English edit by Maria Luiza Albieri and Carla Elliff
Illustration by Joana Ho
Everyone has seen a movie/series/soap opera in which the fingerprints left on an object are used to identify the perpetrator of a crime. But did you know that the petroleum (oil) generated in different oil basins also has unique “fingerprints”?
Yeah... not all oils are the same.
Before understanding the science behind this, it's necessary to know that oil is formed from processes of diagenesis (reactions that occur in the first centimeters of the sedimentary column with the presence of microorganisms and at temperatures of up to 50 °C) and catagenesis (transformations that occur without the presence of microorganisms, at temperatures between 100-150 °C and result in the formation of oil) from organic matter. These two terms are used to explain the formation of rocks, fossils and, of course, oil. When plants, animals, phytoplankton, bacteria, etc. are buried in the sediment, they undergo various chemical and physical transformations. The burial of organic waste accumulated in the sediment increases local pressure and, with sufficient pressure, there is also an increase in temperature. In this process, the organic matter will undergo a series of transformations until, in some cases, it forms oil.
Transformation of organic matter that will result in the formation of oil and gas.
The organic accumulation that gave rise to the oil basins we have today took place millions of years ago in different geological periods. Thus, the composition of the oil in these basins is not the same, as it depends on the type of initial organic matter (for example, oils generated from phytoplankton have different characteristics from oils that arise from bacteria or higher plants), on the thermal evolution, the characteristics of the sedimentary basin where the accumulation occurred, and other primary and/or secondary processes that occurred before and after its accumulation, such as oil migration and contamination by microorganisms.
During diagenesis and catagenesis, many molecules are destroyed and/or transformed, but some either resist these conditions or undergo minimal modifications, losing only a few functional groups. These molecules are, for the most part, lipid compounds that preserve in their structure information about the origin and conditions of formation of that oil. For this reason, they are called biomarkers, or geochemical fossils, and many can be used for what is known as "oil fingerprinting".
For example: Oil derived from the organic matter of higher plants has n-alkanes (linear hydrocarbons formed by carbon and hydrogen) with longer carbon chains than those derived from organic matter derived from phytoplankton.
Molecules of n-alkanes: above heptadecane (alkane with 17 C)
and below heptacosane (alkane with 27 carbons).
Likewise, a series of other compounds, such as terpanes, stereranes, hopanes etc., can be used to characterize each oil. Mapping these compounds also allows to identify whether the oil in question is recent or whether it has been in contact with the environment for more time and has undergone weathering (set of processes that lead to the disintegration of materials). Once the oil reaches the marine environment it can undergo a series of processes: a) spreading caused by winds, currents, and waves; b) evaporation of lighter compounds; c) dissolution of the most soluble compounds; d) dispersion: incorporation of oil particles with water; d) emulsification: mixture of oil and water forming a material similar to mayonnaise; e) adsorption of part of the material to living organisms and suspended particles; f) removal: sedimentation of denser components; g) biodegradation: microorganisms use the oil as a source of carbon, breaking the molecules into smaller ones; h) photooxidation: breakdown of molecules by the action of sunlight.
To identify the fingerprint of an oil sample, gas chromatography is used, in which the components of the sample, after pre-processing, will be separated according to their physical-chemical properties. Thus, its constituents can be identified and quantified. This analysis gives rise to a chromatogram, which is the graphical representation of the equipment signal (x axis: time that the compound takes to pass through the equipment, y axis: signal intensity that is related to the compound concentration). And by observing the chromatogram of different oils it is possible to see that they have very different characteristics. In the image example, samples P1 and P2 show differences both in the composition and in the abundance of their compounds.
Schematic chromatograms of two types of oils.
Schematic chromatograms of the same oil with different degrees of biodegradation.
Therefore, the study of oil “fingerprints” is a very valuable tool for forensic geochemistry. When a spill occurs and no company states they are responsible for what happened, forensic geochemistry can help in the investigation of which basin originated this oil and/or compare the material with oils from different refineries. In the case of oil that has been appearing on the beaches of northeastern Brazil for more than a month, this is the type of analysis that universities are carrying out to identify whether the oil originates from Brazilian basins and to try to assess how long it has been in the environment.