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21.
G. Etiope A. Feyzullayev A.V. Milkov A. Waseda K. Mizobe C.H. Sun 《Marine and Petroleum Geology》2009
The assessment of gas origin in mud volcanoes and related petroleum systems must consider post-genetic processes which may alter the original molecular and isotopic composition of reservoir gas. Beyond eventual molecular and isotopic fractionation due to gas migration and microbial oxidation, investigated in previous studies, we now demonstrate that mud volcanoes can show signals of anaerobic biodegradation of natural gas and oil in the subsurface. A large set of gas geochemical data from more than 150 terrestrial mud volcanoes worldwide has been examined. Due to the very low amount of C2+ in mud volcanoes, isotopic ratios of ethane, propane and butane (generally the best tracers of anaerobic biodegradation) are only available in a few cases. However, it is observed that 13C-enriched propane is always associated with positive δ13CCO2 values, which are known indicators of secondary methanogenesis following anaerobic biodegradation of petroleum. Data from carbon isotopic ratio of CO2 are available for 134 onshore mud volcanoes from 9 countries (Azerbaijan, Georgia, Ukraine, Russia, Turkmenistan, Trinidad, Italy, Japan and Taiwan). Exactly 50% of mud volcanoes, all releasing thermogenic or mixed methane, show at least one sample with δ13CCO2 > +5‰ (PDB). Thermogenic CH4 associated with positive carbon isotopic ratio of CO2 generally maintains its δ13C-enriched signature, which is therefore not perturbed by the lighter secondary microbial gas. There is, however, high variability in the δ13CCO2 values within the same mud volcanoes, so that positive δ13CCO2 values can be found in some vents and not in others, or not continuously in the same vent. This can be due to high sensitivity of δ13CCO2 to gas–water–rock interactions or to the presence of differently biodegraded seepage systems in the same mud volcano. However, finding a positive δ13CCO2 value should be considered highly indicative of anaerobic biodegradation and further analyses should be made, especially if mud volcanoes are to be used as pathfinders of the conditions indicative of subsurface hydrocarbon accumulations in unexplored areas. 相似文献
22.
Alexander Chakhmakhchev Masaru Suzuki Amane Waseda Kuniaki Takayama 《Organic Geochemistry》1997,27(7-8)
Siliceous sourced Tertiary oils from the Circum-Pacific area of Japan, Russia and the U.S.A. have a heavy carbon isotope composition, monomodal n-alkane distributions, and nearly identical regular sterane compositions with a predominance of C27 homologues. These are consistent with open marine depositional environments dominated by diatomaceous organic matter. However, a number of alkane and biomarker parameters such as Pr/Ph, CPI, relative concentration of 28,30-bisnorhopane, and the C35/C34 homohopane ratio indicate more oxic depositional environments for the source rocks of Japan and Russia. In contrast to the California Monterey Formation sourced oils, petroleums with low maturity levels from the North Sakhalin basin, Russia and the Akita basin, Japan have lower concentrations of asphaltenes and sulphur and are characterized by higher API gravities. A correlation of extractable organic matter from source rocks vs the least matured petroleums demonstrates that oil expulsion in siliceous shales of the Akita basin occurs at a maturity level corresponding to Ro≥0.65%, which is in the range of the conventional oil window (Ro = 0.6−1.1%). 相似文献