Environ. Sei. Technol. 1982, 16, 278-282 Systems, Parts I and 11, EPA Reports 60017-77-113 and 600/7-78-074. Arthur D. Little Inc. and Rosenblatt, D. H., “Research and Development Methods for Estimating Physicochemical Properties of Organic Compounds of Environmental Concernn, Report for US. Army Medical Research and Development Command Fort Detrick MD, 1981. Steen, W. C.; Karickhoff, S. W. Chemosphere 1981,10,30.
(21) ‘,Sugiura, K.; Ito, N.; Matsumoto, N.; Mihara, Y.; Goto, M. Chemosphere 1978, 7, 734.
Received for review August 4,1981. Revised manuscript received December 14, 1981. Accepted December 14, 1981. Financial support for this work was obtained from the USEPA and the Inland Waters Directorate of Environment, Canada.
Gaseous Hydrocarbons around an Active Offshore Gas and Oil Field Denls A. Wlesenburg,t James M. Brooks,” and Roger A. Burke, Jr.$ Department of Oceanography, Texas A&M University, College Station, Texas 77843
Low molecular weight hydrocarbons (LMWHs, C1-C4) were measured from the water column and sediments around an oil and gas field. No significant differences in mean methane levels were observed between platforms that were and were not discharging brine. However, in the 20-station grid, the relative standard deviations were greater and the highest individual methane and ethane concentrations were found in surface waters near the platform discharging brine. Higher methane values at all depths observed during summer coinciding with decreased ethanelethene ratios in a near-bottom nepheloid layer provided direct evidence of in situ biological production associated with increases in zooplankton and bacterial biomass in the water column. The sediment LMWHs are predominantly of thermogenic origin probably due to seepage from the subsurface, as evidenced by high levels of methane and elevated ethanelethene ratios. The LMWH input from brine discharge in the field is estimated at 283 g/day. Introduction
As the energy requirements of an expanding society continue to grow, there has been increased offshore drilling for oil and gas to meet those energy demands. With this increasing offshore activity, much recent concern has been focused upon the potential contamination of the marine environment by offshore production operations. An e€fective method for evaluating petroleum-derived hydrocarbon inputs into the marine environment is by measurement of gaseous, low molecular weight hydrocabrons (LMWHs). While LMWHs are not themselves important environmentally, because of their low toxicities, they are of secondary importance since they are introduced along with the light aromatic compounds, the most immediately toxic components of petroleum (1,2).Several studies (3, 4) have shown that LMWHs are excellent tracers of these most toxic components of petroleum. LMWHs are also of significance in geochemical petroleum prospecting, because of the upward flux of gaseous hydrocarbons from subsurface gas and oil reservoirs (5). The natural processes controlling the geochemistry of LMWHs in the water column are largely unknown, except in anoxic areas where obligate anaerobic bacteria produce methane (6). Concentrations of methane higher than atmospheric equilibrium values exist naturally in the aerobic water column because of in situ generation (7, 8) and ‘Present address: Biological and Chemical Oceanography Branch, Naval Ocean Research and Development Activity, NSTL Station, MS 39529. Present address: University of South Florida, Department of Marine Science, St. Petersburg, FL 33701. 278
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diffusion or seepage across the sewater-sediment interface (9,10). The biological processes producing Cz-C4 hydrocarbons are not well defined, although biogenic production of ethene, ethane, propene, or propane have been observed in the water column (11,12) and surficial sediments (10, 13). Most of the natural sources of LMWHs to the Louisiana-Texas shelf, however, are believed to be small compared to the anthropogenic inputs from offshore platforms, transportation, and runoff (14). Other studies ( 4 1 5 ) have reported background levels of LMWHs in the marine environment, as well as the resultant increases in these levels associated with isolated, catastrophic inputs that have resulted from offshore drilling accidents (4, 16). In this paper, we report on measurements of LMWHs around an active, established, gas- and oil-producing field in the northwestern Gulf of Mexico. The effects of anthropogenic inputs upon natural concentration levels are detailed. The Buccaneer Gas and Oil Field (BGOF) is located ca. 50 km south southeast of Galveston, TX (Figure 1)in ca. 20 m of water. The field consists of 17 structures, of which 2 are production platforms, 2 are quarters platforms, and 13 are satellite structures surrounding well jackets. Produced brine from the production operations is discharged into the surface waters at only one of the production platforms (platform 296B). The brine discharge rate has been estimated at 95400 L/day (17).The field has been in production since 1960. A unique feature of BGOF is that it is relatively isolated from other production fields, thus our study was not complicated by other nearby anthropogenic hydrocarbon sources. Experimental Section Samples of the produced brine and oil were collected for analysis of LMWHs along with seawater and sediment samples from a grid of stations around platforms 288A and 296B (see Figure 1). Water column samples were taken from the surface and from 10- and 20-m depths around both production platforms. The 20-station grid for water sampling comprised four lies extending north, south, east, and west of each platform, with samples being collected at 25,50, 100, 150, and 300 m distant from the platform. Sediment samples were taken along the same grid lines at distances of 10, 25, and 50 m. Water column sampling was performed aboard the M/V Tonya and Joe by using standard 12-L Niskin sampling bottles. Samples for LMWHs were transferred into 200mL glass bottles, which were allowed to overflow, poisoned with sodium azide, and capped without a headspace. The samples were analyzed in a shore-based laboratory after the methods of Brooks et al. (8). The method involves stripping the hydrocarbons from solution with a purified
0013-936X/82/0916-0278$01.25/0
0 1982 American Chemical Society
Table I. Light Hydrocarbons (pg/L) in Buccaneer Gas and Oil Field Brine and Oila compound sample 1 sample 2 sample 3 sample 4 Brine methane 630 810 1670 3 220 ethane 170 360 620 830 propane 126 290 580 730 isobutane 55 114 309 451 butane 45 114 288 468 isopentane 120 170 404 646 pentane 121 112 322 600 n-CS-Cl4 420 500 1100 5 100 BXT~ 10000 10300 15300 31100 Oil 8900 135000 141000 126000 methane ethane 8500 275000 169000 351000 propane 60 000 1530 000 802 000 1 8 6 9 000 isobutane 78 000 1 270 000 673 000 1 5 3 0 000 butane 116 000 1 4 5 0 000 791 000 1 6 9 0 000 isopentane 646 000 3 840 000 2 460 000 5 160 000 pentane 661 000 2 950 000 2 050 000 4 130 000 a Brine samples 1, 2, 3, and 4 were taken 9/21/79, 9/22/79, 1/7/80, and 1/8/80, and oil samples were taken 1/5/80, 1/9/80, 2/4/80, and 4/8/80, respectively. BXT = the sum of benzene, toluene, ethylbenzene, and m-,p-, and o-xylene. KILOMETERS
BUCCANEER FIELD
NAUTICAL MILES
Figure 1. Location map of the Buccaneer Gas and Oil Field.
helium stream onto a trap at liquid-nitrogen temperature and subsequent isolation and heating of the trap, followed by injection of the trapped sample into a chromatographic stream via a six-port valve. A Hewlett-Packard 5830 gas chromatograph with a flame ionization detector and a Porapak Q column were used for analysis. Surficial sediment samples were collected by divers using 500-mL jars. The surficial sediment analysis was after the method of Bernard et al. (18). Water column measurements of transmissometry, total suspended matter (TSM), chlorophyll, adenosine triphosphate (ATP), and sediment organic carbon were also obtained. Transmissometry was measured optically with a Martek XMS in situ transmissometer, which also contained a thermister probe for temperature measurements. Total suspended matter was determined by filtering onto 0.40-pm Nuclepore filters (191,chlorophyll by fluorometry (20), organic carbon by the wet oxidation technique detailed by Fredericks and Sackett (21),and ATP by using a SA1 Model 3000 ATP photometer and the method of Holm-Hansen and Booth (22). Results and Discussion Brine and Oil. Table I shows the LMWH concentrations in the produced oil and brine discharged from platform 296B. The daily and seasonal variations observed in the four BGOF oils probably reflect production mixes from the many producing wells in the field. All oils contain appreciable quantities of light hydrocarbons, with concentrations and relative compositions reflecting differing maturation processes (catagenesis) occurring in the sedimentary column (23). The source of the LMWHs in the brine is the oil/gas. The brine and oil/gas mixture rise together through the well pipe and are then separated on the platform. The LMWH concentrations in the brine are controlled both by
the mole fraction of individual components in the oil and individual component solubilities. However, since the produced oil/brine mixture undergoes physical separation, the concentrations of LMWHs in the dischraged brine are also influenced by evaporative losses in which LMWHs with higher vapor pressures and lower solubilities are partitioned to the atmosphere to a greater extent than the more soluble components. All these processes act to produce a discharge brine with high LMWHs and light aromatic (e.g., benzene, toluene, xylenes) concentrations. The data in Table I indicate that n-C5-CI4 hydrocarbons contribute only 4-12 % to the total hydrocarbon content of the produced brine, whereas the light aromatic (benzene-0-xylene) concentrations are much larger than both the LMWHs and the n-C5-CI4 hydrocarbons. These relative distributions reflect the partitioning effects of solubility, vapor pressure, and varying concentrations. Water Column. There were no obvious directional or distance differences among the 20 stations sampled for LMWHs around the platforms. Thus, the data from each platform were averaged for comparison. The mean LMWH concentrations measured in Buccaneer Field (Table 11) are typical of unpolluted waters along the Texas Coast. Since no underwater venting of gas is occurring in BGOF, the only major source of anthropogenic gaseous hydrocarbons is from the discharge of oil-field brine. Surface-water methane levels around the platforms averaged 4-5 times higher than calculated atmospheric equilibrium concentrations (24). This supersaturation is typical for coastal waters which contain comparatively high concentrations of both nutrients and suspended materials (8). A comparison of the mean values of methane (Figure 2) between the platform discharging brine (296B) and the platform where there was no discharge (288A) reveals no significant difference between the stations, all within 300 m of the platforms, that could be attributed to platform effluents (i.e., brine discharges). However, the effect of brine discharge at platform 296B is readily apparent in the relative standard deviation (RSD) and also the maximum values (Table 11). At platform 296B, the RSD is ca. 40% compared to ca. 7% at platform 288A where there is no brine discharge. The maximum methane concentration occurred 100 m to the west of platform 296B during the Environ. Sci. Technol., Vol. 16, No. 5, 1982 279
Table 11. Average Light Hydrocarbon Concentrations (nL/L) around Platforms 296B and 288A during the Summer and Winter Samplings in 1980a ethane/ ethene
methane
platform, month
eth- ethRSD, % max ene ane Surface Water 296B, Aug 198 40.9 517 9.0 5.8 0.64 296B, Jan 148 37.8 302 5.7 5.0 0.88 288% Aug 174 6.9 193 7.6 Cl,) n-alkanes reported by Middleditch et al. (17). The c6-c1, hydrocarbons have the greatest input rates because of the higher solubility of the substituted benzene compounds in the brine. However, these brine discharges are apparently rapidly diluted by the large volume of water moving through the BGOF area, such that elevated LMWH levels were only observed in a few samples, and the average values represent natural variations and not anthropogenic effects. At all sampling depths methane levels were significantly higher in the summer sampling as compared to that of the winter. The reasons for increased levels at the surface are only speculative. It seems likely that the differences are the result of greater turbulent mixing during the winter, which would increase degassing to the atmosphere. The winter water column was characterized by a mixed layer in the upper 12-13 m with a temperature inversion near the bottom due to surface cooling of the mixed layer. The summer surface water was characterized by ca. 6 O C higher temperature, along with ca. 50% of the TSM levels observed during the winter (Figure 3), but with a much greater percentage of the TSM composed of living biota. Thus the lower surface methane in the winter could be the result of greater mixing in the surface layer and/or less abundances of phytoplankton and cellular material (zooplankton and bacteria) than during the summer. No statistical difference in average methane concentrations was observed between the platforms, either at the 10-m or near-bottom sampling depths (Figure 2). The near-bottom methane levels during the summer sampling were over twice the values in the upper 10 m of the water column at both platforms. These higher near-bottom levels seem to be associated with the near-bottom nepheloid layer. The apparent source of this methane is the cellular material (Figure 3), which consists of zooplankton and bacteria. The phytoplankton contribution, as estimated by chlorophyll measurements, to the TSM load of the samples was ca 25% near-bottom compared to ca. 48% near-surface. Thus the summer bottom nepheloid layer was associated with either a high zooplankton or a high bacteria contribution to the particulate biotic load. This is in agreement with other studies which have postulated that methane can be produced in aerobic water in situ, either in reducing microenvironments associated with
0
TRANS M ITT A N C E ( %) 40 60
20
0
I
I
5-
--I
25t 30 1 20
Trans.
100
1
Non-cellular 5%
Surface
Temp.
SUMMER I
S
22
I
,
.
I
1
24 26 28 TEMPERATURE (" C )
.
I
30
1 I
10 -
'15
-
E"
r
I0.
w 20
n
-
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32
Bottom
Phyto
I
TSM Averages - Sfc 481t128 r g / L 10 m: 396t 129 r g l L BT: 1529t3214 r g l L
c
9)
li
TSM Averages Sfc: 233 t 61 fl g / L 10m:219t61 r g l L BT: 323t275 r g / L
I
r
80 2 \'
I
43% Cellular
Surface
i/ Trans.
Phyto 6% Cellular
Temp.
25 -
7%
WINTER Clay 87%
30
suspended particulate material or in the gut of zooplankton (7, 8). The methane concentrations in near-bottom water in the winter were not as great as during the summer, even though bottom TSM levels were ca. 5 times greater (Figure 3). The near-bottom nepheloid layer in the winter was due to resuspension of bottom sediments, with clay making up ca. 87% of the TSM load. Although the percentage of phytoplankton and cellular material was only about 13% of the TSM in bottom waters, the absolute amount of these biotic fractions was almost as high as at the surface levels. Therefore, the winter methane values in the near-bottom waters could have resulted either from bacterial production due to resuspension of bottom sediments or from zooplankton present in the biota. The low ethanelethene ratios in the bottom water indicate that the high methane values found in the summer bottom water could not result from LMWH diffusion across the seawaterlsediment interface into the stable bottom water. The ethene and ethane data for the water (Table 11)and sediments (Table 111)show that if diffusion were contributing significantly to LMWHs in the bottom waters, ethanelethene values would be much higher than was observed. The ethanelethene ratio of the sediment gases was always greater than 1 (see Table 111). In the summer bottom waters, however, the ethanelethene ratio decreased to between 0.09 and 0.15. These low ethane/ ethene ratios at near-bottom levels imply in situ production
Table 111. LMWHs in Surficial Sediments in Buccaneer Field locations below discharge pipe at 296Ba around 296B, summer 197gb around 296B, winter 1980 below discharge pipe at 288A around 288A, summer 1979 normal shelf sediments'
methane, mL/L
ethane/ ethene
carbon, %
%
2.3
3.1
0.23
28
1.3
3.4
0.18
19
1.1
2.2
0.21
14
3.6
6.5
0.27
10
1.9
4.6
0.19
18
0.1
0.5
0%
CaCO,,
a Average of summer and winter samplings. Numbers represent average of 16 stations taken north, east, south, and west at 10, 25, 50, and 100 m, respectively. ' After Bernard ( 1 0).
of both methane and ethene in bottom waters with no significant input of methane or ethane from the sediments. Surficial Sediments. LMWH distributions in nearsurface marine sediments are a result of relatively shallow biological production and consumption processes (6)and in some cases of migration of thermogenic gas produced deep in the sedimentary column (5). Although near-surEnviron. Sci. Technol., Vol. 16, No. 5, 1982 281
face sediment LMWH surveys are used in offshore petroleum exploration, there has been little published evidence that LMWH migration from subsurface accumulations are expressed in surficial sediments. Bernard et al. (18) found in slope and abyssal plain cores from the Gulf of Mexico that upward diffusion from large accumulations of LMWH in zones deeper than 10 m were not detectable in near-surface sediments. Thus, if thermogenic hydrocarbons appear in surficial sediments, their presence must be the result of migration up microfaults or fractures. Table I11 lists average LMWH concentrations and compositions in surficial sediments observed around BGOF, as well as average Texas shelf values (10). Methane levels in BGOF sediments are an order of magnitude higher than levels measured on the Texas shelf by Bernard et al. (18). The highest methane level they reported was 258 pL/L compared to 9.5 mL/L in this study. No discernable pattern was observed in transects north, east, south, and west of platforms 288A and 296B. There were no obvious correlations between the sediment gases and either organic carbon or CaC03content of the Sediments. The levels were influenced by sediment particle size, with the highest values being found in sediments with a high clay content and lowest levels in sandy or shelly sediments (see ref 19 for sediment property details). Ethane and propane showed similar patterns to methane, and no significant differences were observed between platforms. The higher than normal average methane concentrations in the BGOF sediments suggest a relatively large addition of thermally derived deep gas (13,25)to the biologically produced component typically present in shelf sediments. The ethane/ethene ratios for the BGOF sediments (Table 111)are also diagnostic of petroleum origin. Normal marine sediments have ratios generally ca. 0.5 (IO),since ethene is produced in shallow sediments by some biotic process and the biogenic production of >C, homologues of methane is small. Since olefins are nearly absent from petrogenic hydrocarbon sources, ethane/ethene ratios greater than 1.0 are indicative of petroleum-derived hydrocarbons (13). While there is some biogenic gas in BGOF sediments, the LMWHs are dominated by the thermogenic components.
Summary The release of brine from BGOF has little impact on LMWH distributions, except in surface water within a few meters of the discharge. No significant differences in mean methane concentrations around the two platforms were exhibited, although the relative standard deviation and maximum concentration were greater around the platform discharging brine. Anthropogenic LMWH inputs from the brine discharge are also reflected in higher mean ethane concentrations and high ethane/ethene ratios in surface waters around the discharging platform. Observable increases in methane concentrations during the summer in surface waters can be attributed to greater biological activity and/or less turbulent mixing than during the winter. High methane concentrations coinciding with decreased ethanelethene ratios in the near-bottom nepheloid layers provided direct evidence of in situ production of methane in the near-bottom water. This in situ production was associated with increases in both zooplankton and bacterial biomass and with the type of suspended particulate matter (living) rather than the total abundance. Surficial sediments exhibited about an order of magnitude greater methane concentrations and 6-12 times greater ethane/
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ethene ratios than normal Texas shelf sediments, indicating that these surficial sediment gases are controlled by migration of thermogenic gas from deeper in the subsurface.
Acknowledgments We thank B. Gallaway of LGL Ecological Associates
US., Inc., for collection of surficial sediment samples. H. A. Abd el-Reheim, C. R. Schwab, and R. Pflaum helped with seawater collection.
Literature Cited (1) Blumer, M. Environ. Aff. 1971, 1, 54. (2) Baker, J. M. Environ. Pollut. 1970, 1, 27. (3) Sauer, T. C., Jr.; Sackett, W. M.; Jeffrey, L. M. Mar. Chem. 1978, 7 , 1. (4) Brooks, J. M.; Wiesenburg, D. A.; Burke, R. A., Jr.; Kennicutt, M. C. Environ. Sci. Technol. 1981, 15, 951. (5) Horvitz, L. CCOP/SOPAC Tech. Bull. 1980, 3, 261. (6) Reeburgh, W. S. Earth Planet. Sci. Lett. 1976, 28, 337. (7) Scranton, M. I.; Brewer, P. G. Deep-sea Res. 1977,24,127. (8) Brooks, J. M.; Reid, D. F.; Bernard, B. B. J . Geophys. Res. 1981,86, 11029. (9) Bernard, B. B.; Brooks, J. M.; Sackett, W. M. Proc. Am. Offshore Technol. Conf. 1977,435-483. (10) Bernard, B. B. Ph.D. Thesis, Texas A&M University, 1978. (11) Lamontagne, R. A,; Swinnerton, J. W.; Linnenbom, V. J. Tellus 1974, 26, 71. (12) Wiesenburg, D. A., Ph.D. Thesis, Texas A&M University, College Station, TX, 1980. (13) Kvenvolden, K. A.; Vogel, T. M.; Gardner, J. V. J . Geochem. Explor. 1981, 14, 209. (14) Brooks, J. M. Ph.D. Thesis, Texas A&M University, College Station, TX, 1975. (15) Brooks, J. M.; Sackett, W. M. J . Geophys. Res. 1973, 78, 5248. (16) Brooks, J. M.; Bernard, B. B.; Sauer, T. C.; ABD el-Reheim, H. Environ. Sei. Technol. 1978, 12, 695. (17) Middleditch, B. A.; Basile, B.; Chang, E. S. Bull. Environ. Contam. Toxicol. 1978,20, 59. (18) Bernard, B. B.; Brooks, J. M.; Sackett, W. M. J . Geophys. Res. 1978,83, 4053. (19) Brooks, J. M.; Estes, E. L.; Wiesenburg, D. A.; Schwab, C. R.; Shokes, R. F. In “Environmental Assessment of Buccaneer Gas and Oil Field”; Plenum Press: New York, 1981; pp 69-115. ‘ (20) Strickland, J. D. H.; Parsons, T. R. “A Practical Handbook of Seawater Analysis”, 2nd ed.; Fisheries Research Board of Canada: Ottawa, Canada, 1972. (21) Fredericks, A. D.; Sackett, W. M. J . Geophys. Res. 1970, 75, 1933. (22) Holm-Hansen, 0.; Booth, C. R. Limnol. Oceanogr. 1966, 11, 510. (23) Hunt, J. M. “Petroleum Geochemistry and Geology”; W. H. Freeman: San Francisco, CA, 1979. (24) Wiesenburg, D. A.; Guinasso, N. L., Jr. J . Chem. Eng. Data 1979, 24, 356. (25) Cline, J. D.; Holm&, M. L. Science (Washington, D.C.) 1977, 18, 1199. (26) Wiesenburg, D. A.; Bodennec, G.; Brooks, J. M. Bull. Environ. Contam. Toxicol. 1981, 27, 167.
Received for review August 24,1981. Accepted January 15,1982. This work is a result of research conducted under interagency agreement between the Environmental Protection Agency and the Department of Commerce, National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Southeast Fisheries Center, Galveston Laboratory, and under Contract No. NA79-GA-(7-0037. Preparation of this paper was made possible by the Office of Naval Research Contract No. N O 0 0 1 4-80- C-0113.
