Katakolon - a small but perfectly formed oil field in Western Greece

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Fig. 1. The Adriatic-Ionian area and the Cretaceous to Eocene basinal carbonates hydrocarbon province

The Cretaceous to Eocene basinal #carbonates of the Adriatic-Ionian zone host prolific #hydrocarbon resources. These #reservoirs have been exploited onshore in Albania since the late 1920s and offshore in the Italian Adriatic since the 1960s, and continue to produce right upto the present day. Recently, interest in this play has increased, in part due to the Petromanas-Shell success at #Shpiragu in Albania in 2013 (http://youngpetro.org/2013/12/09/new-oil-discovery-in-albania-by-shell-petromanas) and in part due to the awareness that improved #seismic imaging can unlock remaining #exploration potential in the sparsely explored Greek-Albanian segment of the play, resulting in a new wave of exploration particularly onshore and offshore in Greece (http://www.petroleum-economist.com/articles/politics-economics/europe-eurasia/2018/greece-presses-the-ep-button).

The relatively unknown #Katakolon oil and gas field offshore in Greece is interesting both in its own right and because it illustrates some of the general characteristics of fields in this play. This small field, with proven reserves of 10.5 mmbbl, is located in the Ionian Sea some 3.5 km offshore the north western Peloponnesus, Greece. The operator, #Energean, is progressing development of the field, with a development plan approved, an environmental impact assessment ongoing and the drilling of an initial development well planned for 2019 with first oil potentially achieved in 2020.

Figure 2. Top Reservoir structure map for Katakolon field (Xenopoulos, S., & Roussos, N., (2007))

The field is the only discovery in Western Greece, and confirms that the prolific #petroleum systems of the Ionian Zone extend all the way to the southern portion of the external Hellenide thrust belt. Consequently, Katakolon is a key datapoint for evaluating plays and prospects in the area and generally understanding exploration risk in western Greece. Given this, and the widely scattered and relatively inaccessible nature of much of the literature on the field, we thought we’d share a brief summary of the public domain literature with Strata’s friends.

The discovery well WK1 was drilled in 1981 and encountered #gas and then recovered minor #oil with water on test. The well was then sidetracked and WK1a recovered oil and gas from 3 separate intervals. A year later WK2 was drilled as a flank appraisal well and produced oil on test from 2 separate intervals with flow rates of 1,000-1,400 bopd. 3d seismic was acquired in 1984, but the field was considered marginal until Energean’s recent efforts to progress the development through drilling development wells from onshore. The gas cap is reported as being some 120 m thick, with an underlying oil rim some 100 m thick. Oil gravity is ~ 27° API with a GOR of 600-700 scf/ bbl and the gas phase contains significant H2S (5.4%) and CO2 (8.8%).

So let’s take a brief look at each of the key elements of the Katakolon accumulation.

Fig.3. (a): Regional cross-section across W Greece showing the location of the Katakolon field (Maravelis et al (2014)). (b): Detailed cross section of Katakolon field after Kamberis et al (200)

The #trap appears relatively simple at first glance: it’s a simple four-way dip structure in the hanging wall to the Ionian thrust system, formed during the main Oligo-Miocene phase of compression in the #Hellenides. On closer inspection, however, it becomes apparent that the trap is rather more complex: the top carbonate surface, that defines the top of the reservoir, is an unconformity that erosively cross cuts the #stratigraphy of the east dipping reservoir section. Consequently the trap at Katakolon is a buried hill trap with the structural relief on the trap being related to the palaeotopography of an erosional landscape. In essence, the Katakolon structure is similar to the exposed and eroded hanging wall anticlines that form the dramatic mountainous landscape of Greece and Albania: only, in the case of Katakolon, the mountain has subsequently subsided below sea level and then been onlapped by Pliocene shales to form a hydrocarbon trap. Buried hill traps are fairly unusual globally, but are reasonably well documented in the #Mediterranean region, with examples documented in the Gulf of Valencia and in the Pannonian basin.

The #reservoir at Katakolon consists of Eocene-Jurassic #carbonates of the Ionian basin, with most of the resource contained within the Eocene - Upper Cretaceous section. These #limestones are a classic #Tethyan facies, corresponding to the Scaglia of the Umbria-Marche basin in Italy, and an important reservoir throughout the Adriatic with long production histories as mentioned in the introduction. These intervals are dominantly pelagic, but contain some input of shallow water material from adjacent isolated platforms as distal calciturbidites and debris flows. In general, some primary porosity is preserved within the coarser grained facies, particularly thicker units (>35 cm) deposited as grainstones. #Fracturing is important both to provide #porosity and #permeability and, where studied, fractures associated with pre-folding foreland flexure appear to be the dominant set. Consequently, these reservoirs are classic dual porosity-permeability systems. In the case of Katakolon, exposure of the reservoir interval prior to trap formation suggests the possibility that karst process may also have contributed to the creation of the discrete poro-perm system. At Katakolon, average porosity is reported to be ~5% and whilst matrix porosity comprises the bulk of the storage, most of the reserves are envisaged to come from the secondary fracture network.

The Ionian basin contains a number of well described marine carbonate type II-S #source_rocks: shallow water, pre-rift Triassic source rocks associated with evaporites, and deep water, syn and post-rift, Jurassic-Cretaceous organic rich, pelagic limestones commonly associated with ocean anoxic events (OAEs). Geochemical fingerprinting of hydrocarbon samples from nearby seeps – including one of the largest thermogenic methane seeps in Europe - suggest that the Katakolon hydrocarbons are sourced from the Triassic source rocks with the H2S derived by thermochemical reduction of sulphate associated with kerogen. Thermal modelling indicates that the onset of hydrocarbon generation occurs in the Oligo-Miocene and is associated with thrust loading. Due to the low geothermal gradient, the oil window is deeper than 5km and consequently it is likely that the Katakolon hydrocarbons are sourced from a local kitchen area in the footwall to one of the adjacent thrust sheets.

Migration is consequently relatively local and is likely to have occurred along the contact between the carbonate sequence and the overlying clastics.

The last element, seal, is made up of the Late Pliocene clastics that onlap and enclose the topography of the buried hill. It is worth noting that the age of this top seal substantially post-dates peak hydrocarbon migration which is estimated regionally to have occured at the same time as the main phase of #orogeny in the external Hellenides. A similar situation is observed in the Italian S Adriatic. From an exploration risk perspective, the observation that such late traps have received a charge is encouraging for other, untested late traps elsewhere in the Ionian.

Figure 4. The Upper Cretaceous-Eocene reservoirs in the Adriatic-Ionian zone are an interbedded sequence of calciturbidites and rhythmically bedded pelagic limestones and marls. This succession of Scaglia-type facies was deposited in a deep water basin, flanked to the S and E by shallow water carbonate platforms. Two main types of turbidites can be recognized: platform-derived turbidites, easily recognized by the abundant shallow water bioclasts that they contain (A, C, F, G); and pelagic turbidites, composed almost exclusively of planktonic foraminifera (B, D, F, G). Both types are commonly intercalated into normal pelagic micritic-dominated successions (B, E, F, G). Platform-derived turbidites were supplied from productive carbonate ramp systems located to the S (Apenninic and Apulian Platforms) and to the E (Barbara and Adriatic Platforms) of the basin, and carried shallow water sediment for long distances into the basin. Pelagic turbidites are basinal in origin and are associated with spectacular submarine slumps within the basin, suggesting seismic shocks as the triggering mechanism (seismoturbidites). Outcrop analogues for these reservoir intervals can be found in several localities across the Central and Northern Apennines and Southern Italy, exposing distal slope to basin successions. (F) shows an outcrop of an Early Paleocene basinal succession of the Scaglia Rossa Fm. (Umbria-Marche Basin), and the related sedimentary log (G), in the Furlo area, circa 70 km west of the Adriatic coastline. The interval corresponds to the distal position of the Unit R1 of Cazzola & Soudet (1993), one of the main reservoir units in the producing fields in the Central Adriatic, made by lenticular turbidite bodies of limited lateral extent. These units consist of coarse and relatively well-sorted sediments, with good primary intergranular porosity (8-20%) and permeability. Bf: Benthic foraminifera; Br: Bryozoan; CA: Calcareous red algae; Pf: Planctonic foraminifera.

Conclusions

In summary, the Katakolon field shows many of the characteristics of the Scaglia fields elsewhere in the Adriatic region, particularly in the interplay of geological events that resulted in its formation: Tethyan Jurassic rifting created the basin context within which source rock and reservoir were deposited, whilst later Alpine compression led to #hydrocarbon_generation and #hydrocarbon_migration and also resulted in trap creation and reservoir enhancement through fracturing. It’s geographic location as an outlier at the southern end of the Adriatic-Ionian basin offers encouragement for the next round of exploration activity both onshore and offshore in Greece and ongoing development activities mean the field may finally enter production in the next couple of years. It looks as if the Katakolon field may stay in the news for a while yet…

Selected reading

The Energean website contains a lot of material on the Katakolon field and Energean’s development plans (https://www.energean.com/operations/greece/katakolo/) plus material on their exploration ventures in Western Greece and elsewhere.

For an overview of the petroleum systems in Western Greece and the Ionian see:

• Karakitsios, V. (2013). Western Greece and Ionian Sea petroleum systems. AAPG bulletin, 97(9), 1567-1595.

For a description of the sedimentology, diagenesis and reservoir characteristics of the Cretaceous to Eocene basinal limestones in the Italian Adriatic see:

• Cazzola, C., &Soudet, H. J. (1993). Facies and reservoir characterization of Cretaceous-Eocene Turbidites in the Northern Adriatic. In Generation, Accumulation and Production of Europe’s Hydrocarbons III (pp. 191-207). Springer, Berlin, Heidelberg.

For a discussion on post-depositional tectonic and diagenetic modification of similar reservoir sequences to those encountered at Katakolon see:

• Vilasi, N., Malandain, J., Barrier, L., Callot, J. P., Amrouch, K., Guilhaumou, N., &Swennen, R. (2009). From outcrop and petrographic studies to basin-scale fluid flow modelling: The use of the Albanian natural laboratory for carbonate reservoir characterisation. Tectonophysics, 474(1-2), 367-392.

For a detailed geological and geochemical discussion of the different source rocks in Western Greece see:

• Rigakis, N., &Karakitsios, V. (1998). The source rock horizons of the Ionian Basin (NW Greece). Marine and Petroleum geology, 15(7), 593-617.

For a detailed description of the hydrocarbon seepage in the Katakolon area see:

• Etiope, G., Christodoulou, D., Kordella, S., Marinaro, G., &Papatheodorou, G. (2013). Offshore and onshore seepage of thermogenic gas at Katakolo Bay (Western Greece). Chemical geology, 339, 115-126.

Figure References

• Kamberis, E., Rigakis, N., Tsaila-Monopolis, St., Ioakim Ch. and Sotiropolos, S. (2000). Shallow biogenic gas-accumulations in Late Cenozoic sands of Katakolon peninsula, western Greece. Geol. Soc. of Greece, Spec. Pub, 9, 121-138.

• Maravelis, A., Vassiliou, A., Tserolas, P., & Zelilidis, A. (2014). Structural elements and petroleum exploration on the Apulian platform, Hellenic Fold and Thrust Belt, Zakynthos Island (western Greece). AAPG 2014 European Regional Conference & Exhibition, Barcelona, Spain, May 13-15, 2014.

• Rigas, M. & Benos, P. (2018). Energean Analyst Presentation 2018. https://www.energean.com/investors/reports-presentations/

• Xenopoulos, S. & Roussos, N. (2007). Status of Existing and Possible New Production in Greece. AAPG and AAPG European Region Conference, November 18-21, 2007, Athens, Greece.