Is oil derived from plants or animals?

1. Oil – Fossil Fuel

In daily life, we often mention oil, natural gas and coal as the three major fossil energy sources. The "fossil" in the term fossil energy is Fossil. The reason why it is called fossil energy is that these energy sources are transformed from the remains of ancient organisms over tens of millions or even hundreds of millions of years. So, is oil, the fossil energy with the largest consumption in the world, transformed from animal remains, plant remains, or ancient fungi?

The "colorful" oil family

When it comes to oil, what comes to mind is the black, viscous liquid gushing out of the oil well. But in fact, the color forms of oil and petrochemical products themselves are diverse. The main components of oil are hydrocarbon compounds, mainly saturated hydrocarbons, and contain a small amount of unsaturated hydrocarbons such as aromatics. After distillation, the products at different temperatures are separated, and N kinds of crude processed products such as gasoline, diesel, aviation kerosene, paraffin, asphalt, etc. can be initially obtained. Of course, crude oil also contains impurities such as sulfides and nitrides. These impurities will not only generate sulfur oxides and nitrogen oxides during combustion to pollute the environment, but also corrode transportation pipelines. Therefore, crude oil must go through processes such as hydrogenation and desulfurization to remove impurities before it can be processed.

Petroleum can be used not only as fuel, but also as chemical raw materials. The polypropylene meltblown fabric in the masks we use during the epidemic is a petrochemical product. It can be said that petroleum products are closely related to our daily lives.

The black bottle on the far left is crude oil, and the distillation temperature increases from left to right. The second bottle is high-purity gasoline, and the one on the far right is asphalt)

1.2. Oil on the tip of the nose

In the class at school, the teacher brought a set of samples to the classroom, which were crude oil from Daqing Oilfield and its fractions at different temperatures. What overturned my perception was that high-purity gasoline and diesel were actually very clear. Gasoline was almost colorless, while diesel was light brown.

The gasoline sample smelled like a heavy paint smell, which was very pungent. The crude oil smelled like a faint sour smell and the smell of asphalt on the road in summer. It could not be shaken in the sample tube at all, and lay quietly at the bottom. The distilled heavy components (components with high boiling points) were more viscous than honey, and asphalt was just like crude oil, so viscous that it could not be shaken (tip: asphalt is liquid).

PS: I was scolded by the teacher in class because I put my nose directly into the bottle mouth to smell it (pay attention to the operating procedures when doing chemical experiments).

2. Sources of oil:

2.1. Two hypotheses about the origin of oil:

The shape of oil is very different from the solid "fossil" in the rock layer, and its composition is not ordinary ore (can we say that?). So the question is, how do we know what kind of creature was this black thing before?

In academia, there are two theories about the origin of oil. One is the petrochemical oil theory, which roughly means that oil is formed from carbon in the rock layer of the earth's crust and is a renewable energy source. The other is the mainstream view in academia, that is, the biological sedimentation oil theory.

According to the theory of biodepositional oil, petroleum is derived from ancient biomass through complex evolution underground. This sentence contains a key: underground. You read it right, it is this word. This word includes many conditions:

No oxygen, otherwise all the biomass would be oxidized and decayed before it could be turned into oil;

Pressure. With such a thick layer of soil on top of the oil, the pressure must be high enough.

Temperature, roughly speaking, the ground temperature rises by 1 degree Celsius for every 30m you go down from the surface.

There may also be microbial activity...

Think about it again, the rocks of the same age as the oil are likely to be buried at the same depth as the oil, and the rocks may contain fossils of ancient organisms from that time. So should we dig out the rocks in the oil burial area to see if this question can be answered?

2.2. Preliminary evidence provided by geological research:

In the 1970s and 1980s, researchers from Daqing Oilfield conducted geological research on shale and oil rock (in layman's terms, rocks containing oil) in the Songliao Basin. The research found that the Songliao Basin was previously a lake.

Researchers have found fossils of algae and aquatic animals in the rocks. The most typical fossils are two types: charophytes and choanoflagellates. Existing charophytes only live in shallow waters, which require the living water to be weakly alkaline. Most live in fresh water, and a few live in brackish water. In biological classification, choanoflagellates belong to the phylum Arthropoda, subclass Chrysomelidae, and order Saccharidae. Existing species mainly live in terrestrial freshwater waters. If the salt content of the water is between 0.05% and 1% and the pH value is between 6.6 and 9.5, they can reproduce rapidly.

Chara structure diagram

Oriental leaf scleractinian fossil

Onshore waters, fresh water, slightly salty, weakly alkaline. With these conditions put together, can readers guess what kind of geographical environment this is? That's right, a lake. And the living conditions of these biological fossils found by researchers in the rocks are almost completely consistent with those of a lake. Later, researchers discovered that this stratum is from the Cretaceous period. The Cretaceous period was 145 million to 66 million years ago, which is enough time for the formation of oil. Moreover, research shows that this ancient lake had experienced an oxygen-deficient event before, and the conditions for the formation of oil have basically been confirmed.

2.3. Origin of chemical analysis technology

Through this study, we can at least infer that oil should be evolved from the biomass of aquatic organisms. So how do we determine whether it is plant or animal? This is when chemical analysis comes into play.

Let's review first. The three major categories of the biological world are nothing more than animals, plants, and fungi. Fungi evolved chitinous outer walls 100 million years ago, and previous geological studies have not found any traces of fungi. The conjecture that fungi formed oil is basically impossible; and the core difference between the animal kingdom and the plant kingdom is the presence or absence of chloroplasts. There are almost no organisms containing chloroplasts in the animal kingdom, and chloroplasts are rich in chlorophyll, so we can say that animals do not contain chlorophyll, and plants do.

(But there are always exceptions in the biological world)

Some people may say that plants have cell walls formed by cellulose, but animals do not. First, bacteria actually have cell walls, but the main component is peptidoglycan; second, modern research has found that plant cellulose and lignin are the raw materials for the formation of another fossil fuel, coal. The carbon atom skeleton structure of cellulose is very similar to the graphite-like structure of coal (carbon atoms are arranged in regular hexagons and connected in layers), which also confirms the hypothesis that coal is derived from plants.

So we naturally came up with this conjecture: if oil is derived from plants, then the chlorophyll in ancient plants should have transformed into a chemical substance. If this substance is detected, then oil must have evolved from plants, and vice versa, it must have evolved from animals. As luck would have it, there is a chemical substance that meets expectations: maleimide substances.

This is maleimide itself. Its homologous series of substances all have various substituents added to it, such as methyl, ethyl, and phenyl.

This is chlorophyll a. Do you think maleimide looks a bit like some part of it?

The researchers also took samples from shale oil rocks in the Songliao Basin and tested the content of maleimide in the rock samples. Maleimide has been confirmed to be a degradation product or derivative of chlorophyll. Then the matter is much simpler. As long as the samples from the Songliao Basin are tested for this substance, the problem will basically be settled.

The general process is separated like this

After testing, it was found that there were considerable amounts of maleimide and its derivatives in the samples from the Songliao Basin, and the test found that the phenyl-substituted maleimide series accounted for a higher proportion of the total amount, and the deeper the burial depth and the higher the maturity of the organic matter in the rock sample, the higher the proportion of the phenyl-substituted maleimide series. At the same time, the researchers detected several special maleimides: Me,i-Bu maleimide and Me,n-Pr maleimide. These two substances are derived from photosynthetic green sulfur bacteria. The existence of photosynthetic green sulfur bacteria also confirms that the ancient lakes in the Songliao Basin have experienced hypoxic events.

Now, we have basically confirmed that oil is transformed from plant remains, and in this transformation, aquatic plants are dominant. But some people may still say: What you are testing are just piles of stones, where is the crude oil? Well, let's test the crude oil directly:

The sample tested in this inspection report is a crude oil sample from the Pearl River Mouth Basin. The researchers directly detected maleimide substances in the crude oil. As the burial depth increases, the degree of biodegradation gradually decreases. This is why we see a peak in the figure: as the depth increases and the degree of biodegradation increases, the content of maleimide substances increases.

Now, we can basically imagine such a scene: 100 million years ago, the Songliao Basin was still a lake with a large number of algae living at the bottom of the lake. After the hypoxia event and geological movement, these aquatic plants were buried underground, and after tens of millions of years of complex changes, an oil field was eventually formed underground.

Readers may still be wondering, where is the Songliao Basin? Why did we choose this place for samples? Then let's take out the map and look for Daqing Oilfield. Yes, Daqing Oilfield is in the Songliao Basin. Daqing Oilfield is the first oilfield discovered after the founding of New China, and also the first oilfield discovered in the Songliao Basin. The development and construction of Daqing Oilfield has shaken off the label of "oil-poor" in China.

References:

1. Li Jingyi. Formation and distribution of maleimide in petroleum, 2020

2. Huang Qinghua, Chen Chunrui, Wang Pingzai, Han Minxin, Li Xingjun, Wu Daqing. Late Cretaceous biological evolution and paleo-lake anoxic events in Songliao Basin, 1998

3. Gao Ruiqi, Characteristics of Cretaceous continental sediments in Songliao Basin, 1980

Author: Su Xin, a popular science creator

Reviewer: Zhang Xunhua, Researcher at Qingdao Institute of Marine Geology