Biodegradation in the Subsurface as a Selective Filter 

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FINAL TECHNICAL SUMMARY 

STUDY TITLE: Weathering of Aromatic Compounds in the Coastal Marine Environment: 

Quantifying Rates of Microbial Metabolism 

REPORT TITLE: Biodegradation in the Subsurface as a Selective Filter for Natural 

Petroleum Emissions 

CONTRACT NUMBER: 14-35-0001-31063 

SPONSORING OCS REGION: Pacific 

APPLICABLE PLANNING AREA(S): Southern California 

FISCAL YEAR(S) OF PROJECT FUNDING: FY 02/03, FY 03/04, FY 04/05 

COMPLETION DATE OF REPORT: November 2006 

COSTS: FY 02-04 – $83,071 

CUMULATIVE PROJECT COST: $83,071 

PROJECT MANAGERS: Russell J. Schmitt

AFFILIATION: Coastal Research Center, Marine Science Institute, University of 

California, Santa Barbara, CA 93106 

ADDRESS: University of California, Santa Barbara, CA 93106 

PRINCIPAL INVESTIGATOR: David L. Valentine 

ADDRESS: Department of Earth Science and Marine Science Institute, University of 

California, Santa Barbara, CA 93106 

KEY WORDS: oil seep, weathering, aromatic hydrocarbons, biodegradation 

BACKGROUND: Several tons of oil seep from the sea floor into the waters of coastal 

California each day. The oil is converted to tar by an ill-defined combination of chemical, 

biological and physical processes – collectively referred to as weathering. The broad focus of 

this project was to determine the microbial role in the weathering process – particularly as it 

relates to aromatic hydrocarbons. 

OBJECTIVES: The objectives of this work were to investigate the nature of hydrocarbon 

weathering processes, particularly those mediated by microbes. 
Final Technical Summary – Valentine 

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DESCRIPTION: Initially we planned to use a radiotracer approach to investigate the 

consumption of specific aromatic hydrocarbons. Soon after the project was initiated we became 

aware of a new analytical approach better suited to the investigation of microbial hydrocarbon 

weathering: two-dimensional comprehensive gas chromatography (GC×GC). At the same time, 

the instrument we planned to use for the radiotracer work was pulled from the market, 

confirming our decision to switch techniques. 

 

Completed research focused on the detailed compositional changes that occur in petroleum from 

the time of formation, through subsurface migration, to the point of seepage from the seafloor 

and finally to spreading at the sea surface. This was achieved applying the new technique of 

comprehensive two-dimensional gas chromatography to a natural progression of petroleum 

samples collected from subsurface reservoirs, the sea floor and the sea surface. 

STUDY RESULTS: Environmental studies designed to assess the patterns and rates of 

petroleum weathering during migration and seepage has been completed, the samples were 

analyzed using GC×GC, and a manuscript has been submitted for publication. This manuscript 

highlights the power of the approach developed in this study, and shows detailed chemical 

changes in over 4000 specific hydrocarbon compounds. Biodegradation is shown to be the 

dominant process in the subsurface, accounting for a loss about half the total petroleum 

hydrocarbon. Once petroleum reaches the sea floor, evaporative processes act rapidly, causing a 

loss of about 10% of the total petroleum hydrocarbon – on the order of minutes. Other physicalchemical processes, such as adsorption to a thin layer, also seem to drive compositional changes. 

The entire weathering scheme appears to drive the petroleum toward a more even massdistribution of compounds, which may provide a quantitative and robust biodegradation index. 

 With regard to specific compounds and compound classes, striking trends in the chemical 

composition of petroleum are apparent from these studies. During transport from the reservoir to 

the sea surface, a number of compound classes, representing several hundred distinct 

compounds, are removed nearly quantitatively. The compound classes most affected include nalkanes, branched alkanes, isoprenoids, alkyl cyclohexanes, alkyl cyclopentanes, alkyl benzenes, 

and alkyl napthalenes. The application of physical model incorporating evaporation (gas 

washing) and dissolution (water washing) indicates that neither of these processes can explain 

the observed changes in chemical composition – thus indicating these changes are microbial in 

origin. Biomarkers such as hopanes and steranes in these samples remain unchanged. 

Differences are also observed between samples at the sea floor and at the sea surface, though not 

as pronounced as the differences previously discussed. These differences are best explained by 

evaporation, that is, the low boiling-point compounds are preferentially removed. This may 

occur through gas washing at the sea floor or through evaporation at the sea surface. While this 

investigation was geographically limited in scope, focusing on only one seep, the patterns of 

weathering are striking and provide never-before-seen resolution as to the changes in petroleum 

composition due to weathering processes. 
Biodegradation in the Subsurface as a Selective Filter 

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STUDY PRODUCTS: 

One manuscript has been submitted for publication based on this work, and two presentations 

were recently given. Two additional publications are planned for research resulting in-part from 

MMS support. These include the results of long-running biodegradation studies and a 

comparison study of oil composition from various reservoirs offshore reservoirs. 

Publications: 

American Geophysical Union 2005 Fall Meeting, Wardlaw and Valentine (2005) Petroleum 

Weathering Associated with Hydrocarbon Migration and Seepage, a Case Study From the Santa 

Barbara Channel, CA (published abstract). 

Wardlaw G, C Reddy, B Nelson, S Arey and D Valentine (submitted to Nature) Biodegradation 

in the subsurface a selective filter for natural petroleum emissions. 

Research Presentations:

American Geophysical Union 2005 Fall Meeting, Valentine, Wardlaw, Kinnaman, Redmond, 

Kimball, Busso and Larson (2005) Lessons in Microbial Geochemistry from the Coal Oil Point 

seep field: progess and prospects (published abstract).