Methane (CH₄) is an important greenhouse gas, but our understanding of the magnitude of its sources is fraught with considerable uncertainties. Quite neglected in early atmospheric methane budget studies, Earth’s degassing is today considered a major natural source of methane. Geological (natural fossil) methane emissions, including gas seepage in petroliferous sedimentary basins (macro-seeps, microseepage, marine seepage) and geothermal exhalations, were estimated at the global scale by several research groups, based on bottom-up and top-down procedures, and accounting for ~40-50 Tg CH₄ yr^-1 (latest review and discussions on conflicting estimates are in Etiope and Schwietzke, 2019, and Thornton et al. 2021). This value is equivalent to roughly one-third of the average emission attributed to wetlands, and it rivals with the high uncertainty of freshwater sources. Global gridded mapping was developed to provide the spatial distribution of the geological methane sources, as well as their isotopic (¹³C/¹²C) composition and potential intensity (Etiope et al. 2019), and it has been used to refine fossil fuel industry and microbial CH₄ emission budget. Geological emission breakdown at continental scale could be derived (e.g., Petrescu et al. 2023), with the emission values that must be considered only in terms of “order of magnitude”. The geo-CH₄ sources are now included in the methane budget of the Global Carbon Project (Saunois et al. 2020), where they are compared to other natural and anthropogenic sources, including an analysis of double-counting with some biological sources. Correct definitions, source attribution, uncertainties and limits define the roadmap strategy for refining emission estimates.

In spite of the decreasing production, Romania remains one of the most important oil and gas producers in Europe. The European inventories of anthropogenic methane release reveals unexpectedly high emissions related to the oil and gas industry in Romania. The number of drilled wells in Romania is exceeding 60,000, with the majority of them currently being inactive. A first approach to quantify the methane emissions from active wells was conducted within the ROMEO project. The highest emissions were observed at the facilities with poor technical maintenance. As part of the UNEP-funded project Global Analysis of Methane Emissions from Abandoned Oil and Gas Wells, an evaluation of methane release from inactive wells is ongoing. No comprehensive inventory of the inactive wells is available for the moment at a national level. The ENVERUS (DrillingInfo), database provides information on 6348 wells in Romania, out of which 776 wells in the Transylvanian Basin. More than 72% of the reported wells were dry, while different amounts of oil and/or gas have been found in the others. According to the nationally applicable closure procedure, most of the wells are plugged and buried, making their identification and detection of gas leaks difficult. In a few cases, substantial methane emissions have been reported, which may pose a threat to neighboring residents.

Acknowledgment: this contribution was supported by the project Global Analysis of Methane Emissions from Abandoned Oil and Gas Wells, funded by UNEP.

All hydrocarbon reservoirs leak slightly. At sites with only minor leakage the hydrocarbons (HC) infiltrating the sediment from below are completely metabolized before reaching the sediments surface due to microbial activity, thus not creating any surface manifestations, e.g. seeps. Nevertheless, the HC influx will change geochemistry as well as microbial community composition and activity in the affected area because electron donors are added into the system. The PROSPECTOMIS project wants to detect these changes to develop a minimally invasive and low-cost tool do detect HC reservoirs using omics and geochemistry techniques.

In November 2021 we recovered fifty 2-3 m long sediment cores from three minor HC seepage zones and two non-seepage reference zones in the southern Barents Sea and sampled sediment and pore water with high spatial resolution.

While Fourier-transform ion cyclotron resonance mass spectrometry and cell abundances did not show any differences between HC seepage zones and reference zones, multiple pore water concentration profiles e.g. for Manganese, Calcium, Silicon, Strontium, Sulfide revealed differences. Also, higher fluxes of sulfate and alkalinity in HC zones indicate that the sink for sulfate and the source of alkalinity must be shallower in HC zones, most probably caused by anaerobic oxidation of methane (AOM) below the sampled depth interval. Linear sulfate pore water profiles indicate no net turnover of sulfate. Nevertheless, we observed low rates of sulfate reduction using radiotracer incubations, mostly in HC zones.

Combining these parameters with multi-omics datasets will reveal potential indicators for minimal HC seepage.

The crystalline basement below the giant Norwegian Troll field has been subject to research and publication for many decades. Still no well bore has penetrated the deep crystalline basement. An ever-increasing amount of geophysical information, documents pronounced basement heterogeneity. The seismic reflection patterns suggest the presence of mappable fluid reservoirs within metamorphic basement rocks.

High-reflection-low-density geobodies appear to be connected to major detachment zones with varying seismicity, but avoiding younger transform lineaments. The features are considered to have formed during the collapse of the Caledonian mountains, opening the precursor basins of the North Sea and the Northern Atlantic Ocean. The structures are partly inherited from contractional features, formed during the Caledonian continental collision between Baltic and North American plates. Former and present-day fluid migration, using this inherited deformation pattern, is significant for understanding the development of one of the most important energy sources for Europe.

While earlier speculations about origin and significance of the strong seismic reflections had to be based on presumed analogues on the Norwegian mainland, we will present evidence from new, high quality seismic, gravity, magnetic and magneto-telluric data and propose new working hypotheses for their geologic origin. Pointing to low density, low magnetism and low resistivity, our mapped „geo-bodies“, could form fluid reservoirs that have influenced the regional development of the Greater Troll area.