Knowing your gas

ELLINAS

The first serious study to estimate potential hydrocarbon quantities in the East Med, including Cyprus, was made by the US Geological Survey (USGS) in 2010.

Their work in the East Med concentrated on the Levant Basin, which encompasses the region between Cyprus, Eratosthenes Seamount to the west, Lebanon, Israel and Egypt. This assessment was based on published geologic information and on commercial data from oil and gas wells and fields in the region.

The USGS mean estimates were 1.7 billion barrels oil and 122 trillion cubic feet (tcf) gas in the Levant Basin. The USGS also estimated the mean probable resources of natural gas liquids (NGL) to be 3.1 billion barrels. These are significant quantities, but small in global terms, and with about a third of the Levant Basin explored, mainly in Israel’s EEZ, discoveries so far tend to confirm them. In order to understand what this means in terms of exploration so far and the future, it is useful to go through some background information.

Hydrocarbon reservoirs
Hydrocarbon reservoirs develop when oil and gas are trapped in porous rock, known as ‘the reservoir’, by overlying impervious rock, normally a thick salt deposit, which acts like a barrier. Reservoirs are found anywhere from near the surface to 9,000 m depth. Hydrocarbons are trapped mostly in two ways: by structural and by stratigraphic traps.

Common structural traps are formed by folding and faulting of subsurface rock strata and include domes, folds and faults.

Stratigraphic traps are formed as a result of lateral and vertical variations in the thickness, texture, porosity and makeup of the reservoir rock.

The way hydrocarbons are sealed is critical. If the seal is not fully functional then hydrocarbons may escape. In general sealing in structural traps is more effective and less prone to leakages than in stratigraphic traps. Most major hydrocarbon reservoirs are of the structural trap type. In the US close to 80% are structural, 10% stratigraphic and the remainder a combination of both.

The size, depth and trapping potential of the Levant Basin suggest that large quantities of hydrocarbons can be found offshore, characterised by a variety of potential structural and stratigraphic traps. However, all major discovered gas fields, Tamar, Leviathan, Aphrodite and Dalit are of the structural type and they all lie in a NW-SE band, at 5km to 6km below seabed level.

Exploration in the East Med
Potential hydrocarbon deposits are identified through exploration. The initial stages involve identification of large-scale features of the sub-surface geology that might contain hydrocarbons. These are then investigated in more detail through seismic surveys.

These measure the time it takes for reflected sound waves to travel through rock strata of varying densities to create a profile of the subsurface geology. Once potential hydrocarbon reservoir targets are identified, exploration wells are drilled to determine conclusively the presence, or lack, of hydrocarbons. It is only at this stage that the presence of hydrocarbons can be confirmed.

Often reservoirs are not uniform. They may have variable porosities and may be compartmentalised, with fractures and faults breaking them up and complicating hydrocarbon flow. This is precisely the problem with Aphrodite and the reason why the initial estimate of 7 tcf gas dropped to less than 5 tcf after appraisal drilling. And more appraisal drilling may be needed to estimate gas quantities more accurately.

With water depths up to 2000m in the East Med, exploration well drilling is quite expensive and can be up to $100m-$150m per well. Given also that the global success rate is about 40%-45%, oil companies are sometimes reluctant to proceed to this stage unless drilling targets satisfy strict evaluation criteria, especially during this difficult period of low oil and gas prices. This is one of the reasons Total decided not to proceed with drilling in blocks 10 and 11.

Experience in Israel
In Cyprus the exploration process is more recent, but in Israel it took a long time and effort before they had success. Offshore exploration started in the late 1960’s and early 1970’s with a series of seven wells drilled by Belpetco on the shallow shelf off Israel, but all were found dry.

The next exploration campaign during the mid 1970’s to mid 1980’s resulted in some success. Light oil was found but after initial flow, performance rapidly decreased and production was stopped.

A series of four wells were drilled offshore Israel by Isramco during the late 1980’s to late 1990’s at depths of more than 5000 m. All the Isramco wells encountered oil and gas but not in quantities for commercial production.

Exploration activity took off again during 1999-2000 when several, large gas fields were discovered, the Noa, MariB and Gaza Marine, at a shallow depth with gas reserves estimated at about 3 tcf.  Altogether close to 50 wells have been drilled between 1960 and now resulting in seven gas discoveries, with culmination in the more recent giant gas field discoveries of Tamar, 10 tcf, and Leviathan, 22 tcf. In addition, indications from these exploration surveys in Israel’s EEZ support the USGS findings, i.e. that there might be oil at greater depths beneath the gas fields.

Besides natural gas, the Leviathan field is said to potentially contain 600 million barrels of oil beneath the gas layer, at over 7 km below seabed level.Should this be proven, it would also greatly increase the probability of finding additional oil and hydrocarbon resources at similar depths elsewhere in the region, including Cyprus.

Limitations
Cyprus was lucky to hit Aphrodite with the first attempt, without going through the tortuous Israeli experience. But success should not be taken for granted, as Onasagoras has shown in Block 9.

The central part of the Levant Basin, where Block 9 is, does not appear to contain large structural traps. Indications are that the potential gas reservoirs in Block 9 are of the stratigraphic type, which are more risky and could explain the lack of success at Onasagoras. As a result there is a risk that Amathusa may also have limitations.

However, limitations with these two wells do not negate the USGS findings. Israel persevered with failures and half successes for a long time before Tamar and Leviathan were discovered. Cyprus should not be any different.
Its EEZ potentially holds significant quantities of hydrocarbons, as USGS and subsequent analysis of extensive seismic survey data has shown, and these should eventually be discovered through more exploration and drilling. Most of Cyprus EEZ is still an unexplored frontier area.

The discovery of large gas fields offshore Israel and Cyprus in the Levant Basin has attracted the oil industry’s attention to this underexplored part of the East Med.

Recent large 2D and 3D seismic programmes have significantly changed the understanding of the petroleum systems of the region. Once oil prices recover and normality returns exploration should continue, with hopefully more licensing rounds and more successes.

Published in: in-cyprus.com