Abstract

Multiple methods are currently used to collect, prepare, extract, and analyze near-surface migrated hydrocarbons from marine sediments to evaluate subsurface petroleum generation and entrapment. Few have been rigorously tested to evaluate their effectiveness. A Gulf of Mexico field calibration survey over the Marco Polo field was undertaken as part of an industry-funded research project to better understand previously published and unpublished seabed geochemical results and determine which gas and liquid hydrocarbon extraction methods best characterize migrated hydrocarbons in near-surface sediments.

The Marco Polo calibration data set demonstrates the importance of targeted coring and sampling depth. To improve the detection of seabed migrated thermogenic hydrocarbons, core samples should be collected along major migration pathways (cross-stratal leakage features) identified by conventional deep seismic and high-resolution sea floor imaging. Not all targeted cores hit the designated feature, and thus, collecting replicates along key migration features is critical. Collecting sediment samples below the near-surface transition zone known as the “zone of maximum disturbance” is also important to avoid possible alteration effects and interference by recent organic matter.

Geochemical analysis should include a full range of hydrocarbon types: light hydrocarbon gases (C1–C5), gasoline range (C5–C10+), and high-molecular-weight (HMW) hydrocarbons (C15+). The interstitial sediment gas data should be plotted on a total hydrocarbon gas (Σ C1–C5) versus wet gas fraction (Σ C2–C5/Σ C1–C5) chart to identify background, fractionated, and anomalous populations. Compound-specific isotopic analysis on selected anomalous samples is critical to correctly identify migrated subsurface gases from near-surface generated microbial gases. Microdesorption bound gases did not provide gas compositions or compound-specific isotope ratios similar to the Marco Polo reservoir gases, and thus, the bound gas extraction is not recommended. A gasoline range analysis provides a new range of hydrocarbons rarely examined in surface geochemical studies that assist in identifying thermogenic hydrocarbons. Extraction gas chromatography and total scanning fluorescence (TSF) maximum fluorescence intensity provided information on the presence of thermogenic HMW hydrocarbons but did not work as well with the low-level microseepage samples. The TSF fluorogram signature was similar for both seep and regional reference (background) samples and did not help to identify migrated thermogenic hydrocarbons.

The Marco Polo calibration study provides a framework to better understand how best to collect (targeted deep cores) and extract migrated hydrocarbons from near-surface marine sediments and to evaluate the results.

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