European Geologist Journal 46
Oil seep detection based on historical and recent satellite SAR data and manual interpretation: Eratosthenes Seamount (ESM): a very shy oil-trap candidate?
by Clément Blaizot
1 CEO of GEOESPACE
For many years, remote sensing has been more and more widely accepted by geoscientists as a useful tool in the early stages of exploration campaigns to unveil natural oil seepages. Today, European programs such as SENTINEL allow us to retrieve an increasing amount of free and up-to-date SAR satellite data. By combining archives data, recent data and manual interpretation, our seep study discovered evidence of natural seeps present on both edges of the Levantine Basin, still waiting to deliver its first barrels of oil, despite gigantic nearby gas discoveries.
Seep hunting, like many other oil & gas disciplines, is a highly interpretative technique mainly because of the difficulty of discriminating natural seepages rising from the seafloor (seeps) from pollution (spills). Another often underestimated challenge is posed by what shall be nicknamed “lookalikes”: algal blooms, reefs, atmospheric bubbles, oceanic or wave artefacts, etc. Nevertheless, even if natural seepages uncover only a clue rather than the entire crime scene (where to drill!) in the full exploration investigation chain, seeps still remain a trustworthy witness of an active petroleum system and seep locations can indicate the edges of efficient vertical or lateral migration routes.
As described by Pajot (2013), Synthetic Aperture Radar (SAR) data is commonly accepted to be a handy and efficient way to perform seeps studies. One fast and cost-effective solution was to gather archived SAR data from around 1990 to about 2010. However, today’s European programs such as SENTINEL enable us to reach another landmark by adding recent data to baselines belonging to the past. Provided one is capable of processing the raw images delivered by the European Space Agency (ESA), it is almost free of charge to quickly collect a large set of different SAR images on an area of interest for the last 25 years.
This innovative approach relies on the coupling of archive data – which can first highlight small specific zones of potential within very large areas of up to 1,000,000 km² or more – with brand new data acquired just days or weeks ago, which can confirm the initial potential.
Seep interpretation heavily relies on the person seated behind the computer. In a time of Big Data, where the oil industry is benefitting from the progress of Artificial Intelligence and Machine Learning, there is still a point to be made by promoting the benefits of manual interpretation. Seep detection has little to do with binary concerns, it is never all true or all wrong: it is often “in-between” and in the end, it is all in the eyes of someone who has seen seeps around the world in several different contexts and that person’s ability to observe “weak signals”, where seeps might not be obvious at first glance. If we are not able to trust computers to make automatic interpretations, how can we help humans to become more accurate? One of the answers might be found in the accumulation of large sets of SAR data alongside a methodology that emphasises the recurrences of active seeps over time rather than the seep itself.
“The Devil is in the detail”
Seep hunting is like every classical hunt or angling expedition. There will be the easy catches and the fierce prey. Some seeps bear very significant features (morphology, radiometry, contours and context) whereas others are very tricky to detect. In addition, pollution and lookalikes play their parts in making the interpretation harder. Uncertainty is part of the game. However, seep interpreters have got two main assets at their disposal: their eyes and geo-statistics.
- Their eyes, because over the years, the interpreter gets used to favourable and unlikely contexts surrounding the seeps.
- Geo-statistics, because it helps to reduce the risk of misclassification of a seep, a spill and a lookalike. If it might sound erroneous to pretend that lightning never strikes twice in the same place, it is reasonable to claim that one will not misinterpret two or three times in the very same place. Interpreters should allow themselves some boldness in seep selection (which does not mean turning an obvious occurrence of pollution into a seep).
To be fair to the remaining dubious seep study detractors among the geoscience community, seep studies sometimes fail to make the mark because of two connected reasons: lack of data coverage and spills (pollution) being ranked as natural oil leakages.
Some seep detection methodologies rely on very small datasets (0-10 images for every X/Y of study zone). These methodologies have some limits, as it is doubtful practice to claim to give a reliable assessment of the potential of an area based on such a sparse dataset and it can also lead to a second issue: pollution being confused with natural oil seepages. Due to missing data coverage, the interpreter will not be fully aware of the natural conditions and local context of the interpreted SAR images on the study zone, be it natural phenomena such as swell, algae or bathymetry or anthropogenic factors such as boat presence, shipping routes or rigs.
The methodology of this study is based on the gathering of a large dataset and therefore it helps to flag shipping routes. Then, when there is an identified shipping route on the surface, the interpreter wisely tends to be more cautious in his/her interpretation.
Chase recurrence and weak signals
By selecting large sets of SAR images taken at different dates, with more than 100 images for every X/Y location of the study zone, a reliable analysis of the potential of an area becomes possible and the recurrence of the seepages in the same place can be highlighted.
SAR imagery can detect objects with low roughness on the sea surface. However, many external factors can interfere with seep interpretation: currents, waves, swell, pollution. Our expertise is based on the manual discrimination between seeps, spills and lookalikes by paying close attention to the context of the interpreted objects.
Spills (pollution) often appear with very low radiometry on an SAR image (SAR (radar) data, unlike optical data, contains only black, grey and white pixels; low radiometry characterises an object having black pixels, while high radiometry will tend to feature white pixels). Indeed, ships tend to discharge significant amounts of oil directly on the sea surface, without any travel between sea surface and seafloor. In contrast, seeps rise through the whole water column: the journey between oil’s origin on the seafloor and where it lies on the sea surface was documented in great detail by Jatiault et al. (2018). Considering the distance between the departure point and the destination point, it should be no surprise that seepages sometimes appear with higher radiometry and more diffused edges compared to pollution.
In short, one should not be misled by the radiometry of a seep: it’s all about the context, sea conditions on the image and the presence of lookalikes.
Overview of the history of the Levantine Basin Seeps and its present contribution
Long before satellites were launched or even thought about, the Levantine region (Figure 1) appears to have hosted one of the first human encounters with seepages, dating back as far as 2000 BC and the famous bitumen from the Dead Sea, mainly used for boat caulking.
Figure 1: Levantine Basin, Eratosthenes Seamount (ESM) and discoveries.
At the beginning of the 20th century, the comprehensive Report III of the Federal Oil Conservation Board of the United States (FOCB, 1928) had already traced records of onshore oil seepages in Syria. Very close to the sea between the towns of Latakia and Aleppo, asphalt made with Upper Cretaceous limestones and marls was found by locals. Furthermore, some petroleum appeared to seep out near the city of Latakia and live oil has been found in the Upper Jurassic to Upper Cretaceous carbonate series of Latakia 1 and 2 wells drilled close to the city (Bowman & Jensen, 2011). Roberts & Peace (2007) report plenty of offshore seepages in the East Mediterranean Sea linked to oil migration pathways. Bowman (2011) mentions repeated oil seeps on the Latakia Ridge, the Thrust and Fold Belt limiting the northern European plate.
It is therefore no real headline that natural oil is heavily seeping offshore on the eastern edge of the Levantine Basin. The Aphrodite, Leviathan and Tamar discoveries stand out as massive reminders that the centre of the basin is clearly gas-prone. Those discoveries have regenerated vivid interest in the region (Skiple et al, 2012).
This study confirms the presence of numerous (35) recurrent (28 different dates from 2008 to 2017) and concentrated natural oil seepages forming a “star-shape” anomaly within the Latakia Ridge area (Figure 2).
Figure 2. Seep anomaly on the eastern side of the Levantine Basin around the Latakia Ridge.
This seep anomaly on the eastern flank of the Levantine basin is a striking example of the Holy Grail of seep interpretation: numerous, concentrated and recurrent seeps at the same location, various orientations, superimpositions, characteristic morphologies, fair bathymetry, low direct pollution context and a “star-shaped structure” with one clear emission point.
The present study also identifies very interesting concentrated seeps on the Western side of the Levantine Basin (Figure 3). Here, the seep anomaly is more enigmatic and relies on the importance of collecting many different SAR radar scenes. It highlights the advantages of the methodology based on the coupling of archival data and new Sentinel data. Archive data had already delivered evidence of some seeps that paved the way for an anomaly. SENTINEL data strengthened the first interpretation and boosted confidence in it. This anomaly also confirms how instrumental weak signals can be.
Figure 3: Seep anomaly on the western side of the Levantine Basin.
Seeps are numerous (24) and recurrent (17 different dates between 2008 and 2017). There is no clear emission point such as in the eastern side anomaly; however, seeps are concentrated and superimposed with various orientations, while the nearby local context is different to the otherwise relatively intense pollution observed within this part of the Mediterranean Sea. Some of the seep properties are weak and less distinct (Figure 4).
Figure 4: Selected illustrations of seeps on the Western side of the Levantine Basin (above: processed SAR image; below: SAR image with seep contours vectorised).
Unlikely from afar but far from being unlikely
There is no real definition of a beautiful seep in seep hunting. Only concentrated and accumulated seeps at many occasions in the same place matter. The seep on the left side of Figure 5, located within the Eastern Levantine seep anomaly, displays all the classical characteristics of a seep and looks rather easy to detect compared to the tenuous and less perceptible seep on the right side of Figure 5, located within the Western Levantine seep anomaly. What counts most: having one or two very characteristic unrepeated single seeps or having ten to twenty uncharacteristic repeating seeps? As seepages are part of active hydrocarbon systems, they should be repeating themselves over and over for an area to be rated as a hot prospect, regardless of the aesthetic side of the seepages.
Figure 5: Selected illustrations of a seep in the Eastern (left) and the Western (right) Levantine Basin.
Each seep is unique. Even by feeding computers with every seep characteristic or every image library, the issue of the context of a seep will remain the most decisive part of interpretation.
Eratosthenes Seamount (ESM), a sleeping giant?
According to the discoveries, at least two petroleum systems seem to be present in the Levantine Basin, which has been a major subsidence area since Triassic times and which could be roughly similar stratigraphically to the Palmyrides area in Syria from Triassic to Eocene. It is only in recent times (Neogene) and thanks to the progressive implementation of the Cyprus and Latakia accretion ridges that the Levantine basin became an important foredeep depocentre. One can note that apart from the above-mentioned onshore and offshore oil seeps, two kinds of fluids have been discovered and produced so far:
1) on the eastern and southern edges, the light oils of Mango 1 well in the North Sinai Egyptian waters and of Yam-Yafo wells in Israeli waters have produced 8000 bopd in Cretaceous sands and 800 bopd, respectively. These oils are correlated either to the Triassic Amanus shale or the Upper Cretaceous shales (Barrier et al., 2014).
2) very large gas discoveries have been made in the last decade in the centre of the basin, offshore of Israel, Cyprus and Egypt, leading to the rapid production of the Tamar and Zohr fields. This gas is thought to be of biogenic origin from the Mio-Pliocene rapidly subsiding series of the Levantine Basin in a low thermal flow context. However, one cannot rule out the possible existence of thermal gases generated from deep Mesozoic source rocks.
In fact, in such a context and whatever the origin of the gas might be, these extremely large quantities of recently generated gas will displace the oil from the centre of the basin to its very margins. This would be the reason why we see oil seeps on the eastern and southern edges of the Levantine basin. And what about the western edge? That is clearly where the prominent Eratosthenes Seamount proudly stands –one of the biggest structures in Europe, more than 4,000 square kilometres of structural four-way dip closure, and never drilled for hydrocarbons (Figure 6). This figure also hints at the similarities of gravity anomalies observed around the Calypso, Zohr and Leviathan discoveries and also at the Eratosthenes Seamount.
Figure 6: Bouger anomalies in Milligals (mGal) around the Eratosthenes Seamount (ESM), based on Chamot-Rooke et al., (2005)and retrieved from the Republic of Cyprus 3rd Licensing Round, and discoveries (yellow stars).
For the time being, only ODP leg 160 has begun to reveal the very nature and stratigraphy of this huge closure (Robertson, 1998). ODP leg 160 is made of 3 penetrations (sites 965-966-967) which have encountered shallow water carbonates in Santonian passing to deep sea carbonates in Maastrichtian and Eocene; then, an important uplift took place leading to an Oligocene hiatus, a Miocene shallow carbonates deposition and the subsequent very thin or non-deposition/erosion of Messinian salt followed by shale deposition in the Plio-Quaternary.
According to the few seismic sections available, it is clear that below the Upper Cretaceous series there still exist bedded seismic markers, witnesses of a possible thick (up to 3,000 m) Triassic to Lower Cretaceous sequence, which could host a similar petroleum system to the one identified offshore and onshore Syria, Israel or north-eastern Egypt.
Consequently, within the massive Eratosthenes structural closure, the main target might therefore be related to the efficient, Palmyrides-like, Triassic petroleum system that contains the Lower Triassic Amanus shale source-rock, the Middle Triassic Kurrachine dolomite reservoir capped by the Upper Triassic evaporites, such as that described by Barrier et al. (2014).
The presence of potential oil seeps in the vicinity corroborates the reality of today’s oil migration from the adjacent Levantine Basin, as indicated in structural sketch Figure 7.
Figure 7: W-E structural sketched cross-section of the Levantine Basin, modified from Skiple et al., (2012) and according to Bouger anomalies from Chamot-Rooke et al. (2005).
The importance of (slightly) going against the flow
If nowadays machine processing seems to be stuck on every geoscientist’s lips, then seep studies remind us of the importance of the human process that lies in hesitation, going back and forth, pondering and deciding.
Up to this day, how can a program make the distinction between a natural leakage and a lookalike when even a human interpreter feels unsure about it? Artificial Intelligence is an amazing tool perfectly suited to the detection of anthropogenic events, where the context is not as crucial as it is for natural events such as seeps.
Seep studies require various ingredients: gathering large sets of data, paying close attention to recurrence, welcoming “weak signals”, performing manual interpretation and combining archival and recent data.
Data from geology and geophysics, as well as the presence of recurrent seeps within the Levantine Basin, confirm the Eratosthenes Seamount structure as an oil prospect. Based on this evidence, with potential confirmation from some more 3D geophysics surveys, the Eratosthenes Seamount structure deserves the drilling of exploration wells to see whether the nearby Leviathan gas-giant could be overshadowed by an Eratosthenes oil-monster.
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This article has been published in European Geologist Journal 46 – Oil and gas exploration in Europe: innovation and new technologies
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