At the end of last year, Per Arne Bjørkum – Scientist at Equinor and Professor emeritus at University of Stavanger – published a new line of thinking as to when oil is formed and how it migrates out of the source rock. In this way, he casts doubt on whether geologists have been led in the right direction by today’s petroleum geochemists in their search for oil. This was further illustrated in an article that was published on geoforskning.no.
Dag Karlsen, professor at the Department of Geosciences at the University of Oslo, has followed Per Arne Bjørkum’s theory. In an extensive contribution on geoforskning.no, he discusses Bjørkum’s claims thoroughly. Here is a short summary of some of the main points made in the article, which was written in Norwegian.
“Many believe that Bjørkum is in very deep water – he throws himself like Kierkegaard “out at 10,000 fathoms deep” – and it is questionable whether he and his theories will be able to float for a particularly long time,” Karlsen states.
“Much of what is being put forward is quite startling, such as being of the “new solutions to non-existent problems” type. Philosophically, it regularly happens that new “prophets” promote new explanatory models by first pointing out that “the ancient gods and theories” no longer apply – and historically we have had similar events in geochemistry,” he adds.
The story of CO2
The traditional theory that oil is formed deep in the subsurface – leading to a reduction in TOC (total organic content) through the oil and gas window – and then migrates to lower pressure and temperature conditions, is a model that Bjørkum strongly opposes.
“Instead, he promotes a model with low-temperature formation of oil and gas where CO2 is key in enabling migration. More precisely, Bjørkum believes that oil is able to migrate out of the source rock because CO2 is formed at the same time. CO2 is then able to reduce the viscosity of the heavy oil up to 1,000 times because gas bubbles are formed, which also causes the volume of the mixture to increase by up to 30 percent,” Karlsen further explains.
In contrast, the University of Oslo scientist explains that the largest amounts of CO2 are released long before the rocks reach the oil window in depth, and also before the kerogen is formed. “Then it goes without saying”, according to Karlsen, “that CO2 is not very important for oil migration, neither for primary migration nor for secondary migration: first the kerogen must be formed!”.
Karlsen further adds that it is only in special cases that one finds a lot of CO2 in connection with petroleum fields. If the substance had been important for oil migration, it would have been found in all oil and gas fields.
TOC and burial depth
Another central part of Bjørkum’s theory is the distribution of TOC with depth. According to the old theory, it should decrease with depth (in the oil and gas window), but Bjørkum writes that he sees no signs of this.
Karlsen states: “As for the alleged lack of reduction in TOC through the oil window (North Sea), it is very important to understand that there are no samples from the oil-generating deep sub-basins in any shale database from the North Sea. This is because not a single well has been drilled in the actual generating source rocks for the oil and gas on the Norwegian shelf.”
“This may be a surprise to many who do not know that drilling is only on structural highs – not in the deepest parts of the basins. It is classic «Demaison & Moore»: those areas where the source rock is better – both qualitatively and quantitatively – these are the areas we have never drilled in.”
“It is in the deep basins where the source rocks probably have much higher hydrogen index (HI) and TOC. We thus do not have samples of what Bjørkum believes he has – samples from the rocks that have generated oil and gas on the shelf. What we have are in fact the worst possible analogues for these,” Karlsen continues.
“To claim that these 871 samples (ref. Bjørkum’s figure 1) give a representative picture of reality therefore strands on a lack of understanding… ,“ Karlsen concludes.
Another view
A slightly different observation was put forward by Øyvind Sylta from Migris in his response to Per Arne Bjørkum’s publication. Sylta writes: “If I plot all the data we have in our database for the Draupne Formation in the North Sea, then I do observe a decreasing trend of the maximum value for TOC towards depth in the depth range from approximately 3,000 m to 4,500 m.”
“I also see an increasing TOC from approximately 1,200m (TOC approx. 4%) to approximately 2,800 m (TOC ca. 12%), which I do not think is consistent with Per Arne’s theories. It would be interesting if he could explain this connection,” he says.
Although Sylta fully supports the fact that Bjørkum took the initiative to challenge existing believes, which Karslen also emphasised, the director of Migris does conclude that current models based on published kinetic models for oil and gas formation, together with their own methods for modelling of 3D migration of oil and gas from source rocks to accumulations, work well.
The original theory
In a textbook, written more than 40 years ago (Hunt, 1979), the author claims that oil is formed in the temperature range of 50 to 130 ° C. Later, a common understanding was reached among geochemists that most of the light oil is formed after the temperature has reached 100 ° C, and that it is expelled from the source rock in the temperature range 120 to 150 ° C.Furthermore, there is a consensus that wet gas in the reservoir is broken down to dry gas (with more than 90 percent methane) at temperatures above approx. 200 ° C.It also means that the formation of heavy oil (API weight less than 20 ° (heavier than 0.93 g / cm3)) is today seen as residues of light oil because light components are eaten up by bacteria when temperatures drop below 70 ° C .
Bjørkum’s theory
Heavy oil is formed in and emerges from source rocks at small depths and at low temperatures, less than 30 – 40 ° C, or approx. 500 meters. Long before the source rocks come down to the depths where the so-called oil window is located.The oil is able to migrate out of the source rock because CO2 is formed at the same time and makes it easily liquid. CO2 can reduce the viscosity of heavy oil up to 1,000 times because gas bubbles are formed which causes the volume of the mixture to increase by up to 30 percent.The most important implication is that good quality oil can be found in basins where source rocks have not reached temperatures higher than 120 ° C, and geologists can look for oil in areas where source rocks are currently considered immature.
Translated from the original articles published on geoforskning.no by Ronny Setså with some final editing by Henk Kombrink.
