Despite the many large gas discoveries in the Levant Basin in the Eastern Mediterranean, not much has been disclosed about its offshore sequence stratigraphy, mainly because of the proprietary nature of industrial data.
Noble Energy played a large role in the discoveries of the Tamar, Leviathan and other sizeable gas fields beginning in 2009, enabling significant advancements in natural gas exports and LNG production for the first time in the region.
Chevron acquired Noble Energy in 2020 and announced last August plans to increase production in the Leviathan field from 12 to 21 billion cubic meters per year and to build a floating LNG plant, suggesting the basin has much more to give.
Andrew Madof, a senior geologist at Chevron’s Eastern Mediterranean Business Unit who is working on the Leviathan project, recently shared a rare offshore stratigraphic framework of the southern Levant Basin at the International Meeting for Applied Geoscience and Energy in Houston. Building on previously unpublished geological and geophysical data, he offered new ideas about the basin’s Oligo-Miocene deepwater sands.
“We are looking at an area that is absolutely fascinating,” Madof said. “It has received very little attention in the public to date.”
Frontier Play
It has been widely noted that the Levant Basin has a complex history of rifting and tectonic activity, thought to be caused primarily by the opening of the Tethys Ocean, a predecessor to the Indian Ocean during the Mesozoic era. The basin is known for significant gas reservoirs located in structural and stratigraphic traps in multiple Mesozoic, Cenozoic and Tertiary stratigraphic intervals.
After the 2009 discovery of the Tamar gas field, which opened a new frontier play, Noble Energy and partners drilled six successful sub-salt exploration wells totaling more than 35 TCF of biogenic gas in the Oligo-Miocene reservoirs, according to a 2013 AAPG Search and Discovery publication titled, “The Tamar Field from Discovery to Production,” authored by Dan Neeham et al.
The Oligo-Miocene sands reservoir section, also known as the “Tamar Sands,” consists of deep seafloor fan sandstones with thin beds of silts and mudstones, according to a 2017 publication also authored by Needham titled, “The Tamar Giant Gas Field: Opening the Subsalt Miocene Gas Play in the Levant Basin,” an excerpt from the book, “Giant Fields of the Decade 2000-2010: AAPG Memoir 113.” Needham said the high-quality reservoir sands at Tamar had thick deposits of more than 250 meters of gross pay. The gas – more than 98-percent methane – was thought to be sourced from the interbedded shales. The traps were noted as faulted, four-way closures reaching more than 100 square kilometers at Tamar and roughly 330 square kilometers at Leviathan, the largest gas discovery in the basin to date.
Stratigraphy Rearranged
While the stratigraphy of the region is well understood in onshore localities, its offshore architecture and sedimentary infill is much less constrained.
Taking into account the spatiotemporal depositional architecture, sediment transport directions and controls on accumulation, Madof made a case for a new view of the Tamar Sands in a bio-stratigraphically constrained sequence stratigraphic framework. While he based the framework on integrated, multi-scale geological and geophysical data, he used both independent validation and multiple working hypotheses to shed new light on the Levant Basin.
To start, Madof and his team created a sequence stratigraphic framework using core, well and mineralogic data from the basin. They carried out a similar exercise using multiple seismic datasets. Subsequent to the picking of reflections, they relied on spectral decomposition and variance attributes to constrain depositional units and their bounding surfaces.
Based on numerous multi-scale datasets, the team identified the presence of four large-scale, deepwater depositional sequences named the A, B, C and D Sands – underlain by sequence boundaries. Because the three lowest sequences (the B, C and D Sands) contained only early and late lowstand accumulations, they were considered “incomplete sequences”. Yet the upper sequence, the A Sand, which contained lowstand, transgressive and highstand accumulations, was fully preserved and therefore considered the only “complete sequence”, Madof noted.
Madof relied on this complete sequence as a clue to how the depositional environment was changing.
Sitting directly on top of the A Sand is the Levant Mudstone, a carbonate-rich shale that is representative of the transgressive surface. (Madof pointed out that it was once considered the maximum flooding surface, meaning it marked the transition between transgression and regression in the basin.) Overlaying the Levant Mudstone is a section of Hard Lime, the most carbonate-rich interval and a more accurate representation of the maximum flooding surface, Madof stated. “By far it is the best candidate that can be defended scientifically.”
This required Madof to reorder the sequence stratigraphy.
A deep dive into the complexity of the Levant Mudstone also revealed two distinct cycles of increasing carbonate presence, each followed by a reset. On top of the Levant Mudstone, traction-based sedimentary structures could be seen in core.
Yet when biostratigraphy from multiple wells was integrated to date the timing of the deepwater depositional units, the bio-stratigraphic ages varied. As a result, a “robust interpretation” could not be made to tie the system to any eustatic cycle, Madof said. Instead, the team relied on a time-probabilistic framework to quantify age-based uncertainties across multiple wells for the same stratigraphic interval. In doing this, new hypotheses arose about the depositional timing and formation of the Tamar Sands.
A New Perspective
While it is customary for geologists to look at continents when interpreting sequence stratigraphy, there are other forces at play in the deepwater Levant, Madof said. “Deepwater sands are often thought to be deposited when sea levels drop and beaches are exposed,” he explained. However, “we don’t have the resolution to make this determination.”
Furthermore, noting a change in the shales in the Levant Mudstone from land-based muds to carbonate shells, Madof surmised that an oceanographic event was contributing to the deepwater sequences. “When there are warmer temperatures in the ocean, we see more carbonate deposits. This is a pattern that is derived from the ocean as opposed to onshore,” he said.
The intervening shales between the sandstones showed a marked increase in calcium carbonate through time. “What makes this interesting is that the Eastern Mediterranean was not closed off to the Indian Ocean at this time,” Madof said.
Madof proposed that the Hard Lime in the sequence is actually representative of the onset of the Middle Miocene Climatic Optimum. The MMCO is a global warming trend caused by increased levels of carbon dioxide.
“I think this has a really important effect on this system. If you think paleo-geographically, the Persian Gulf was not yet formed, and the Indian Ocean was open, so this system had direct communication to the open ocean,” Madof said. “We see an increase in carbonates globally, and currently I think it’s a good working hypothesis that the upward increase in carbonates (in this system) could be representing a global oceanographic signature.”
He added: “The MMCO has not necessarily been tied to these rocks before. It makes us think differently and does not require us to rely solely on a pre-existing paradigm, especially when age control does not work. Our conclusion is that we don’t need to throw the whole sequence stratigraphic model away, but only use what works.”
He noted that the Hard Lime is also representative of a large-scale change from growth stratigraphy below, to non-growth deposition above, signifying a tapering of regional deformation.
In the eyes of Nicolas Hawie, a geologist and senior technical adviser at Halliburton who has authored numerous papers on the Levant Basin, said the Hard Lime’s characteristics – including composition and sedimentary features – certainly provide clues about the marine environment under which it was deposited. It tends to indicate relative increase in the sea level and by that, a retreat of terrigenous sediment input landward.
He added, “This could also imply that the Hard Lime is not just a marker of sea level rise but also a record of enhanced biological activity, potentially leading to higher organic matter accumulation in the deeper basin, which is critical for biogenic hydrocarbon generation.”
“Thus, this unit could present source rock characteristics if anoxia is achieved as well as possible enhanced sealing capacity for underlying Miocene sandy reservoirs,” Hawie said.
The idea that the Hard Lime could represent the onset of the Middle Miocene Climatic Optimum has significant implications that still needs to be further investigated for both geological understanding and hydrocarbon exploration in the whole Levant Basin, he added.
As the MMCO is characterized by increased marine productivity and significant biogenic carbonate deposition, one might think that carbonate reservoirs could have also developed at the shallower flanks of the basin or along paleo-topographic highs – which could open up an additional uncertain play to investigate in the near future, Hawie said.
Future of Levant
If the MMCO hypothesis proves correct, geologists would have a better predictive tool for exploration in the Levant Basin and perhaps far more prospectivity than once thought.
If the Hard Lime is established as the onset of the MMCO, it could very well lead to new exploration strategies, Hawie said. “We might focus on identifying other areas of similar sedimentation patterns or characteristics associated with that climatic period,” he explained, “potentially revealing new play types in the basin.”
If anything, there appears to be a noteworthy relationship between changes in deepwater depositional architecture and the rise of authigenic carbonates. Madof emphasized that the competing terrestrial and oceanographic controls on deepwater stratigraphic systems suggest that such intricate relationships may be more prevalent than previously thought.
“When making an interpretation, you should not be pinned down to one deterministic idea,” Madof said. “It’s important to think about more than one hypothesis and to keep the probabilistic explorer mindset.”