Feyzullayev, A.

Methane (CH4) flux to the atmosphere was measured from gas vents and, for the first time, from soil microseepage at four quiescent mud volcanoes and one ‘‘everlasting fire’’ in eastern Azerbaijan. Mud volcanoes show different activity of venting craters, gryphons, and bubbling pools, with CH4 fluxes ranging from less than one to hundreds of tons per year. Microseepage CH4 flux is generally on the order of hundreds of milligrams per square meter per day, even far away from the active centers. The CH4 flux near the everlasting fires (on the order of 105 mg·m22·d21) represents the highest natural CH4 emission from soil ever measured. The specific CH4 flux to the atmosphere, between 102 and 103 t·km22·yr21, was similar to specific flux from other mud volcanoes in Europe. At least 1400 tons of CH4 per year are released from the investigated areas. It is conservatively estimated that all onshore mud volcanoes of Azerbaijan, during quiescent activity, may still emit ;0.3–0.9 3 106 t of CH4 per year into the atmosphere. The new data fill a significant gap in the worldwide data set and confirm the importance of geologic sources of greenhouse CH4, although they are not yet considered in the climate-study budgets of atmospheric CH4 sources and sinks.

The assessment of gas origin in mud volcanoes and related petroleum systems must consider postgenetic 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 б13 CCO2 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 б13 CCO2>+5‰ (PDB). Thermogenic CH4 associated with positive carbon isotopic ratio of CO2 generally maintains its б13C-enriched signature, which is therefore not perturbed by the lighter secondary microbial gas. There is, however, high variability in the б13 CCO2 values within the same mud volcanoes, so that positive б13 CCO2 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 б13 CCO2 to gas–water–rock interactions or to the presence of differently biodegraded seepage systems in the same mud volcano. However, finding a positive б13 CCO2 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.

A global database of gas composition and methane stable isotopes of 143 terrestrial mud volcanoes from 12 countries and 60 seeps independent from mud volcanism from eight countries, was compiled and examined in order to provide the first worldwide statistics on the origin of methane seeping at the earth’s surface. Sixteen seep data were coupled with their associated subsurface reservoirs. The surface seepage data indicate that at least 76% of the mud volcanoes release thermogenic gas, with only 4% biogenic and 20% with mixed character. The average (201 data) of methane concentration and methane carbon isotope ratios (δ to the power of 13 C1) of mud volcanoes are 90% v/v and -46.4‰, respectively. The other types of seeps, which are independent from mud volcanism, have an average δ to the power of 13 C1 value that is slightly higher (-42.9‰). Gases from mud volcanoes are generally lighter (more methane, less ethane and propane) than their associated reservoir gases, suggesting a molecular fractionation during advective fluid migration. Other types of seeps, especially "dry" seeps, maintain the reservoir C1/(C2 + C3) "Bernard" ratio. Mud volcanoes behave like a "natural refinery" and the origin of gas more isotopically enriched than -50% and with C1/(C2 + C3) >500 should be attributed to a thermogenic source, rather than partial oxidation of biogenic gas. Some data that appear biogenic in the "Bernard diagram" can be explained by molecular fractionation of mixed gas. Consequently, the "Bernard" parameter may be misleading when applied to mud volcanoes since it does not always reflect the original gas composition. The mechanisms of the molecular advective segregation should be studied quantitatively by specific models and experiments.

On Earth, burial of fine-grained sediments in offshore passive margins (e.g., underwater fans and deltas) commonly results in fluid expulsion features including large-scale polygonal fractures, mud volcanoes, and pockmarks. Comparison of resulting offshore polygons and mud volcanoes with giant polygons and high-albedo mounds in the Chryse–Acidalia region of Mars shows the terrestrial and martian features to be similar in size, morphology, geologic context, and general co-occurrence within the same basin. These similarities suggest that the process of terrestrial fluid expulsion may provide an analog that could link the giant polygons and mounds in Chryse and Acidalia to a single process. Moreover, while the terrestrial offshore polygons and mud volcanoes commonly develop in the same basins, these features do not necessarily occur in exactly the same locations within those basins, as they are independent responses to compaction and dewatering. Thus, the fluid expulsion analog does not require that the martian giant polygons and mounds have identical distributions. This is the situation in Chryse and Acidalia where the giant polygons and mounds are extensively developed and generally have overlapping distributions, but where each set of features may occur in places without the other. This fluid expulsion analog is enhanced by the fact that giant polygons and mounds in Chryse and Acidalia cooccur in a regional sense and in a geologic setting that is consistent with a fluid expulsion model of formation. Implications of this analog may impact our view of the role of water in the depositional history of the martian lowlands.