AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Analysis and Interpretation of Biomarkers from Seafloor Hydrocarbon Seeps
(1) Geological Technology, Anadarko Petroleum Corporation, Houston, TX.
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 non-polar solvent, such as hexane, followed by asphaltene precipitation and separation of saturate and aromatic hydrocarbon fractions prior to analysis of the hydrocarbons by 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 is usually observed to increase, often to the point where the biomarkers can be altered. Hopanes and steranes are the biomarkers most susceptible to microbial alteration, while the tricyclic terapnes, 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.