Recognizing and compensating for interference from the sediment's background organic matter and biodegradation during interpretation of biomarker data from seafloor hydrocarbon seeps: An example from the Marco Polo area seeps, Gulf of Mexico, USA

摘要

A suite of seep samples from the vicinity of the Marco Polo field in the Green Canyon area of the Gulf of Mexico provides an exceptional opportunity to study the impact of interference from sediment background organic matter and alteration by biodegradation on thermogenic hydrocarbons from seafloor seeps. These seep samples contain a range in both the concentration of seeped hydrocarbons present and level of biodegradation experienced. In addition, the subsurface oil that sources the seepage was available for comparison.The results of the study show that it is essential to have the biomarker data from the sediment's background organic matter when dealing with low concentration seepage. These data provide a means to characterize the contribution of background organic matter to the biomarker distributions. This, in turn, allows the identification of thermogenic hydrocarbons when the concentration of seeped oil in seafloor sediments is low.In high concentration seepage, the hopanes and regular steranes were found to be more susceptible to near-surface microbial alteration, while tricylic/tetracyclic terpanes, diasteranes, monoaromatic steroids, and triaromatic steroids were more resistant. If biodegradation is severe enough, all biomarker compounds are vulnerable to alteration and very little information is preserved.In an effort to reconstruct the altered distributions, low temperature pyrolysis of the seeped oils' asphaltenes was done to liberate occluded biomarkers compounds. Although biomarkers were recovered, they did not resemble the parent oil. Characteristics of some of the recovered biomarker distributions suggest alteration by biodegradation. This implies the three dimensional structure of the asphaltenes may not offer complete protection from microbial attack.These results demonstrate why interpretation of biomarker data from seafloor seeps can be a difficult task. Care must be taken to recognize contributions from the sediment's background organic matter as well as identify alteration by biodegradation. Failure to understand these processes can lead to erroneous interpretations being made about the seep's parent oil and source rock.