#source rock

How much oil can we actually extract?

When you hear about a company discovering 500mmbo (million barrels of oil) you assume they must be getting pretty much every last drop of oil out of the ground. However, this is very far from the truth as the example below shows.

If you start with 1 billion barrels of oil, only 600-700mmbo(60-70%) will ever be able to leave the source rock (the rock producing the hydrocarbons) as 30-40% will be trapped within the pore space. This remainder is why source rocks are also being targeted as reservoirs (rocks that hold oil and gas) and why fracking shales for oil has become such a big business in the past few years. While a lot of oil has remained trapped, you're still not doing too badly as you have two thirds of the oil left to extract right? Unfortunately not, only 10-20% of the oil ever reaches the reservoir due to processes that occur during migration, such as loss of oil to the surface as seeps or destruction by bacteria. Altogether, from the original billion barrels you now only have 120-140mmbo remaining.

This is about the size of a small oil field and depending on the field's location may still be economical to extract. However, most companies only achieve a 40% recovery rate, leaving you with just 48-56mmbol, a fairly small reserve by most people’s standards. This means in a supergiant (>500 billion barrels of oil) field, such as Ghawar in Saudi Arabia which has between 100 -200 billion barrels of recoverable resources, the amount of hydrocarbons initially produced by the source rock must have been immense. For Ghawar alone that figure is between 40,000-80,000 billion barrels of oil, while current global reserves together only account for 1687.9 billion barrels.

Recoverable reserves are only 4% of the total oil that was ever produced from the source rock(s). This is why many companies are researching into technologies that will allow them to recover a much greater proportion of hydrocarbons from their reservoirs. For example, BP has developed a product called LoSal, a type of EOR (enhanced oil recovery) technology designed to free oil bound to a rocks surface. The oil is bound to clay minerals by divalent cations such as calcium and when highly saline water is pumped into the reservoir these bonds are compressed forcing the oil closer to the clay surface. By reducing the salinity of the water, the molecules can expand exposing the bond between the oil and clay, allowing divalent cations to be replaced my monovalent ions (e.g. Sodium) and freeing the oil.

As fewer large oil fields are being discovered, it is improvements to current technology allowing greater recovery rates that will determine how much oil the world has left to extract.

Watson