![]() Once available dissolved sulphate is exhausted, generally at a depth of 1 to 4 m (varies by ocean conditions and sedimentation rate), methanogenesis starts using competitive substrates. Sulphate-reducing bacteria metabolise available labile carbon. ![]() ![]() After oxygen is fully consumed by aerobic respiration, sulphate reduction becomes the dominant form of respiration (Sulphate Reduction Zone) 2CH 2O + SO 4 2−→H 2S + 2HCO 3 −. As oxygen is depleted, nitrate, iron and manganese reduction occurs within the Suboxic Zone when these substrates are present: NO 3 −→N 2 Fe 3+→Fe 2+ and Mn 4+→Mn 2+. The first marine sediment biosystem is just below the water–sediment interface, where sufficient oxygen is present for aerobic respiration (Aerobic Zone) CH 2O + O 2→CO 2 + H 2O. The generation of microbial gas, also known as biogenic gas, in shallow marine sediments is the result of a succession of interacting microbial organisms and their physical environment related to oxygen conditions and competitive substrates. These laboratory and field studies confirmed the published empirical observations: different methods provide different results, but provided calibration data to better understand the compositional and isotopic differences. In order to better understand the published results, and determine which sediment gas extraction methods best characterise migrated near-surface sediment gases, a series of laboratory experiments and field studies were conducted by the University of Utah’s Energy and Geoscience Institute. 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. Numerous publications have shown that different sediment gas extraction procedures provide different results on replicate samples. Many of the sediment gas extraction procedures currently used by industry were based on sampling and laboratory protocols designed for well cuttings and not rigorously tested to evaluate their effectiveness with unconsolidated marine sediments. There are multiple protocols to collect, prepare, extract and analyse near-surface gases from marine sediments. He describes a contiguous coating or network of structured water whereby thermogenic hydrocarbons will migrate vertically within the sedimentary column in a contiguous structured water network known as “handshake migration”. Whiticar describes how structured water creates a relatively impermeable membrane of organised water molecules that entrap the migrated thermogenic gases and the in situ generated interstitial microbial gases will have little or no exchange with the entrapped/sorbed phase. Others have suggested that the binding process is related to structured water. He also believed there was a preferential adsorption of the migrating thermogenic hydrocarbons relative to the in situ derived microbial gases. It was Horvitz’s belief that migrated thermogenic hydrocarbon gases readily bind to near-surface fine grained sediments (clays) due to the highly adsorptive nature of the clays towards hydrocarbons. Horvitz pioneered the concept of bound sediment gas analysis with his acid extraction adsorbed gas analysis. The bound gases are believed to be attached to organic and/or mineral surfaces, entrapped in structured water or entrapped in authigenic carbonate inclusions. 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. ![]() 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. ![]() The interstitial sediment gases are contained within the sediment pore space, either dissolved in the pore waters (solute) or as free (vapour) gas. Sediment gases can reside in the interstitial spaces, bound to mineral or organic surfaces and/or entrapped in carbonate inclusions. 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. 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. 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. ![]()
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