Assessment of the Potential for CO₂ Storage and Utilization in the Fractured and Porous Reservoir of the Cambrian Sandstones in West Lithuania’s Baltic Basin

Abstract

Cambrian sandstones comprise a large and saline-only aquifer that can be utilized for CO₂ geological storage in the Baltic basin, including Lithuania. The two prospective storage sites with the most potential are located in west Lithuania. Despite the larger area of the Gargždai elevation (233 km²), the Syderiai uplift (62 km²) is characterized by the largest storage volume. The most significant difference between the studied structures is primarily related to the much higher reservoir quality of the Cambrian sandstones at the Syderiai site. The sandstones’ average porosity is 16% and their permeability measures 310 mD, while the Gargždai site is characterized by poor reservoir quality (average porosity of 7% and permeability as low as 10 mD in the sandstone). The main controlling parameter for the sandstones is authigenic quartz cementation. The reservoir type is classified as the porous sandstone type for the Syderiai site and as the fractured reservoir type for the Gargždai site. The storage volumes of CO₂ of the sites were assessed as 56.7 Mt and 31.3 Mt, respectively. The present study determined that the Syderiai uplift was the prospective site with the most potential for the geological storage trapping of CO₂, owing to its high reservoir quality, while the Gargždai elevation is characterized as a potential alternative for CO₂ storage combined with EOR technology for oil exploitation, despite its poor reservoir quality.

1. Introduction

The geological storage of CO₂ emissions as a measure for the mitigation of climate change has been actively considered since the 1990s [1]. Geological storage in saline aquifers has been implemented in several large-scale carbon capture sites, e.g., the Sleipner sites (Norway), which have been operating since 1996 [2] and In Salah (Algeria), which opened in 2004 [3]. Before the industrial storage of captured CO₂ was used as a solution, natural CO₂ accumulations were used in the USA and Canada. The world’s first large-scale CO₂-EOR project was implemented by Chevron in an oil field in Texas in 1972 [4]. CCUS for Enhanced Oil Recovery (CCUS-EOR) has the double benefits of greatly enhancing oil recovery and carbon storage and reducing emissions; therefore, it is the most practical and feasible option [5]. Since the 1980s, the industrialization of CCUS-EOR in the USA has expanded rapidly, allowing annual oil production to exceed 1500 × 10⁴ t in 2012 and remain stable ever since.

In Europe, the largest CO₂ storage capacity is located in the North Sea [6]. Project Greensand should be noted as having been the first in the world to demonstrate that CO₂ could be transported across national borders and stored offshore to mitigate climate change. The project entered its demonstration phase in 2023 [7].

This project is of particular significance due to the limited storage capacity of countries located in the Baltic Sea region and urges countries in the region to combine their efforts [8]. Storage potential varies considerably in countries situated in the Baltic Sea region. Specifically, CO₂ storage capacities in Finland and Estonia rely on mineral carbonation in the Fennoscandian Shield [9,10,11] and the shallow periphery of the Baltic basin [12,13,14]. This large Baltic sedimentary basin can potentially sequester CO₂ emissions in this highly industrialized region. Despite the protracted geological history of the basin, the Deimena Regional Stage (RSt.) saline aquifer, within a Cambrian sedimentary unit, is the only potential reservoir that has been assessed for its CO₂ storage capability [15]. For the sake of comparison, the overlying large Lower Devonian (Kemeri RSt.) saline aquifer comprises the only low-amplitude tectonic uplifts in the region, with amplitudes less than 10 m, due to essentially low tectonic structuring after the secession of the Caledonian tectonic phase in the Lower Paleozoic in the Baltic basin [16].

The reservoir with the largest storage potential for CO₂ is located in Latvia, which has an excellent reservoir quality and abundant tectonic uplifts [17]. The prospective area is confined to the Liepaja–Saldus Tectonic Ridge that runs across the entire territory of Latvia and extends from the central Baltic Sea to northeastern Belarus [18,19,20]. Its recently reassessed total storage capacity, in its Cambrian structures, was reported to potentially range from 600 Mt to 1000 Mt (Figure 1) [21]. At the current rate of production of capturable CO₂ emissions by energy and manufacturing industry-based emitters, these Latvian structures could provide storage for at least 150–250 years.

The tectonic structures were established in the latest Silurian/earliest Devonian age (Gargždai RSt.), at the end of the Caledonian tectonic stage. Furthermore, some faults were strongly reactivated at the end of the Variscan tectonic stage in the Upper Devonian/Permian in the Latvian territory [22]. Despite extensive deep drilling, seismic surveys have been of low quality in Latvia, with most dating to the 1960s and early 1970s.

The amplitude of the largest onshore Dobele uplift, located in central Latvia, measures 140 m [21]. Twenty-two deep wells were drilled in the 1980s to assess the UGS capacity of this prospective site. The current economic strategy of Latvia does not consider the Dobele structure as a potential site for UGS application. On the other hand, the storage capacity was recently reassessed and measured as high as 150 Mt CO₂, owing to its high reservoir quality (average porosity 23% and permeability 500 mD) and large 70 km² area. The thickness of the Deimena RSt. ranges from 48 m on the crest of the structure to 60 m on the flanks of the uplift. It is notable that the structure was active during the Upper Cambrian era, which led to the erosion of the central part of the structure (the Ablinga Fm., which is truncated at 15 m thick). The largest depth measured in the structure is about 950 m.

The lowest reservoir quality was found in the largest offshore uplift E6 in Latvia [20]. The uplift is bounded by two major faults and therefore is classified as an increased-risk storage site; essentially, the oil detected in the Middle–Upper Devonian sandstones (found using drill core sampling) implies the vertical migration of hydrocarbons from the Cambrian reservoir. The reservoir properties of the sandstones were reportedly good: a porosity of 14%–24% (mean: 21%) and permeability of 10–300 mD (mean: 140 mD). The minimum depth of the structure is −850 m and the maximum thickness reaches 52 m.

The prospective Dalders structure was identified in this tectonic ridge in the deepest part of the Baltic Sea, close to the state border between Sweden and Latvia [23,24,25]. No drilling was carried out in the structure; it was detected based on an industrial seismic survey. The amplitude of this NE–SW-oriented uplift reaches c. 150 m, and it has no obvious tectonic fault control. Its CO₂ storage capacity measures 85 Mt [24]. The extrapolation of the reservoir parameters to the adjacent E7 uplift suggested a moderate reservoir quality (minimum porosity 12%, permeability 50 mD, 57 m thick sandstones, and maximum depth −1362 m). The Dalders uplift represents the easternmost margin of the Faludden pinch-out monocline with fault control, which has a large storage capacity and is located in the Swedish offshore sector of the Baltic Sea. It comprises a number of smaller uplifts, and the Faludden monocline is classified as a stratigraphic trap, sealed by a thick sequence of Ordovician and Silurian limestone and marlstone.

To the south, the Łeba Ridge is the prospective structure with the most potential in the Polish sector of the Baltic Sea (Figure 1). Four gas condensate deposits (B4/1991, B6/1982, B16/1985, and B21/1996) and three oil deposits (B3/1981, B8/1983, and B24/1996) were discovered, with the total reserves measuring 10 Gm³ of gas and approximately 30 Mt of oil [26] (not indicated on the map). The depleting oil fields B3 and B8 are considered to be first-order targets for CO₂ injection in the Cambrian sandstones [27,28,29,30,31].

The largest oil production reserves were found in the Kaliningrad District (Russia), including the adjacent offshore sector. These oil fields are aligned along several major tectonic faults. About 40 oil fields were discovered in both onshore and offshore territories. The depletion of oil reserves measures c. 80% and 20%, respectively [32].

The Cambrian reservoir is overlain by thick shales containing subordinate carbonates. The primary sealing formation is composed of the Ordovician carbonates and shales. The former lithologies are predominant in the shallow periphery of the Baltic basin and gradually give way to shales and intercalated limestones. The thickness varies from 40 m to 250 m [33]. There is scarce information available on the petrophysical properties of the Ordovician rocks. The limestones from the transitional zone between the Cambrian reservoir and Ordovician cap rock might be readily subject to carbonate alteration [34].

The thickness of the Silurian shales comprising the central part of the basin ranges from 350 m in central Lithuania and central Latvia to more than 3500 m in the basin margin in central Poland [35]. The petrophysical properties of the Silurian shales are related to shale gas prospects in the basin [36].

The secondary cap rocks are represented by the Narva RSt., which are 70–120 m thick [26]. They are composed of dolomitic marlstones with subordinate dolostones. It should be noted that the Lower Paleozoic succession was intensively faulted in the Baltic basin, while the Narva confined unit and overlaying deposits were only occasionally subject to faulting, mainly in the Latvian territory. The primary and secondary cap rock formations are separated by a Lower Devonian saline aquifer measuring approximately 150 m thick. The high salinity of the aquifer is due to the effective isolation of the hydrochemical system.

Based on extensive seismic survey and drilling activities, several different-scale uplifts have been identified in the Cambrian reservoir in Lithuanian territory, both onshore [37] and offshore [38]. About 20 oil shows and oil fields were discovered in the western part of the country. The depth of the oil fields is about 2 km, which proves the isolation of the reservoir. Only three offshore and two onshore structures were selected as prospective sites for the geological storage of CO₂ emissions in the central and western parts of Lithuania (Figure 1) [39]. The Vaškai, Syderiai, and D11 structures are linked along the Telšiai fault, which is west–east-oriented. The westernmost uplift E2 is bounded by the Liepaja–Saldus Ridge, which controls almost all of the potential storage sites in Latvia, as discussed above. The largest site, the Gargždai elevation, is located in the second-largest fault zone in west Lithuania.

This report focuses on the evaluation of two main potential CO₂ storage structural traps in west Lithuania. The reservoirs show very different reservoir parameters, owing to the different burial depths of the Cambrian sandstones. Originally, quartz arenites were deposited in the shoreface facies (Syderiai) and graded to the inner shelf (Gargždai) facies. Their protracted diagenetic history has led to very different reservoir qualities for the studied sites. The late-diagenetic authigenic quartz cementation controls the wide variation in the reservoir properties, which is typical for high-temperature dissolution and the reprecipitation of sandstones. The Syderiai structure was classified as a porous reservoir type, while the deeply buried Gargždai site was classified and studied as a fractured sandstone reservoir type. In addition to the reservoir properties for both sites, two-dimensional (2D) and three-dimensional (3D) seismic survey data were incorporated into the study to assess the reservoir’s CO₂ storage capacities.

2. Major CO₂ Emission Sources in Lithuania

Economic collapse in Lithuania resulted in a drastic reduction in CO₂ emissions, from 10.0 t per capita in 1991 to 4.3 t in 1993 (https://ourworldindata.org/co2/country/lithuania: 21 October 2024). The lowest emissions were recorded in 2000, totaling only 3.3 t per capita. The total emissions rate has gradually recovered during the past 20 years and was recorded as moderate, measuring 4.6 Mt, in 2022. The annual CO₂ industrial emissions, listed in the EU Emissions Trading System (ETS), were reported to be 5.61 Mt in Lithuania in 2020. More than 50% of emissions are produced by the energy sector. Ten sources produced 4.51 Mt, a much larger amount than the 100 kt of CO₂ viable for capture. There was little impact of the closure (in 2009) of the Ignalina NPP on CO₂ emissions in the country, as most of the electricity deficiency was compensated for by imported electricity. The three largest emitters—AB Achema (2.52 Mt—46.4%), AB “Orlean Lietuva” (1.48 Mt—29.9%), and AB “Akmenės cementas” (0.87 Mt—14.7%)—contribute to 85% of the ETS market. Notably, the AB “Orlean Lietuva” oil refinery and the AB “Akmenes Cementas” cement plant are located 40 and 50 km from the Syderiai structure, respectively, and are therefore close to the town of Telšiai (Figure 2). The Gargždai structure is located 110 km to the southwest. The Hoegh LNG and “Fortum Klaipėda” CO₂ sources have only a minor impact in terms of greenhouse gases (contributing 0.15 Mt and 0.20 Mt, respectively).

It should be noted that Latvia is the smallest CO₂ emitter in the Baltic region. Its emission rate of 3.6 tons per capita reported in 2022 is comparable to Lithuania’s (https://ourworldindata.org/co2/country/latvia: 21 October 2024). By contrast, the Estonian emission rate was reported to be as high as 11.1–14.9 tons per capita from the 1990s to 2010 [15]. This amounted to a total of 15–20 Mt per year. The Estonian emission rate per capita decreased considerably to 6.9 tons in 2020 (https://ourworldindata.org/co2/country/estonia: 21 October 2024), owing to the declining percentage of oil shale used in the energy sector (10.31 Mt in 2022).

3. Cambrian Saline Aquifer of the Baltic Basin

The Cambrian succession comprises the basal section of the sedimentation cover of the Baltic basin [40], overlying the western periphery of the Baltic Craton of the Precambrian consolidation [41]. Its sands, silts, and clays were deposited in the shallow marine basin on the passive margin of the craton [42,43]. Cambrian rocks appear along the northern margin of the basin (i.e., Estonia and Sweden) and the basin’s thickness increases systematically to the southwest, reaching up to 5 km in the western part of the basin, in central Poland [44].

The Cambrian incipient marine basin shifted into the Baltic region from the east [43,45]. It comprises the regional-scale confining unit of the Moscow basin. Only the westernmost periphery of the basin (i.e., Lithuania, Latvia, and Estonia) grades from clays to an alternation between quartz arenites and clays.

The rearrangement of the sedimentation pattern is marked by the widening of the marine transgression and the establishment of the Baltic sedimentary basin. The narrow depositional locus located along the present Baltic Sea is attributed to the Dominopole RSt., composed mainly of siltstones.

The wide transgression is documented in the sedimentary record of the basin, which marks the vast flooding in the region, including the entire territory of Lithuania. The basal section of the new transgression was confined to the Holmia kjeulfi trilobite biozone, defined as the Vêrgale RSt., in Lithuania [32]. Most of the basin was filled with mud that graded to fine-grained arenites in the eastern periphery of the basin.

The Deimena RSt. is the most widely documented reservoir in the Baltic basin. The sharp change from mud-dominated facies (Vêrgale–Kybartai Fms.) to overlying fine-grained sands (Deimena RSt.) reflects the basin’s recession. This regression was forced by the deacceleration of tectonic subsidence on the passive margin. In the global stratigraphic chart, the Deimena RSt. correlates with the Wuliuan Stage (the Eccaparad oelandicus trilobite zone). The Pajūris, Ablinga, and Giruliai Fms. are located in the Deimena RSt. and are discussed in a later section.

The Deimena RSt. is found in central and west Lithuania. The thickness systematically increases to 76 m in the west. The sandstones in the shallow periphery of the basin are cemented by scarce carbonates (calcium and dolomite) and clay minerals (mostly illite). The major diagenetic boundary of the reservoir is located at c. 1 km depth. Quartz cementation is the basic parameter controlling the reservoir properties of the sandstones in the central part of the basin, including west Lithuania [34,35]. The porosity of the sandstones ranges from 22% in central Lithuania and to approximately 6% in the westernmost part of Lithuania. Accordingly, the permeability varies dramatically, from 500–2800 mD to 1–10 mD.

The temperature in the Cambrian aquifer varies from 10–14 °C to 65–95 °C. The increase in temperature shows control by the chemical composition of the Cambrian formation water. A low salinity of 1–8 g/L occurs in the eastern periphery of the aquifer, which is strongly affected by the intense infiltration of meteoric water [36]. The hydrodynamic boundary is marked by a rapid increase in water salinity, from 10 g/L to 100 g/L in a 40–50 km wide zone in central Lithuania. High-salinity (120–200 g/L) brine is distributed throughout the western half of Lithuania. The chemical composition of the formation water varies between the shallow and the deep sections, as the following: (1) Ca–HCO₃ (1–2 g/L); (2) Ca–Na–Cl–SO₄ (2–100 g/L); and (3) Na–Cl (100–200 g/L).

The Cambrian aquifer is overlain by up to 40–250 m thick alternating layers of Ordovician shale and carbonate and a 350–750 m thick layer of Silurian shale, which represent the regional confining units of the Baltic artesian basin. The thickness and burial depth of these systematically increase from the east to the west. The high thickness of this sealing layer of shales is considered a major positive criterion when rating the Cambrian saline aquifer for its potential for CO₂ geological storage.

4. Methods and Data Sources

Two-dimensional and three-dimensional onshore seismic data from the Gargždai area were collected in west Lithuania. The new seismic data, mostly acquired in the 2000s, were preferred to older data because the old data tended to have issues, such as incorrect near-surface static corrections, low fold, and a low signal-to-noise ratio. Eleven 3D seismic surveys, which in total covered an area of 550 km², as well as nearly 300 2D lines, which had a total length of just less than 2000 km, comprised the study dataset. Approximately 25% of the study area was covered by 3D seismic data, and the density of 2D seismic lines was 1–4 km of the profile length per square kilometer. The Gargždai elevation includes 6 oil fields that are aligned along the Gargždai fault zone and are complicated by the occurrence of smaller-scale faults.

The Syderiai area was covered by a 3D seismic survey in 2012. This study was originally focused on the eastern part of the Syderiai uplift, which is complicated by small-scale faults intersected by the major Telšiai fault, as mentioned above. Additionally, eight 2D seismic profiles were reinterpreted in the western part of the study area.

The spatial precision of the interpreted horizon was determined by the type of seismic data (2D or 3D) and by bin size, which was usually 12.5 m for 2D seismic lines, and 25 × 25 m for 3D seismic volumes. The interpreted time horizons were converted to depth horizons using the site-updated models of horizon velocity (the model of average-velocity-to-horizon). Seismic-to-well ties were used to relate well logs representing the geological section to seismic sections and to identify rock interfaces at which seismic reflections originated. The seismic-to-well ties provided a synthetic seismic trace, which visually related to the real seismic data; thus, this demonstrated the relationship between stratigraphic and lithological boundaries and an actual seismic reflection section. The well Purmaliai-2 was selected for the seismic-to-well tie in the Gargždai area and the well Syderiai-1 was used as the reference well in the Syderiai area. The velocity model was obtained using reference wells in the two studied areas. The seismic interpretation software LMKR Geographix 2017.3 was used for the seismic data interpretation, which included the horizon interpretation, seismic-to-well tie, digitization of the scanned map, and creation of the final map. LMKR Geographix and Matplotlib were used for drafting the figures [46].

Two key regionally recognizable reflecting surfaces were interpreted, i.e., the top of the Ordovician and the top of the Precambrian crystalline basements. The top of the lowermost Silurian layer (Stačiūnai Fm. of the Llandovery stage, measuring approximately 2 m thick) was the strongest regional reflector. The micritic limestones are underlain by Ashgill wackestones and overlain by Upper Llandovery black shales. The top of the crystalline basement was the second regional reflector and underlies the Cambrian succession.

The seismic survey was combined with extensive drilling in the Cambrian reservoir in west Lithuania, the purpose of which was oil exploration. The study of the Gargždai structure was carried out over more than 50 years. More than fifty wells were drilled, which led to the discovery of six oil fields and the creation of a database comprising 2724 samples compiled from industrial reports and used for the present study (Lithuanian Geological Archives).

In contrast to an abundant database of oil exploration wells, only well Syderiai-1 was drilled in 1991. The well was situated east of the edge of the oil field. Twenty-three samples of sandstones were collected from the upper part of the Deimena RSt. and five samples were obtained from the shale layer of the Kybartai and Rausvė RSts.; the porosity, horizontal and vertical permeability, and bulk density were measured (Lithuanian Geological Archives).

A unified Soviet methodology, including the Lithuanian Geological Survey (formerly referred to as the Lithuanian Geological Board), was applied to measure the petrophysical parameters, and the results were presented in industrial reports. A comprehensive description of the methods is given in [47,48]. A total of 2724 samples were collected from the wells in the Gargždai area and a smaller database was collected (27 samples) from the well Syderiai-1. Also, the grain sizes of sandstones and siltstones were measured using the standard sieving method (9 fractions). Few wells were studied in the Gargždai zone, and the research was therefore expanded to west Lithuania (collecting 1661 samples).

Conventional well logging provides basic information on the stratigraphical interpretation of drilled sedimentary layers, which is essential in intervals with no drill coring available. The porosity was defined by combining the gamma-ray and sonic velocity well logs.

The injection capacity for CO₂ of a specific area is controlled by two key parameters that should be considered when evaluating its potential for geological gas storage. The temperature and pressure of the injected gas limit the amount of CO₂ that can be injected [49]. Temperature logging through sedimentary cover, down to the Cambrian succession, was carried out in the well Syderiai-1 and in 4 wells in the Gargždai area. This is an essential parameter for calculating the CO₂ density in an aquifer. Furthermore, the pressure of the Deimena reservoir measured 152 bar in the Syderiai structure and varied from 212.5 to 201.6 bar in the Gargždai elevation.

The CO₂ sequestration capacity of deep saline aquifers is commonly derived using a volumetric, top–down approach [50,51]. The basic formula used for capacity estimation in deep saline aquifers is given in Equation (1), as the following:

MCO₂ = A × h × NG × Φ × ρCO₂r × Seff

(1)

where MCO₂ (measured in megatons) denotes the storage capacity of the trap aquifer; A (km²) denotes the area of the structural trap (i.e., the aquifer or oil field); H (m) denotes the average thickness of a terrigenous reservoir; NG denotes the average net/gross ratio of the trap aquifer; Φ (%) denotes the average reservoir porosity of the trap aquifer; ρCO₂r (kg/m³) denotes the CO₂ density under reservoir conditions; and Seff (shown as a fraction) denotes the storage efficiency factor.

The interaction between pore water and rock might initiate significant alternations in reservoir mineral transformations and in associating petrophysical properties in injected sandstones and overlying cap rocks. Old hydrochemical data were collected from oil exploration reports. The chemical composition of the aquifer water was analyzed for samples from well Syderiai-1 and from 29 wells in the Gargždai area.

5. Results

5.1. Syderiai Structure

The Syderiai uplift was discovered following a 2D seismic survey carried out in 1971 and was studied in more detail during 1982 and 1988. The well Syderiai-1 was drilled in a crestal part of the largest uplift, located on the hanging wall of the Telšiai fault, and associates with the parallel 20 km wide tectonic Telšiai elevation, which comprises the northern flank of the fault (Figure 3) [37]. The depth of the well reached 1589 m. The borehole intersected the boundary of the Cambrian deposits and the Paleoproterozoic crystalline basement, representing the old Telšiai shear zone [41].

Detailed 3D seismic studies were carried out in the Syderiai area in 2011. This structure was originally considered to be a prospective site for natural underground gas storage (UGS). The main target of the seismic study was to prove the tectonic tightness of the faults, which is of paramount importance for the safe storage of gas (Figure 3). Furthermore, the analysis of wells Pabalvė-1, Tryškiai-73, Tryškiai-74, and Žarėnai-1 provided important information on the Cambrian reservoir’s properties close to the Syderiai site. The amplitude of the Telšiai fault measures 250 m, making it the largest tectonic structure defined within the sedimentary cover in Lithuanian territory (Figure 1).

This tectonic structure was classified as a right lateral strike–slip fault induced by NW–SE propagation of the transgressional forces from the Caledonian orogen [28]. The major fault was complicated by stepover, i.e., the Luokė and Rūdupis local faults (which are oriented NE–SW), which was associated with contractional push-up in the Syderiai uplift. The amplitudes of the two paralleling fractures were assessed at about 100 m and 90 m. The well Syderiai-1 penetrated the top of the Cambrian at a depth of 1458 m (−1320 m b.s.l.) and the thickness of the reservoir is 50 m.

Cambrian sandstones are overlain by the Letse Fm. (Lower Ordovician), measuring 1.0 m thick, and are composed of dolostones, with abundant glauconite grains also deposited in the shallow marine environment, which marked a wide new transgression in the Baltic region. The stratigraphic break lasted for approximately 20 Ma. The Letse Fm. grades to an intercalation of marlstones and limestones in the Volchov Fm., representing the basal section of the cap rocks [33,52]. The total thickness of the Ordovician succession measures 128.3 m. The ratio of limestones and marlstones with some interlayers of organic-rich shale was assessed at 58%. Limestones are strongly cemented by (mainly early) diagenetic calcite and have a nodular structure, which suggests the relative tightness of the Ordovician cap rock. The Silurian thickness of the overlying shales is 430 m and contains only scarce limestone interlayers; therefore, it is considered essentially a high-quality cap rock. Still, no specific petrophysical measurements were available.

The Deimena sandstone reservoir is subdivided by the 29 m thick Upper Sandstone I and 15.5 m thick Lower Sandstone II, which are separated by 6.5 m thick shales (Figure 4). The Deimena RSt. is underlain by the Kybartai and Gėgė shales, which were deposited in dysoxic and oxic environments, respectively [53]. These shales are considered to be source rocks enriched in organic matter [38]. It should be noted that most hydrocarbons generated from Alum Shale are distributed in the westernmost part of the basin and migrated along the Cambrian sandstones.

The Giruliai Fm. (thickness: 6.8 m) represents the uppermost part of the Deimena reservoir and is characterized by the lowest reservoir quality, a porosity ranging from 7.5% to 17.5%, and a permeability that varies from 0.1 mD to 513 mD (Figure 5). This formation is topped by mostly quartz-cemented thin-bedded sandstones (thickness: 2.5 m). The sandstones of the Ablinga Fm. show the best reservoir properties (mean porosity: 17.8%; permeability: 475 mD). The middle and lower parts of the reservoir were drilled with no coring. The well logging recorded significant variations in the porosity of the sandstones. The undrilled lower part of the Ablinga Fm. shows a transgressive sedimentation trend, grading from high-porosity sandstones at the base (23%) to lower-quality sandstones at the top (17%). The Pajūris Fm. represents the lower part of the Deimena RSt. and shows similar porosity values, in the range of 14% to 18%. The porosity and permeability indicate a positive statistical trend, from c. 8%/30 mD to 19%/500 mD. However, wide scattering should be noted, which points to a more complex correlation between the two parameters [54].

Quartz arenites were deposited in the proximal shoreface and storm-dominated distal shoreface environments [39]. Massive sands and thick-bedded sands accumulated in the shallow marine basin. The sedimentation facies correlated with vertical variations in sandstone porosity. Notably, the sampled sandstones in the upper part of the reservoir showed a strong anisotropy of vertical vs. horizontal permeability, calculated as 0.70, which is an important parameter that should be stressed in the Deimena reservoir model (Figure 5).

The density of sandstones is commonly ignored as a parameter of minor importance. The average density of sandstones is 2270 kg/m³, which is typical for moderately buried compacted sandstones. The strong statistical correlation found between sandstone density and porosity was striking, and a strong negative statistical correlation was found for the sample set (the R-squared value was 0.99) (Figure 6). In turn, this correlation pointed to the superlative control of the quartz cementation over the sandstones’ porosity. Also, the studied clayey siltstones attributed to the Kybartai and Rausvė RSts. showed a systematic increase in density of approximately 30 kg/m³, compared to the sandstones (Figure 6).

Temperature logging was carried out in the well Syderiai-1, from the surface to a 1510 m depth. The bottomhole temperature was 51 °C in the Deimena reservoir and the average thermal gradient was calculated to be 3.6 °C/100 m in the sedimentary deposits in the well.

The salinity of the formation water of the Deimena RSt. measured 122 g/L (

Table 1. Chemical composition of formation water of wells Syderiai-1 (depth 1454.2–1464.0 m, Ablinga Fm.) and Gargždai-5 (depth 1982–1988 m, Giruliai Fm.).

Well	TDS *	pH	Cl	SO4	HCO3	Na	K	Ca	Mg	Fe	Si
Syderiai	122.4	5.75	76,452	67	64	29,027	383	12,726	3128	31	8.4
Vlkyč-5	166.5	5.10	103,468	225	18	34,094	90	24,187	2702	60	31.1

* TDS salinity g/L and concentration mg/L.

). The formation water was classified as Na–Cl. It is notable that the Cambrian aquifer of west Lithuania is significantly depleted in SO₄, which is shown by the high levels of diagenetic pyrite precipitation, as documented in sandstones and shales in west Lithuania [55,56]. For the sake of comparison, the Na–SO₄–Cl zone is located in central and eastern Lithuania, correlating to a shallow-burial, coupled water–rock interaction in the aquifer.

5.2. Gargždai Elevation

The Gargždai elevation is the largest tectonic uplift in Lithuania. The first discovery of oil was recorded in the well Šiūpariai in 1968 (Figure 7). More than 50 wells were drilled at this elevation, leading to the discovery of six oil fields in the 1970s. The northernmost Vėžaičiai oil field is only partially connected to the major elevation to the south (which contains Šiūpariai, South Šiūpariai, Diegliai, Pociai, and the largest oil field, Vilkyčiai). The Ablinga oil field is a small, isolated structure, terminating the Gargždai elevation in the north (Figure 7).

The intricate pattern of the local uplifts in the Gargždai elevation and the adjacent structures was defined using 3D seismic data. Different seismic blocks were unified into a common structural model by Ref. [57]. The depth of the Cambrian reservoir varied from −1935 m (Vilkyčiai uplift) to −1977 m (Vėžaičiai uplift) and reached as low as −1996 m in the Ablinga oil field. The bottom of the Gargždai elevation is located at a −2015 m depth. It is notable that the oil fields in the underlying residual oil zones (ROZs) were found to be only partially saturated due to the partial release and south-facing leakage of hydrocarbons into the rapidly uplifted Mazury–Belarus High (Anteclise) during the Late Carboniferous–Early Permian.

The seismic stratigraphic interpretation argues that the Gargždai zone was established in the earliest Devonian (Gargždai Fm.). Accordingly, the Telšiai and Gargždai faults were formed during the very short tectonic phase initiated by the collision of the Baltic and Laurentia continents [42]. Furthermore, no discernible deformation along the Caledonian faults was documented in the seismic profiles crossing the Gargždai Fault Zone. Only weak activity was recorded, comprising few-meter-long amplitude deformations, along the secondary faults during the Upper Paleozoic and Mesozoic eras [57].

By contrast to the reservoir studied in the Syderiai, the Deimena RSt. in the Gargždai area showed more complicated reservoir architecture. The persistent quartz cementation considerably reduces the reservoir quality in the deepest part of the basin [55]. Furthermore, the inhibiting role of quartz cementation should be considered in the context of the oil fields [58], which were formed as long ago as the Middle Devonian [22].

Five reservoir compartments were defined in the vertical section of the reservoir (Figure 8). In terms of the stratigraphic nomenclature for Lithuania, the Deimena RSt. is subdivided into stratigraphic units attributed to, respectively, the Giruliai, Ablinga, and Pajūris Fms. The Ablinga and Giruliai Fms. represent the common upward-deepening sedimentation trend (Figure 7), similar to that at the Syderiai site. The thickness of the Ordovician cap rock ranges from 58 to 91 m and the thickness of the Silurian shales varies from 670 to 710 m, which are thicker layers than those found in the Gargždai elevation. The lithological composition of the cap rock is similar in both sites.

Most of the samples were collected from the Giruliai and Ablinga Fms., which comprise the most prospective part of the reservoir within the oil fields (Figure 9 and Figure 10). The authigenic quartz cementation reduces the reservoir quality in west Lithuania. The average porosity, horizontal permeability, anisotropy of permeability, and rock density of different formations are presented (

Table 2. Reservoir properties of Cambrian sandstones in the Gargždai elevation.

Index	Porosity %	Maximum Permeability mD	Anisotropy Permeability mD	Density kg/m3
Giruliai Fm.	4.74	9.33	0.74	2530
Ablinga Fm.	6.10	6.88	0.88	2490
Pajūris Fm. (I)	5.33	12.33	1.00	2480
Pajūris Fm. (11)	6.35	10.30	1.00	2470
Vargalė RSt.	12.20	0.30	no data	2400

). The average porosity is as low as 4.74% and the permeability is 9.33 mD in the Giruliai Fm.; the sandstones of the Ablinga Fm. have a slightly higher porosity of 6.10% and a lower permeability of 6.88 mD.

The permeability–porosity relationship in sandstones is subjected to various processes [59,60] and has been tested in sedimentary rocks in relation to petroleum and reservoir characterization [61]. The effects of grain size, packing, compaction, and solution/dissolution processes related to the development of primary and secondary porosity can lead to a wide variation in permeability. A common cluster can be observed on the plot of porosity vs. permeability (Figure 10). Furthermore, the diagram hints at two subclusters, showing permeability in the order of 1 mD (subcluster I) and sandstones of moderate reservoir quality (subcluster II). Two sandstone lithofacies (thin-bedded and thick-bedded sandstones) were deposited in the proximal and distal shorefaces recognized in west Lithuania.

The authigenic quartz cementation is one the most important parameters that controls the porosity–permeability relationship [55], while secondary porosity is only selectively distributed in scarce areas [62]; this is likely related to the hydrocarbon generation phase, which was associated with the partial corrosion of quartz grains and the precipitation of carbonate cement. There is no discernable grain mineral composition effect on the petrophysical parameters of sandstones. The chemical composition of sandstones varies from 97% to 99%. The increase in aluminum and associated bulk elements was related to fine clay lamina (including sub-millimeter-scale stylolites) incorporated into the studied samples. The amount of carbonate cement is as low as <0.8%. Therefore, the mineral composition of the sandstones exert only a negligible effect on the reservoir quality [63,64].

The mean grain size of sandstones varies from very fine sand to medium sand. The grain size discriminates two clusters and conforms to subclusters that can be recognized on the porosity vs. permeability diagram (Figure 10). The fifth-degree polynomial function approximated the pattern of the grain size of the studied sandstones. The average grain size is 0.16 mm in west Lithuania. The best-sorted sandstones are in the diameter range of 0.16–0.18 mm and classified as very-well-sorted sand (sorting value: 1.24) [65]. The modal grain size of Cluster I was 0.12 mm and that of Cluster II was 0.25 mm, which correlated with the poorly sorted 1.56 and 1.52 sandstones, respectively; they were classified as moderately well sorted.

The porosity of the Pajūris sandstones is compatible with the porosity values found in the upper part of the reservoir (5.33% in the Upper Giruliai and 6.35% in the Lower Giruliai sandstones). The increased permeability of the sandstones in the lower part of the reservoir is primarily related to more extensive fracturing of the sandstones (12.33 and 10.30 mD) (Table 2).

Thick-bedded storm deposits and thin-bedded inner shelf deposits alternate in marine basin sediments. Similarly, the permeability of most of the sandstones was restricted to 0.1–1 mD, which correlated with the low porosity of the sandstones, while an increased permeability of 10–200 mD was recorded for some few-meter-thick layers. Similar to the Syderiai area, the stability of the reservoir properties of the lithofacies is suggested, which may be indicated by two petrophysical clusters in the Deimena sandstones.

The highest density of sandstones is reported in the Giruliai Fm. (2530 kg/m³), which showed a minor decreasing trend in density (from 2490 to 2470 kg/m³) in the Ablinga and Pajūris Fms. This is likely related to the variable quartz cementation, which is the main parameter controlling reservoir quality. It has been extensively studied in the west Lithuanian oil fields [58,66]. Generally, three clusters were discernible in the plot of density vs. porosity. The most quartz-cemented, Cluster 2, had the highest-density sandstones (Figure 11). Cluster 1 had the low-density trend, implying the admixture of a clay fraction in the sandstones (possibly from drilling mud). Cluster 2 was classified as fractured samples. Also, the discordance in the statistical correlation of two parameters is a common phenomenon in large databases.

The high anisotropy of vertical vs. horizontal permeability should be stressed as an important feature of the Gargždai elevation reservoir, similar to that of the Syderiai reservoir, as discussed above (with an anisotropy ratio of 0.70). The ratio of vertical vs. horizontal permeability is 0.74 in the Giruliai Fm. and 0.88 in the Ablinga Fm. Abundant stylolites, nucleated along the bedding surface and spaced at 2–4 cm, contributing significantly to the anisotropy of the reservoir structure [35].

The isotropic structure of the lower reservoir (Table 2) can reasonably account for grain size and sorting, which strongly affects permeability in the Pajūris Fm. [44]. No dedicated study was carried out on the grain size sorting of Cambrian sandstones in west Lithuania.

The temperature of an aquifer is a paramount parameter for CO₂ geological storage. The Gargždai site is located within a geothermal anomaly in west Lithuania. Temperature logging was carried out in four wells, which were drilled to depths of approximately 2100 m. The bottomhole temperature varied from 80 °C (Diegliai-2) to 88 °C (Vilkyčiai-5); the thermal gradient varied, respectively, from 3.7 °C/100 m to 4.1 °C/100 m.

The chemical composition of the Deimena aquifer water was analyzed using 29 wells. The average mineralization of the brine was 170 g/L (Table 1). The chemical composition of the aquifer was classified as Na–Cl. Despite the limited study area, the concentration of analyzed chemical elements showed a rather wide variation, as the following: Cl = 83,600–112,700 mg/L; SO₄ = 109–952 mg/L; HCO₃ = 21–220 mg/L; Na = 26,593–32,913 mg/L; and K = 611–1368 mg/L. No systematic lateral variations in water composition were observed, as only the concentrations of sulfate, bicarbonate, and potassium were considered. The iron concentration was 60 mg/L and the silica content was 31 mg/L in well Vilkyčiai-5.

6. Discussion

Several tectonic structures (uplifts) are defined in the Cambrian sandstone reservoir in Lithuania. Because of Lithuania’s relatively low tectonic activity, the amplitude of most structures varies between 20 and 30 m. The calculated geometric volume of local structures (N = 116) ranges from 5 to 10 million m³ (79%) and from 10 to 20 million m³ in 17% of defined uplifts in Lithuania [16]. As mentioned above, only three onshore structures were considered to have potential for CO₂ underground storage. The Vaškai uplift was rated as a high-risk storage site due to its thin Silurian cap rock and deformations along the Telšiai fault (amplitude: 250 m), which were traced from the Cambrian succession to the Upper Devonian and were covered by thin Quaternary deposits [25].

The most prospective Syderiai structure was also influenced by the Telšiai fault in west Lithuania. This structure is located within the hanging wall of the fault. The amplitude of the brachyanticline reaches 70 m (Figure 3). According to the seismic data, the Telšiai fault cuts through the Lower Paleozoic deposits. Notably, the Cambrian sandstones are overlain by alternating 118 m thick Ordovician carbonates and shales and 435 m Silurian shales which comprise the most reliable cap rock. No deformation of the post-Gargždai deposits was documented in the seismic profiles.

The geometry of the Telšiai fault has a flower structure and was formed in the Late Silurian (Jūra Fm.) and Early Devonian (Gargždai RSt.). The local Luokė and Rūdupis faults represent the most important boundary of the Syderiai uplift (Figure 3). Based on the seismic data, the brittle faulting was associated with only minor bending deformation along the fault. The clay smearing along the fault should account for fault tightness [67], due to the relatively high amplitude of the Luokė and Rūdupis faults, which are tilted at a high angle of approximately 80° (Figure 12). The amplitude of the Rūdupis fault is about 100 m and the magnitude of the smaller Luokė fault is about 80 m. The total vertical offset was estimated to be 180 m, which represents their link to the major Telšiai fault. It should be noted that the faults discussed isolate the hydrodynamic systems of storage sites in the east and the south and reduce the storage capacity of the Syderiai site. Fluid and gas flow across fault zones might significantly affect the hydraulic behaviors and flow pathways [68,69,70]. The shale gouge ratio was estimated to be only 10% for the Syderiai and Rūdupis faults, when applying the approach from Ref. [67]. The pressure of CO₂ in the storage site will support a cross-fault pressure difference of >8 bar. Regarding the Silurian cap rock, the permeability of argillite was assessed to generally range from 10⁻⁶ to 10⁻⁹ mD [50].

The kinematic model of the Gargždai fault zone, oriented roughly as NE–SW, was classified as a reverse fault with an amplitude of about 100–150 m. In closer view, individual segments of the fault zone are defined on the map (Figure 7). The zone represents the orthogonal system of faults oriented as NNE–SSW (major) and WNW–ESE (subordinate) and shows the complex pattern of the tectonic blocks. The Pociai and Diegliai structures, protruding to the east, are compressed by WNW–ESE faults. The Gargždai fault zone terminates in the strongly fragmented Ablinga horsetail splay into the north.

As discussed above, the Gargždai fault zone was established in the Early Devonian, during a short time following the protracted Caledonian tectonic stage. No discernible reactivation of tectonic deformation was recorded since the Caledonian tectonic stage. Only some amplitude deformations a few meters in size along the accessory faults were observed in the seismic profiles, implying weak activation during the Devonian and Mesozoic eras. The stability of the Gargždai zone during the sedimentation of the carbonate platform sharply changed to deeper water anhydrites during the Zechstein (Upper Permian), which was studied and described by Ref. [57].

The reservoir properties within the Syderiai and Gargždai sites are significantly different, due to their different burial depths (c. 1460 m and 2000 m, respectively) and temperatures (51 °C and 85 °C). The quartz cementation of the sandstones controls the reservoir properties of the Deimena RSt.; the quartz cementation is approximately 10% in the Syderiai site and 30% in the Gargždai site.

The area of the CO₂ trap was estimated at 62 km² for the Syderiai site, and a much larger area was estimated (233 km²) for the Gargždai site (Figure 13). For the sake of comparison, the area of the largest offshore structure, E6, is 88 km² [24], the area of the Dalders structure is 161 km² [53], and the acreage of the largest Dobele structure is about 70 km² [21].

The average thickness of the Deimena RSt. reservoir is 50 m at the Syderiai site and 75 m at the Gargždai site (

Table 3. Input parameters for CO₂ geological storage at the Syderiai and Gargždai sites.

Volume	Syderiai	Gargždai
Closure, m	−1385	−2015
Structure amplitude, m	75	90
Thickness, m	50	75
Area, km2	62	233
Volume, mln·m3	1900	6980
Net-to-gross ratio	0.80	0.30
Porosity, %	0.16	0.07
CO2 density, kg/m3	710	610
Storage efficiency (fraction)	0.35	0.35
CO2 storage capacity, Mt	56.7	31.3

). The different net/gross ratios of the reservoirs are related to a higher content of intercalating shales and more damage in the open pore space due to strong quartz cementation in the deeper Gargždai site (with a ratio of 0.35), while a high ratio (0.80) was calculated for the Syderiai reservoir. The porosity of the sandstones averaged 16% and 7% in the Syderiai and Gargždai areas, respectively.

The temperature and burial depth (hydrostatic pressure of the aquifer) are significantly different in the two studied sites. The difference in the density of the trapped CO₂ between the Gargždai reservoir (610 kg/m³) and the Syderiai reservoir (710 kg/m³) is essentially important [71,72].

The considerable difference in the supercritical CO₂ gas phase is related to the temperature difference between the deeper and shallower reservoirs (87 °C and 51 °C, respectively). This higher temperature significantly reduces the storage capacity of the large Gargždai structure.

The efficiency factor Seff (storage efficiency) is the most uncertain parameter when assessing the CO₂ storage volume of a structure. In the literature, the efficiency factor is reported to vary between 1% and 40%, but the processes underlying its derivation are not always clear [54]. In Lithuania, the efficiency factor (sweep efficiency) could be reasonably estimated to be 35%, compared to 34% for oil extraction from the oil fields [73]. This value was assumed to be the maximum storage threshold for CO₂ storage.

An important parameter is the partial hydrodynamic isolation of both structures by tectonic faults. This fault restricts the hydrodynamic system of the storage site, as quoted by Ref. [51], assuming 0.30 efficiency of a one-fault bounded trap. The Gargždai elevation is bounded by the Gargždai fault, located to the east of the elevation. A similar restriction in the storage capacity should be considered for the Syderiai uplift, which is isolated due to the small-scale Luokė and Rūdupis faults.

Despite the larger area of the Gargždai structure, the Syderiai sandstones can store a larger volume of CO₂ due to their considerably higher reservoir quality. A storage capacity of 56.7 Mt was estimated for the Syderiai uplift and a capacity of 31.3 Mt was estimated for the Gargždai elevation.

The long-term storage of CO₂ can significantly modify a reservoir’s properties. An accurate prediction of CO₂ solubility over a moderate range of temperature and pressure values is important in studies of geological sequestration [74]. In the present study, thermochemical modeling was carried out to determine mineral phases, which were dissolved in equilibrium with formation water. The following main mineral phases were considered for both sites and showed rather compatible hydrochemical compositions (e.g., Na–Cl formation water, iron enrichment, and low sulfur concentration, among others):

• Sulfates (e.g., gypsum and anhydrite);
• Carbonates (e.g., calcite and dolomite);
• SiO₂ phases (e.g., chalcedony);
• Iron hydroxides (e.g., ferrous hydroxides).

The modeling was performed using the PHREEQC program [75]. The saturation index (SI) was calculated for individual mineral phases at reservoir temperature. SI > 0 indicated that water was supersaturated for the mineral phase concerned; thus, the respective mineral precipitates followed the trend that the higher the SI, the higher the saturation. In the case of SI = 0, the water and the mineral phase were in equilibrium, while SI < 0 indicated that the water was undersaturated with regard to the relevant mineral phase, so the corresponding minerals were dissolved.

Two saturated mineral phases, namely, ferrous hydroxides and pyrite, were considered to be high-risk parameters. The introduction of oxygen facilitated the precipitation of ferric minerals, which had previously been strictly avoided in the aquifer (the precipitation of ferrous hydroxides was modeled as the following: SI = 1.3). The iron concentration measured as high as 31 mg/L in the formation water. Also, silica was slightly oversaturated (SI = 0.3) in the formation water. By contrast, the carbonate minerals and gypsum were undersaturated and were prone to dissolution under the intense injection of CO₂ gas.

There was a minor influence of injected CO₂ on the Cambrian sandstones from Latvia, for which changes were reported that were similar to those in the modeling experiment; these were interpreted as the negligible dissolution of carbonate cement, feldspar, and some accessory minerals (e.g., illite, barite, and anatase/brookite), followed by some precipitation of amorphous silicas, clays, and carbonate minerals [34].

7. Conclusions

The storage capacities of the two largest onshore sites located in the deep part of the Baltic basin in west Lithuania, the Syderiai uplift and the Gargždai elevation, were determined for the evaluation of their potential for the underground storage of CO₂. The calculated storage volumes amounted to 56.7 Mt in the former site and 31.3 Mt in the latter site, which is compatible with the CO₂ emission rate in Lithuania. The large storage volume in the Syderiai structure is due to the much higher reservoir quality of the Cambrian sandstone reservoir, despite its smaller storage area. The thickness of the Deimena reservoir measures 50 m in the Syderiai uplift and 66–80 m in the Gargždai elevation. The primary cap rock is composed of a layer of Ordovician carbonate and a layer of Silurian organic-enriched shale, measuring 60–129 m and 430–710 m thick, respectively.

The regional-scale Telšiai and Gargždai fault zones control the largest studied uplifts. Based on the seismic survey, these major faults and associated smaller-scale faults were established during the Late Silurian and the Early Devonian and are classified as transpressional and compressional tectonic features. The paramount feature observed in the seismic profiles was only negligible recent geological activity; therefore, the Late Devonian and succeeding tectonic stages pointed to the reliable isolation of the reservoir.

Different burial depths and temperatures correlated with considerable differences found in the porosity and permeability of sandstones. The major parameter controlling the reservoir properties was authigenic quartz cementation. The average porosity of the sandstones was 16% and their permeability was 450 mD at the Syderiai site, while the porosity was as low as 7% and the permeability ranges from 5 to 12 mD at the Gargždai site.

The temperature of the Deimena reservoir varied from 51 °C in the Syderiai uplift to as high as 80–87 °C in the Gargždai elevation. The temperature controls the chemical composition of an aquifer. The hydrochemical simulation suggested only a minor effect of the CO₂ on the reservoir quality, e.g., a positive solubility index for iron minerals and quartz, while the carbonate minerals and gypsum were undersaturated mineral phases.

The present study argues that the Syderiai uplift is the site with the best potential for the geological storage of CO₂, owing to its high reservoir quality and the large geometric volume of the structure. The Gargždai elevation in the westernmost part of Lithuania is an attractive alternative for CO₂ storage combined with EOR technology for oil exploitation, despite its poor reservoir quality. These characteristics might motivate the exploitation of the thick ROZ underlying the exploited oil fields.