**1. Introduction**

Regarding the distinctive characteristics of the subsurface geological setting, Croatian territory is usually subdivided into three large provinces—Pannonian basin, Dinarides and the Adriatic offshore. Only the first and the third province can offer locations with favourable conditions for the geological storage of carbon dioxide. The Dinarides can be ruled out due to several reasons. Firstly, this mountain range in Croatia is largely composed of Mesozoic carbonates that are strongly karstified to depths exceeding several kilometres. The karst hydrogeological system and its vulnerable groundwater resources effectively prevent any type of CO2 geological storage there. The other reason is generally moderate to locally strong seismic activity [1,2], which would put both the surface installations and subsurface storage objects at risk. Thus, in prospecting for geological conditions favourable for a safe and prospective CO2 geological storage in Croatia, one is directed both to the south-western part of the Pannonian basin and to the Adriatic offshore, the latter being far less explored but still covered by a comprehensive geological dataset, adequate for screening. This work is focused on the initial assessments of CO2 storage potential of this extensive offshore area, based on the regional-scale knowledge of subsurface geology; i.e., the distribution and composition of lithostratigraphic units and architecture of regional-to-local structures. Why is the storage potential of the Adriatic offshore so important for Croatia? It is because almost half of the greenhouse gas (GHG) emissions from large

stationary sources in the country occur along the coastline (Figure 1)—most notably in the industrial regions of Split and Rijeka, and in Istria where two large cement plants and the largest CO2 source in Croatia, the Thermal Power Plant Plomin, are situated. Thermal Power Plant (TPP) Plomin alone is the largest single source of CO2 in the country, exceeding 2 Mt/year according to Croatian Environmental Pollution Register [3].

Another important aspect for prospective CO2 geological storage in the Adriatic offshore in Croatia is the decline of gas production on existing offshore gas fields in the Northern Adriatic. Consequently, these fields might be used in the future to decarbonize not only stationary CO2 sources located along the coast, but also for inland CO2 sources closely located or already connected by the existing pipeline network (Figure 1). Moreover, there is professional expertise of and technical potential of the otherwise declining upstream part of national petroleum industry that might be used for developing of a carbon capture and storage (CCS) system, but it will not be there for a long time. Use of this expertise for deployment of CO2 geological storage would have unprecedented economic and environmental effects.

**Figure 1.** Location map of large stationary CO2 sources (Croatian Environmental Pollution Register [3]), main pipeline network (after [4,5]), contours of the potential CO2 geological storage objects in the Adriatic offshore and the peak ground acceleration values with a return period of 475 years (after [6]).

The first regional screening of CO2 geological storage potential in Croatia was performed within the scope of the two FP6 projects—CASTOR (CO2 from Capture to Storage) and EU GeoCapacity. This resulted in a database of the potential CO2 storage objects, containing their geological descriptions and numerical estimates of theoretical storage capacities [7]. This database was later actualized through

the FP7 project CO2StoP [8] with the purpose of making this information uniformly structured and accessible on a European scale.

#### **2. Geology and Petroleum Exploration of the Adriatic O**ff**shore in Croatia**

There would be no possibilities for considering the offshore CO2 geological storage without previous HC (hydrocarbon) exploration activities that acquired data on the subsurface geological structure and lithology of rock formations in the Croatian Adriatic offshore. Interpretations evolved during five decades of intensive petroleum-geological exploration, firstly in the Northern Adriatic in the 1970s and then in other sectors southwards in 1980s. Results of initial explorations were not particularly promising [9], although some hydrocarbon shows and a few potentially economical accumulations were discovered. Major progress was made in the middle of 1990s that resulted in gas production from the Northern Adriatic offshore [10]. Several gas fields were discovered here in 1970s, first the Ivana field and later Ika and Ida fields (Figure 1) with reservoirs in Pliocene-Pleistocene clastic deposits [10–12]. Traps were formed by differential compaction, resulting in small structural closures with numerous isolated sand bodies within a progradational Plio-Pleistocene turbiditic sequence [10,13]. These thin sandy layers are characterized by intergranular porosity and markedly irregular distribution of reservoir properties [14], together with a low level of cementation. One reservoir was discovered in the underlying karstified Upper Cretaceous carbonates [10–12]. The structures are relatively shallow (from −500 to −1000 m) [10], practically meaning that only some of them might be used for CO2 geological storage and that their storage capacities will be small. Locations of the three gas fields in the northern Adriatic offshore that were included in the EU GeoCapacity database, i.e., the Ida, Ika and Marica gas-fields, are presented in Figure 1.

The oldest rocks drilled in the Adriatic offshore are of the Permian age. According to [15], these rocks have only been drilled in two locations—one in the Italian offshore (well Amanda-1bis [16]) and one in the Croatian part (well Vlasta-1 [17]; Figures 2 and 3). Permian rocks have heterogeneous lithologic composition, comprising clastics, carbonates and evaporites [18,19]. The Lower Triassic is also characterized by mixed carbonate and clastic sediments, with both siliceous and carbonate sandstones and dolomites indicating shallow water depositional environment. Middle Triassic unit is characterized by shallow-water carbonates; however, with widespread occurrences of andesite and pyroclastics [20–23]. Evaporites can be locally found in the basal part of the Upper Triassic, more frequently in the Central and Southern Adriatic [24,25], while dolomites prevail in the Northern Adriatic area (Figure 2, with the column locations marked in Figure 4). Generally, the shallow water carbonate sedimentation in platform conditions began in the Late Triassic, on a large Southern Tethyian Megaplatform (STM) [22]. Tectonic disintegration of this megaplatform commenced by Early Jurassic rifting that resulted in formation of several smaller carbonate platforms separated by deep marine troughs and basins, giving a way to the formation of the Adriatic Basin and the Adriatic Carbonate Platform (AdCP), characterized by pelagic and platform carbonate sedimentation throughout Jurassic and Cretaceous, respectively [22]. Towards the end of Cretaceous the AdCP gradually disintegrated and emerged but carbonate sedimentation was locally restored by Paleogene transgression with the Foraminiferal limestones deposited mainly during Early to Middle Eocene when the carbonate platform sedimentation on the AdCP terminated [22]. The total thickness of the AdCP succession amounts more than 8000 m with average thickness of around 5000 m [22].

Following the lithostratigraphic subdivision generally accepted in petroleum geological exploration of the Adriatic offshore in Croatia, that is hindered by relatively scarce distribution of deep wells and seismic lines, hereafter, we will use the term "carbonate complex" for an informal lithostratigraphic unit that includes (a) Lower Jurassic (post Pliensbachian) to Middle Eocene carbonate platform succession (the AdCP succession, sensu [22]), (b) the Lower Jurassic to Middle Eocene pelagic carbonate succession of the Adriatic Basin, and (c) the underlying Upper Triassic (post Carnian) to Lower Jurassic shallow marine carbonate and clastic succession assigned by [22] to the AdCP basement or to the STM. Thus, the "carbonate complex" of the Adriatic offshore consists prevailingly of carbonate rock formations of basinal and carbonate platform origins, deposited since the Late Triassic to Middle Eocene time. In most of petroleum exploration studies (e.g., [17]) this complex is bounded on top by the Top carbonate complex horizon mapped throughout the Adriatic offshore in Croatia and shown in Figure 3.

**Figure 2.** Schematic cross-section A–B of the Adriatic offshore (NW–SE, modified after [26–29]; locations in Figure 4).

**Figure 3.** Structural map of the Top of carbonate complex with marked locations of potential storage objects within five structural traps. Map compiled after [25,28,30–33].

During the Middle–Late Eocene and Lower Oligocene the Adriatic offshore in Croatia was partly affected by compressional tectonics and a SW-directed propagation of thrusts that resulted in the formation of the External Dinarides fold-thrust belt, exposed along the eastern Adriatic coast and its hinterland, but also partly present in the Adriatic offshore (e.g., [30,34,35]). In the course of a SW-propagating thrust system, a large part of the AdCP succession and its basement were imbricated into a set of NW–SE striking, fault-related anticlines and synclines, that gradually led to the formation of a contemporaneous foreland basin system characterized by deposition of syntectonic flysch sediments mainly of Middle–Upper Eocene, locally of Lower Oligocene and in places, up to Lower Miocene aged sediment [22]. The continued SW-propagation of frontal thrusts locally overrode through the AdCP margin and reached up into the Adriatic basin, while more internal foreland basins gradually evolved into piggy-back basins that were filled up with a 2 km thick syntectonic clastic-carbonate succession of the Promina deposits composed of marls, calcarenites and carbonate conglomerates, at first of marine, and then of lacustrine, delta-fan and alluvial-fan origin [22,36,37]. Locally preserved Miocene deposits

exposed on Pag island and in coastal hinterland of the External Dinarides, assigned to the so-called Dinaride Lake System, are exclusively of lacustrine origin. They are prevailingly composed of marls with occasional occurrences of coal seams [38,39], thus they could be considered as a post-tectonic cover in the coastal hinterland area. In contrast to these lake deposits, in contemporaneous offshore basins, Miocene deposits are represented by marine hemipelagic marls and turbidites composed of alternating marls, and calcareous and marly siltites, interbedded with sandy limestones and sandstones deposited on top of the Eocene–Oligocene marine turbidites. Based on their petrophysical characteristics, the clastic deposits of Middle Eocene to Miocene age in the Adriatic offshore are considered to have both the reservoir and sealing capabilities favourable for a regional deep aquifer formation. As a rule, the transition from Miocene to Pliocene sediments in the Adriatic offshore is marked by a regional Messinian unconformity well recognized in reflection seismic sections [17,40]. Pliocene sediments resulted from a subsequent transgression and include clays, marls and sands. In most of the offshore area there is depositional and lithologic continuity from Pliocene into Pleistocene deposits composed of sands, silts and clays with lignite interbeds, except locally where transition from Pliocene into Pleistocene is marked by transgression [41]. In the central Adriatic offshore Pleistocene and Holocene deposits can, in places, reach the thickness of 2000 m, with the total thickness of the Eocene to Holocene sequence being up to 6000 m in the deepest sub-basins. In the Northern Adriatic, the thickness of the same sequence frequently exceeds 2000 m (Figure 4).

**Figure 4.** Thickness of an Eocene to Holocene sequence of clastic sediments with the regional, deep saline aquifer outlined as a potential CO2 storage object. Thickness is derived based on depth of the top of the carbonate complex (Figure 3) and sea bottom depth [42].
