Methane emission from natural gas seeps and mud volcanoes in Transylvania (Romania)

Gas seepage from petroleum basins is the second largest natural source of methane to the atmosphere, after
wetlands. The uncertainty in global emission estimates should be reduced by extending the flux database which
is fundamental for defining the emission factors and the actual area of seepage adopted for up-scaling. As a contribution
to this goal, we report a new seepage data-set for the Transylvanian Basin, one of the largest natural
gas producing regions of Europe, that is characterized by the widespread occurrence of natural leakages of gas at
the surface, including at least 73 mud volcanoes and gas seeps. In this study, methane flux was measured using
closed-chambers, from 12 seepage sites, in correspondence with focused gas vents (mud volcano craters, bubbling
pools, and flammable gas leaks), in the soil surrounding the vents, and at 15 sites located far from macroseep
zones but close to gas fields. Fluxes from individual vents (macro-seeps) were found to reach orders of
kg CH4 m)2 day)1 (up to 12 kg m)2 day)1) and diffuse fluxes from soils (miniseepage) were found to be up to a
few g CH4 m)2 day)1. Far from seep zones, positive CH4 fluxes (microseepage) may occur locally, typically on
the order of tens to hundreds of mg m)2 day)1. The values, as well as the occurrence of seepage even far from
vent zones and in mud volcanoes that are apparently extinct, are coherent with results obtained in other countries.
Gas fluxes from macro-seeps and soils may change seasonally, but the interannual variation of the average
emission factor was found to be minimal. The total CH4 output for Transylvania macro-seeps is estimated conservatively
to be around 680 t year)1; the total geo-CH4 seepage emission from the Transylvania petroleum system
could be approximately 40 · 103 t year)1, and at least 100 · 103 t year)1 for all Romanian petroleum systems,
that is roughly 10% of the total anthropogenic CH4 emission in the country.

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