A magical journey: Oil's autobiography, My legend part 2

Ancient life came from the vast ocean and returned to the cold underground. What was left between coming and going? Oil, the inheritance of the reincarnation of life. I am the death of life, but I am unwilling to die. Holding my breath, I endure the high temperature and high pressure in the dark underground for billions of years.

Decay is my past life, black gold is my present life. In the oxygen-free space-time tunnel, I carry the belief of the afterlife, silently gather positive energy, and finally turn decay into magic, supporting the modern civilization of mankind.

Life in the "Cosmic Egg"

The original source of the universe is a so-called "cosmic egg", which condenses all the space-time mass and energy, and breeds everything in the future material world, including celestial bodies and life. About 15 billion years ago, the "cosmic egg" gave birth to the universe in an unparalleled big bang. The remnants of the explosion in the universe interacted with each other, and while forming planets, they also gradually formed galaxies. One of the galaxies is called the Milky Way. There is a star in the Milky Way called the Sun. There are also eight planets orbiting the Sun, one of which is the home of mankind - the Earth.

When the Earth was first created, there was no life on earth. At that time, the Earth was far away from the Sun. As the Earth got closer and closer to the Sun, it was in the most suitable position for the emergence of life in the solar system. The materials that made up the Earth met the requirements for the emergence of life. After receiving the participation of external materials in the universe, the Earth began to have primitive oceans.

The primordial ocean gave birth to primitive life over a long period of time, beginning a long evolution of life. It was not until the Cambrian period 540 million years ago that a great explosion of life occurred. The creatures on Earth suddenly became rich and colorful, and the number of species increased exponentially.

Bacteria or green plants convert carbon dioxide and water into organic matter through photosynthesis. As the bottom of the food chain, various animals feed on it and gradually prosper. In the cycle of flowers blooming and falling, life and death, how does life leave traces on the earth and last forever?

Where is my home?

"Underground oil sea" and "underground oil river" are words often used by scholars to describe underground oil. Over time, many people think that underground oil is like a sea or a lake. It seems that if you make a hole in the ground and get a water pump, you can easily pump oil up.

This is not the case. Don't forget that the oil industry is not only the most capital-intensive industry in the world, but also the most technology-intensive industry.

Petroleum, as the name suggests, is the oil in the stone. It is like water in the sponge, soaking into the stone. However, the stone is so hard, can the oil penetrate into it?

Oil and gas generation and preservation

In fact, the stones in nature are not solid and seamless. People who often do outdoor activities should know that the rocks on the mountain have various cracks and holes of various sizes. Even the stones that have not been weathered or damaged have various cracks and holes inside them, but some are relatively large and can be seen with the naked eye, while others are very small and can only be seen with the help of a magnifying glass or microscope. For example, if you pour a few drops of water on the sandstone for sharpening a knife, the water will disappear after a while, leaving only a wet mark on the surface. This proves that a stone as solid as a grindstone also has many pores that cannot be seen by the naked eye.

The cracks and holes in the rock are my home. The more and bigger the cracks and holes are, the more oil the rock can hold; the better the cracks and holes are connected to each other, the easier it is for me to flow in the rock.

The existence of natural gas underground is the same as that of oil, stored in the cracks and holes of rocks. The difference is that natural gas is a gas, and it is more "penetrating" than liquid oil. Rocks that can penetrate oil can also penetrate natural gas. Even some rocks that oil cannot penetrate are unobstructed for natural gas.

Schematic diagram of oil and water distribution in rock pores

The oil industry calls all rock layers that can store oil and natural gas reservoirs. Obviously, a reservoir refers to a rock layer with storage capacity, but it does not mean that a rock layer with storage capacity will necessarily contain oil or natural gas. It has the storage capacity, but oil or natural gas must be drilled in to store it before it can be called an oil (gas) reservoir. The space that can store oil and gas is called a trap, and the trap after storing oil and gas is called an oil and gas reservoir.

Underground trap

The quality of oil and gas traps is mainly reflected in three aspects: first, the size of the trap, that is, the maximum effective volume of the trap, which determines the size of the place where the oil and gas is stored. Of course, the larger the better; second, the thickness and porosity of the reservoir in the trap. Similarly, the thicker the reservoir is and the more and larger the cracks and holes there are, the more oil can be stored; third, the tightness of the cap rock and the updip shielding of the trap. Having enough storage space is only a necessary condition for the formation of a good oil and gas reservoir. If the surrounding rocks are not well sealed, the oil and gas will migrate into the trap and then run away, and a good oil and gas reservoir cannot be formed.

My Colorful Past Life

Every year, from early spring to late autumn, the earth we live in is colorful, and various plants and animals decorate the earth in a colorful way. Have you ever thought that the yellow, black, and brown oil that is closely related to modern life can be made from them!

Carboniferous biological restoration map

Throughout geological history, there has been an incredible abundance of life on land, ranging from dinosaurs and primitive mammals to plants and flowers, and even pollen from angiosperms and spores from gymnosperms that are invisible to the naked eye; life in water is also colorful, including fish, shellfish, snails, plankton and plants, which are the main bodies that decorate the land and ocean, and are also the basis of organic matter on Earth.

On land, after various animals and plants die, they are often eaten by other animals or quickly rot and decompose due to wind and rain. In water bodies, due to the protection of the water layer, the quiet environment at the bottom of the water, and the lack of oxygen, the decay of organisms can be greatly alleviated or even prevented.

In the water that accounts for 75% of the earth's surface, there live a large number of microorganisms and micro-organisms. It is estimated that a diatom can reproduce to the same size as the earth in one day without any obstacles! It is estimated that the annual death of plankton in the ocean can reach 550 billion tons. They live in large numbers in seawater, which is called "biological rain" by scientists, and after death, they fall to the bottom of the sea to form "corpse rain". There are also a large number of biological remains that are continuously carried to the ocean and lakes by rivers and winds, and continue to sink to the bottom. They are sedimentary organic matter.

Various algae that form oil and gas

Kerogen: The gateway to oil

In water bodies, after the death of organisms, the main components of their bodies, such as carbohydrates, proteins, lipids, and lignin, will successively undergo different degrees of decomposition and destruction. Part of the decomposition products become "snacks" for other organisms, and the other part is decomposed into carbon dioxide and water. The remaining part, which is a very small part of the original number of organisms, does not undergo complete biological recycling and physical and chemical decomposition and enters the sediment, becoming sedimentary organic matter.

Early sedimentary organic matter was continuously buried in deeper places. Under normal geological conditions, it takes 5,000-10,000 years to form a 1-meter-thick sediment layer. During this period, fermentation caused by bacterial activity degrades the cellulose, protein and polysaccharide macromolecules in the organic matter. Most of the organic matter aggregates into organic matter that is insoluble in organic solvents at this stage, which geologists call "kerogen".

Kerogen

Schematic diagram of kerogen formation

Kerogen, the word is a transliteration, which comes from Greek. It was previously translated as "kerogen", which is a literal translation. Kero means wax, and its original meaning is "a substance that can produce oil or waxy substances". Later, the meaning was confused again... It was not until the 1960s that it was clearly defined as insoluble organic matter in sedimentary rocks. It refers to all organic matter that can produce oil, gas or coal. It is not a substance with specific ingredients, but an organic high molecular polymer. The carbon content of kerogen is 70% to 90%, the hydrogen content is 3% to 11%, and the oxygen content is 3% to 24%. Geochemists divide kerogen into type I, type II and type III according to its elemental composition.

Kerogen Biological Sources and Classification

Kerogen is the most important form of organic carbon on Earth, and the total amount of kerogen in the Earth's crust is about 300 trillion tons. This number may not be intuitive, so the following data is more vivid: the total amount of kerogen is roughly equivalent to 1,000 times the total coal reserves and 16,000 times the total oil reserves in the world!

Kerogen in source rocks decomposes to produce oil and gas

So why study something so wide-ranging? Because kerogen is the precursor to coal, oil, and natural gas. In other words, it can generate coal, oil, and natural gas!

Despite this, before its huge role was discovered, little research was done on it, because it was basically insoluble in organic solvents and was quite complex. Later, people discovered that this thing could generate petroleum, and then they began to study it with great enthusiasm.

From organic matter to kerogen, it is mainly due to the degradation of bacteria, and the further evolution of kerogen into oil is mainly due to temperature. In shallow strata where the ground temperature does not exceed 60°C, the content of oxygen and sulfur elements in kerogen decreases, the amount of liquid hydrocarbons formed is extremely small, and the kerogen forms some extremely complex macromolecular fragments; after the ground temperature exceeds 60°C, the macromolecular fragments formed in the previous stage form smaller fragments due to the further breakage of their chemical bonds, and hydrocarbons begin to be produced in the kerogen, entering the main stage of oil generation; with the increase of burial depth, or due to magma activity and other reasons, when the ground temperature reaches above 120°C, the hydrocarbon groups in the kerogen almost disappear, the liquid hydrocarbons are destroyed, and a large amount of low-molecular methane gas is produced...

It can be said that the formation of oil is a process in which kerogen constantly adjusts and transforms itself to adapt to the environment.

Bumpy transportation road

Now, we already know that oil is generated and stored in underground rocks. So are the rocks that produce oil and store oil the same rock body?

Primary oil and gas are squeezed into the reservoir

This is not the case under normal circumstances. Rocks that produce oil are professionally called source rocks (also called hydrocarbon source rocks), and rocks that store oil are called reservoir rocks. Generally, after oil and gas are produced from source rocks, they need to migrate for a certain period of time and a certain distance before they can reach the reservoir rock layer and accumulate there. The process of oil and gas escaping from source rocks is called primary migration. For most oil and gas reservoirs, the oil and gas accumulations in them do not flow directly from the source rocks. Many of them have undergone secondary, tertiary or even more migrations, from one oil and gas reservoir to another.

During the migration period, some oil and gas can successfully reach the next stronghold and be stored to form new oil and gas reservoirs, while some of them escape and disappear without a trace for various reasons. To form a high-quality oil and gas reservoir, the "hardships" experienced during the process are really beyond the reach of outsiders.

A wonderful combination

The oil and gas reservoir is my "home". To form this complete "home", six conditions are required: first, there must be a master in the home, that is me, so there must be an oil-producing layer for me to exist; second, the home must have hardware facilities that satisfy me. I have come all the way from the oil-producing rock, how can I not be given a good place? That requires a reservoir layer; third, there must be walls and a roof to protect me from wind and rain, that is, a cap layer and a shielding layer are needed to prevent me from escaping; fourth, there must be movement conditions, so that I can enter the "home" after a long journey from the oil-producing layer; fifth, the space at home must be large enough, with enough cracks and holes for me to live in, that is, the enclosure conditions must meet my size, otherwise I will continue to run forward; sixth, the place where I live must be stable. If there are earthquakes every two or three days and geological movements, I can't stay at home honestly. I either run to someone else's house or I will escape directly.

In order to form oil and gas reservoirs and give me a complete home, these six conditions are indispensable. These are what petroleum geologists call the six elements of oil and gas reservoir formation - generation, storage, capping, transportation, enclosure, and protection.

Main conditions for the formation of oil and gas reservoirs

A century-old oil case

In daily life, various media refer to oil as "fossil fuel". This is actually based on the understanding that oil is generated by ancient organisms (including animals and plants, especially plankton). This is the "organic origin" theory of oil, which guides the development of the modern oil industry.

For a long time, there has been a view in the petroleum geology community that petroleum is of inorganic origin. If we count from the famous Russian chemist Mendeleev, the hypothesis of the inorganic origin of oil and gas has been proposed for more than 100 years.

The inorganic origin view of petroleum holds that in the process of petroleum formation, the magma that rises first undergoes a large amount of thermal expansion due to the extremely small pressure it is subjected to in the cracks in the earth's crust, forming a large amount of magma gas, which is composed of gas molecules according to certain components, such as acetylene, water, etc.

The gas that is constantly released from the magma not only increases the pressure and temperature in the cracks, but also increases the density of the hydrocarbon molecules formed in the cracks. Their cohesive force is constantly strengthened, causing the hydrocarbon molecules to tend to form a complex structure. That is, acetylene → ethylene → methane → ethane → propane → butane. When the concentration of hydrocarbon gas in the cracks increases further, low-carbon hydrocarbons will polymerize into high-carbon alkanes, and then a phase change will occur. In other words, gaseous hydrocarbons become liquid hydrocarbons - petroleum. In the early stages of the formation of petroleum, because the particles are extremely small, they can move upward with the heat. They aggregate in large quantities above the cracks and fuse into larger oil droplets. When the dense oil droplets further fuse, their weight will be greater than the thrust generated by the thermal expansion of the magma gas, so they fall or flow along the crack wall to the bottom of the crack and overflow the magma.

Since oil can only be formed when the pressure, temperature and hydrocarbon gas concentration in the cracks reach a very high standard, the gas separated from the magma will encounter very high pressure as soon as it leaves the magma, not only forming a stable structure at the atomic level, but also quickly combining into hydrocarbons. Therefore, part of the magma gas will combine into oil during the floating process, and it will continue to increase, and gradually an oil reservoir may be formed.

First, it has been confirmed that there are a large amount of carbon dioxide-rich fluids deep in the earth. The carbon dioxide emitted by these fluids can react with hydrogen during the rising process to produce hydrocarbons through the famous Fischer-Tropsch reaction (a reaction that uses carbon monoxide and hydrogen as raw materials to produce hydrocarbons under appropriate conditions).

Secondly, there are also a large amount of hydrogen-rich fluids deep in the earth, and the earth is constantly expelling hydrogen. Inorganic hydrogen deposits have been discovered in many places, and the deeper you go into the earth, the higher the hydrogen content. In the middle and lower mantle and the core, there are mainly hydrogen and hydrides, which provides material guarantee for the occurrence of the Fischer-Tropsch reaction.

Thirdly, catalysts that can promote the smooth progress of the Fischer-Tropsch reaction are widely present in many sedimentary basins, making the reaction possible.

The "inorganic generation of petroleum" argument holds that, whether on land or on the seabed, as long as there are geological conditions for the formation of cracks deep in the earth's crust, there may be oil-generating structures there. The generated oil and natural gas can migrate up along the cracks and gather into large oil and gas fields.

Although the organic origin theory is recognized by most geologists and guides most oil and gas exploration practices around the world, with the increasing difficulty of oil exploration worldwide and people's deepening understanding of oil fields, more and more phenomena cannot or are difficult to explain with the theory of organic origin of petroleum. The inorganic origin theory, which has long been out of favor, has regained the attention of some petroleum geologists. The main reasons are as follows:

First, oil was found in some areas that was formed about 1.5 billion years ago. What does this mean? According to traditional petroleum geology and biology theory, the biomass at that time did not seem to be enough to form oil. Why can oil be found in strata without organisms?

Second, why are the world's large and super-large oil and gas fields mostly concentrated? Are those the only places on Earth with life at that time?

Third, why is there a mantle plume deep in the strata of most large oil and gas fields? This is evidence that the oil reservoir is connected to the deep underground.

Fourth, traditional petroleum geology theory holds that it takes at least millions of years to generate oil. However, research on organic matter in the hot springs of Yellowstone National Park in the United States shows that it only takes thousands of years to generate oil! What's more, algae floating in the waters of the Gulf of Mexico can produce liquid oil droplets after being exposed to the sun for several weeks.

Faced with these phenomena that challenge traditional petroleum geological theories, people seem to have reason to believe that: the oil in some oil fields in the world seems to be replenished continuously; some oil and gas may come from deep in the earth's crust; the generation, migration, and accumulation of oil may be related to earthquakes, and earthquakes are precisely the manifestation of crustal movement. Can they "send" oil and gas from deep underground?

In the great debate between the organic and inorganic origins of petroleum, the "organic origin theory" has always held an absolute advantage, because so far, all the large oil and gas fields in the world have been found under the guidance of this theory. Of course, on some specific issues, this theory also has its own shortcomings.

At present, these two theories have not been well integrated. Correctly understanding the source of oil and gas is not only a theoretical issue, but also has practical significance: its determination will bring strategic adjustments to the deployment of oil exploration. Moreover, once the relationship between the accumulation of industrial oil and its inorganic origin is determined, the scope of oil exploration will be greatly expanded, and the global oil reserves will also increase greatly, thereby maintaining a steady growth in the world's crude oil production. This is also the reason why many scientists have devoted a lot of energy to this major academic issue.

Author: Ma Xinfu