|So-called “unconventional” deposits of oil and gas are found in shale, a type of layered, fine grained rock. Dana283/Shutterstock|
Two to three kilometres beneath the surface of Australia’s Northern Territory sits buried energy. The layered rock formations known as the Velkerri Shale were recently estimated to contain over 118 trillion cubic feet of gas.
While these gas reserves are clearly large, what is really remarkable is their age. These rocks were deposited 1,400 million years ago, in an ocean known as the Roper Seaway. These rocks, and their encased oil and gas, are about a billion years older than rocks where oil and gas are usually found.
The molecules that make up the oil and gas – called hydrocarbons because they consist of hydrogen and carbon atoms – are the long-decomposed remains of dead bacteria that inhabited ancient oceans.
Recently we have learned a lot about Earth’s ancient marine environment.
This has been achieved by analysing rare elements, particularly cerium (Ce) and molybdenum (Mo) extracted from the once-living organic matter within the Velkerri Shale. Ce and Mo act as indicators of how much oxygen was available in the oceans 1,400 million years ago. The studies reveal an ocean starved of oxygen, even in surface waters. At deeper depths this ocean was completely toxic, rich in hydrogen sulfide.
These results indicate that Earth’s atmosphere at this time was oxygen-poor. In fact, it is likely that it contained less than 3% oxygen. Currently we enjoy 21% oxygen in Earth’s atmosphere and a largely oxygenated ocean.
Further work by our colleagues has focused on extracting molecules of biological origin from the same rocks. These “biomarkers” have revealed an ocean dominated by bacteria.
But why is the age of the Velkerri Shale so unusual? It’s got a lot to do with the unique sequence of events that need to occur to produce and preserve oil and gas.
The Velkerri Shale contains enough gas to power Australia for more than 90 years, at current consumption rates. While only a proportion of this gas would be recoverable, a resource estimate of 118 trillion cubic feet forms a large onshore unconventional shale gas reserve in a country soon to be the largest gas exporter in the world.
Oil and gas are non-renewable resources. The concept of “Peak Oil” or more broadly “Peak Hydrocarbons” refers to the point in time when the maximum rate of oil and gas extraction is reached. However, the technological development of unconventional oil and gas, and now the realisation that gas can be sourced from extremely old rocks, such as the Velkerri Shale, means that the arrival of “Peak Hydrocarbons” may be further delayed.
The development of unconventional oil and gas remains contentious, and well-informed public debate will ultimately decide whether such shale gas resources are developed.
If the Velkerri Shale moves from exploration to production, we will be making use of gas produced in a “slime world” that existed nearly a billion years before the first complex life on Earth evolved, where bacteria ruled the seas and the atmosphere was largely devoid of oxygen.
The above story is based on materials provided by University of Adelaide.