Natural Laboratories for the Study of CO2 Geological Storage in Romania

Abstract

The study of natural laboratories for CO2 geological storage is still important as it can reveal important aspects related to environmental impacts of a potential CO2 leakage from an anthropic reservoir and can contribute to further understanding of CO2 migration pathways. Furthermore, natural laboratories are a good opportunity for testing innovative monitoring methods. Although not specified in any database of natural laboratories for the study of CO2 geological storage, Romania has many areas with natural CO2 emissions that can be used for this purpose. In the framework of a national project (financed by the Ministry of Education and Research), we have started to inventory, evaluate and make a selection of best candidate sites for further study in two zones, Banat (western part of Romania) and Harghita (eastern part of Romania). The two zones have different geological structures, and the origin of CO2 emissions in the two areas is related to the magmatic activity, the magmatism of the Eastern Carpathians in Harghita and the neogene magmatism of the Apuseni Mountains in Banat. The structural model of Banat is closely correlated with the strong tectonization of the sedimentary cover (with a maximum thickness of 5000 m). The overall picture is that of horsts and grabens. The tectonic blocks are separated by active, deep faults, confirmed by the occurrence of thermal and mineral water springs (Biled, Calacea, Timișoara, Buziaș, Pișchia, Lipova, Ivanda) and mofettes (Buziaș, Seceani), with releases of CO2, radon, helium, argon and other gases of crustal origin. Harghita zone is included in the mofettic aureola of the Eastern Carpathians. The deep geological structure of the zone is dominated by three fault systems comprising crustal, regional and local faults. These fault systems facilitate the migration and distribution of post-volcanic emissions, reaching the surface as mofettes or in mineral water springs. Based on review of existing literature and previous studies on greenhouse gases natural emissions, we made a preliminary inventory of relevant sites that can constitute natural laboratories for the study of CO2 geological storage. In order to confirm sites suitability, we conducted two field campaigns, one for each zone, in September and October 2019. Within these campaigns, apart from localization of the sites, we also made soil gas and high precision GPS measurements for the assessment of CO2 emissions. In Harghita zone, we investigated the following sites with natural CO2 emissions: Harghita Bai, Selters, Sântimbru Bai, Baile Apor, Sulphur Cave, Lazarești and Tușnad. Apart from Baile Apor (Balvanyios) and Lazarești, all the other sites presented covered mofettes, built as dry spas. It is worth mentioning that these mofettes constitute important points of touristic attraction within the spas developed over centuries around them and the carbo-gazeous springs. The CO2 concentration in these mofettes, as measured during the field campaign, exceeds 98% CO2. In Baile Apor (Balvanyios) site, the mofettes are not covered and present high CO2 concentrations, but the site is an important touristic attraction of the area and of the country and is highly frequented by tourists, therefore its use as laboratory can be problematic. The only site that we found suited as natural laboratory in Harghita is Lazarești, in the Cozmeni commune. Within this site we have localized several dry and wet mofettes, with CO2 concentrations exceeding 40%. For Banat zone, we investigated only three sites: Buziaș, Sacoșu Mare and Seceani. For Buziaș, the two mofettes are included in the spa facilities of a hotel and used for therapeutic purposes, under strict medical supervision. At Sacoșu Mare, although preliminary indicated as natural CO2 emission site, we were unable to localize any mofette. South of Seceani village (north of Timișoara city) we have identified as potential natural laboratory a partially marshy area with a large areal extent. On this area we have found several mud volcanoes and dry mofettes. The CO2 emissions are low, mainly due to the seasonal character of mud volcanoes activity which, according to previous findings, has its peak in the spring and a low in autumn, when the field measurement campaign took place. The activity of the mud volcanoes and therefore the mofettic emissions are strongly connected to the hydrological regime. In the selected natural laboratories, Lazarești for Harghita zone and Seceani for Banat zone we intend to make in the next two years a complex geological, geochemical, geophysical and environmental study. The objectives are to define the local geophysical model and improve the geological model, identifying all the migration pathways for the mofettic gases, to make an integrated environmental impact assessment and to identify early indicators for the presence of CO2.