Abstract
Seafloor sediment samples collected from southwestern offshore Taiwan have been analyzed for hydrocarbon gases resident in samples by using the technique of headspace gas analysis. The results reveal that the gas content is in tens to thousands ppm (vol. in wet sediments). Both microbial gas (usually called biogenic gas) dominated with methane and thermogenic gas containing C2(superscript +) hydrocarbons were detected, inferring that the gases involved in the potential gas hydrate occurrence in the study area may have multiple origins. The microbial gas generated by methanogenic archaea in immature sediments is more widely distributed than thermogenic gas generated in sediments of the catagenesis stage. The presence of thermogenic gas infers an effective petroleum system, which may favor the formation of gas hydrate as well as for oil and gas exploration.
Highlights
Gas hydrates occur in sedimentary deposits under specific conditions of high pressure and low temperature present in permafrost regions and beneath the sea in outer continental margins, usually at water depths greater than about 500 meters
The presence of gas hydrates in offshore continental margins has been inferred mainly by anomalous seismic reflectors known as bottom-simulating reflectors (BSR) that coincide with the predicted phase boundary at the base of the gas hydrate stability zone (GHSZ)
4.2.1 Gas Source and Gas Hydrate Distribution. This survey reveals the occurrence of gas anomalies of both microbial and thermogenic origins in the seafloor sediments, inferring that the potential gas hydrate in the study area may have multiple gas origins, with microbial gas more widely occurring than thermogenic gases
Summary
Gas hydrates occur in sedimentary deposits under specific conditions of high pressure and low temperature present in permafrost regions and beneath the sea in outer continental margins, usually at water depths greater than about 500 meters. BSR has been reported in the seismic survey carried out in southwestern offshore Taiwan, indicating potential occurrences of gas hydrates (Chi et al 1998; Schnürle et al 1999; Liu et al 2006). Beyond this geophysical evidence, any geochemical information concerning the gas system may become important to support the presence of the gas hydrates. The chemical and isotopic composition of the hydrocarbon gas, e.g., the methane/ethane ratio and δ13CCH4 are a diagnostic to the gas origin. Hydrocarbon gas of the methanogenic origin is exclusively composed of methane, which is light in carbon stable isotopic composition, usually with δ13CCH4 lighter than -50 000. The thermogenic methane is heavier in carbon stable isotopic value (Schoell 1984)
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 callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
