**5. Conclusions**

It is important for the Republic of Croatia to consider the possibilities of reducing their emissions by making use of CCS technology. This is the only way to achieve the Paris agreement targets in time, simply because the existing large stationary industrial sources from the energy and other industrial sectors can be cost-effectively, safely and quickly decarbonised, before the uptake of renewable technologies really starts "kicking in." Timely preparation of this will positively influence energy prices and save many jobs, not to mention that every nation should take care of its contribution to the global effort to reduce GHG emissions. Croatia is at present, still far from phasing out its fossil fuel energy sources and has a comparably high proportion of industrial emissions that will not just disappear in the near future. This can all be dealt with by evaluating the new "geological storage resource" to make use of the deep subsurface rock formations by the building of carbon capture and storage (CCS) systems.

There are favourable conditions for geological storage of CO2 in Croatia, both in the southern part of the Pannonian basin and the Adriatic offshore [7]. The capacity declared for hydrocarbon fields is better defined than estimates for aquifer formations which still need detailed exploration in order to define the structures for storage. Regarding the Adriatic offshore, it is far less explored than Pannonian Basin, but it has a considerable dataset, enough for screening in terms of the basic characteristics of subsurface geology.

Since almost half of emissions occur in the coastal areas, the Adriatic offshore's CO2 storage potential gains importance now that even exceeds petroleum exploration. That is simply because one can import oil or natural gas, but one cannot export CO2. There is still the significant professional knowledge and technical potential from the otherwise declining upstream part of national petroleum industry. It simply must be put into use while the window of opportunity still exists.

The most prospective CO2 storage objects are the small gas fields in the Northern Adriatic. Total storage potential in their reservoirs is not large (32 Mt) but it is available, and there are installations on exploitation sites which would significantly reduce investments. Another group of objects are deep saline aquifers. They offer much larger potential but with large uncertainty; that is why this is called "theoretical storage capacity". We think we have demonstrated the two most important "CO2 storage plays" based on the regional geological data—Miocene sandstones in the Dugi otok basin (as a regional deep saline aquifer, DSA Dugi otok) and Triassic to Eocene carbonate rock formations, whose paleotopography is covered by thick impermeable layers of Miocene to Early Pliocene age, so five structurally defined aquifers were delineated, each of them representing a site where CO2 storage capacity could be investigated on a local scale. They are all distant from sources of carbon dioxide and maybe the two of them will eventually prove to be too small to be economical, but the other three show significant potential that should not be overlooked.

**Author Contributions:** Conceptualization, B.S. and I.K.M.; methodology, B.S., I.K.M., M.C. and D.V.; software, I.K.M., M.C. and D.V.; validation of regional geology, J.V. and B.T.; formal analysis, I.K.M. and D.V.; investigation, B.S., I.K.M. and M.C.; resources, B.S. and M.C.; data curation, I.K.M. and M.C.; writing—original draft preparation, B.S., I.K.M. and M.C.; writing—review and editing, J.V. and B.T.; visualization, I.K.M. and M.C.; supervision, B.S.; project administration, B.S. and M.C.; funding acquisition, B.S.

**Funding:** The publication process was supported by the Development Fund of the Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb. The authors would like to acknowledge that they have had support from European Commission (in a number of FP6, FP7 and Horizon 2020 projects), from the Croatian Environmental Protection and an Energy Efficiency Fund, from HEP (Croatian power utility who once ordered a professional study) and from the University of Zagreb with which they are affiliated.

**Conflicts of Interest:** The authors declare no conflict of interest.
