Although Estonia's bedrock contains organic-rich rocks that could theoretically serve as the source material for oil, the country's geological conditions rule out both the formation and accumulation of petroleum. According to Leho Ainsaar, professor of geology at the University of Tartu, Estonia lacks the necessary sediment depth, sufficient temperature and the geological structures known as traps that hold oil in place.

The formation of oil is a geochemical process that takes millions of years and begins with organic material. "Oil originates from organic matter that gets buried with sediments underground. This material is typically marine in origin, meaning it comes from oceanic sediments," Ainsaar explained. The best source rocks are organic-rich clays, known as black shales, and the organic material itself mostly derives from algae and plankton.

A very large portion, if not the majority, of the world's easily accessible oil comes from organic matter that settled on ancient seafloors during the age of dinosaurs. However, oil did not form from the dinosaurs themselves. Dinosaur carcasses that ended up in the sea either decomposed or were consumed by other creatures. "Fish and animals make up less than one percent of preserved organic matter," Ainsaar emphasized.

When deeply buried, organic material gets compressed into a dense substance made of organic molecules known as kerogen, which also serves as the precursor to oil shale.

The critical factor in oil formation, however, is temperature. If kerogen is exposed to sufficiently high temperatures underground, its complex organic compounds begin to break down into simpler ones and become liquid — that's how oil forms. "If kerogen reaches a higher temperature — not necessarily high in everyday terms, but within the range of 50 to 150 degrees Celsius — it starts to break down," Ainsaar said.

This specific range of depth and temperature is called the "oil window." In typical continental crust, that corresponds to depths of roughly two to four kilometers.

Estonia's problem: too shallow and cool

The absence of an oil window is the primary reason oil cannot form in Estonia. According to Leho Ainsaar, the issue lies in the thickness of Estonia's sedimentary cover, which is too thin to support oil formation. "In northern Estonia, it's about 100 to 150 meters thick and beneath that lies the crystalline basement — granites and gneisses — which contain no organic material. In southern Estonia, the sediment layer reaches a maximum of 500 to 700 meters, but even that is not deep enough to reach the oil window," the professor explained.

Nevertheless, the potential source material does exist here. "For example, the Dictyonema shale along the northern coast would be an excellent source rock for oil if only it had been subjected to the right conditions," Ainsaar noted.

Statistically, most of the world's oil reserves — more than two-thirds — formed during the Mesozoic era, between 200 and 250 million years ago. Older rocks have often already broken down or evaporated, while newer ones have not yet had time to generate oil.

Second issue: Lack of oil traps

But oil formation deep underground is only part of the story. Since both oil and the natural gas that forms alongside it are lighter than water, they begin to slowly migrate upward through porous rock layers under the pressure of groundwater. This movement can take millions of years.

For oil to accumulate into a deposit, it needs an oil trap — a geological structure that halts its upward movement. According to Ainsaar, two conditions must be met for such a trap to form. First, the porous rock layer, such as sandstone, must be covered by an impermeable layer like clay, which does not allow oil or water to pass through. Second, these layers must be folded into a dome shape.

"In order for oil to actually accumulate somewhere, the layers have to be bent in a way that creates a dome — a geological structure where oil can become trapped," Ainsaar explained. "If the layers are arched upward, oil can no longer move up or to the sides."

The professor added that oil collected in such traps is often under high pressure because it is being squeezed from below by groundwater and from above by heavy rock layers. "If you then drill a well, the pressure may be higher than the surface resistance and oil can gush out of the ground — just like we see in the movies. That's a completely realistic scenario," Ainsaar said.

This is also the second reason why there is no oil in Estonia — even if some were to migrate here from Lithuania or beneath the Baltic Sea. "Estonia has no oil traps. Our geology is too simple, the layers gently tilt southward and there are no folds where oil could become trapped," the professor noted.

Signs from the past

While Estonia has no industrial oil reserves, there is evidence that oil once moved through the region. According to Ainsaar, the Kärdla meteorite crater on Hiiumaa is a "nicely dome-shaped formation" that, due to the ridges formed by the impact, could theoretically have functioned as a trap.

"Traces of asphalt have been found there and asphalt is the final fraction of oil — it doesn't evaporate. It was likely oil that originated in Courland [western Latvia] and migrated here, leaving small remnants in Estonia," he said. The professor added that the quantities involved are insignificant.

Gas, on the other hand, has been found in Estonia. Although most natural gas forms in a similar way to oil — through the breakdown of organic material at high temperatures — the gas discovered in Estonia is of a different type.

"There are known cases in Estonia involving biogenic or bacterial gas. For example, gas leaks have been detected on Keri Island," Ainsaar explained. There, the gas formed through bacterial activity as organic material decomposed at shallow depths. "It's essentially the same process used to produce biogas at landfills," he said.

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