It is very rare for geologists to get the opportunity to "witness" in almost real time the genesis of a potential oil and gas reservoir, but that is exactly what happened offshore Central Africa last year.
In January 2020, huge flooding in the mighty Congo River hit a massive pile of sediment that had accumulated at the head of the river’s offshore canyon, sending a one square kilometre avalanche of sand — called a turbidity current — down the continental slope and into the depths of the Atlantic Ocean.
Carrying 15% of the sediment that all the world’s rivers deposit into the oceans annually, the current hit speeds of 8 metres per second, equivalent to the pace of Kenya’s David Rudisha, the men’s 800-metre world record holder.
It took two days for the sediment to settle on the seabed in 4500 metres of water, more than 1130 kilometres from the mouth of the Congo River.
Over millions of years, under the right pressure and temperature conditions and with just the right amount of organic material, these sediments could house hydrocarbons.
Known as turbidite reservoirs, these structures were difficult to map until 25 to 30 years ago when advances in seismic technology revealed their abundance in the deep waters of the Congo basin.
In Angola, the huge multi-billion-barrel Girassol, Dalia and Kizomba discoveries epitomise the fecundity of these turbidite sands.
Visualised from above, they look like meandering river channels, but instead of water, many of them hold hydrocarbons in the pore spaces between sand grains.
Scientists only know about the January avalanche after it damaged submarine cables and because African and European researchers had — fortuitously — installed instruments in the Congo canyon to measure current and sediment velocities.
In a few million years, the 2020 turbidite deposit may also hold hydrocarbons. Of course, the world certainly will not need oil and gas by then!
