Surface sediment gases as indicators of subsurface hydrocarbons – examining the record in laboratory and field studies

Near-surface sediment gases are commonly examined in surface geochemistry surveys to evaluate subsurface hydrocarbon generation and entrapment. Sediment gases reside in interstitial spaces, bound to mineral or organic surfaces, and/or entrapped in carbonate inclusions. Characterizing migrated sediment gases with minimal compositional and isotopic fractionation is challenging, and different methods are currently in use to extract and analyze surface sediment gases. These methods have not been compared in controlled laboratory experiments. The key to calibration experiments is to know the charge gas composition and thus have a basis to evaluate the sediment gas extraction procedure.For this laboratory study, marine sediments were homogenized and cleansed of in-situ light hydrocarbons and bacteria. Replicate sediment samples were prepared and charged with two different hydrocarbon gas compositions and two concentration levels, along with the requisite blank control samples. The sediment gases were characterized by four laboratories using a variety of gas extraction procedures; three headspace (interstitial), two adsorbed (acid extraction), and one ball mill (occluded). The results have been compared to the charge gases and each method evaluated.The laboratory calibration experiments demonstrated that selected interstitial gas extraction methods examined do provide sediment gas compositions and compound specific isotopes similar to the charge gases. However, some of the results indicate that systematic sampling and/or laboratory procedures associated with the adsorbed and ball mill gas extraction methods have altered the original charge gas compositions. Thus it is our recommendation to use a headspace interstitial gas extraction method in conjunction with a tight sealing storage container and water preparation procedure to best extract and analyze migrated sediment gases.