The chemistry of marine petroleum seeps

Abstract

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 compositions 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 δ13C for various liquid-solid chromato-graphic fractions. The range in δ34S covered by the California oils is −4 to +15.9%., and the δ15N range is from +5.6 to +11%.. Carbon isotopic ratios for total oils exhibit a relatively narrow range (−22.1 to −27.3%.); however, chemical fractions show a relatively greater variation (−21.5 to −31.5%.), 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 δ13C of chromatographic fractions, serve to distinguish 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 cycloalkane 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 equivocal 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 thereby alter the distribution pattern. However, marine sediments receiving petroleum contributions can be differentiated from those in which only biogenic hydrocarbons are present.