Abstract
Gases contained within near-surface marine sediments can be derived from multiple sources: shallow microbial activity, thermal cracking of organic matter and inorganic materials, or magmatic-mantle degassing. Each origin will display a distinctive hydrocarbon and non-hydrocarbon composition as well as compound-specific isotope signature and thus the interpretation of origin should be relatively straightforward. Unfortunately, this is not always the case due to in situ microbial alteration, non-equilibrium phase partitioning, mixing, and fractionation related to the gas extraction method. Sediment gases can reside in the interstitial spaces, bound to mineral or organic surfaces and/or entrapped in carbonate inclusions. The interstitial sediment gases are contained within the sediment pore space, either dissolved in the pore waters (solute) or as free (vapour) gas. The bound gases are believed to be attached to organic and/or mineral surfaces, entrapped in structured water or entrapped in authigenic carbonate inclusions. The purpose of this paper is to provide a review of the gas types found within shallow marine sediments and examine issues related to gas sampling and extraction. In addition, the paper will discuss how to recognise mixing, alteration and fractionation issues to best interpret the seabed geochemical results and determine gas origin to assess subsurface petroleum gas generation and entrapment.
Highlights
Seabed surface geochemical surveys are undertaken to investigate evidence of subsurface petroleum generation and entrapment [1,2]
The interstitial sediment gases are contained within the sediment pore space, either dissolved in the pore waters or as free gas
The purpose of this paper is to provide a review of the gas types found within shallow marine sediments and examine issues related to gas sampling and extraction
Summary
Seabed surface geochemical surveys are undertaken to investigate evidence of subsurface petroleum generation and entrapment [1,2]. Examination of seabed geochemical signatures from worldwide surveys suggest the near-surface sediment gas and isotopic signatures are different than expected due to in situ microbial alteration, partitioning between vapour, solute, and sorbed phases, mixing with gases from multiple origins, degassing fractionation from sample collection and fractionation related to the gas extraction method [2,3]. Numerous publications have shown that different sediment gas extraction procedures provide different results on replicate samples [1,3,5,6,8,11] Few of these published studies compare the surface sediment gas compositions and compound-specific isotopes to the subsurface reservoir gases or conducted laboratory experiments to test their effectiveness. The paper discusses how to recognise mixing, alteration and fractionation issues to best interpret the geochemical results and determine gas origin with a focus on petroleum exploration
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
