The uniquely unique uniqueness of Ionian Zone Geology in Greece and Albania: Untested plays of a Tethyan-Margin Petroleum-System Epic

Contents

Geologists and Hyperbole; Lithospheric Linguine; Sit back and enjoy the ride; A pinch of salt please; Pushed over or pulled under and does it matter? The evaporitic inchworm; Let's twist again, like we did last aeon; Tectonic stacking; Can we ever know where to drill; Yeah, but...; Onshore vs Offshore; Well, well...a bit more detail; The exploration cavalry; Go-on you know you want to; References.

Geologists and Hyperbole

The claim that a place has a unique geology is made often, and never incorrectly. There are places for which the assertion is true, and there are places for which the assertion is dynamically, majestically, unforgettably true.  I suppose every geologist makes this argument for their own field areas – but I want to make the case for mine here, and I think I can convince you. I have been and I still am gutturally amazed by this place! And while yes, it is prospective for hydrocarbons, you don’t need to know that to appreciate it!

Before I go any further, apologies for the smallness of the figure text & keys here, but you should be able to save the image & re-open if you want a closer look – failing that I should have copies on the website www.paetoro.com shortly, and failing that get in touch for copy. 

Lithospheric linguine

Subduction zones along continental margins are already a special part of the world’s surface – responsible for some of its most dramatic scenery and some of its most dramatic tectonic events. There is an even rarer subset of places where such subduction zones are pinned laterally at their terminations by continent-continent collision. These places are sufficiently rare enough in the world that we can easily list them:

1.      Hikurangi Trench pinned by the Pacific-Australia collision in the Southern Alps astride “Zelandia” continental crust in central region of New Zealand.

2.      The opposite verging Puseygur Trench also pinned as above, but at the opposite end of New Zealand’s South Island.

3.      Eastern Papua-New Guinea, now becoming pinned by incipient Australia-Asia collision, and 

4.      Java -Banda arc by the same collision in southern Indonesia and Timor.

5.      The Andaman Arc pinned by the India-Asia collision in Myanmar.

6.      The Makran arc of southern Iran and Pakistan, pinned both by collision of Arabia and Asia in the Straits of Hormuz area, and by collision of India and Asia in SW Pakistan.

7.      The Hellenic & Cyprus Arc, pinned in the east by Arabia-Eurasia collision in Syria and Turkey, and in the west by collision of the continental Adriatic promontory of Africa with Eurasia.

There are other places where subduction zones have collided or are colliding with continental crust – as illustrated by yellow belts in Figure 1.  Those listed above however, and shown as red circles in Figure 1, are different – they are where continents are also colliding with other continents at the ends of arcs.  We are not yet in “unique” territory – given there are a number of them. Yet these areas, where rapidly migrating subduction zones are pinned intransigently at their terminations by continental collision, experience some of the fiercest dynamics of anywhere on the planet. I have been fortunate enough to grow up on one of them and to spend a good deal of my life studying another. They rock. Literally.

Sit back and enjoy the ride

In some of “the seven”, like the New Zealand ones, the rates of convergence involved mean that the terminating ends are dominated by contraction everywhere. This is the “norm” if it is sensible to talk of such things amongst such a small sample size. However, in subduction zones that are pinned at both ends, there is potential for something even stranger to go on. Now we are down to a non-unique, but very special three – just the Makran arc and the Hellenic-Cyprus arc & the wider Andaman-Banda arc.

Old cold heavy Mesozoic oceanic crust, once it starts to sink at a subduction zone– likes to really get going & sink big time, and the inflection point of the subduction zone can move away from the area of collision – back towards the oceanic plate - this is known as “roll-back” (see Figure 7). A professor of mine once likened it to the effect of paint skin in a pot of (quite thin) paint. It sits there atop the liquid happily. But if you tear it along a big enough tear and give it a bit of a shove downwards, its own weight continues to pull it down into the paint, migrating away from the point of the initial tear.  

When this happens with continental margin subduction zones, it effectively sucks the continent along behind it, stretching and extending it, in vigorous “back-arc” extension – so called because it often occurs in conjunction with curvilinear volcanic arcs (as it indeed does in Greece). Indeed the Hellenic arc is responsible for one of the biggest eruptions in historic times. It is this back-arc extension which has caused the majesty and beauty of the Aegean Sea – as Eurasia is stretched and thinned behind the Hellenic arc, while it migrates southward towards Africa, subsiding in a topographic drama to challenge any Atlantean saga Plato could produce. 

To give you further sense of this, the collision between a point in the Libyan Sahara, and a point in central Romania say – is happening at roughly 10 mm a year. But Crete is moving south towards Africa at something more like ten times that rate - 100 mm a year, because the Hellenic subduction zone to its south is sinking fast and pulling it along for a ride.  That difference of 90 mm – is tearing apart the south-eastern corner of Europe. In the Makran, the collision is maturing as Arabia gets closer, so it is now more “scrunch” than stretch. The subduction zone inflection point (i.e. where it bends downwards) is no longer free to migrate so there is no “pulling” as in the Aegean. Note that sometimes when the subduction zone really does “freeze-up” because of the continental collision – it can sink and ultimately detach – causing an uplift rebound in the orogen above when the surface lithosphere releases the heavy burden (see Figure 7). That might have happened for a now “closed” earlier phase of subduction beneath the Dinarides, Albanides and Hellenides. It might be why extension is affecting the high Pindos and Albanides, and why an inter montane basin - the Meso-Hellenic Trough, once marine, now sits so high. 

The Aegean and the Hellenic arc are indeed unique then - in being a subduction zone pinned at both ends by continental collision - and yet free to continue migrating in its central portions near Crete, hence taking a big chomp out of the SE European continent. We have arrived at base camp on our assault on the Ionian Zone mountain of uniqueness. 

A pinch of salt please

We are not finished our unique journey yet. We have only just begun. Another set of ultra-dramatic tectonic regimes exist where thick evaporitic basins – typically formed during rifting – are reactivated in contraction. Those who have studied such belts will roll their eyes skyward in marvel at the scale, weirdness, and force of such structures.  Again, there are only a limited number of these around the world – a bit too many to list easily this time – but Figure 2 shows some of them. The most famous of them however is the Zagros in Iran. Others include the Verkhanoyansk Range in Siberia, the cunningly named Salt Range in Pakistan, The Tunisian Atlas, the Jura in the Alps, the Sverdrup Basin/Parry Island fold belt in the Canadian Arctic, the Colombian Pre-Cordillera, and the McKenzie Range in Canada. Old Tethyan ocean margins are particularly favourable for their occurrence, and Figure 3 shows a generalised schematic for such belts.

Such evaporitic fold and thrust belts generate big, big, structures, and are also notable for providing very effective seal for any overthrust areas containing a petroleum system. They were early favourites at the very beginning of oil exploration – but have become somewhat neglected in recent years because of the industry’s increasing reliance on seismic acquisition for investment decisions. Imaging the subsurface with seismic in such belts – is tough. Not impossible, but tough. That’s not a criticism, because modern offshore seismic is beautiful – and who wouldn’t want it where there is a choice. Yet we do sometimes forget the ways we (as an industry) were able to find oil before the dominance of seismic in the industry’s exploration tool-kit.  

Those of you already acquainted with the Ionian Zone will see where this is headed. Of areas in the world where there are thickly developed evaporitic basins, which are also caught up in fold and thrust belts of continent-continent collision, lying at the terminations of rapidly migrating subduction zones, let’s think, ummmm.... - how many are there? 

One. 

Just one. The Ionian Zone of northwest Greece (Epirus) and Albania. This makes it genuinely unique on the planet today. And as abundantly clear in Albania particularly – but also in Greece – it has a working petroleum system, just to add to the intrigue.  See Figure 4.

Pushed over or pulled under and does it matter? The evaporitic inchworm.

What can we expect from such a crustal convulsion? One useful analogue may exist in the Guadalquivir Basin of SE Spain. The late Chris Banks at Royal Holloway drew my attention to Berástegui et al. (1998 - his pre-prints at the time), and in particular, the Triassic-evaporite-soled fold and thrust belt section shown in Figure 5. Several things are striking – a relatively unfolded carbonate thrust sheet with thick evaporitic decollement has climbed over the foreland and simply kept going to give massive displacement, demonstrated by the RGH-1 well sub-thrust penetration, with upwards of 50-80 km on this single large thrust when restored – despite the relatively undeformed hanging wall.  

People often have problems with such mega-displacement thrust sheets, as it seems counter intuitive that such a thin sheet could be pushed over its foreland without much greater degrees of faulting at the surface. It just wouldn’t be strong enough surely. But this is where we need to think about the dynamics. What is driving the collision? It’s not (so much) the overriding plate being pushed over by some force in the hinterland, though topography and gravity contributes a bit. It is the attachment of the under-thrust continental crust to a subducting oceanic plate which is being pulled under the overriding plate by the tug of a heavy subducting slab. See Figure 6. Of course it's all driven by gravity to some extent - contraction at the planet's surface is ultimately just an effect of things variously piling up, falling into a hole and rubbing up against each other on rather a large scale - but the key point is how far these stresses are transmitted within the thrust sheets.

Think of a cloth tablemat on a slippery table and a ring binder resting on the table, with a gentle incline at its leading edge. If you try and push the mat over the binder – many times you’ll have an almighty crunch where the rug and binder meet, and then the mat fails behind the collision, near where you are pushing. If in contrast you gently, slowly, slide the ring binder under the table mat, there might be a bit of a bump at first collision, but once it’s under, the over-thrust mat just keeps going, for quite a long way, before any failure occurs internally. 

Perhaps also with evaporitic thrust sheets. Maybe it’s like a gentle inchworm movement on the very efficient salt decollement over the first obstacle, and then it relaxes and lets the sub-thrust slide under. The foreland is pulled under. There is no figurative bulldozer pushing the thrust sheet over the foreland and building up internal stresses within it. With all due respect to critical wedge theory etc, we need to get those bulldozer analogies out of our head.

The reason there is a difference in these models comes from thinking about how and where the stress is transmitted from – the big picture as per Figure 6 and Figure 7. In the hinterland-shove model – the weak deforming thin skin has to carry stress, without breaking, all the way from the hinterland to the frontal thrust – which seems inherently improbable. In the foreland undertow model, the frontal thrust is just reacting almost passively to tugs and obstacles from the under-thrust units just below it. No long- distance stress transmission is required, and if the thin over-riding skin does manage to surmount any obstacles, the lack of friction on the evaporite detachment can just keep it going. In many ways, the frontal thrust is responding to a void in front of it, rather than just a shove behind it. When that void disappears, internal thrusting and back-thrusting becomes important.

If the analogy & model has any merit, the implications are quite profound. Once a very large evaporite soled thrust sheet surmounts a frontal obstacle of a subducting foreland, there may be very little to stop it from keeping going - for very large displacements. This idea may be right, it may be wrong, but the possibility is intriguing and deeply interesting for sub-thrust hydrocarbon potential.  See Figure 3, Figure 4, and Figure 6. More to the point, whatever the theoretical justification, places like Guadalquivir Basin seem to suggest it can happen, somehow, whether we understand it or not...

In NW Greece, a well similar to that in the Guadalquivir basin, but in the frontal parts of the belt (Filiates-1) has already illustrated what seems to be 40-50 km displacement on the similar Ionian thrust since the Miocene – see this well in Figure 11. But the scale of the collision in NW Greece’s Ionian Zone is greater and has been going on since the early Oligocene - and even earlier. Clastic turbidites make an appearance in the Eocene (Figure 4). Could shortening therefore be much be more than what is demonstrated by this post-late Miocene displacement? It’s not even half the lifetime of the Ionian Zone deformed belt.

Quite noticeably, very fragmented Ionian zone thrust sheets of southern Albania - with numerous large thrusts - slowly merge or fade in NW Greece into the Epirus fold and thrust belt – with its single dominant large thrust – the Ionian thrust. Other lesser thrusts do occur, but lateral terminations place limits on their possible displacement and folds dominate – like the Zagros.  Is this a transition between areas where the frontal indenting obstacle is and isn’t surmounted? I was a structural geologist for the team at Enterprise Oil that helped drill internal parts of the belt in 2001 (Demetra-1) to test the model and target internal duplexes – but for operational reasons the well didn’t reach the sub-thrust depths, so lots of the questions like this remain unanswered. 

Let’s twist again, like we did last aeon.

The Ionian Zone of Greece and Albania has another claim to uniqueness. It is one of the best studied areas in the world for paleomagnetic rotations – measured throughout the stratigraphic section from the Jurassic to the Pliocene, to provide a uniquely thorough record of tectonic rotation. These involve 45 degrees rotation clockwise of the whole of NW Greece & Albanian Ionian zone relative to surrounding stable continental areas - from the Oligocene to the recent – brought about precisely because the Hellenic arc subduction has been pinned by continental collision occurring in Northern Albania and NW Greece.  Some even faster rotations – some of the fastest tectonic rotations recorded on the planet - exist near Levkas and Kefallinia, where the subduction zone proper begins and is really beginning to wrench Eurasia apart in earnest. This is near to a pronounced wrench known as the Kefallinia Line, - it is here that some of the largest and most dangerous earthquakes in Greece have occurred historically.

It is a cliché that we often need to step back from postage stamp geology and look at the big picture, but it really does help here. The scale of what might be occurring is nicely illustrated by the “isopic zone” map produced by the Geological Survey of Greece (IGME) that has been around for many years. Isopic zones are a bit like terranes in that they have different stratigraphic associations. They frequently are bounded by major faults as per terranes, but not always – they can delineate basin/platform transitions and the like. These zone maps are not sensu-stricto geological maps but they are a powerful intuitive tool. I have taken the map produced by IGME, and combined it with geology from Albania, Montenegro, the Republic of Macedonia, and Kosova, to produce an extended isopic zone map over the whole Hellenide-Albanide orogen.  See Figure 8.

Tectonic stacking.

What is apparent from this map, is that as we travel from northern Albania to Crete, we see a number of transitions:

1.      In Central Albania, the Mesozoic carbonate backbone of the Ionian Zone emerges out from underneath Tertiary clastics into a series of thrust sheets with moderate displacement.

2.      In Epirus and South Albania, transition occurs to a fold dominated belt with some thrusts - but these are almost all laterally terminating – limiting their possible displacement – except for the frontal Ionian thrust.

3.      In Akarnania and the Ionian Islands south of the Gulf of Arta, the Ionian zone becomes increasingly fragmented with major thrusts and folds. From this point southward, oceanic subduction and associated back arc extension becomes increasingly important.

4.      In the Peloponnese, south of the Gulf of Corinth, the widespread thrust stacking of other zones over the top of the Ionian zone is increasingly apparent, leading to extensive allochthons (thin skinned thrust sheets). This leads to tectonic windows & klippen of various isopic zones where erosion has dissected the thrust sheets. Major “back-arc extension” related graben such as the Gulf of Corinth cut through it. 

5.      Finally, in Crete, a complex jumbled stack of thrust emplaced isopic zones is present, and dissected for good measure by normal faulting induced by subduction associated underplating and uplift.

A general trend of increasing surface shortening, structural complexity, and tectonic stacking is apparent heading SE along the Ionian zone - apart, crucially, from S Albania & Epirus, where the measurable surface shortening apparently decreases. One possible explanation is that the shortening does increase consistent with the rest of the belt, but is hidden from our view by an almost total double-decking of the Ionian zone along the Ionian thrust, similar to Figure 6 (lower). This would be more consistent with the expectation that tectonic stacking increases as a function of distance travelled SE along the Ionian zone.

The implication is that a large degree of multiple stacking can be expected along the thrust belt, increasing from the NW to the SE. We then find ourselves asking, what is underneath the evaporite decollement? Is it something similar to platform carbonates of the Monti Alpi and Tempa Rossa fields nearby in southern Italy, and in the Apulian zone of the westernmost Ionian Islands? Is it more Ionian zone stratigraphy? 

If so, Mesozoic fractured carbonate and Tertiary turbiditic reservoirs are possible in the sub-evaporitic zone. Given the likely depth, reservoir quality may be an issue, but no-one has ever drilled that deep to know in NW Greece. It might be less of an issue in the most likely gas scenario – but overpressures induced by tectonic loading and a thick evaporitic seal could be significant, so it’s not for the faint hearted. Previous thermal modelling efforts have suggested the play should be gas-prone – especially in internal areas, but rapid tectonic stacking will also have the effect of depressing thermal gradients, so there are still question marks over the most likely phase, especially in more external zones.

Probably the most frontal areas of the onshore thrust belt are the most prospective. The scale of active tectonics and presence of major transverse shear zones raises questions about seal however, even with an evaporitic seal. Salt can fault. It heals quickly, but a lot of fluid can travel in a short time. 

It’s not easy. There is no paucity of risk. In many ways it is an easy play to nay-say – reservoir quality risk, model driven, difficult imaging, salt, and so on. If those things scare you, then there are better places to go. But it could be big, multi-TCF, and until we get down there into the sub-thrust a few times with a drill bit, we are kidding ourselves to think we really know how good or bad it could be.

Can we ever know where to drill?

Critics may argue that we can never fully discern the sub-thrust structures in such regimes. There is a partial truth in that – the onshore seismic is rarely very good in such mountainous belts, and non-seismic geophysical techniques, while getting better all the time, only take you so far. Yet it is disingenuous to say there is no information about the sub-thrust. Structures at the surface are affected by structures at depth. Nearby onshore or offshore areas where seismic imaging is better can give important information about structural style. The presence of major duplexes (see Figure 3 and Figure 6) sub-thrust can be inferred from models of structural geology, and these are often highly prospective. Arguably, they may even help displace the more “squishable” evaporites in any passive roof complex out of the way, rather than accumulate them in thick piles.

Critics are right in that we can never know exactly what’s down there pre-drill, but structural geology done well can tell us enough to figure out some end-member models – and knowing what they look like can help us to differentiate them with other techniques. A key risk in such an area is encountering huge thicknesses of evaporite and/or non-reservoir units in the sub-thrust. Yet experience in Albania suggests there is at least some correspondence of sub-thrust and surface highs, including from areas of central Albania where Tertiary cover provides flatter onshore terrain and enables better seismic. Shell has used this to good effect in the Shpiragu discovery. It might not be obvious everywhere, but a good structural geologist can hunt for hints of the largest areas of sub-thrust duplexing, fine-tuned by structural styles and rheological models from surrounding areas.

However, in such areas, if you wait for seismic to define an “X” on the map, you may be disappointed. The scale of the structures though, may be very large. You may not need an “X” on the maps as much as a generously sized “O”. Ultimately, being prepared to drill a well in a suspected area of a sub-thrust high, with contingency to side-track up and down dip, may be a better strategy than over-investing in limited value-of-information seismic. Some seismic is needed of course, but knowing when to leave geophysics behind and hit the drill bit operates on a different paradigm in such onshore areas to that of the offshore. Realistically the chances of getting the right spot in this play on a one-well programme are limited. In an ideal world 3 or 4 might be a great programme, but hey one is better than nothing. I shouldn’t be greedy…

Yeah, but…

So, what are the risks really? At present any deep onshore sub-thrust exploration is largely model driven – given that the sub-thrust in more internal parts of the belt is untested. In my 1995 thesis (if you’ll forgive the shameless plug), I argued that paleomagnetic and structural geology arguments support an extensive zone of sub-thrust petroliferous Ionian zone. Other models exist, but the scale of required shortening is less easy to reconcile with other observed structures in the Adriatic area, where the very reliably documented rotations between Greece and autochthonous areas of Italy have to be accommodated somehow. As explained more fully in my thesis, anyone who can extract meaningful paleomagnetic rotation data from the island of Paxos will help to answer this key question – it would help localise the shortening associated with the rotations even more and give further clues to the magnitude of shortening in NW Greece.  Unfortunately, the Apulian Zone carbonate lithologies on Paxos don’t yield themselves easily to paleomagnetic analysis, but the technology is improving all the time, so it may be ripe for another go, if it hasn’t happened already. Even if the model is successful though – the reservoirs will be deep, so quality is a risk, and the active tectonics raise questions about seal, even in the presence of the evaporites.

If the model were incorrect, either non-reservoir or unknown reservoir lithologies could be present in the sub-thrust. Leading candidates would be metasedimentary clastics (see alternative scenarios in Figure 11) as observed further north in the Dinarides, or ophiolites as observed further east in the Hellenides.

 On the positive side, gravity and magnetic modelling tends to argue against the latter or other igneous basement options, but on the down-side it is possible to see ophiolitic xenoliths within the Ionian zone evaporites sometimes. More study of these is needed to ascertain their character, weathering, and distribution. It is worth remembering, that the last stages of deformation in the Pindos zone east of the Ionian zone, involved late Cretaceous thrusting of massive ophiolite sheets (Figure 4) – so early deposition of ophiolitic olistostromes and conglomerates into the Eocene and Oligocene of any Ionian Zone sub-thrust is highly likely. Are these sedimentary units the source of xenoliths rather than any sub-thrust “plucking” of solid in-situ ophiolite? It’s something that could be studied further for someone with a sharp eye for xenoliths in the field. The average rounding and weathering characteristics of such clasts might just be useful model differentiators.

Onshore vs Offshore? 

The following criteria are important for deciding a desirable well/prospect location:

1.      Maximise chance of significant sized structure – i.e. sub-thrust duplex.

2.      Proximity to known areas of HC seeps and/or accumulations.

3.      Minimise depth to top reservoir.

4.      Maximise chance of evaporitic top-seal.

5.      Maximise chance of sub-salt reservoir lithologies.

6.      Minimise chance of active tectonics & related breach.

7.      Minimise well costs.

8.      Minimise environmental & tourist industry impact.

9.      Minimise safety hazards.

10.  Minimise legal/border disputes.

Given the above criteria, while the offshore definitely has its own & somewhat different attractions, for me onshore NW Greece is more tantalising. The risks are different, more, or less, depending on your view of breach and reservoir quality - but the potential for a very large reward seems greater. It is possible to fine-tune the sub-thrust play maps further in terms of knowing where best to go onshore, but that is not a conversation for here.  

Don’t let me dissuade people from the offshore – it’s just that seal and source might be a bit more of an issue there, if Triassic evaporites cannot be placed as a seal to the petroleum system (Figure 4). Offshore you can image, but there is less chance of evaporitic seal coming into play and so the relentless active tectonics of the area creates issues for breach – similar to exploration efforts off the east coast of New Zealand. Not impossible, but tricky.   Messinian evaporites might play a similar role to the Triassic ones in places, but they are thinner and hence more susceptible to breach. 

Nevertheless, the offshore has important lessons to yield about structural style if it can be imaged well, and if Triassic evaporite can be inferred in offshore thrust stacks, then it bodes well for prospectivity there also. That is most likely to happen along the frontal marine parts of the Ionian thrust, though it has to be said that in such a location the under-thrust units will most likely be Apulian Zone and won’t quite pack the Posidonia Shale source rock punch that the Ionian Zone itself has – but maybe there are others at Cretaceous, Tertiary, and Triassic levels (Figure 4). 

Well, well…a bit more detail

Figure 9 and Figure 10 show some of the HC & well history in the Ionian Zone and surrounding areas. 

Filiates-1 in the external (near coastal) part of the Ionian Zone is interesting (Figure 11, left side, location on Figure 10). There is Triassic evaporite at surface - sometimes known as “Vrisela Diapir”. The well drilled through 3770 m of the evaporites before coming out – presumably through a low angle thrust – into 1 m of (overturned?) Eocene limestone, and then “Helvetian” marls underneath (Langhian-Serravallian ~ 12-16Ma) - to quote IFP-IGRS (1966). Although many of the evaporites occurring at surface in the Ionian Zone are described as diapirs, like the Guadalquivir basin (Figure 5) and the Dumrea area in Albania, they are more of an evaporitic “pile” related to thrusting – so the thickness is unlikely to be representative of the original depositional thickness. It is possible however that some existed as diapirs before contraction affected the Ionian Zone. 

As already touched upon, the fascinating thing about the Filiates area, is that although significant thrust faults occur there, the only one that seems to have enough lateral continuity to impart this kind of displacement lies seaward of Kerkira (Corfu) some 40-50 km to the west. This needs to be further checked out in the field. Bear in mind that the age of these marls at 12-16 Ma is only about half way through the history of convergence in the fold belt – so you wonder about the earliest Oligocene sediments that were first overthrust. If later ones are overthrust here, as demonstrated by Filiates-1, the older ones must be too - so just where are they sitting in any sub-thrust environment?   Presumably they have to be under the evaporites somewhere even more internal than Filiates-1. Presumably the facies also become more proximal and coarse grained as they head more internally in the sub-thrust – increasing Tertiary sub-thrust reservoir potential. The implication would be of a simply huge displacement on the evaporitic Ionian thrust. If demonstrated it would certainly consititute one of the largest and furthest travelled thin-skinned detachements on the planet. That's why it is important to understand just how unique the Ionian Zone's tectonic setting is. It might seem fantastical, but as we have seen, the tectonics of this part of the world is already well and truly zany. If weird and wonderful can be true anywhere - it's here.

Enterprise had a good go at trying to find out in 2001 with the much more internal (eastern) Demetra 1 well (Figure 11, right side, location on Figure 10)– but as with experiences at Dumrea in Albania – those evaporites are thick and challenging. The well was abandoned in evaporites due to high overpressures before ever reaching the sub-thrust – and already it was at just under 4000m. Yet there are reasons to be cheerful – for one - seismic was able to discern some structure pre-drill. Not a huge amount, but enough. The fact that high overpressures were encountered (see Figure 11) is entirely compatible with extremely rapidly tectonically loaded Tertiary sediments underneath a near perfect evaporite seal and thrust stack, having problems de-watering fast enough. Of course, carbonate rafts within mobile salt can also often have high overpressures, by virtue of deformation and uplift - and so while its’s tempting to think the kicks indicate the base of the evaporite was being approached by the drill bit - it’s not guaranteed by any means. 

The primary sub-thrust targets, ideally, are repeated Ionian Zone Tertiary mass flow deposits and Mesozoic fractured carbonates – including fractured Cretaceous calciturbidites and older Jurassic Pantokrator platformal limestones (Figure 4).  

An alternative scenario would place Permo-Triassic clastics underneath the evaporites – depending on how you feel about the tectonic models as described (both models are shown in Figure 11, at right). 

Given the efficiency of the evaporitic decollement/detachment in NW Greece, older Pre-Triassic Ionian Zone units are never seen at the surface - so their nature can only be inferred – mostly from analogues in more internal zones of Greece, Albania, and Montenegro. Gravimag modelling before the drilling of Demetra-1 tended to argue against crystalline basement sub-thrust, but could not uniquely differentiate models of sub-thrust metasediments from models of sub-thrust Ionian Zone stratigraphy.  Even in a success case any reservoir quality at those depths is always going to be a big issue, and likely to depend on fractures. If there is a bit of overpressure, while difficult operationally, it might help with the porosity preservation. However, safety is always paramount and with significant H2S issues on top of overpressure, the well and acreage was abandoned around the time of Shell’s takeover of Enterprise, and never really picked up again until Energean acquired the acreage in recent years. There is local prospectivity in the thin skinned supra-thrust strata as well, where live seeps abound (Figure 10), but I’d wager the big potential is in the sub-thrust.

It’s always a game of holding nerve when these thicknesses of evaporite are encountered during drilling, and anyone who has worked with salt related structures knows they are capable of humbling us all – but it is telling us something structurally that so much evaporite was encountered in this very internal part of Epirus at Demetra-1. It’s a region that if there was no sub-thrust stacked Ionian zone, you might expect evaporite facies to be diminishing, not having thicknesses of this order. 

Another puzzling footnote is the presence of the Triassic evaporites at Peshkopi in Albania (see inset Figure 8), on the eastern side of the Pindos-Albanides. What on earth are they doing there? Is it a separate and distinct Triassic basin, or are the Ionian Zone evaporites continuous under the entire Albanides and the Albanian equivalent of the Pindos Zone (Krasta-Cukali Zone). The extent of these Triassic evaporites may be greater than we think. Perhaps some Albanian geologists on linked-in can shed more light on this issue. It will likely fall to non-seismic geophysical techniques like gravity and magnetics and other new EM technologies to progress this and other questions.

The exploration cavalry

Many companies it seems, notably Energean in partnership with Repsol, and separately, Hellenic Petroleum, seem to be taking a good crack at the onshore. Shell, as already mentioned, has made some interesting new discoveries and great advances with seismic onshore Albania in recent years (Shpiragu), and a shallower sub-thrust accumulation exists just north of the border at Delvine Field. Admittedly, flatter-terrained Tertiary cover over the fold and thrust belt makes seismic acquisition somewhat easier in Central Albania, but it is a foolish geoscientist who writes off what seismic acquisition and processors can do these days. Their magic puts Merlin to shame. 

It will be increasingly challenging for onshore seismic, the further south you go from central Albania, but to my mind this is where the elephants, if they exist, are lurking. That said, even further south, Energean has very recently gained approval for development plans of Katakolon field, a short distance offshore Peloponnese, so even the shallow thin-skinned belt, onshore and offshore, can work for smaller accumulations. Other players are working the deeper offshore, which at time of writing seem to include Total, Energean, Edison, and Esso. Correct me if wrong. 

So, HC exploration & development is happening in the Ionian Zone, but the deep sub-thrust play essentially remains untested in NW Greece. Only the two wells in Figure 11 have ever come close – Filiates-1 drilled by BP in 1965, and Demetra-1 drilled by my prior employer Enterprise in 2001. It’s good to see exploration progressing, but the sub-thrust play continues to taunt from behind its evaporitic decollement curtain. 

A whole host of disciplined, focussed and determined companies are intrepidly exploring in the Ionian Zone though and I wish them every success.  The ones I know best are committed to transparency, safety, the environment and local communities. That’s important, because the area is one of pristine beauty, stunning coastlines, and fervent tourism. Environmentally it remains home to some of the Balkan’s last wild bear and wolf habitats, and archeologically the area is teaming with important archaeological and historical sites dating right from the Palaeolithic to the present. Everyone wants this area preserved for posterity. No-one wants to endanger the wondrous assets it already has.

Go-on, you know you want to…

In summary, it would be a lie to suggest the onshore of NW Greece is anything other than high risk, - moving at best to moderate risk after a lot of de-risking work. Yet it is potentially high reward, and:

1)     Seeps are abundant and petroleum systems proven in the area.

2)     Sub-thrust duplexes are indicated by structural geology arguments.

3)     Paleomagnetic arguments for amounts of shortening support this, and furthermore that petroliferous Ionian zone stratigraphy could be present within them.

4)     Some simple studies could help de-risk the models further.

5)     The play is largely untested and structures in any major sub-thrust duplex are likely to be large. The play opening potential is good in the success case with multiple along strike upsides. Once down there any calibration made possible with various geophysical methods will likely ease the next step.

6)     Good structural geological understanding of the area, coupled with non-seismic geophysical techniques, and the best that seismic can do, can make progress despite the uncertainties.

7)     The area is in one of the economically fastest growing areas of Europe, with governments keen to support exploration.  

Interested? Don’t hesitate to get in touch. I’m always happy to talk more about Greece and Albania, just for the sheer love of it. I’ve already had one crack at play-mapping the sub-thrust for Enterprise many moons ago – it feels a good time for an update if anybody wants a go.

Whatever the outcome(s) in the Ionian Zone in coming years, with all the various factors which influence drilling activity, there is reason to be optimistic. Advances in technology, a cost-driven renaissance in onshore drilling, rebounding economies thirsty for energy, and new pan-Eurasia gas pipeline infrastructures (e.g. TAP) – all suggest it is a play which will be tested sooner or later. 

More generally - sub-salt sub-thrust plays in onshore fold and thrust belts globally – will almost certainly be back in focus in the 21st century.  The most prospective belts drive straight through some of the fastest growing economic regions on the planet. They don’t yield their secrets easily, but the promise seems worth the effort.

My thanks go to all the people, including some fantastic geoscientists, that have discussed this tantalising play with me over the years, and in particular Angelos Mavromatidis, the rest of the Enterprise Oil Greece Albania team, and many Greek geologists both in academia and industry. Views and any errors are mine. 

References: 

A lot of references have gone into this article – too many to list here so if you want to know more details, don’t hesitate to message me or contact me via www.paetoro.com. 

The maps and diagrams in this article are drafted by Paetoro. Figure 3 and Figure 6 derive from some initial drafts of mine at Enterprise, but hopefully they are generic enough for nobody to mind modified versions being presented here. Data is collated from many different sources of varying confidence, and it is difficult to always stay totally up to date with all changes, so: 1) let me know if you spot any inaccuracies, 2) if it isn’t already obvious, this isn’t intended to be an investment level document, and 3) delineation of borders and use of country names remains passionately controversial in many parts of the Balkan Peninsula – so I’ve had to use something. I haven’t intended to imply any particular solution where there is dispute. That said, the new generation of energy co-operation and transparency that is emerging from the Balkan Peninsula is heartening, and though there are things to sort out, the geopolitical trends seem good ones. A growing will to move forward is unmistakable. 

Dave Waters, August 2017.