Section 2: Science of Seepage — Methodology
Microbial communities inhabiting prolific hydrocarbon seeps in the Gulf of Mexico have been characterized by culture-independent DNA profiling of 16S ribosomal RNA (rRNA) genes. High-throughput 454 pyrosequencing was combined with serial analysis of ribosomal DNA (SARD) to vastly increase the number and, hence, sensitivity and accuracy of microbial 16S rRNA gene detection. This approach enabled the detection of more than 5 million ribosomal sequence tags and revealed that approximately one-third of the sequence tags showed a similar distribution among the sediment piston-core samples compared to the major hydrocarbon constituents present in the seeps. Numerous correlated distributions or associations were found between particular microbial DNA sequences and specific hydrocarbons, suggesting a biochemical role in the transformation of these compounds. Quantitative polymerase chain reaction (qPCR) primers were designed to target these 16S rRNA gene sequences and were found to accurately detect and serve as sensitive bioindicators for these hydrocarbons in blind tests. The results underscore the need for a thorough characterization of geochemistry and microbiology to fully understand the dynamics of these biogeochemical associations.
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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.