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.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California