The reproducibilities in Table 1 are from 2-4 repeat analyses and range between 0.1 and 2.9X, indicating that factors other than data reduction procedures affect the reproducibility (e.g., peak intensity and incomplete peak resolution). Stable carbon isotope analysis on a bulk oil sample was performed by the USGS, Denver. RESULTS AND DISCUSSION

The results of CSIA for normal, isoprenoid and polycyclic aromatic hydrocarbons are given in Table 1. The n-alkane data for the hydrothermal petroleums range from Cts to Cs7 and examples of

gas chromatograms for both aliphatic and aromatic (Fl and F2) hydrocarbon fractions are found in prior publications (e.g., Kvenvolden and Simoneit, 1990; Kvenvolden et al., 1986). The CPI values (calculated over the carbon ranges from Cza to C34, Cooper and Bray, 1963 and CZs to Css, Marzi et al., 1993) for the n-alkanes range from 1.O--1.2. The 613C values of the n-alkanes in the hydrothermal petroleum samples range from -24.9 to -30.4%0 (vs. PDB), and the data are plotted in Fig. 2. In genera1 the data fit within or close to the lo error of the overall mean of -27.5 + 1.3L and are typical isotope ratios for terrestrial organic matter. However, the trend to less negative values (heavier) with decreasing carbon number may indicate an increasing contribution of marine derived or-

Note

514

-15

.‘..I...“..‘.I....I....‘...

’

32D-2

-20 t

-25 -

2

bulk

-

659-m

oil

-

14D-6H

+

*-

z

-30 -

-35 -

t ,l....‘ ....‘ ....‘20 ....‘....‘ ....I30 10 15 25 C number

Fig.

35

number for the n-alkanes in the hydrothermal Escanaba Trough (the value of a total oil is also indicated).

2. Plots

of 613C vs. carbon

ganic matter to the lower molecular weight nalkanes or alternatively may be the inherent characteristic isotope signature observed for deltaic oils (Chung et al., 1994).The greatest 613C deviations in each sample are observed for the lowest and highest homologs of the n-alkane series. This deviation is probably due to the low n-alkane concentrations as was discussed in the case of similar samples from Guaymas Basin (Simoneit and Schoell, 1995). The differences in the means of the three samples (Table 1) are not significant. These mean values are significantly different from the bulk isotope composition of an Escanaba Trough hydrothermal petroleum (613C = - 24.8%0, Table 2). The bulk isotope value may reflect a higher content of marine organic matter or enrichment with polar NSO and asphaltic products. The isoprenoid alkanes (Cts-CzO) of these samples have a 613C range from -24.5 to -26.9%0, with an overall mean of -25.6 + 0.6%0 (Tables I and 2). These values are heavier than the n-alkanes

40

petroleums from

and may reflect a stronger component of organic matter from marine sources or alternatively represent the inherent organic matter character as described by Chung et al. (1994). The polycyclic aromatic hydrocarbons (PAH, F2), products from high temperature hydrothermal alteration processes, have a 6°C range of -23.1 to -28.4%, with an overall mean of -24.7 k 0.6%0 (Tables 1 and 2). The PAH isotope ratios are also heavier than the n-alkanes and similar to the bulk oil value. This similarity supports an enrichment of 13C in the polar components. A comparison of the mean isotope values with those reported for hydrothermal petroleum samples from Guaymas Basin is given in Table 2. The mean isotopic composition of the n-alkanes from Escanaba Trough is 4.4%0 lighter than that for Guaymas Basin and the differences for the isoprenoid alkanes and PAH are similar (4.0 and 3.53/00, respectively). Thus, these two locales are distinguishable based on the molecular and compound specific isotope compositions.

Table 2. Summary of carbon isotope data for Escanaba Trough samples and overall comparison with Guaymas Basin. CONCLUSION 8°C Organic

matter type

n-Alkanes, CSIA lsoprenoid alkanes. CSIA PAH, CSIA Bulk oil Guaymas n-Alkanes, CSIA lsoprenoid alkanes. CSIA PAH, CSIA Bulk oil

(overall mean,% vs. PDB)

Number

Escanaba Trough -27.5 * 1.3 -25.6 +_0.6 -24.1 k 0.6 -24.8 Basin, Gulf of California* -23.1 f 0.9 -21.6 f 1.0 -21.2+_0.6 -22.5 + 1.1

*Data from Simoneit and Schoell (1995).

of samples (n)

3 3 2

I 5 5 2 IO

We determined the carbon isotope compositions of individual alkanes and PAH and of a bulk oil from the Escanaba Trough hydrothermal system. The data support a dominant origin from terrestrial organic matter with a minor marine component also recognizable. Acknowledgements-We thank the Chevron Petroleum Technology Company for permission to publish this work, C. N. Threlkeld and A. Warden, U.S. Geological Survey, Denver for carbon isotope data for bulk samples, R. 6. Dias and R. N. Leif for technical assistance, and H. M.

Not .e

Chung and J. Curiale improved this Note.

for

useful

comments

which

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