Abstract
Romania is one of the European countries with the most vigorous natural seepage of methane, uprising from pressurised natural gas and petroleum reservoirs through deep faults. The largest seepage zone is represented by large mud volcanoes, with CH4 >80% v/v, occurring on the Berca-Arbanasi hydrocarbon-bearing faulted anticline, in the Carpathian Foredeep. Smaller mud volcanoes have been identified in other areas of the Carpathian Foredeep, in the Transylvanian Depression and on the Moldavian Platform. New surveys carried out in Transylvania allowed us to discover the richest N2 mud volcano zone in the world (N2>90% v/v), with a remarkably high He content and a helium isotopic signature which highlights a contribution of mantle-derived source. The large mud volcanoes are generally quiescent, with rare explosive episodes and provide a methane flux in the order of 102-103 t km?2 y?1. Independently from mud volcanism, a remarkable dry macroseepage, however, has been found, with a degassing rate up to three orders of magnitude higher than that of mud volcanoes (i.e. 103-105 t km?2 y?1). The total gas flux from all investigated macroseepage zones in Romania is estimated in the range of 1500-2500 t y?1. The emission from microseepage, pervasively occurring throughout the hydrocarbon-prone basins, has yet to be assessed and added to the total gas output to the atmosphere.
Highlights
Romania hosts Europe’s largest mud volcanoes and the densest methane-seep population. This seeapage is the result of active compressional tectonics and neotectonics perturbing deep natural gas and petroleum reservoirs, similar to what happens in many other hydrocarbon-prone basins which developed along active and seismic tectonic belts (Link, 1952; Etiope and Klusman, 2002)
The N2-rich gas composition of Homorod mud volcano is completely different from other Romanian CH4-dominant seeps
N2-dominant mud volcanoes are known in Papua New Guinea, and others may occur in areas close to recent magmatic activity (e.g., Alaska; Motyka et al, 1989), but to our knowledge, none of them reaches N2 levels >90%
Summary
All macro-seeps are generally CH4-dominant (> 90% v/v), with low CO2 and N2 concentrations and generally low 3He/4He ratios, indicating the absence of significant gaseous contributions from the mantle (Etiope et al, 2004a; Yang et al, 2003): most of the gas seeps, especially mud volcanoes, are generated along accretionary prisms dominated by crustal sediment sources. Those seeps having a high N2 and low CH4 content (Motyka et al, 1989; Baylis et al, 1997) are quite rare and considered unusual.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
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 call
