Preliminary assessment of CO2 geological storage opportunities in Greece

International Journal of Greenhouse Gas Control 3 (2009) 502–513

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Preliminary assessment of CO2 geological storage opportunities in Greece N. Koukouzas a,*, F. Ziogou b,1, V. Gemeni a,2 a b

Centre for Research & Technology Hellas/Institute for Solid Fuel Technology & Applications, Mesogeion Ave. 357-359, GR-15231, Halandri, Athens, Greece Centre for Research & Technology Hellas/Institute for Solid Fuel Technology & Applications, 6th km. Harilaou, Thermi Road, P.O. Box 361, GR-570 01, Thermi, Thessaloniki, Greece

A R T I C L E I N F O

A B S T R A C T

Article history: Received 22 May 2008 Received in revised form 26 October 2008 Accepted 27 October 2008 Available online 17 December 2008

This paper provides a preliminary assessment of the suitability of Tertiary sedimentary basins in Northern, Western and Eastern Greece in order to identify geological structures close to major CO2 emission sources with the potential for long-term storage of CO2. The term ‘‘emissions’’ refers to point source emissions as defined by the International Energy Agency, including power generation, the cement sector and other industrial processes. The Prinos oil field and saline aquifer, along with the saline formations of the Thessaloniki Basin and the Mesohellenic Trough have been identified as prospective CO2 geological storage sites. In addition, a carbonate deep saline aquifer occurring at appropriate depths beneath the Neogene-Quaternary sediments of Ptolemais-Kozani graben (NW Greece) is considered. The proximity of this geological formation to Greece’s largest lignite-fired power plants suggests that it would be worthwhile undertaking further site-specific studies to quantify its storage capacity and assess its structural integrity. ß 2008 Elsevier Ltd. All rights reserved.

Keywords: Ptolemais-Ardassa unmineable lignite seams Prinos saline aquifer Mesohellenic Trough CO2 storage Greece

1. Introduction The development of CO2 capture and storage (CCS) technologies is considered as a potential option in the portfolio of required measures to stabilize atmospheric greenhouse gas concentrations. Other options include energy efficiency improvements, the switch to less carbon-intensive fuels, renewable energy sources, enhancement of biological sinks, and reduction of non-CO2 greenhouse gas emissions (IPCC, 2005). Greece, as the fifth largest world producer of lignite and the second largest within the European Union, generates almost 93% of its electrical power requirements from fossil fuels, with lignite accounting for about 64% of the total. In 2005, the total CO2 emissions of all sectors (excluding the contribution of Land Use, Land Use Change and Forestry) amounted to 112,000 kt showing an increase of approximately 33% from 1990 (84,000 kt) (EEA, 2007). Of this, the stationary emissions of 36 industrial facilities, emitting over 100 kt CO2/year, accounted for 68,531 kt CO2 in 2005, representing 61% of net national emissions (CITL, 2005). The power sector is responsible for approximately 76% of the stationary CO2 emissions followed by the cement sector (16%) and oil refineries (5%).

* Corresponding author. Tel.: +30 210 6501771; fax: +30 210 6501598. E-mail addresses: [email protected] (N. Koukouzas), [email protected] (F. Ziogou), [email protected] (V. Gemeni). 1 Tel.: +30 2310 498391; fax: +30 2310 498392. 2 Tel.: +30 210 6501771; fax: +30 210 6501598. 1750-5836/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2008.10.005

Greece is committed under the European Community BurdenSharing agreement, to limit its GHG emissions increase for the period 2008–2012 to +25% compared to base year emissions (1990 for CO2, CH4 and N2O emissions, 1995 for F-gases) (Hellenic Ministry for the Environment, Physical Planning and Public Works, 2004). Taking into account the expected increase of electricity demand and the continued high fossil fuel dependency of the Greek power sector (increased power generation capacity of some 9.6 GW in the period of 1995–2020 will be mainly new natural gas combined cycle power plants), the potential for CCS opportunities within Greece should be investigated as a way of mitigating the greenhouse gases, in line with other options. This paper assesses the suitability of the Tertiary and NeogeneQuaternary sedimentary basins in Northern, Western and Eastern Greece for CO2 storage. The fossil-energy potential (hydrocarbon and coal) and the exploration and production maturity of these sedimentary basins provide a first indication for the selection of CO2 geological storage sites (Bachu, 2003). Moreover, the proximity of large emission sources to these storage structures (within area of radius less than 100 km) constitutes an additional advantage. According to previous work in the GESTCO Project (GESTCO, 2004) storage options for CO2 in Greece exist in the depleted oil field of the Prinos Basin offshore, in deep saline watersaturated reservoir rocks of the Mesohellenic Trough (MHT), the Thessaloniki Basin and the Prinos Basin Miocene sediments (Fig. 1). The proven lignite reserves of Greece are estimated at approximately 6.7 Gt, of which 4 Gt are considered economically recoverable (Koukouzas and Koukouzas, 1995). Almost 64% of the

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Fig. 1. Stationary CO2 emissions in relation to potential storage basins in Greece.

mineable reserves are located in the Florina-Ptolemais-Kozani Basin. In this intramontane basin, unmineable lignite seams at depths of 500–1000 m (Ardassa area) have been identified but the potential for enhanced coal bed methane (ECBM) recovery and CO2 storage is likely to be low. The presence of inorganic matter (ash; average of 15.1%), water (average 52.6%) and the high inertinite content (average 17%) in these seams (Koukouzas and Koukouzas, 1995) result in reduced adsorption capacity (Clarkson and Bustin, 1997). Within this screening region an extended carbonate aquifer exists below the lignite seams, which may provide suitable storage for CO2 emitted from the lignite-fired power plants located nearby. The lack of reliable and valid geological information at great depths restricts a complete and comprehensive evaluation of the reservoir quality. The suitability of the selected sedimentary basins for effective CO2 storage in Greece is assessed based on the criteria developed by Bachu (2003) and modified by Gibson-Poole et al. (2006). However, this assessment is preliminary and requires a more detailed assessment and characterisation of the particular basins due to the tectonic instability, the high faulting intensity and the geothermal conditions.

A volumetrics-based CO2 storage volume estimate for Prinos depleted oil field is presented while the estimations of the CO2 storage capacity for the identified deep saline aquifers are based on the methodology suggested by the GESTCO Project (GESTCO, 2004). Therefore, in order to ensure consistency with the GESTCO Project, a constant CO2 density of 750 kg/m3 was considered across all the basins despite the fact that this density value is likely high for Greece’s geothermal conditions. 2. Stationary emissions An inventory of major CO2 point sources in Greece identified 20 power plants, 8 cement kilns, 4 refineries, 2 iron and steel plants, 1 aluminium plant and 1 ammonia production plant. A CO2 ‘supply curve’ for the identified stationary CO2 emission sources is shown in Fig. 2. Around 68% of stationary CO2 emissions originate from plants with emissions ranging between 1 and 10 Mt CO2, 20% come from one power plant with annual emissions above 10 Mt CO2 while the remaining 12% originate from point sources emitting less than 1 Mt/year (Table 1).

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Fig. 2. Large stationary source emissions and number of emission sources in Greece.

The majority of stationary CO2 emissions are generated in the region of Western Macedonia (Florina-Ptolemais-Servia area) where the largest of the Public Power Corporation’s lignite-fired power plants are located representing 70% of the country’s total power and heat production. As a result, 50% of stationary CO2 emissions in Greece derive from this region (Tables 1 and 2). An indication of the location of CO2 point sources relative to Tertiary sedimentary basins with CO2 storage potential in Greece is also shown in Fig. 1. 3. Selected sedimentary basins of Greece 3.1. Prinos Basin The Prinos-Kavala sedimentary basin in the North Aegean Sea covers an area of 800 km2. The thickness of sediments within it

Table 1 Contribution of each sector (Mt CO2) in the different ranges. Sector

0.1–0.2 Mt

0.2–0.5 Mt

0.5–1 Mt

1–10 Mt

Above 10 Mt

Power Cement Refineries Iron and steel Aluminium Fertilizer

0.3 0.2

0.7

4 0.5

33 10 3

14

Total %

0.6 0.7

0.7 0.1

0.9 0.5 0.3 1.7 2.5

5.9 8.7

68

14 20

Table 2 Installed capacity and CO2 emissions of main power plants in the Western Macedonia Region (Psomas, 2006). Power plant PPC PPC PPC PPC PPC PPC

S.A., S.A., S.A., S.A., S.A., S.A.,

Total

TPS TPS TPS TPS TPS TPS

Ag. Dimitriou Kardias Ptolemaidas Amyndaiou Florinas Liptol

Installed capacity

Emissions (t CO2/year)

1595 MW 1250 MW 620 MW 600 MW 330 MW 43 MW

13,629,229 9,815,429 3,487,897 5,124,545 1,955,721 358,515 34,371,336

exceeds 6 km (Harker and Burrows, 2007; Koukouvelas and Aydin, 2002). To date, it is the only geological area in Greece, where oil and gas has been produced; production has now been taking place for more than 20 years. Therefore, sufficient data are available to make an in-depth evaluation of the storage potential. In total around 110 million barrels of oil (Prinos/Prinos North oil fields) and approximately 600 Mcm3 of natural gas (South Kavala) have been produced (Papavasileiou, 2007). 3.1.1. Structural and stratigraphic evolution of Prinos Basin The Prinos Basin is a fault controlled rift basin, trending NE–SW. Although the area is characterized by faults with high strike slip component which developed within the N–S extensional regime of the Aegean region, the Prinos Basin and the adjacent Miocene Basins have not been affected (Higgins and Higgins, 1996; Koukouvelas and Aydin, 2002). The basement consists of metamorphic rocks, mainly gneiss, quartzite and dolomitic marble (Proedrou and Papaconstantinou, 2004; Maltezou and Brooks, 1989). These rocks have no CO2 storage potential. The basin fill comprises three main series which occur throughout the basin with very distinct boundaries between them (Koukouvelas and Aydin, 2002; Proedrou and Papaconstantinou, 2004): The pre-evaporitic series, the evaporitic series and the post-evaporitic series (Fig. 3). 3.1.2. Prinos Basin CO2 storage potential The reservoirs in the basin consist of sandstones and some siltstones with an aggregate thickness of around 260 m (Proedrou and Papaconstantinou, 2004). Stratigraphic and structural traps (e.g. anticlines) can provide safety in the CO2 storage. Containment is provided by salt and evaporite deposits and overlying clastic unconsolidated sediments, which cover the whole basin and are up to 2300 m thick (Harker and Burrows, 2007). They are an excellent seal, as they have proven capability to retain the hydrocarbons. The reservoir intervals have an average permeability of 50 mD and porosity ranging from 15% to 20% indicating fair injectivity potential. The depth from the surface to the top of the reservoir varies from 1 to 3.5 km, which is adequate to maintain CO2 in its supercritical state. According to geothermal borehole data and due to the increased regional heat flow, the temperature of the basin is around 122 8C at a depth of 1377 m corresponding to an average gradient of 78 8C/km (Boulvais et al., 2007; Kolios et al., 2005).

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oil recovery is currently applied (Papavasileiou, 2007). The theoretical storage capacity has been estimated by converting the known ‘‘ultimately recoverable reserves’’ (URR) pore space (past production plus remaining reserves) to CO2 stored. However not all the (hydrocarbon-saturated) pore space will be available for CO2 storage because some residual water may be trapped in the pores due to capillary forces, viscous fingering and gravity effects (IPCC, 2005). The theoretical storage capacity of the Prinos oil field is estimated at 19 Mt CO2 (IEA, 2005) based on 19.9 Mm3 ‘‘ultimately recoverable reserves’’ (Papavasileiou, 2007) and assuming that CO2 injection will be terminated when the initial reservoir pressure will be reached (Bachu and Shaw, 2003).  Kallirachi unexploited oil field: The Kallirachi field occurs between two major normal faults and is another potential site for future CO2 storage, either when the hydrocarbon is depleted or through the application of CO2 storage combined with EOR (Proedrou and Papaconstantinou, 2004). The field is a combined stratigraphic and a structural trap similar to the rest of the oil-bearing traps in the basin. The thickness of the Kallirachi reservoir sandstones is estimated to be over 300 m. Based on 3D seismic data, a major fault provides lateral seal while vertically the reservoir is sealed by 900 m of salt deposits. The probable and possible oil-in-place volume is expected to be up to 650 MMstb at a total depth of 2555 m (Mavromatidis et al., 2004). The theoretical storage capacity based on 240 MMstb recoverable oil reserves is estimated to be around 32 Mt CO2 (IEA, 2005).  Prinos Basin as a saline aquifer: The depth to the top of the Prinos saline aquifer is about 2 km assuring the supercritical state of CO2 for such a warm basin (Bachu, 2003). The average thickness of the reservoir is estimated to be 260 m with a net/gross of 0.8 resulting in approximately 208 m of net sand (Hatziyannis and Xenakis, 2003). The storage capacity of the saline water-bearing reservoir rocks in the offshore Prinos Basin has been estimated at 1350 Mt CO2 (GESTCO, 2004). Fig. 3. Stratigraphy of Prinos Basin.

The offshore Prinos Basin appears to be relatively tectonically stable although more than 60% of the European seismicity is released in the Aegean area because of the interaction of the North Anatolian Fault (NAF) with the North Aegean Trough (Higgins and Higgins, 1996). The seismicity map (Fig. 4) shows that most of Greece is dominated by shallow and intermediate depth earthquakes. They occur principally along the Hellenic Trench and in the North Aegean (Higgins and Higgins, 1996; Papanikolaou and Papanikolaou, 2007). However, the existence of oil fields in faultbounded traps indicates that the influence of earthquakes on CO2 storage in the Prinos Basin could be limited. Potential CO2 storage opportunities exist in Prinos oil field, in the unexploited Kallirachi oil field and in the saline water-bearing reservoir rocks of the basin as a whole. However, there is potential for conflict of interest with future hydrocarbon discoveries due to the high prospectivity of these Miocene sands. 3.1.3. CO2 storage opportunities and storage capacity estimation  Prinos oil field at depletion stage: The Prinos oil reservoir, with a thickness of over 260 m, is located on a gravity-slide anticline in the North-central part of the basin and as a fault-bounded trap presents a potential CO2 storage site. Enhanced oil recovery (EOR) and CO2 storage can not be considered as a concept for that field since at least 20–30% of the recoverable reserves of the field still remain to be produced (IEA, 2004). In addition, tertiary

3.2. Ptolemais-Kozani Basin From an economic point of view the Ptolemais-Kozani Basin in NW Macedonia is the most interesting in Greece because of the exploitation of the lignite reserves. This large Neogene Basin is a part of a tectonic trench over 250 km in length that extends from northern Greece into the Former Yugoslavian Republic of Macedonia. The basin is divided into two elongated grabens that are characterized by different stratigraphic evolution and subsurface morphology. 3.2.1. Structural and stratigraphic evolution of Ptolemais-Kozani Basin The formation of the basin occurred at the end of the Tertiary era and its creation is considered to be a consequence of subsidence in large NW–SE fault zones (Fig. 5). Internally, the structural elements of the basin are bounded normal faults that uplifted Mesozoic limestone, gneiss, and Mio-Pliocene lake deposits and divided the basin into sub-basins which trend in the same direction (Pavlides and Mountrakis, 1987). The basin was subject to gradual subduction during the Middle–Upper Miocene that led to the sedimentation in the basin and the formation of the Ptolemais Lignite Formation. Another tectonic episode during the Pleistocene and the Holocene resulted in further subsidence, with sediment accumulations in places more than 350 m thick. The repetition of a lacustrine-swamp environment allowed the formation of the lignite (Kotis et al., 2001). The basin fill consists of the Ptolemais Lignite Formation (Upper Miocene), the Proastio Formation (Early Pleistocene) and a formation similar to the Perdika, the Ardassa Lignite Formation,

506

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Fig. 4. Seismicity map in the Aegean area (mb 4.5).

of Upper Pleistocene age. The basement beneath the basin consists of Palaeozoic and Mesozoic metamorphic and plutonic rocks underlying Cretaceous limestone and flysch (Pavlides and Mountrakis, 1987) (Fig. 6). 3.2.2. Ptolemais-Ardassa Basin CO2 storage potential Although the basin is described as a seismically inactive area (Pavlides and Mountrakis, 1987), geological data, geophysical studies and GPS measurements in Western Macedonia denote ongoing tectonic activity in the upper crust (Diamantopoulos, 2006). For example, the recent 1995 Kozani-Grevena earthquake

(Ms = 6.6) was related to the reactivation of Aliakmonas fault, which is the most significant fault zone in Western Macedonia (Mountrakis et al., 2006) (Fig. 4). Potential CO2 storage opportunities exist in the aquifer occurring in the sedimentary sequence below the Ptolemais Lignite Formation, extending from the Greek-Former Yugoslavian Republic of Macedonia border towards Elassona. This uniform carbonate sequence, which consists of limestones and marbles, occurs at depths greater than 800 m with a thickness which in places exceeds 1000 m. The carbonate sequence is quite extensive, surrounds the basin and seems to be highly fractured.

Fig. 5. Tectonic map of Ptolemais-Kozani graben.

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Fig. 6. Geological cross-section of the Ptolemais-Ardassa Basin showing the stratigraphy of the basin and the potential CO2 storage sites and the cap rock formations.

Both lignite formations (of Ptolemais and of Ardassa) could be cap rocks as they consist of clays, marls and mud and have an overall thickness of approximately 150–200 m. The capacity of the carbonate aquifer has not been estimated yet due to lack of available data related to reservoir and seal properties (porosity, permeability, seal integrity and salinity). Moreover, a geological and geo-mechanical characterisation, including the existing natural fault and fracture systems and 3D reservoir modelling, are required to assess the storage potential of this reservoir. Possible existing resources that might be affected by CO2 storage could include lignite and groundwater; CO2 storage may further alter the hydrological budget of the region, which is already affected by over-exploitation of the lignite. Finally, the Proastio-Komnina-Mesovouni fault zone, which occurs in the proximity of the identified potential storage site, should be considered in order to assess the tectonic stability of the area. This fault zone consists of the 10 km long Proastio fault and the 20 km long Komnina-Mesovouni fault and has an extensional stress field with a NW–SE axis of least principal stress (Mountrakis et al., 2006). 3.3. Mesohellenic Trough The Messohellenic Trough, the largest and most important ‘molasse-type’ basin of the Hellenides (Vamvaka et al., 2006), is located on the western border of the central Hellenic thrust belt. It is 40 km wide and 300 km long, extending from Albania to Thessalia in a NNW–SSE direction (Ferriere et al., 1998). 3.3.1. Structural and stratigraphic evolution of the Mesohellenic Trough The Mesohellenic Trough developed from middle Eocene to middle Miocene time as a piggy back basin along the eastern flanks of a giant pop-up structure bounded by the Eptachori thrust. The MHT shows a tectonically controlled variation in its evolution along its axis (Doutsos et al., 1994) (Fig. 7). According to Brunn (1956), the sedimentary fill of the basin is divided into five siliciclastic formations (Fig. 8), which are from bottom to top: the Krania Formation (Middle–Upper Eocene,

maximum thickness: 1500 m), the Eptahori Formation (Middle– Upper Oligocene and estimated thickness 1000 m), the Pentalofos Formation (of Aquitanian age), around 2500 m thick, the Tsotyli Formation of Late Aquitanian-Tortonian age and thickness of 1500 m. In places, this sequence is overlain by the Ondria Formation (Middle–Upper Miocene), which is restricted to a few areas in the Mesohellenic Trough probably because of erosion (Vamvaka et al., 2006). 3.3.2. Mesohellenic Trough CO2 storage potential Seismic interpretations suggest that the Mesohellenic Trough is highly faulted (Kontopoulos et al., 1999). Some of the normal faults, generally those oriented E–W, are believed to be still active (Vamvaka et al., 2006). Despite that, the basin appears to be fairly tectonically stable at present with limited earthquake activity in the area. According to the results of geochemical analyses, the Mesohellenic Basin contains organic matter in shales suitable for the generation of gas and wet gas (Avramidis et al., 2002; Kontopoulos et al., 1999). The Mesohellenic Trough can be classified as a cold sedimentary basin with a geothermal evolution similar to that of the Alpine Molasse Basin (Gerolymatos et al., 1988). Geothermal gradients for the basin were estimated by Fytikas and Kolias (1979) as 35 8C/km, increasing both the storage capacity and safety due to the strong effect of temperature on CO2 density and therefore its buoyancy with respect to formation water or reservoir oil (Bachu, 2003). Potential CO2 storage opportunities exist in the Eptahori and Pentalophos saline formations of the Mesohellenic Trough and, specifically, between the east margin of the Trough and the Theotokos-Theopetra uplifted structure as the fault zone on the east results in contact between the basin filling sediments and impermeable rocks (ophiolites and schists) (Fig. 8). Moreover, some rollover anticlines developed along the faults of the Theotokos thrust have created local structural traps (Kontopoulos et al., 1999). The Eptahori Formation (Fig. 9), with an estimated thickness of 1200 m in the south of the Trough, consists of conglomerates and sandstones while the seal is provided by marine turbiditic shales

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Fig. 7. Geological map of the Mesohellenic piggy-back basin (Zelilidis, 2003).

(thickness about 250 m) with interbedded lignitic horizons (Vamvaka et al., 2006) as well as the mudstone interbeds which occur between the sandstone beds of Pentalofon and Tsotilion formations (Doutsos et al., 1994). The base of the Pentalophos Formation consists of conglomerates, followed by alternating turbiditic sandstones and shales, with minor conglomerates (Doutsos et al., 1994; Vamvaka et al., 2006). The estimated thickness is 2500 m. The porosity of the turbidite sandstones of the Eptahori and Pentalophos Formations varies between 15% and 25% (Kontopoulos et al., 1999) while the average permeability is estimated to be low (Hatziyannis and Xenakis, 2003) indicating poor to moderate injectivity potential. The depth to the top of the geological

structures varies from 1 to more than 2 km, which is adequate to maintain CO2 in a supercritical state. Insecure containment is a possible risk due to the existence of normal faults cutting the Pentalophos and Eptahori Formations. Further north, the contact between the Pentalophos Formation and the Tsotyli Formation may be also characterized as tectonic (Vamvaka et al., 2006). Possible impact on existing resources should also be considered since the samples from the Eptachori and Pentalophos Formations indicate gas potential (Avramidis et al., 2002; Kontopoulos et al., 1999). The storage potential of the closures in the MHT have not yet been estimated due to the low level of exploration activities carried out in the area and the resulting limited availability of geological data.

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Fig. 8. Geological cross-section of the Mesohellenic Trough.

3.4. Thessaloniki Basin The Thessaloniki Basin is located to the west of the city of Thessaloniki and is bordered to the north by the mountains of Paikon and to the west by the mountains of Vermion, both consisting of Mesozoic limestones. The basin is bounded to the south by the Thermaikos Gulf. The basin covers an area of 4200 km2 onshore and 4000 km2 offshore. The Axios, Aliakmon, Loudias and Gallikos Rivers run into the basin. 3.4.1. Structural and stratigraphic evolution of Thessaloniki Basin The Thessaloniki Basin has a NNW–SSE trend and constitutes a complicated tectonic graben which developed from the Eocene until the Quaternary (Lalechos, 1986). The tectonic rifting occurred during the Upper Oligocene–Early Miocene period and created marginal faults (Ghilardi et al., 2007). The sediments comprise mainly clastic units (conglomerates, sands, clays) and locally calcareous sediments (limestones, marls). The basement below the basin contains high grade metamorphic rocks mainly of the Axios geotectonic zone. The Paleogene sediments are of mollasse and flysch type sandstones in alternation with conglomerate beds. The maximum thickness of

Fig. 9. Stratigraphic column of Eptahori formation in the Mesohellenic Trough.

the sedimentary sequence is in the centre of the basin and sediment thickness decreases at the margins (Kolios et al., 2005). Due to the presence of hydrocarbon accumulations near Epanomi, the Greek Public Petroleum Company has carried out detailed exploration in the area. According to the results from seismic reflection surveys and deep boreholes the basin fill comprises Pliocene-Quaternary sediments with an average thickness of 600 m, overlying Miocene clays 750 m thick that grade downwards into Miocene conglomerates (thickness 250 m). Underlying these strata is the Oligocene flysch with a thickness of around 500 m. 3.4.2. Thessaloniki Basin CO2 storage potential Potential storage opportunities exist in saline sandstone aquifers of the Western Thessaloniki Basin, around structures such as the Alexandria Anticline and the Loudias and Agriossykia Synclines. The Alexandria Anticline is characterized as a structural trap, while the faults that bound the synclines bring the possible storage site into contact with impermeable rocks. The geophysical surveys that were done in the basin showed that the faults do not reach the surface, a factor that minimises the risk of stored CO2 leaking into the atmosphere. The thickness of the reservoir formations exceeds 500 m (Kolios et al., 2005), providing opportunities for CO2 storage. The age of the sandstones is Eocene. The overlying sediments comprise primarily Oligocene flysch with a thickness of around 1200 m (in the West Thessaloniki area), forming a major cap rock (Kolios et al., 2005). The geothermal gradient in the Alexandria region is estimated as 27 8C/km. The water of the aquifer is brackish (Kolios et al., 2005). The sand/clay ratio in the aquifer formation ranges between 40% and 90% sand and is strongly affected by the formation of the salt structures. The depth to the top of the saline formations varies from 900 to 1200 m (Alexandria), and 2400 m (West Thessaloniki), respectively, meeting the optimum depth that maximizes the CO2 storage capacity in cold basins (Bachu, 2003). The injectivity varies from very poor to poor, with porosity ranging from 5% to 20% and permeability ranging from a few mD to 120 mD (Hatziyannis and Xenakis, 2003; Kolios et al., 2005). The fault pattern of the broader area is complicated, including NW–SE, NE–SW, E–W and NNE–SSW striking faults (Fig. 10) (Paradisopoulou et al., 2006). The latest 1978 Thessaloniki earthquakes (Ms = 6.5 and 5.5) are attributed to the reactivation

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Fig. 10. Stress pattern in the N. Aegean domain from structural analysis of faults of Middle Pleistocene—present (modified from Mercier et al., 1989).

of the NW–SE and E–W faults in the Mygdonian Basin. The studies up to now show that the E–W-striking faults are the most active and are associated with the current seismic activity (Paradisopoulou et al., 2006) (Figs. 4 and 10). Therefore, the tectonic stability of the identified structural traps should be further examined in order to minimise any containment risk, as the area is characterized by complex active fault architecture. Possible impact on existing resources should also be considered as the Basin is considered prospective for geothermal energy production. 3.4.3. CO2 storage opportunities and storage capacity The total storage capacity of volumetric traps in the onshore Thessaloniki Basin is estimated at 604 Mt CO2 for Loudias and Agriossykia areas while the storage capacity of the Alexandria saline aquifer is estimated at 35 Mt CO2 (GESTCO, 2004).

4. Discussion and conclusions Carbon dioxide capture and geological storage has the potential to make a large reduction in the CO2 emissions from power plants in Greece given the high dependency on fossil fuels for the bulk of the national electricity generation and the aim to fulfil Greece’s share of the overall European energy policy targets. The emissions map shows that 50% of stationary CO2 emissions in Greece, solely produced through power and heat generation, derive from the region of Western Macedonia. On the other hand, Greece is a quite tectonically active country (Fig. 11). The geotectonic regime of the Hellenic Arc creates an extensional pattern in front of it resulting in the creation of normal faults and grabens. Moreover, Greece has, on the whole, favourable geological conditions for geothermal resources, especially in the active volcanic arc of the South Aegean. Due to active intensive tectonics that facilitated the rise of deeper hotter fluids to the surface, there are numerous areas with positive geothermal anomalies including reservoirs of hot fluids at relatively shallow depths that are commercially exploitable (Fytikas, 1988). The strong seismicity and the associated high heat flow anomalies in the tectonic framework of Greece indicate the need

for detailed site characterisation of any prospective CO2 storage site (Kaldi and Gibson-Poole, 2008). Using the Techno-Economic Resource-Reserve Pyramid for CO2 Storage Capacity (Bachu et al., 2007; CSLF Task Force, 2007), the CO2 storage capacity estimates for the depleted and unexploited oil fields in the Prinos Basin correspond to theoretical capacity or to the total pore volume of the pyramid as modified by Kaldi and Gibson-Poole (2008). Based on the volumetric method that all the pore space freed up by the production of recoverable hydrocarbon reserves will be replaced by CO2 (Bachu and Shaw, 2003), the theoretical storage capacity of the depleted oil field in the Prinos Basin is around 19 Mt and that of the unexploited Kallirachi field is around 32 Mt CO2 but reservoir heterogeneity, irreducible water saturation, and also CO2 mobility and buoyancy should be taken into account in order to calculate their effective capacity (Bachu and Shaw, 2003; CSLF Task Force, 2007). The aquifer potential of Prinos and Thessaloniki Basins, falling into the theoretical category of the Techno-Economic ResourceReserve Pyramid for CO2 Storage Capacity (Bachu et al., 2007; CSLF Task Force, 2007), produces a figure of 1990 Mt total CO2 storage capacity based on structural traps with well-defined spill points (GESTCO, 2004). It is important to note that the CO2 storage capacity of saline water-bearing reservoir rocks (saline aquifers) in Greece is overestimated because in reality CO2 density will be much lower than 750 kg/m3, as assumed in the GESTCO (2004) Project. It is believed to be around the 400–500 kg/m3 range due to high geothermal gradients values and temperatures at storage depths of the specific basins (Bachu, 2003). Thus, a more realistic preliminary estimate would be in the order of 1400–1500 Mt. Based on currently available information, the following outcome can be derived from the above Basin-Scale Assessment (Bachu et al., 2007; CSLF Task Force, 2007) of prospective sedimentary successions in Greece (Table 3):  The tectonically stable offshore Prinos Basin has favourable characteristics for CO2 geological storage as well as sufficient storage potential to take in the total amount of CO2 produced by the nearby Komotini gas fired power station (0.7 Mt/year CO2) for several decades. In addition, the offshore location of the

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Fig. 11. Geotectonic regime of Greece.

potential reservoir and seal units increases the transportation costs but it is countered by the well established infrastructure framework within 30–40 km of the coast (pipelines, wells and platforms).  The onshore Thessaloniki Basin appears to have very good technological and economic potential for CO2 storage. Favour-

able factors include limited faulting, optimal depth range for CO2 storage capacity, and relatively low drilling costs within the closures identified. It appears to have the capability to store all the regional stationary CO2 emissions (one cement plant and one refinery with its 400 MW Combined Cycle Gas Turbine Unit emitting in total around 1.9 Mt CO2/year) or the total lifetime

Table 3 Evaluation of prospective sedimentary basins for CO2 geological storage in Greece. Prinos Basin

Thessaloniki Basin

Tectonic stability Size Depth

Stable Small (<1000 km2) Intermediate (1500–3500 m)

Reservoir–seal pairs Faulting intensity Geothermal Hydrocardon potential Maturity Coal Coal rank Onshore/offshore Accessibility Infrastructure CO2 sources

Excellent Limited Warm basin (80 8C/km) Large Mature

Stable Medium (1000–5000 km2) Shallow (<1500 m) and intermediate (1500–3500 m) Excellent Limited Cold basin (<30 8C/km) Small Unexplored

Offshore Easy Extensive Few

Onshore Acceptable to easy None Moderate

Ptolemais Basin

Mesohellenic Trough

Stable ? ?

Intermediate Large (5000–25,000 km2) Intermediate (1500–3500 m)

Intermediate Moderate ?

Intermediate Extensive Moderate (30–40 8C/km) Small Unexplored

Mature Very shallow (<300 m) Lignite Onshore Easy None Major

Onshore Difficult None Major within 100 km

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output of a lignite-fired power plant located in the region of Western Macedonia (around 100 km distance).  The aquifer properties and structure of the carbonate reservoir beneath the Ptolemais Basin requires further detailed site exploration to assess the basin’s suitability due to the significant point sources of CO2 in the region. In addition, a new coal-fired power plant is planned to be commissioned in the area in the next 2 years providing a further opportunity for developing a large-scale CO2 storage project.  The CO2 storage potential of the Mesohellenic Trough is unclear due to sparse drilling across the basin although suitable reservoir and seal units appear to be present at appropriate depths, but the extensive faulting should be considered as a potential risk. To conclude, the large-scale implementation of CCS at seven large point sources of CO2 in NW Greece (five power plants, one refinery and one cement plants), which emitted around 35 Mt CO2 in 2005, would reduce national CO2 emissions by 25–28%. The geological settings of the Tertiary and Neogene-Quaternary sedimentary basins in Greece appear to provide a promising option for CCS implementation. The identified potential reservoirs and overlying seal units occur within approximately 100 km of the significant stationary CO2 emissions in NW Greece, which is favourable in terms of infrastructure costs. The theoretical storage capacity of the offshore depleted Prinos oil field is currently limited but appears to be sufficient quantified CO2 storage capacity in the associated saline water-bearing reservoir rocks. This justifies a detailed investigation in order to define their practical and matched storage capacity as well as a proper characterisation and site screening particularly in regard to containment and risk of leakage. This would be a step forward in evaluating the feasibility of CCS as a tool towards the mitigation of the greenhouse gas emissions within the overall strategy for the promotion of sustainable development in Greece. Nonetheless, it should be taken into consideration that the actual conditions for CO2 storage in Greece, which do not appear to be so favourable, compared with other regions in Europe such as the North Sea or in Germany because of high tectonism and faulting, high geothermal gradients and low injectivity in some basins. Such an investigation would be aligned with the fact that CCS technology is seen as a high R&D priority in Greece’s 2007–2013 Energy Programme. Acknowledgments The authors would like to thank the native English speaker Mr. Colin R. Ward, Professor of Geology, in the University of New South Wales, Australia, for providing guidance and constructive comments to the grammar and typographical errors of the manuscript. References Avramidis, P., Zelilidis, A., Vakalas, I., Kontopoulos, N., 2002. Interactions between tectonic activity and eustatic sea-level changes in the Pindos and Mesohellenic basins, NW Greece: basin evolution and hydrocarbon potential. J. Petrol. Geol. 25 (1), 53–82. Bachu, S., 2003. Screening and ranking of sedimentary basins for sequestration of CO2 in geological media in response to climate change. Environ. Geol. 44, 277– 289. Bachu, S., Bonijoly, D., Bradshaw, J., Burruss, R., Holloway, S., Christensen, N.P., Maathiassen, O.M., 2007. CO2 storage capacity estimation: methodology and gaps. Int. J. Greenhouse Gas Control 1 (4), 430–443. Bachu, S., Shaw, J.C., 2003. Evaluation of the CO2 sequestration capacity in Alberta’s oil and gas reservoirs at depletion and the effect of underlying aquifers. J. Can. Petrol. Technol. 42 (9), 51–61. Boulvais, P., Brun, P., Sokoutis, D., 2007. Fluid circulation related to post-Messinian extension, Thassos Island, North Aegean. Geofluids 7, 159–170. Brunn, J.H., 1956. Etude geologique du Pinde septentrional de la Macedoine occidentale. Ann. Geol. Pays Hellen. 7, 1–358.

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