Earth's movements in space could tell us where to look for oil underground. Recent work highlights the role of Earth's orbit in the formation of shale oil, thus proposing a new method for targeting these deposits.
This type of hydrocarbon differs from conventional oil, as it is trapped within clayey rocks called shales. The latter are formed from fine sediments that accumulated at the bottom of ancient lakes or seas. In these oxygen-poor environments, organic matter was able to concentrate before transforming, over millions of years, into oil directly within the source rock.
The study focused on Milankovitch cycles, periodic changes in our planet's orbit and tilt. Spanning hundreds of thousands of years, these cycles affect the long-term climate by altering the distribution of received solar energy. Among them, orbital eccentricity transforms the shape of Earth's ellipse, which goes through phases that are more or less rounded.
When eccentricity is high, seasonal contrasts intensify, generating warmer and wetter conditions. In ancient lakes, this stimulates nutrient input and biological productivity. This results in the deposition of muds rich in organic matter, which will later evolve into rocks favorable for the genesis of shale oil.
Conversely, lower eccentricity leads to a drier climate. Lake levels then drop, and the nature of the sediments changes: more sands are transported by gravity flows towards the slopes and deep areas of the basin. This rhythmic alternation between wet and arid periods produces a predictable sequence of rock layers, identified in regions such as the Sichuan Basin in China.
By examining rock cores and geochemical data, scientists have reconstructed these environmental upheavals. They correlated the sedimentary layers with the orbital cycles, observing an average accumulation exceeding four centimeters per millennium (about 1.6 inches). This precision allows for the development of a model to locate areas where good quality shale reservoirs are most likely to develop.
This method thus combines astronomy and geology to optimize oil exploration. Although the extraction of shale oil involves hydraulic fracturing – a technique raising ecological questions – it remains an important energy source. This work, published in the
Journal of Paleogeography (Chinese edition), could enable more effective exploration campaigns.