Sediment Gases as Indicators of Subsurface Hydrocarbon Generation and Entrapment - Examining the Record Both in Laboratory and Field Studies

Abrams, Michael A.¹, Nicola F. Dahdah² (1) University of Utah Energy & Geoscience Institute, Salt Lake City, UT (2) Energy & Geoscience Institute at the University of Utah, Salt Lake City, UT

Near-surface marine sediment gases are commonly examined in surface geochemistry surveys to evaluate hydrocarbon generation and petroleum entrapment. Sediment gases can reside in interstitial spaces; bound to mineral or organic surfaces; and/or entrapped in carbonate inclusions. Removing sediment gases with minimal composition and isotopic fractionation can be a relatively difficult process. Multiple methods are currently available to remove and characterize surface sediment gases. Contractors provide field studies which indicate their sediment gas extraction method is best but few have rigorously tested their methods in a laboratory under controlled conditions. The key to calibration experiments is to know what ground zero is (charge gas composition) and thus have a basis to gauge the gas extraction effectiveness.

Marine sediments were cleansed of in-situ light hydrocarbons and bacteria, then homogenized. Replicate sediment samples were charged with different gas types and concentration levels. Some replicate samples were not charged as control samples. The sediments samples were sent to several laboratories for gas extraction and characterization using different analytical procedures; 2 different headspace (interstitial) procedures, 2 different adsorbed (acid extraction) procedures, ball mill (occluded), and a new interstitial gas extraction method (disrupter). The results were compared to charge gases and each method evaluated.

Based on the laboratory calibration experiments, we can show several extraction methods provide sediment gas measurements very similar to the charge gases both compositionally and isotopically. But several sediment gas extraction methods provided results with systematic errors indicating the pre-processing and analytical procedures have altered the original gas compositions.