the chemistry of marine petroleum seeps

Prévia do material em texto

Journal of Geochemical Exploration, 7 (1977) 255--293 255 
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands 
THE CHEMISTRY OF MARINE PETROLEUM SEEPS * 
W.E. REED and I.R. KAPLAN 
Department of Earth and Space Sciences and Institute of Geophysics and Planetary 
Physics, University of California, Los Angeles, Calif. 90024 (U.S.A.) 
(Received September 20, 1976; accepted October 19, 1976) 
ABSTRACT 
Reed, W.E. and Kaplan, I.R., 1977. The chemistry of marine petroleum seeps. J. Geochem. 
Explor., 7: 255--293. 
Natural oil seepage contributes approximately 10% (0.6 × 106 metric tons per year) of 
the annual input of petroleum hydrocarbons to the marine environment and half of that 
arises from the continental shelves of the circum-Pacific. Chemical properties of submarine 
petroleum seeps are reviewed, focusing on those of the Southern California Borderland, 
in relation to crude oil produced in the same geographic area, the compositional changes due 
to weathering in the marine environment, and the hydrocarbon and trace element compo- 
sitions of the surrounding seawater and adjacent sediments. One purpose has been to 
determine whether the hydrocarbon compositions of seawater or surface sediments reflect 
the composition of nearby seepage, and can be used to identify previously unknown 
petroleum sources. 
The analytical procedures included identification of saturated hydrocarbons by gas 
chromatography and gas chromatography/mass spectrometry;stable isotopic ratios of 
sulfur, nitrogen and carbon on gas and oil, and the 513C for various liquid-solid chromato- 
+ o graphic fractions. The range in 534 S covered by the California oils is --4 to 15.9/oo, and the 
515 N range is from +5.6 to +11°/oo. Carbon isotopic ratios for total oils exhibit a relatively 
narrow range (--22.1 to --27.3°/oo ); however, chemical fractions show a relatively greater 
variation (--21.5 to --31.5°/oo), with the hexane fraction lighter than the total oil, and the 
n-alkanes representing the lightest of all fractions measured. The content of nitrogen and 
sulfur, their isotopic ratios, and the ~13C of chromatographic fractions, serve to distin- 
guish oils and seeps derived from different formations. Comparison of gas chromatographic/ 
mass spectrometric data from a seep and from a crude oil produced nearby show cyclo- 
alkane compositional differences attributable to origins, rather than to differences in 
weathering history. 
Data presented demonstrate that analyses of the water column for enrichment in high- 
molecular-weight hydrocarbons or trace metals adjacent to seep areas may produce equi- 
vocal results. Sediment analysis appears to give a more reliable indication of petroleum 
contribution. Anthropogenic hydrocarbons and trace metals, derived from shipping lanes, 
harbor activities, or sewage disposal systems, can exceed natural concentrations and there- 
by alter the distribution pattern. However, marine sediments receiving petroleum contribu- 
tions can be differentiated from those in which only biogenic hydrocarbons are present. 
* Publication No. 1626: Institute of Geophysics and Planetary Physics, University of 
California, Los Angeles, California 90024, U.S.A. 
256 
INTRODUCTION 
The worldwide energy supply situation, coupled with an unprecedented 
degree of direct governmental intervent ion in virtually all aspects of the 
international pet ro leum industry, has exploded in the most unstable economic 
and monetary condit ions the world has known since World War II. One of 
the most evident by-products of this unique situation, which will a f fect all 
economic, social and political institutions, is the burgeoning offshore search 
for oil and gas, a deve lopment destined to cont inue with unabated intensity 
during at least the next two decades. One result of such activity could be 
catastrophic release of potent ia l ly toxic components of crude oil and its 
alteration products. This report considers some problems relating to 
pet ro leum in the marine environment, with part icular emphasis being placed 
on the chemical propert ies of natural pet ro leum seeps. 
Offshore oil product ion by 1980 is expected to equal or exceed 1970's 
total onshore and offshore free-world product ion of more than 40 mill ion 
barrels per day, and deep-water "pay" may contain more oil than has been 
discovered to date, both on land and offshore. The capabi l i ty for drilling in 
water depths of 3000 m or more already exists; and the technology is well on 
its way for truly deep-sea product ion and transportat ion. 
TABLE I 
Budget of petroleum hydrocarbons introduced into the oceans (National Academy of 
Sciences (U.S.), 1975) 
Source Input rate (106 metric tons/year) 
best estimate probable range 
Natural seeps 0.6 0.2-- 1.0 
Offshore production 0.08 0.08--0.15 
Transportation : 
LOT iankers 0.31 0.15--0.4 
non-LOT tankers 0.77 0.65--1.0 
dry docking 0.25 0.2--0.3 
terminal operations 0.003 0.0015--0.005 
bilges bunkering 0.5 0.4--0.7 
tanker accidents 0.2 0.12--0.15 
non-tanker accidents 0.1 0.02--0.15 
Coastal refineries 0.2 0.2--0.3 
Atmosphere 0.6 0.4--0.8 
Coastal municipal wastes 0.3 ..... 
Coastal, non-refining, 
industrial wastes 0.3 
Urban runoff 0.3 0.1--0.5 
River runoff 1.6 - 
Total 6.113 
257 
Oil pollution of the marine environment can be caused by atmospheric 
fallout; leakage from vessels, whether afloat or not; marine collisions (vessel/ 
vessel, vessel/fixed natural object, vessel/fixed platforms or other offshore 
oil facilities); bilge pumping, tank cleaning, deballasting of vessels; loading 
and unloading of tankers; rupture of submarine pipelines and storage facilities; 
operational mishaps in offshore drilling and production including well blow- 
outs; leakage from subsea strata; and drainage for onshore facilities (for 
example, from rivers or from dumping of waste oil into sewers). 
The quantity of petroleum hydrocarbons entering the ocean today has 
been variously estimated to range from 5 to 10 million metric tons per year. 
One estimate recently published by the U.S. National Academy of Sciences 
(1975) is shown in Table I. 
Of this inventory, the best data come from records on release during trans- 
portation. The reason is that monitoring and documentation is imposed on 
tankers at terminals and during ship operations. Presently, transit losses rep- 
resent the major source of pollution. 
Runoff from land, either representing rivers supplying water and sediment 
as a natural erosion cycle, or sewage outfalls, numerically represents a close 
second source. However, monitoring here is quite inadequate for most 
municipalities, and the data available are sparse and probably inaccurate on 
a global scale. 
Special problems associated with petroleum exploration on the outer con- 
tinental shelf 
Operational pollution effects. During exploration and production, pollution 
will occur primarily through four sources: 
(1) Drilling mud released from drilling platforms which will reduce optical 
transparency of the water and may form a dense layer overlying the natural 
sediment surface. In such a circumstance, anoxic conditions may be establish- 
ed in the sediment causing production of hydrogen sulfide and elimination of 
benthonic animals. This should only cause local disturbance. 
(2) Crude petroleum released from drilling mud and other drilling proce- 
dures. The amounts here are probably very low and would not constitute 
any appreciable hazard, especially in deep water far from shore. 
(3) Waste gasoline and exhaust fumes may be expected from normal traffic 
of launches, tugs and other service vessels. Much of these waste products will 
be volatile and will only have a short duration in the sea. 
(4) Heavy metals will be released from drilling rigs,formation water, crude 
oil and gasoline. Probably the most hazardous will be lead, which might be 
relatively soluble in saline water. Vanadium and nickel contents might also 
represent a hazard, which will have to be evaluated. 
Accidental spills. Probably the greatest danger to outer continental shelf (OCS) 
operations will come from a single large spill. The reason for this is that the 
petroleum could form an impermeable layer on the ocean surface, preventing 
258 
Fig. 1. Black and white print from color infra-red photograph taken from aircraft at 
2000 m elevation off Isla Vista, California. The textured material near shore is kelp. The 
"stringers" flowing in a southwest direction represent oil from a natural seep (courtesy of 
D. Hodder, Esca-Tech, Playa del Rey, Calif.). 
exchange of gases with the atmosphere and also absorbing visible light. It 
should be recognized, however, that for many reasons each geographic locality 
is vulnerable in a di f ferent sense. For example, the OCS of southern Califor- 
nia is swept by the souther ly moving California current, which tends to move 
the surface oil f i lm away f rom the coast (see Fig. 1). Topographic features of 
the shelf, such as areas overlying banks, are subjected to relatively swift 
bot tom currents and substantial swells. Hence, water movement would 
accelerate dispersal and oxidat ion of the oil. However, the topography also 
introduces a potent ia l for damage in two environments. One is the coast 
259 
along the offshore islands which could be exposed to oil. Intertidal pools, 
nesting grounds, etc., could be endangered. Second, there are numerous basins 
within the shelf where water movement is very slow, and the oxygen content 
of water is greatly reduced. If petroleum does settle there, it may cause 
further removal of oxygen and create temporary stagnant anoxic conditions. 
The biota in these basins is probably not as rich as on banks and platforms, 
and damage would probably not be significant. 
Longevity of pollution effects. The longevity of an oil spill cannot be easily 
estimated. Because petroleum degradation is controlled by both biological 
and non°biological processes, it will depend on the following environmental 
conditions. Roughness of sea and wind velocity, temperature of sea surface, 
nutrient content of surface waters, particulate content of water column and 
chemical nature of the crude oil. Small spills may be dispersed in a matter of 
hours in the open sea. 
Probably the greatest danger comes from oil being washed on beaches at 
high tide and smeared along an entire beach front. Oil slicks are frequently 
formed from sudden releases of natural seeps between Ventura and Santa 
Barbara. One such slick which drew public attention in October, 1974, was 
virtually unnoticeable within two days and only left small tar residues on the 
beaches. The effects of the 1969 Santa Barbara catastrophy had essentially dis- 
appeared from the beaches one year after the accident. 
Seep occurrences on continental shelves of the world 
Wilson et al. (1974) estimate that of the 0.6 X 106 metric tons/year of 
natural marine oil seepage contributed to the world's oceans, the continental 
shelves of the circum-Pacific represent the area of greatest seepage, contribut- 
ing approximately 50% of the total {Table II). These authors were able to 
find documented reference to only 190 marine seeps for the continental shelves 
of the world, reflecting the incomplete exploration of the offshore areas, and 
the difficulty of observing and documenting seeps in the marine environment. 
There is a correlation between the distribution of seeps, the quantity of oil 
emitted by the seeps, and geologic structure. High petroleum seepage occurs 
within regions currently undergoing tectonic deformation. There is no correla- 
tion with the size of petroleum reserves. Wilson et al. (1974) point out that 
the largest oil field in the world, the Ghawar field on the stable Arabian Plato 
form of the Persian Gulf Basin, has little associated seepage. In contrast to that, 
the continental borderlands of southern Alaska and southern California, 
which are currently undergoing tectonic deformation as evidenced by recent 
seismic activity, are areas of considerable petroleum seepage. Because little 
direct information exists, petroleum seepage in offshore areas can only be 
grossly estimated. The geological criteria suggested by Wilson (1973) and 
Wilson et al. (1974) to estimate seepage potential of offshore areas may be 
summarized as follows. 
260 
TABLE II 
Summary of probable seepage rates into the world's oceans (Wilson, 1973) 
Ocean Estimated seepage rate Seepage rate 
(106 metric tons/year) (% of total) 
Pacific 0.269 48 
Atlantic 0.196 35 
Indian 0.089 16 
Arctic 0.0023 0.4 
Southern 0.0017 0.3 
Total 0.558 
High seepage potential areas: (1) recent deformation consisting of high seis- 
mic activity along strike-slip faults, often with tight compressive folding in 
associated sedimentary basins; and (2) thick Tertiary sedimentary sequences 
which are geochemically mature. 
Low seepage potential areas: (1) trailing edge continental margins with 
little evidence of recent deformation; and (2) old, geochemically mature, 
sedimentary basins, or geochemically immature younger sedimentary sequen- 
ces. 
Seep occurrences in the Southern California Borderland 
The geology of the Southern California Borderland was cited by Wilson 
et al. (1974) as characteristic of areas with high seepage potential. It is struc- 
turally complex, and is presently undergoing deformation as witnessed by 
recent seismic activity, and by the deformation of late Pleistocene marine 
terraces. The onshore Los Angeles and Ventura Basins and the offshore Santa 
Barbara Basin are proven oil-producing provinces (Fig. 2). The Los Angeles 
Basin represents, on a sediment volume basis, one of the world's richest 
petroliferous areas. The geochemical evolution of the thick Tertiary and 
Quaternary sediment sequences in these basins ranges from "mature" in the 
deeper, older sediments, to " immature" in the younger, shallower beds 
(Philippi, 1965). These sediments are complexly folded in places and broken 
by historically active strike-slip and thrust faults (see Fig. 3). 
Estimates of seepage rate in the 2600-km 2 offshore area from Point 
Conception in the north to Long Beach (Fig. 2) in the south range from less 
than 16 ma/day to more than 160 m3/day (Wilson, 1973). Within this area, 
Wilkinson (1972)has located approximately 60 zones of seepage; some are con. 
tinuously active, e.g., Coal Oil Point, emitting petroleum rapidly enough to 
form an obvious slick, while other zones are apparently inactive or only 
sporadically active. The locus of seepage in most cases is along structural 
trends such as faults (Fischer and Stevenson, 1973). The cause of the spora- 
261 
I I 
B. ~2°°w ~47"w 
- - FAULT • PRODUCTION ObL SAMPLE 
o SEEP (OIL 8, TAR) • OIL SEEP SAMPLE 
O SEEP (GAS ~ OIL) • GAS SEEP SAMPLE 
Point __ % 
"~ Conce pt ion .~_~. . . _~ Coal Oil Santa ! 
~L~:.-,o--~-'%_ ~ ~, ,~o o -~- - - -~Po in t (z ) Barbara '193"I}, ,.Santa Barbara 
vo o - / /v ~ ~oJI::~C,C,c~.~ . . . . . . . . . . . . . z 0 ~ - ~ - ~ 
A ° Son - 
i 5 ~ e (~( ~. . . _ _ - - - 
~o\ \Redondo~ / 
S C A L E ~ o ' 
c mi~es IO 
Fig. 2. A. Generalized location map of oil and gas seeps, Southern California Borderland 
(modified from Wilkinson, 1972). 1 = Coal Oil Point seep, 2 = Isla Vista seep, 3 = La Goleta 
seep. B. Collection stations for BLM Baseline studies. Numbered stations correspond to 
data in Tables VIII and IX and Fig. 9. 
dic nature of some seep activity is unknown, although it may be related, 
at least in part, to deformation. For example, Wilkinson (1972) documented 
the suddenoccurrence of seep activity at Malibu following the San Fernando 
earthquake in February, 1971. 
Seep occurrences in Gulf of Alaska 
By comparison with California, there appear to be no reliable data on sub- 
marine seeps from the Gulf of Alaska or the Arctic. Recent measurements of 
dissolved hydrocarbon gases in the water column by Joel Cline (NOAA, 
Seattle, Wash.) indicate high anomalies in the area between Kayak Island and 
Hinchinbrook Island in the northeastern Gulf of Alaska. Reports of oil and 
gas seeps on or near the Alaskan and Arctic coasts extend to the year 1850 
and stimulated petroleum exploration from about 1900 onward. Descriptions 
of seep occurrences are given by Miller et al. {1959), Johnson (1971) and 
Blasko (1976). 
Most of the known seepage areas occur along the eastern Alaska Peninsula 
extending from Puale Bay in the south along Shelikof Strait to Oil Bay in 
the north along the western Cook Inlet (Blasko, 1976 and personal communi- 
262 
South North 
Holocene - {Q -Quaternary Sediments Pleistocene 
{Pp -"P ico" Fm. ?~ Seeps: 
-Gas, oil I~ tar Pliocene Pr -"Repetto" Fm. ,ll~ M 
Fractures 
f 
Ms - Sisquoc Frn. 
Miocene Mm- Monterey Fm. 
Mr - Rincon Frn. 
Fig. 3. Schematic north-south structural cross-section through Coal Oil Point seepage area 
(modified from Fischer, 1975). 
cation). The area of greatest activity appears to be Iniskin Peninsula. Many of 
the seeps are ephemeral, and their flow intensity changes with time. As a 
result, Blasko believes it is not valid to assign a numerical quantity to the 
seeps with any confidence. 
According to Blasko {1976, and verbal communication), short streams from 
glacial ice melt fed by snow and rain carry the bitumen from nearshore seeps 
into the ocean. Concentration of petroleum as high as 750,000 mg/liter 
are found in streams adjacent to seepage, but the content may decrease to 
~0.1 mg/liter at the mouth of the streams 1--2 km from the source. Tar on 
rocks or on vegetation overhanging streams is no longer visible at distances of 
-1 km from the seep source. Tar is rarely observed on beach sands of the 
Alaska Peninsula. Few analyses of these oils have been published, but Blasko 
(1976) quotes a range of values on five captured petroleum seep samples 
from four different sites to be: API gravity, 8.9--21.4 ° ; total sulfur content 
0.12--0.59%. 
With the exception of one sample from Mt. Demian, the gases are composed 
mainly of methane and only show traces of ethane and higher hydrocarbons. 
Nitrogen and CO 2 are often common constituents. 
263 
COMPOSITIONS OF GAS SEEPS 
Compositions of submarine seepages of liquid or semi-solid petroleum from 
southern California have been more thoroughly investigated (Allen et al., 
1970; Chapin, 1972; Mikolaj et al., 1972; Sivadier and Mikolaj, 1973) than 
gas seeps. Because there is virtually no published information on the composi- 
tions of the gas seeps, we collected and analyzed samples of two such seeps 
from the Santa Barbara area (Fig. 2). 
Collection and analysis 
SCUBA divers collected samples from the rising train of bubbles emitted 
from the gas seeps by inverting glass jars and allowing the gas to displace the 
water. Evacuated glass sample tubes (Vacutaners) were opened to the trapped 
gases, filled, and returned to the laboratory for analysis. An aliquot of the gas 
sample was set aside for gas chromatographic analysis, while the remainder 
was introduced into a glass vacuum line. Methane was isolated from the other 
gases, and combusted to CO2 for isotopic measurement. 
Gas chromatographic analysis of the samples was conducted on an nCs-im- 
pregnated Porasil column (3 m X 0.9 mm) installed in a Hewlett-Packard 
Model 5830A instrument. The oven was cryogenically cooled to --50°C and 
held isothermally at that temperature for 2 minutes after injection, then 
programmed at 10 ° C/minute to +100°C and held at that temperature for 20 
minutes. Injector temperature was 175°C and the detector temperature was 
250°C. 
Combustion technique and measurement of the stable carbon isotopic 
composition of the methane isolated from the seep gases by mass spectrom- 
etry followed procedures similar to those described by Claypool and Kaplan 
(1974), Sackett and Brooks (1975), and Bernard et al. (1976). 
Results 
The methane content of the hydrocarbon fractions from the Coal Oil 
Point and Carpinteria gas seep samples is 94.8% and 87.7~o, respectively. 
Stahl (1974) has shown that gases generated by biochemical processes are 
dominantly methane (C1/ZC, > 0.99), whereas gases associated.with petroleum, 
or those not associated with petroleum but which are probably generated 
though a similar process of maturation, are richer in higher-molecular-weight 
hydrocarbons (C1/ZC, < 0.95). Both of the gas seeps which were sampled 
are closely associated with petroleum or tar seeps (Fig. 2), and their composi- 
tions (C1/EC, = 0.948 and 0.877) are consistent (Table III) with the asser- 
tion of Stahl (1974). 
The isotopic composition of the Southern California Borderland samples 
is also consistent with the interpretation that the gas seeps represent petroleum 
associated gases. The seep gases contain predominantly methane, which ex- 
264 
TABLE III 
Composition of gases from Coal Oil Point and Carpinteria seeps, Santa Barbara area, Cali- 
fornia 
Component Relative hydrocarbon composition (%) 
Coal Oil Point Carpinteria 
Methane 94.76 87.70 
Ethane 0.40 3.40 
Propane 2.27 5.00 
/-Butane ]. 03 1.20 
n-Butane 0.92 ]. 70 
2,2-Dimethylpropane 0.01 0.00 
2-Methylbutane 0.54 0.70 
n-Pentane 0.0 ] 0.30 
Hexanes 0.07 0. ] 2 
C1/ZC u 0.948 0.877 
C2/C 3 0.176 0.680 
iC4/nC 4 1.12 0.71 
~p1B3C(CH4) , °/00 --38.7 --40.3 
hibits 13 pDBC values of--38.7%o and --40.3%o {Table III). Methane produced 
by biochemical processes is strongly depleted in the heavier isotope, and 
~PDBC values from --60%0 to --90°/oo (Oana generally exhibits a range of 13 
and Deevey, 1960; Nakai, 1960; Colombo et al., 1965; Sackett, 1968; Clay- 
pool and Kaplan, 1974). Silverman and Epstein (1958) and Silverman (1964) 
have shown that the gas phase separated from petroleums have markedly 
lower 5laC values than the associated liquid hydrocarbons. In a crude oil from 
(bPDBC = --22.4%o), Silverman (1964) de- West Coyote field, California 13 
termined the methane isotope ratio to be --38.0% o, or 15.6% o lighter than 
the isotopic composition of the whole crude oil. 
At Coal Oil Point the gas seep apparently arises from the Miocene Monterey 
Formation, while the Carpinteria gas seep is from the Pliocene Pico Forma- 
tion; thus, the sources of the two gases probably differ. The relationship 
between thermal maturity of the source rocks and the composition of the 
gases generated has been discussed by Stahl {1974), Bailey et al. {1974) and 
Orr (1974). In general, thermal alteration of organic matter in sediments 
results in bond cleavage such that 12C--12C bonds are more readily broken than 
12C--13C bonds; hence, isotopically light methane is initially produced (Frank 
et al., 1974). Simultaneously, the gases are initially enriched in C2--C7 
hydrocarbons, and during progressive metamorphism, C2--C7 hydrocarbons, 
in addition to the higher-molecular-weight constituents, are thermally de- 
graded to methane. With progressively more severe thermal maturation, the 
methane concomitantly becomes isotopically heavier. 
-20 
-3O 
a~ 
-4o 
E 
-50 
-60 
-70 
CI/C I -C 4 Gas Ratio 
1.00 0.95 0.90 0.85 0.80 0.75 
l I I I I 
_~ jTer res t r /a / Source r as~igon 
~/,// ~Biogen/c Gas 
Fig. 4. C1/C n and 513C data from Coal Oil Point (1) and Carpinteria (2) plotted on a 
diagram modified from Stahl (1974). 
265 
The relationship between the isotopic values of methane, and the hydro- 
carboncomposition, is illustrated in Fig. 4. According to this model, the 
Coal Oil Point and Carpinteria seep gases are from thermally mature source 
rocks, with the Monterey Formation source rocks of the Coal Oil Point gases 
being somewhat more mature than the Pico Formation source of the 
Carpinteria seep. 
WEATHERING OF EXPOSED CRUDE OILS 
We might define environmental weathering of petroleum and petroleum 
products in a natural environment as constituting the tendency toward a 
composition which is in equilibrium with the atmosphere, hydrosphere, and 
biosphere. Necessarily, this is a loose definition, encompassing biological as 
well as abiological processes. Weathering of petroleum in the marine environ- 
ment has been intensively investigated (for example, Blumer and Sass, 1972; 
Ehrhardt and Blumer, 1972; Floodgate, 1972; Atlas and Bartha, 1973; Blumer 
et al., 1973; Butler et al., 1973; Cundell and Traxler, 1973; Rashid, 1974; 
Farrington and Medeiros, 1975; Butler and Harris, 1975; National Academy 
of Sciences (U.S.), 1975). In general, such alterations may be summarized: 
(1) Evaporation and dissolution primarily affects lower-boiling-point 
components, removing them from the oil. 
(2) Oxidative processes promote a loss in saturated and aromatic hydro- 
carbons with a concomitant increase in asphaltenes. 
266 
(3) Bacterial degradation proceeds in the order n-alkanes > branched 
alkanes > cycloalkanes > aromatics. 
(4) Effects of weathering on peripheral substituents (OH, C-O, COOR) is 
unclear. For example, Blumer et al. (1973) reported an increase in carbonyl 
content with progressive weathering, while Rashid (1974) found a decrease. 
Each competingdegradative process is necessarily modified by environ- 
mental conditions of temperature, incident light, wave energy, Eh or avail- 
ability of molecular oxygen, and suspended particulates. The weathering 
must also be modified to some degree by the composition of the petroleum 
(Koons, 1972). It is tacitly assumed that the requisite heterotrophic hydro- 
carbon-oxidizing microorganisms are ubiquitously present (Ward and Brock, 
1976). 
Microbial degradation 
In aerobic bacterial catabolism of crude oils, it has been found that n-al- 
kanes are degraded first (Kator et al., 1971; Atlas and Bartha, 1973; Nelson- 
Smith, 1973; Rashid, 1974), with shorter chain lengths preferred over longer- 
chain molecules. Byron et al. (1970), found that nC12 was preferentially 
utilized by various strains of marine bacteria, principally pseudomonads, from 
a hydrocarbon pool consisting of nC12, nC15 and nC16, followed by catabolism 
of nC15 and nC16, in that order. When Louisiana crude oil was used as sub- 
strate, initial oxidation was attributed to the breakdown of n-alkanes smaller 
than nCls (Kator et al., 1971). The soil bacterium, Micrococcus cerificans is 
able to grow on nC29 as its sole source of carbon, however, when a pool of 
nC16 and nC29 together is used as growth medium, nC16 is preferentially 
utilized by the bacterium (Hankin and Kolattukudy, 1968). 
Jobson et al. (1972) examined compositional alterations as a result of 
microbial degradation in two crude oils, one a paraffin-base, and the other 
an asphalt-base oil. They found that both oils could be degraded, although 
the rate of degradation in the asphalt-base crude oil was much lower than 
for the paraffin-base oil. 
It is difficult to compare directly the rapid microbial degradation rate in 
laboratory simulations to the probable preservation or loss of crude oil 
constituents in natural settings without regard to specific environmental 
characteristics. Few studies of actual petroleum spills are strictly comparable. 
The wave energy at the individual spill site, the resident biota, the air and 
water temperature, the supply of nutrients other than petroleum, particulate 
material which can serve both as an absorbant for oil and as a solid substrate 
for microbial attachment, and the composition of the spilled oil differ from 
case to case. Blumer and Sass (1972), in their study of a No.2 fuel oil 
(diesel) spilled in a protected coastal environment, showed that microbial 
oxidation, even of the easily oxidized n-alkanes, was slower in nature than in 
laboratory simulations. Rashid (1974) examined a spill of No. 6 fuel oil 
(BUnker C) and reported that the spilled oil exposed to high-energy coastal 
267 
environments showed substantial compositional alteration after 3~- years, 
whereas the oil in low- and moderate-energy environments exhibited only 
slight modification. In their detailed investigation of Lake Mendota, Ward 
and Brock (1976) were able to show that the rate of hydrocarbon oxidation 
varied seasonally, probably largely as a function of temperature. However, 
during the summer when temperature for bacterial growth was optimal, 
nutrient deficiencies limited the biodegradation of oil; higher degradation 
rates could be achieved by the addition of nitrogen and phosphorus. 
Environmental extremes also inhibit the biodegradation of petroleum. 
Ward and Brock (fn preparation, personal communication, 1976) have been 
able to document a marked reduction in bacterial metabolic rate at high 
salinities. Micro-organisms capable of using hydrocarbons as the sole source 
of carbon and energy could be successfully enriched from natural sources 
with a salinity <20%, but not from brines with higher salinities (i.e., south 
vs. north, Great Salt Lake). 
Chemical degradation 
Most petroleum or petroleum products spilled into the ocean will spread 
into thin films. This spreading characteristic is primarily due to surface active 
components, such as oxygen-, sulfur-, or nitrogen-containing molecules 
in the oil (Garrett, 1972, 1973). Auto-oxidation, defined as the spontaneous 
reaction of a compound with molecular oxygen, is enhanced in oil spread as 
a thin film on the sea surface because of the large surface area, which allows 
maximum exposure to incident sunlight and promotes intimate contact 
between the oil film and the atmosphere. 
The actual importance of auto-oxidation in nature is unclear (Blumer et al., 
1973), largely because of lack of data. Koons (1972) has pointed out that the 
rate of auto-oxidation should vary according to molecular structure; a tertiary 
hydrogen is more reactive than a primary or secondary hydrogen. Aromatic 
hydrocarbons with branched alkyl substituents are most susceptible to photo- 
induced auto-oxidation. Burwood and Speers (1974) examined the seawater- 
soluble extracts of crude oil during a four-week simulated weathering study. 
Development of the characteristic unresolved envelope (Farrington and Me- 
deiros, 1975) in the gas chromatographic trace of the extract was previously 
shown (Boylan and Tripp, 1971) to be time-dependent, suggesting its con- 
stituents may be oxidation products of the crude oil. Gunnar Aksnes (written 
communication, University of Bergen, 1976) suggests that heteroatomic sub- 
stitution in aromatic compounds function as photochemical sensitizers, 
which absorb light in the ultra-violet region, and form diradicals capable of 
abstracting a hydrogen atom from a hydrocarbon (steps 1 and 2): 
S: h~ S-H + R. \ 
RH • ~ + 0 2 
R- + ROOH ROO. 
268 
Burwood and Speers (1974) were able to document in their experiments the 
production of phenols and of thiacyclane oxides (sulfoxides). The phenolic 
content, after one month of simulated weathering, accounted for approximate- 
ly 15% of the seawater-soluble fraction, 7% consisted of higher-molecular 
weight aromatic hydrocarbons, and the unresolved complex mixture amounted 
to 75% of the extract. The bulk of the latter fraction was shown to consist 
of a series, with extensive isomerism, of alkyl substituted mono-, di-, tri-, and 
tetra-thiacyclanes. While it seems likely that autocatalytic oxidation, as 
documentedby Burwood and Speers (1974) occurs in nature, it may only 
represent a quantitatively unimportant process, as only 0.02~/¢~ of; the original 
oil appeared in the aqueous phase as thiacyclane oxides. 
Evaporation and dissolution 
The processes of evaporation and dissolution may be treated together for 
petroleum on water, because the constituent compounds are sufficiently in- 
soluble that partitioning favors the non-degraded oil to be transferred into the 
vapor phase. McAuliffe (1973) points out that the lower-molecular-weight 
compounds of petroleum or petroleum products are subject to dissolution 
in water. Following dissolution, however, most of the dissolved molecules will 
then partition into the atmosphere. Partitioning into the vapor phase decreases 
in the order: alkanes, alkenes, cycloalkanes, and aromatics. For a given com- 
pound class, partitioning into the vapor phase increases with molecular weight, 
because solubilities in water decrease more rapidly than do the vapor pressures 
(McAuliffe, 1973). 
Bentz (1976) states that weathering changes in petroleum composition of 
spilled oil are most pronounced in the first 24--48 hours. Although he does 
not define this more rigorously, the changes thus encountered are probably 
the response of the oil to evaporation and dissolution. Evaporation rates from 
very thin films on water over extended periods have not been adequately 
studied. ~4reider (1971) has shown that evaporative losses from an unspecified 
crude oil after 21 days affected hydrocarbons through nCls. Moreover, 
thinner films on water had significantly greater losses than thicker films. In 
a 24-hour experiment, with a film thickness of 1.9 mm, evaporation affected 
hydrocarbons through nC15; whereas, with a film thickness of 0.1 mm, 
evaporation losses extended through nC20. Butler (1975) has presented a 
detailed model for evaporative weathering. Butler's model assumes that the 
ra~ of such weathering is proportional both to the equilibrium vapor pressure 
of compound i (P,-) and to the fraction remaining (xJxio, where xi is the amount 
of compound i remaining after time t, and xi0 is the amount initially present): 
dx i /d t = - -kP i (x i/o( io ) 
The rate coefficient is determined empirically. His solution to this first- 
order rate equation: 
xi/x i0 = exp [--(k t/xio ) exp(10.94--1.06 N) ] 
269 
where N is the number of carbon atoms in compound i, predicts that the frac- 
tion weathered per unit time decreases more than exponentially with increas- 
ing carbon number. A potentially useful permutation of his equation is that, 
from a knowledge of the initial carbon number distribution, the length of 
time that the oil has suffered evaporative weathering can be estimated. 
COMPOSITIONS OF PETROLEUM SEEPS 
Surprisingly few marine oil seeps are well characterized. Of the 190 oil 
seepages cited by Wilson et al. (1974), 60 occur in the Southern California 
Borderland (Wilkinson, 1972). It is natural that the focus of attention should 
be centered on the composition of seep oils in the Santa Barbara area follow- 
ing the blowout at the Union Oil Company Platform A in January, 1969 
(Allen et al., 1970; Chapin, 1972; Delaney, 1972; Mikolaj et al., 1972; 
Ampaya and Mikolaj, 1973; Fischer and Stevenson, 1973). 
At Coal Oil Point, Allen et al. (1970) estimate that natural seeps introduce 
approximately 8--16 m 3 of oil per day into the marine environment. Some 
compositional data are available for nickel and vanadium concentrations in the 
seep oils (Delaney, 1972), and some on the hydrocarbon compositions 
(Sivadier and Mikolaj, 1973). Because there is no comprehensive compositional 
information on natural submarine petroleum seeps, we collected oil samples 
from the Coal Oil Point and Carpinteria seeps for detailed analysis. Preliminary 
results of our study are reported here. 
Collection and analysis 
At the sampling locations, Vernon and Slater (1963) have previously noted 
that the older tar "f lows" are strongly weathered and encrusted by marine 
organisms. For our study, SCUBA divers were able to collect samples (loca- 
tions shown in Fig. 2A) from fresh vents. The samples are black, viscous 
liquids with specific g~avities near 1.0. 
Column chromatography 
Preliminary fractionation was accomplished by liquid-solid chromatography 
on silica gel, using a substrate/sample ratio of approximately 100 : 1 successive- 
ly eluting with hexane, benzene and methanol, using a solvent/sample ratio of 
200 : 1. Removal of solvent to approximately 2 ml volume was achieved with 
a rotary evaporator. The samples were then transferred, using glass syringes, to 
tared vials, with final solvent removal accomplished using a stream of purified 
N2 gas. 
Molecular sieve occlusion 
Separation of n~alkanes from the branched and cyclic alkanes was accom- 
plished using molecular sieve. An aliquot of the hexane eluate, dissolved in 
270 
benzene, was refluxed for 24 hours in the presence of 100/120 mesh 5 2, 
molecular sieve, using a sieve/sample ratio of approximately 50 : 1. After reflux, 
the benzene was decanted, the sieve thoroughly washed, and fresh benzene 
added. The sieve was again refluxed for 24 hours and washed as before. All 
benzene washes, containing the non-occluded branched and cycloalkanes, were 
combined. The solvent was removed as previously described, and the non- 
occluded branched plus cyclic fraction was weighed. 
The occluded n-alkanes were recovered by reacting the sieve with HF for 
6 hours under a layer of benzene with agitation. Liquid-liquid extraction of the 
HF with benzene was repeated five times. After extraction, the benzene solution 
was dried with pre-cleaned anhydrous Na2SO4. The solvent was removed in the 
usual way, and the occluded fraction, n-alkanes with small concentrations of 
3-methyl- and 2-methyl-alkanes, was recovered and weighed. 
Carbon isotope analysis 
Aliquots of each liquid-solid column chromatographic fraction of the oils 
were analyzed for stable carbon isotopic composition. For isotopic analysis, 
the samples were placed into vacuum (cold) for several minutes to remove sol- 
vents and dissolved gases, combusted with oxygen at 850 ° C, and the produced 
CO2 gas was purified by the technique described by Kaplan et al. (1970). The 
isotope ratios are presented in the "~" notation (see below) using the PDB 
belemnite carbonate standard. 
Sulfur and nitrogen elemental and isotopic analysis 
Sulfur was oxidized to sulfate using the Parr oxygen bomb technique (Parr 
Instrument Company, 1964). Sulfate was precipitated by adding excess 
barium chloride and the quantity determined by gravimetric measurement. 
Purified SO2 gas was prepared for isotopic analysis by direct combustion of 
BaSO4 with quartz powder (Bailey and Smith, 1972). Canyon Diablo sulfur 
standard was prepared by the same method to eliminate corrections for 
different oxygen isotope ratios between standard and samples. 
Nitrogen was extracted from the oils by reaction in closed tubes with 
sulfuric acid, following the method of Bremner and Edwards (1965). The 
quantity of nitrogen obtained was measured volumetrically using the tech- 
nique described by Cline and Kaplan (1975). 
The isotopic results are presented in the "6" notation, compared to 
Canyon Diablo meteorite sulfur and air: 
5 sample -~ [(Rsample - - Rstandard)/Rstandard] X 1000 (°/oo) 
where Rcarbon = 13C/12C; Rsulfur = 348/328; Rni t rogen -- 15N/14N" 
271 
Gas chromatography and combined gas chromatography~mass spectrometry 
analysis 
Aliquots of the total hexane eluates, the molecular sieve occluded fractions, 
and the molecular sieve non-occluded fractions of the oil samples were 
analyzed by gas chromatography (GC) and by combined gas chromatography/ 
mass spectrometry.(GC-MS). Analytical gas chromatographic conditionsusing 
a Hewlett-Packard Model 5830A instrument equipped with capillary injector, 
linear temperature programmer and flame ionization detector, were as 
follows: 15 m X 0.1 mm support-coated open tubular (SCOT) column coated 
with OV-101; helium carrier gas flow was 3.4 ml/minute; oven temperature- 
programmed from 100 to 280°C at 4 ° C/minute; injector temperature was 
210°C and detector temperature was 310 ° C. 
Combined GC-MS was conducted using a DuPont 21-492-1 mass spectrom- 
eter interfaced with a Varian Aerograph Model 204 gas chromatograph. Gas 
chromatographic conditions for the GC-MS analysis were: 10 m X 0.75 mm 
glass column, packed with 80/100 mesh Gas Chrom Q coated with 1% OV-1, 
temperature-programmed at 4 ° C/minute from 100 to 280 ° C. The total effluent 
from the column entered the mass spectrometer through a heated inlet with no 
molecular separator. Mass spectra were determined at an ionizing voltage of 
70 eV, and a source temperature of 250°C. The mass spectrometric datawere 
acquired and processed using a DuPont Model 21-094 data system. Mass 
fragmentograms were used as a preliminary screening and classification proce- 
dure. 
Results 
Distribution of sulfur, nitrogen, 34S, 15N and 13C. The absolute and relative 
contents of sulfur and nitrogen have been used for correlation between dif- 
ferent crude oils of a common origin (Barbat, 1967; Miller, 1973). Ball et al. 
(1959) have shown that sulfur and nitrogen are not necessarily lost in the 
same proportions during weathering of oils. An additional parameter to the 
amount of sulfur and nitrogen in crude oils is their isotope composition. 
Vredenburgh and Cheney (1971) have summarized measurements o~" sulfur 
isotope ratios for many crude oils of different locations and ages, including 
the numerous data of Thode et al. (1958), and report an isotopic range of --8 
to +23%0. 
Though there is only one known study of nitrogen isotopes in crude oils 
(Hoering and Moore, 1958), recent measurements of the distribution of nitro- 
gen present in marine sediment (Miyake and Wada, 1967; Cline and Kaplan, 
197 5; unpublished data, this laboratory) indicate that a range from +2 to 
+10%o may be expected for source organic compounds. Because it is believed 
that secondary addition of nitrogen does not occur (Barbat, 1967), nitrogen 
isotope composition may remain characteristic of the original source beds. 
Thode et al. (1958) concluded that the sulfur isotopic ratio of crude oils 
t-
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['
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274 
is unaltered during thermal maturation. Subsequently, Thode and Monster 
(1970) used the similarity in the sulfur isotopic composition of Paleozoic 
and Cretaeeous crude oils in northern Iraq to conclude that they were derived 
from a common source bed. More recent studies (Vredenburgh and Cheney, 
1971; Orr, 1974) have shown that thermal alteration of the sulfur content and 
isotopic composition of Paleozoic oils from Wyoming has occurred. 
It has been suggested that bacterial degradation of crude oil utilizing 
dissolved sulfate will significantly alter the composition of crude oils 
(Vredenburgh and Cheney, 1971; Bailey et al., 1973; Orr, 1974) but no mea- 
surable isotopic alteration has been reported for altered or weathered petro- 
leum. Either the sulfide produced is not incorporated into the oil or the 
quantity is small compared to the initial amounts of sulfur present. These 
investigations have been made in continental petroleum provinces. Crude oil 
exposed for long periods of time to seawater may have more significant 
addition of biogenically produced hydrogen sulfide. 
In a recent study in our laboratory, eight California Tertiary crude oils, 
and five natural seep samples were analyzed for sulfur, nitrogen and carbon 
isotopic composition (Tables IV--VII and Figs. 5 and 6). The sites of the 
produced oil and seeps are shown in Fig. 2A. In addition, five crude oil 
standards were obtained from H. Coleman (ERDA, Bartlesville, Okla.) and 
analyzed for comparison with the California oils obtained in this study. 
Characterization and differentiation of California crude oils and seeps. The 
range in sulfur isotope ratios covered by the California oils is --4 to +15.9%0. 
If 5348 is plotted against the sulfur/nitrogen ratio (Fig. 5) it can be observed that 
the California oils can be distinctly separated from the Wyoming oils of Big 
Horn Basin (Orr, 1974) and the four non-California standard oils measured 
in this study. The Wilmington, California, standard does correspond to the 
634S sulfur/nitrogen distribution for the set of California oils. The reason for 
the distinctive distribution of the plotted parameters for each set of crude 
oils may. be due to alteration of 534S and differential loss of sulfur and nitro- 
gen during maturation. The California oils are unusual because of their relative- 
ly high nitrogen content and therefore, low sulfur/nitrogen ratio. 
A plot of 615N against 634S (Fig. 6) shows that the California oils and seeps 
from different formations can be differentiated. It appears that in a regional 
sense, the 615N of oils may reflect the dominant source of sedimentary organic 
matter from which they formed (R. Sweeney, unpublished data). For example, 
in tropical areas, nitrogen fixation is dominant among plankton and the 615N 
values are close to the atmospheric N2 standard (~0°/oo), whereas, in the mid- 
latitudes, where NO3- assimilation is considered dominant among plankton, 
515N values are close to +10°/oo (higher 515N). 
Carbon isotope (613C) measurements were conducted on the total crude 
oil, the hexane and methanol fractions, in addition to the n-alkane and branch- 
ed + cyclic alkane fraction of the hexane eluate (see Tables VI and VII). It is 
apparent from the data that the hexane fraction in the production oils is 
275 
-I0 
I00 
50 
IO- 
SIN 
5 
LO- 
.05 
O ~ S(%,) +I0 +20 
, , , I . . . . I , , , - r - - - 
~ • • ' / 
Gach Saran a~ ;an , 
Prudhoe Boy / 
Alaska . / 
(~) Wilmington ~ " 
California ~" S. Swan Hills 
Reclu3 ®Z o. ~onado 
Wyoming / 6• ~) 
I i I I , l , , i 
Fig. 5. Plot of sulfur isotopic composition and the ratio of sulfur to nitrogen concentration 
for various areas. The solid circles are points taken from Orr (1974), the crossed circles are 
samples received from H. Coleman (ERDA, Bartlesville Energy Research Center, Okla.), 
and the solid trangles are seep oils, and crude oils from Southern California Borderland (see 
Fig. 2A). 
8 s5 N (%0) 
0 +2 4 6 8 [0 12 14 
I I I 1 I | 
Coal Oi l Po in t Seeps 
L. Miocene Monterey Fm. 
15 ile~.Beoch Tor Somples 
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Corp ln ter io Beach Seep 
I L. Piiocene Pioo Fro. 
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GO 
+5! 
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crude oils from Southern California Borderland. 
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t~
 
278 
always isotopically lighter than the total oil and methanol fraction. The 
n-alkane fraction is the lightest of all the fractions measured. 
By contrast, the methanol fractions of the seeps are isotopically similar 
to the total oil. Thus, the isotopically lightest material in the crude oil appears 
to be the first to have been destroyed during microbial degradation. 
It is presently not known whether the distribution pattern of 51~C is 
constant for the same fractions of an oil coming from a particular reservoir. 
However, it is evident from the data presented in Tables VI and VII, that 
the numerous fractions represent a sufficiently unique pattern to differen- 
tiate oils from various sources. 
Characteristic of the gas chromatographic trace for the total hexane eluate, 
and of the branched + cyclic fraction of these seep samples (Fig. 7) is an un- 
resolved envelope above the baseline, with small resolved peaks superposed. 
The molecular sieve occluded fraction for the Carpinteria seep (Fig. 7A) ex- 
hibits a relatively clean chromatogram of n-alkanes ranging from nC20 through 
nC29, with minor contaminants, probably representing branched components 
with long alkyl side-chains. The same fraction from the Coal Oil Point seep 
exhibits a large unresolved envelope with n-alkane peaks from nC2o through 
nC29 resolved above the hump. The branched + cyclic fractions of both 
samples show clearly defined peaks attributable to the C19 and C2o isoprenoids, 
and to several cycloalkanes (Fig. 7). 
Weathered petroleum is generally impoverished in n-alkanes, and, if 
weathering is sufficiently lengthy, also in isoprenoid alkanes (Blumer et al., 
1973). The unresolved baseline "hump" has been observed in waste water 
effluents (Farrington and Quinn, 1973a), in sediments and clams (Farrington 
and Quinn, 1973b; Zafiriou, 1973), where it was interpreted as petroleum 
contamination from the discharge of hydrocarbons in sewage effluents and from 
weathered oil spills. To our knowledge, no work has been reported on the 
composition of this unresolved complex mixture, although the study of Bur- 
wood and Speers (1974) on a similar feature in the predominantly aromatic 
seawater-soluble fraction of oil may be applicable. 
Use of mass fragmentography from combined GC-MS analysis provides a 
"fingerprint" technique apparently not previously considered (cf. Bentz, 1976). 
Fig. 8A illustrates five mass fragmentograms derived from analysis of the 
branched + cyclic fraction of the Coal Oil Point seep. Ion fragments character- 
istic of diterpane and triterpane hydrocarbons (m/e 191, Kimble et al., 1974a; 
Reed, 1977; Simoneit, 1977), of sterane hydrocarbons (m/e 217, Henderson 
et al., 1969), and of methylsteranes (m/e 231, Kimble et al., 1974b) are 
shown, together with mass fragmentograms representing the molecular ions for 
the C27H4s sterane (m/e 372), and for the C30H~2 triterpane (m/e 412). Use 
of mass fragmentograms provides a technique by which acompositional 
comparison of strongly weathered oils is possible. 
Van Dorsselaer et al. (1974) indicate that degraded and extended hopane 
derivatives are virtually ubiquitous in their occurrence in sediments, and 
possible in oils. Reed (1977) studied the composition of seep oils in Utah, 
279 
A 
I10 
CARPINTERIA SEEP 
L. PLIOCENE. PICO FORMATION 
Totol Hexone Fraction 
ones 
~ ~ h e d ~ Cyclic Froction 
-(Mol, Sieve Non-occluded) 
nC2l [ 
ZJ 1 
[~ nC25 
n-AIkone Froction 
" '~(Mo l . Sieve Occluded) 
i r ~ 
200 280 
TEMP ('C) 
I 
B 
COAL OIL POINT SEEP 
L. MIOCENE. MONTEREY FORMATION 
~ 1 Hexone Frootion 
v | [JILL I , ~_Cycfic 
~ / ~ ~"(Mol. Sieve Non-occluded) 
onched 8 Cyclic Froction 
~' "~........_~Ikone Fraction 
e / " --(Mol Sieve Occluded) 
IIO 200 280 
TEMP (°C) 
Fig. 7. Gas chromatographic traces of the total hexane eluate, the branched + cyclic alkanes 
(molecular sieve non-occluded fraction), and the n-alkanes (molecular sieve occluded 
fraction) for (A) Carpinteria seep oil and (B) Coal Oil Point seep oil. Locations of these 
seeps is shown in Fig. 2B. 
280 
m/e 191 A 
1:2550 H I 
Slit ]j j~ 1 
Change i // ~l I 
I:3522 ~I 
m/e 231 l 
m/e :572 
I=837 
m/e 412 ~ ~ ~ 
1=268 
I I I I ' 0 50 I00 150 200 
m/e 191 B 
I=5067 I 
s,,, i[ 
Ch0nge ~]i[ 
m/e 217 ,, 
I = 7087 i ]~n 
m/e 231 . ~ ~n 
m/e 372 
1=1476 
m/e 412 I1 
5'0 ,;o 4o 2bo. 
SCAN NUMBER 
Fig. 8. Mass fragmentograms from GC-MS analysis of the branched + cyclic fractions of 
(A) Coal Oil Point Seep and (B) Platform A crude oil. The ion fragments illustrate the 
presence of terpanes (m/e 191), steranes (m/e 217), methylsteranes (m/e 231), cholestane 
(m/e 372)and a pentacyclic triterpane (m/e 412). 
281 
and documented that tri-, tetra-, and pentacyclic terpenoid hydrocarbons 
dominate the alkane fractions of those oils. To date, there has been no sys- 
tematic study of the stability of cycloalkanes during weathering, although 
empirically it appears that these constituents accumulate as a result of the 
degradation of more labile compounds. Brown and Huffman (1976), for 
example, found in open ocean wa~rs along tanker routes that the relative 
persistence of hydrocarbons is: cycloalkanes > branched alkanes > aromatics 
n-alkanes. Thus, the compound distribution pattern of cycloalkanes, which 
appear to be resistant to weathering, may prove useful as a characteristic 
compositional parameter of oil, and possibly of the source rocks of the oil 
(Reed, 1977). 
COMPARISON OF WEATHERED AND UNWEATHERED OILS 
The 1972 U.S. Federal Water Pollution Control Act Amendments established 
the authority of the U.S. Coast Guard for oil spill cleanup. Cost of the cleanup 
is to be collected from those responsible, if such liability can be established. 
The U.S. Environmental Protection Agency {EPA) and the U.S. Coast Guard 
(USCG) are required, under the provisions of these amendments, to develop 
forensic oil identification procedures so that liability may be assessed in oil 
spill cases. To determine the origin of beach tars or oil slicks in the coastal zone, 
compositions of local sources must be known. It is of primary importance to 
determine the compositions of the known oil seeps in the general area, and to 
compare or contrast the seep compositions with production crude oils. The 
Southern California Borderland provides a rigorous testing ground for such a 
comparison. Wilkinson {1972) has located approximately 60 natural submarine 
oil seepages in the same local geographic area where there are approximately 40 
nearshore and offshore oil and gas fields, most of which have multiple produc- 
tion zones. 
The chronic "low-level" pollution of the beaches in this area by tar lumps 
may arise from illegal bilge pumping or deballasting of offshore tankers, or 
from careless production practices on the offshore production platforms, or 
from natural seepages. 
Comparison of mass fragmentograms of the branched + cyclic alkane frac- 
tions of the Coal Oil Point seep oil with that of the Dos Cuadras production 
oil show marked differences (Fig. 8). The m/e 191 mass fragmentogram in- 
dicates that the Coal Oil Point seep oil contains a greater abundance and a 
wider variety of diterpanes (Fig. 8A, scans 90--120) and of triterpanes (Fig. 
8A, scans 140--180) than the Dos Cuadras oil (Fig. 8B, scans 100--120 and 
150--170, respectively). The m/e 412 mass fragmentogram shows that the 
triterpanes of the Dos Cuadras oil are dominated (scan 176) by a single con- 
stituent (identified by its mass spectrum as 17~(H)-hopane); the same 
fragmentogram for the Coal Oil Point seep oil shows only the high back- 
ground "hump". The sterane and methylsterane distribution for the two 
samples are similarly distinct. The Dos Cuadras oil exhibits no resolved methyl- 
282 
steranes, while the seep oil exhibits clearly resolved peaks dominated (scan 
139) by a single component (4-methylergostane). Both oils show the presence 
of the C27H48 sterane (identified by its mass spectrum as 5a-cholestane). 
It is possible that the diterpanes and methylsteranes noted in the Coal Oil 
Point seep sample are the result of bacterial action. However, although partial 
degradation in the molecular structure of several triterpanes, interpreted to 
be due to microbial activity, was noted by Reed (1977) in his study of oil 
seeps in Utah, neither diterpanes nor 4-methylsteranes were apparently gener- 
ated. In those oils, the degradation consisted of partial or complete removal 
of alkyl substituents on the ring structures. The rate, extent, and biochemical 
mechanisms for partial microbial degradation of complex molecules, such as 
triterpanes, remain to be investigated. 
HYDROCARBONS AND TRACE METALS IN THE WATER COLUMN AND 
SEDIMENT FROM SEEPAGE AREAS IN SOUTHERN CALIFORNIA 
As part of a study to determine background levels of hydrocarbons and 
trace metals in the Southern California Borderland, the U.S. Bureau of Land 
Management supported a study during 1975--1976 on these parameters under 
the management of Science Applications, Inc., La Jolla, California. A sum- 
mary of some unpublished data is given below on the distribution of trace 
metals and hydrocarbons as indicators of petroleum contribution to the 
marine environment. 
Hydrocarbons 
Water samples were collected (locations shown in Fig. 2B) in a 90-liter 
modified Bodman sampler (designed and constructed by J. Payne and R. Rise- 
brough, University of California Bodega Bay Marine Laboratory). The water 
was filtered on board ship to remove particulates. 
The filtered seawater was stored for later hydrocarbon analysis in pre- 
cleaned glass bottles to which approximately 400 ml of chloroform had been 
added. In the laboratory, the water sample was acidified to pH 2, and the 
container was pressurized for upward flow (approximately 300 ml/minute) 
through three liquid-liquid extractors connected in series. The extraction sol- 
vent was chloroform. 
Sediment samples were obtained (locations shown in Fig. 2B) using an all- 
aluminum box-corer designed and constructed by A. Soutar (Scripps Institu- 
tion of Oceanography). In all cases samples of the sediment-water interface 
(0--1 cm) were collected for analysis from identical positions within the coring 
apparatus using pre-cleaned aluminum templates and scoops. The samples 
were placed in glass jars and immediately frozen. In the laboratory, the sedi- 
ment samples were thawed, washed with distilled water to remove salts, 
freeze-dried, then extracted in a Soxhlet extractor (300 cycles) using a toluene- 
methanol (3 : 7) solvent mixture. After removal of extraction solvents in a 
283 
rotary evaporator, the total organic extract was saponified by refluxing the 
sample with 1 : 1 : 1 mixtureof 0.5N KOH in toluene-methanol-water over- 
night. 
Fractionation of the seawater extracts and the non-saponifiable fraction of 
the sediment organic extract was accomplished by liquid-solid chromatog- 
raphy. Subsequent identification followed the procedure described earlier. 
Sample station 207, 5 km northwest of Santa Cruz Island (Fig. 2B), was 
taken to represent a background sample, stations 193, 204, and 775 are from 
the general vicinity of the Coal Oil Point seep area, and station 727 is near a 
probable, although l~reviously unreported, submarine seep. Two water samples 
were collected from each station, 3 m below the sea surface and 10 m above 
the sea bottom. Amounts of hydrocarbons, quantified by gas chromatography 
(Table VIII), are reported as "resolved" compounds, and "unresolved" 
components representing the elevated baseline hump (Farrington and 
Medeiros, 1975). 
The water samples from the seep areas generally showed elevated hydrocarbon 
concentrations above that of the background Sample; total alkanes and alkenes 
in the hexane eluates from extracted seawater samples are 2.5--18 times more 
concentrated in the seep area as compared to the background station, while 
the concentrations of aromatic hydrocarbons range from equal to that of the 
background to 4.6 times more concentrated (Table VIII). Hydrocarbon 
analyses of sediments from the same stations show a more dramatic effect. 
Alkane and alkene concentrations in sediment extracts range from a factor 
of 22 to approximately 1000 times greater than the background station, while 
aromatic hydrocarbons range from 34 to 220 times more concentrated than 
background levels (Table VIII). 
The clearly evident ocean-surface slick emanating from these seeps (Fig. 1), 
which no doubt causes some of the oil to be transported away from the seep 
area, apparently does not show a coincident aureole of dissolved hydrocarbons 
in the water column. However, it is clear that the sediments in the general 
vicinity of the seepage are highly enriched in hydrocarbons. This enrichment 
may be due either to the settling of partly oxidized oil from the slick, or to a 
thorough infusion of the surrounding sediments by migration of oil through- 
out the sediment. The gas chromatographic traces of both the hexane and 
benzene eluates from these sample extracts exhibit the distinctive broad, 
unresolved hump characteristic of oxidized seep oils (cf. Fig. 7). Thus, although 
the amounts of hydrocarbons recovered from the sediments and water column 
samples vary widely, their gas chromatographic traces consistently exhibit a 
similar character. 
Anthropogenic sources of hydrocarbons in the form of sewage effluents and 
harbor activities will tend to be largely confined to nearshore areas, particularly 
those areas adjacent to cities. Sewage treatment plants in the Los Angeles area 
utilize submarine canyons as disposal sites for sludge, which is then transported 
by natural processes downward into the Santa Monica Basin. Moreover, the 
region of Los Angeles Harbor is a heavily industrialized, major port, with its 
normal compliment of oil spills. 
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