From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, November 2, 2012
Best practices for the collection, analysis, and interpretation of seabed geochemical samples to evaluate subsurface hydrocarbon generation and entrapment
Dr. Michael Abrams, Manager Geochemistry Department, Apache Corporation
There are multiple methods currently applied by industry contractors to collect, prepare, extract, and analyze near-surface migrated hydrocarbons contained within marine sediments. To improve the detection of seabed migrated thermogenic hydrocarbon seepage, core samples should be collected along major migration pathways identified by conventional deep seismic and high-resolution seafloor imaging technology. Not all targeted cores will hit the designated feature and thus collecting replicates within key features is important. Collecting sediment samples below the Zone of Maximum Disturbance to avoid possible transition zone alteration effects and ROM masking problems is also critical. The sediment sampling device (corer) chosen should be suited for local seabed conditions to maximize both penetration and sample recovery. Real time imaging provides greater detail to confirm feature and provides a specific feature to target.
Multiple sections per core should be collected at variable depths. Geochemical analysis should include a full range of hydrocarbon types; light hydrocarbon and selected non hydrocarbon gases (C1 to C5, CO2, and N2), gasoline range (C5 to C12), and high molecular weight hydrocarbons (C15+). Two types of samples should be collected; one to capture the volatile light hydrocarbons (C1 to C12) and non-hydrocarbon gases; and a second for the higher molecular weight hydrocarbons (C15+). The light hydrocarbons require special handling that should include containers which limit volatile loss and protocols which prevent post sampling microbial alteration. Bulk sediment measurements such as quantity of organic matter and lithology type will be very helpful in evaluating the results to make sure the variability is not related to local near surface sediment conditions.
The identification of background versus anomalous populations is very important when evaluating anomalous seabed hydrocarbons. Note that near surface migrated sediment extracted hydrocarbons are normally highly altered and may not resemble conventional reservoir oil or gas compositions. Mapping thermogenic hydrocarbon oil and gas seeps relative to key cross-stratal migration pathways via fluid flow modeling and seismic attribute analysis provides an effective petroleum systems evaluation tool to better understand the seepage relative to subsurface hydrocarbon generation and entrapment.
