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.
These are the most unconventional hydrocarbons yet discovered; “unconventional” because of the type of rock they’re contained in, and their age. This antiquity gives us a rare chance to use the remains of the bacteria to examine the chemistry of the ancient oceans, the composition of the ancient atmosphere, and the nature of life 1,400 million years ago.
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.
How oil and gas forms underground
To convert organic matter to oil or gas requires heat, but not too much: you need to delicately cook the rocks to temperatures between 60℃ and 160℃ for a few million years. You do this by burying the rocks.
Earth gets hotter as you go deeper, something miners know well. In fact, the deepest mine in the world (in South Africa) goes down 3.4 kilometres, and the rock temperatures down there are an incredible 60℃.
Oil and gas form when the rocks get a little deeper than this. These temperatures are often referred to as the oil and gas “window”. If the organic matter does not reach this temperature window, oil and gas are not produced.