Whenever you log onto a news website, there is inevitably an argument by some politician, social group, or just normal people about how petroleum is ruining/running/destroying the world (especially when it comes to fracking). When you start to read their comments and arguments, as a geologist you start to see an extremely disturbing trend: probably 95% of the people that are complaining about petroleum, how it is extracted and how it runs the world have no idea how petroleum is found, formed, and processed.
I know it’s probably not the most important part of the debate in the aspect of world economics, but it should be a requirement to understand the basics of petroleum systems before you try to regulate the oil industry.
I don’t care much about the economic impacts, so what we will do over the course of the next few days is describe, slightly above laymen levels, the basics of petroleum systems, and at the end we will have a guest scientist explain how petroleum is refined. Lets get started:
A petroleum accumulation is the result of a number of processes and a massive amount of time. Basically it comes down to this: You have a source rock that is usually a black shale (meaning that there is a large organic carbon contribution to a shale bed), it is cooked, petroleum is released, the petroleum travels to another place (because it is more buoyant), and it gets stuck somewhere until someone drills a hole and pumps it out.
Let’s talk about what may arguably be one of the most important factors: The source rock.
Most of the organic carbon comes from algae (although people always love to say “you’re burning dinosaurs, but that’s just not correct), but there can also be contributions from terrestrial plants (such as trees, leaves, seeds, etc.). One problem that we immediately see, is that if the organic matter rots, or oxidizes, it can no longer be considered useful for petroleum generation as the porphyrins are extremely reactive with oxygen. Therefore, the organic carbon needs to be deposited in an anoxic environment, such as deep seas or sometimes very large deep lakes, or it needs to be buried in a place that has an extremely large sedimentation rate, so that the organic carbon gets buried and compacted faster than it can be oxidized.
Once this organic material is buried to a sufficient depth, the organic material begins to decompose and degrade (please note that ‘decompose and degrade ’ in this context does not mean rotting, or oxidation!), and partially separates into its constituent parts, namely biopolymers from proteins and carbohydrates. These constituent parts begin to form new polymers named geopolymers. Given enough time, pressure, and temperature, these geopolymers create a substance named kerogen which is composed mainly of carbon, hydrogen, oxygen, nitrogen and sulfur.
The type of kerogen depends on the type of organic material that went into making it. Please note that kerogen is not a chemical compound, but more of a mixture of organic material, so the precursor material is extremely important to understand when trying to understand the type of kerogen that will be formed, and subsequently what kind of petroleum is formed.
Types of kerogens:
- Type I: (Sapropelic) This type comes from the deposition of lacustrine algae in anoxic conditions. Because algae has a very large percentage of lipids (in comparison to terrestrial plants), it has been show to have a great affinity to form liquid hydrocarbons such as crude oil.
- Type II: This type contains more oxygen than type 1 and tends to produce a mixture of gas and oil. It is mainly created from Plankton in marine settings. Unlike in type I kerogens, the organic material is deposited in a reduced environment instead of purely anoxic (although, there are currently arguments that state there is no such thing as purely anoxic conditions, just massively reduced). Type II-S is just like Type II, just with an increased sulfur content
- Type III: (Humic) Terrestrial plants compose most of this category which includes wood, leaves, and other fibrous material from land. This material produces coal and gas as it lacks the necessary amount of hydrogen to form hydrocarbon chains. This is not to say that type III is not capable of creating liquid hydrocarbons, it is just exceedingly rare and only occurs under very specific conditions. Terrestrial plants have the lowest concentrations of lipids of all the precursor kerogen types.
Figure: Modified Van Krevelen diagram which displays the primary composition of the differing types of kerogens and the levels of maturity with associated hydrocarbon types.
Now that we have the organic carbon buried and starting to rearrange into kerogen types based upon their precursor organic carbon type, we need to pressure cook the material to obtain the hydrocarbons from the ‘cracking’ of kerogens.
This increase in temperature from burial (the rate of temperature increase in a ‘normal’ basin is around 30°C per kilometer, but in basins where there is extension, it can be more around 40°C per kilometer) will begin to break the chemical bonds of the kerogen and produces smaller molecules which compose oil and gas. The main window for this breaking of kerogen into smaller molecules requires a temperature around 80-150°C and on a time scale of 1-100 million years (although this number has been fluctuating and narrowing quite a lot in recent times due to the increasing types of methods to determine the production of hydrocarbons).
There are many ‘windows’ of temperature ranges that will produce differing types of hydrocarbons, such as seen in the Van Krevelen diagram. The oil window is typically from 100-150°C which corresponds to around 3-4 kilometers of depth. Note that the temperature can be altered by a raised geothermal gradient that can be due to an upwelling of the asthenosphere, proximity to plutons or volcanic activity, or the amount of radioactive material within the under/overburden or it can be lowered by being located in a colder craton such as a shield. Anything above this temperature will produce gas, or if raised for extended periods of time, the kerogen composition will be gradually depleted until there is a pure carbon residue left over (graphite).
The rate of petroleum generation follows the Arrhenius function:
; where R is the gas constant, T is the temperature, and Ei is the activation energy. Ei varies between 50 and 80 kcal/mol or approximately 200 kJ/mol. A is an exponential constant that is dependant on the type of kerogen (I,II,II-S, III).
Because the temperature is going to be fluctuating through time as burial progresses, if you have uplift, or changes in geothermal gradients, the effect of temperature is required to be integrated over the range of temperatures. This is called the TTI, or Time Temperature Index
; where (T) is the integrated temperature over time from deposition time (t0) to the present day (tx). F represents a factor that is dependent on the activation energy (Ei) for the reactions. Based upon this calculation, we can conclude that the rate of the reaction approximately doubles for each 10°C increase of temperature.
If we wanted to understand the maturity of the source rock at any given time and with the already calculated and known inputs, we can create a theoretical maturity parameter (P) in which temperature is integrated with respect to time (t):
Information and associated references taken from Bjørlykke, 2010