Chapter 9: Analysis and Interpretation of Biomarkers from Seafloor Hydrocarbon Seeps
Harry Dembicki, Jr., 2013. "Analysis and Interpretation of Biomarkers from Seafloor Hydrocarbon Seeps", Hydrocarbon Seepage: From Source to Surface, Fred Aminzadeh, Timothy B. Berge, David L. Connolly
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After seeped thermogenic hydrocarbons are discovered in seafloor sediments, biomarker analysis of the seeped oil is typically done to gain information about the contents of the subsurface accumulation. The analytical protocol usually consists of solvent extraction of the sediment with a nonpolar solvent such as hexane, followed by asphaltene precipitation and separation of saturate and aromatic hydrocarbon fractions prior to analysis of the hydrocarbons by gas chromatography–mass spectrometry (GC-MS). If the biomarker distributions obtained are unaltered, they can be used to correlate the seep oil to previously discovered oils in the basin or to deduce the characteristics of the source rock that generated the oil using conventional biomarker interpretation schemes. Unfortunately, these compound distributions are frequently compromised, which limits the information that can be discerned. When the concentration of seeped oil is low, the presence of background organic matter may mask the source-controlled geochemical information in the oil. Biomarker analysis of the background organic matter in the vicinity of the seepage is essential for distinguishing the thermogenic input from the background organic matter. Occasionally, reworked organic matter from eroded source rock, coal, or transported hydrocarbon seepage in the sediments may also contribute misleading information. As the concentration of seeped oil increases, biodegradation usually increases, often to the point where the biomarkers are altered. Hopanes and steranes are the biomarkers most susceptible to microbial alteration, although tricyclic terpanes, diasteranes, and aromatic steroids are more resistant and may provide useful information. In some sediments, biodegradation can be severe enough to alter the distributions of all compound types. Because the intensity of biodegradation can vary greatly within an individual seep feature or between a group of related features, biomarker analysis of multiple seeped oil samples provides the best opportunity to obtain useful data.
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With the increased resolution power of many geophysical methods, we are seeing direct evidence of seeps on a wide variety of data, including conventional seismic. New methods and technology have also evolved to better measure and detect seeps and their artifacts and reservoir charge and to map migration and remigration routes. In addition, detection of seepage is important for minimizing the risks associated with shallow gas drilling hazards, ensuring platform stability, and preventing well blow-outs.