Principles of hydrocarbon generation
Thread Content
Oil and gas: Before discussing the formation of oil and gas, it is necessary to first clarify what oil and gas are. The term \"oil\" is fairly familiar to most people, but what exactly is oil? The answer to that question is probably not so clear. To address this question, it can be expressed in one sentence: “Oil is a flammable mineral that forms in underground rocks, is in liquid state, and consists mainly of hydrocarbons.” Another thing that deserves special mention here is natural gas. As a companion to oil, it also consists mainly of hydrocarbons, but it comes to the surface from underground rocks in gaseous form. In a broad sense, natural gas refers to any gas that is produced underground; aside from combustible gases composed of hydrocarbons, any such gas can be considered natural gas. Such as carbon dioxide gas, hydrogen sulfide gas, etc. However, when people talk about natural gas, they usually refer to flammable gases. Oil extracted from underground is commonly referred to as crude oil until it is processed and refined into various products. The main chemical elements in crude oil are carbon, hydrogen, oxygen, nitrogen, and sulfur, among which carbon and hydrogen account for the largest proportions – 84–87% carbon and 12–14% hydrogen. The remaining 1–2% consists of elements such as oxygen, nitrogen, sulfur, phosphorus, and vanadium. Most of these elements occur in the form of compounds. We can divide the numerous compounds found in oil into two main categories: those composed of carbon and hydrogen, namely the compounds commonly known as hydrocarbons, such as alkanes, cycloalkanes, and aromatic hydrocarbons, which are the main components of crude oil. Another category consists of non-hydrocarbon compounds containing oxygen, nitrogen, and sulfur, such as oxygen-containing phenols, aldehydes, etc.; nitrogen-containing pyridines; and sulfur-containing thiols, thiophenes, etc. The most obvious physical property of crude oil is its diverse range of colors, ranging from light to dark – white, pale yellow, brown, dark green, and even black. The oil that we commonly see is usually black. The intensity of the color is related to the amount of non-hydrocarbon substances it contains; the higher the amount, the darker the color. In addition, the physical properties of crude oil measured by instruments include density, viscosity, freezing point, and the various colors of fluorescence it emits under fluorescent lights. These various physical properties are all related to its chemical composition and content. For example, the freezing temperature of crude oil is related to its wax content; the higher the wax content, the higher the freezing temperature, and vice versa. We understand the chemical composition and physical properties of crude oil in order to locate it, modify it, and utilize it. Some crude oils require artificial methods to modify their properties; for example, high-viscosity crude oils that are difficult to flow need to have their viscosity reduced by raising the temperature, so as to make them easier to flow and enable more oil to be extracted. The chemical composition and physical properties of crude oil more directly indicate that it can be widely utilized as a resource; for example, the \"calorific value\" of crude oil shows that it is an important energy source. A wide range of chemical components means a broad spectrum of industrial raw materials. Therefore, the exploration and utilization of oil have become a symbol of human civilized society. Under what conditions is oil formed? Specific geological conditions are required for organic matter in sediments to be preserved. Everyone knows the principle that \"water flows downhill.\" Sediment and organic matter are deposited in low-lying areas carried by water. Therefore, the primary geological requirement is the presence of a low-lying terrain. Such low-lying terrain is referred to as basins, depressions, troughs, etc., depending on its size, and it has been constantly changing throughout various geological periods. If it continues to sink as a result of crustal movements, it can maintain its low-lying topography and continue to receive sediments, thereby increasing the thickness of the strata. If it rises due to crustal movement, the degree of depression gradually decreases, resulting in less sediment being deposited and thus a thinner thickness in the deposited strata. If it rises above the water surface, it loses its low-lying shape; not only does it stop accepting sediment, but it also causes the sediment that was deposited earlier to be weathered and eroded away. It can be seen that basins or depressions that continue to sink are favorable for the accumulation of organic matter. Two factors are mentioned here: one is strata deposition, and the other is basin subsidence. During their formation, both have a factor related to speed: the former is called the \"deposition rate,\" which is related to the availability of sediment sources; the latter is called the \"settling rate,\" which is related to the intensity of tectonic movements. It is most ideal for the two to work together properly. If the deposition rate is lower than the settlement rate, it will cause the depth of the water in the depressions to increase relatively, thereby prolonging the distance that organic matter needs to settle to the bottom. In this way, the sediment is exposed to oxygen in water for a longer period of time, which has a destructive effect on organic matter. If the deposition rate is greater than the settlement rate, the water in the low-lying areas will become shallower, or even dry up and turn into land, exposing the organic matter to the atmosphere and oxygen, thereby causing further damage. Therefore, another geological condition conducive to the preservation of organic matter is that the two rates should be roughly equal, that is, as much settles, as much sediment is replenished. This is known as the “compensatory deposition rate”. Another essential geological condition for oil formation is an oxygen-poor “reducing environment”. This means that the water in the depressions where sediments accumulate needs to remain enclosed or semi-enclosed, or else the sediment-rich areas need to be quickly covered by subsequent sediments, thereby isolating them from oxygen and preventing the oxidation and release of organic matter. Modern oil generation theory also holds that the organic matter in organisms first needs to be converted into a special type of organic substance, known as \"kerogen,\" and it is then from kerogen that oil is formed. This transformation can only occur under certain physicochemical conditions, the main one of which is underground temperature. The temperature range at which kerogen begins to transform into oil is roughly 100–130°C. As the temperature increases from shallow to deeper underground levels, earlier sediments are gradually covered by subsequent sediments, resulting in greater burial depths; organic matter can be converted into oil only when it reaches a certain depth of burial. In addition to temperature, it is also related to the length of time it has been buried; these two factors, temperature and time, can complement each other. In other words, whether the temperature is lower but the burial time is longer, or the temperature is higher but the burial time is shorter, both situations have the same effect on the conversion of kerogen into oil. It is evident that the geological conditions necessary for oil formation are comprehensive; it requires not only conditions that maintain a \"compensatory sedimentation rate\" during the sedimentation process, but also an \"anoxic reducing environment\" for the sediments. Additionally, factors such as appropriate stratigraphic temperature (that is, a certain depth of burial of the strata) are needed in order to enable effective oil formation. The formation of natural gas – the companion of oil. As a companion to oil, natural gas also consists mainly of hydrocarbons, but the conditions under which it forms are more diverse than those for oil. In terms of the formation process, oil needs to be present at certain depths in order to be produced in large quantities, while natural gas can form at various depths. As for the source of these substances, oil is primarily formed from planktonic plants and animals in water, or what is known as humus-type organic matter; whereas for natural gas, in addition to this, higher plants or humus-type organic matter can also play a role. In terms of the causes of formation, there are both organic and inorganic factors, and these diverse conditions for gas formation provide us with a wider range of areas to explore for gas reserves. Based on the conditions under which natural gas is formed, it can be roughly divided into five types: biogas – in modern sedimentary sludge that has not yet solidified into rock, organic matter, under the action of bacteria, can produce natural gas primarily composed of methane, commonly known as marsh gas. Early diagenetic gas – Before the sedimentary organic matter reaches a depth sufficient for oil formation, part of the humic-type organic matter can begin to produce methane gas. Oil-type gas – when organic matter reaches the depth where oil is formed, in addition to the large amount of oil that is produced, natural gas is also generated as a byproduct. As the burial depth increases, the amount of natural gas produced also increases, while the amount of oil produced decreases. Oil production ceases when only dry gas, namely methane gas, is produced. Coal-bed gas – Sedimentary rock layers that contain coal are called coal measures, and coal-bed gas refers to natural gas that is formed in coal measures as a result of time and temperature effects; its main component is also methane. In terms of oil exploration, coalbed methane is not a target for exploration; however, when it comes to finding combustible gases as a source of energy, coalbed methane should not be ignored, as the methods used, the techniques employed, and the geological conditions that give rise to gas reservoirs are largely similar to those in oil and oil-bearing gas exploration. The Ordos gas field discovered in recent years may belong to coal-bed methane. Inorganically derived natural gas – gases produced by igneous rocks or geothermal activity, such as carbon dioxide, methane, hydrogen sulfide, etc. How can we trace the conditions for oil and gas formation? As mentioned earlier, there are many conditions necessary for the formation of oil and gas, and these conditions existed only during past geological eras; they are now part of history. How can people in modern times understand the history of oil and gas formation that dates back millions of years or even further? If historians understand human history by studying historical records and various artifacts, then geologists, in order to trace the conditions under which oil and gas are formed, must examine various information contained in the oil-bearing strata. The oil-bearing strata referred to here are the dark shale layers or carbonate rock layers within the sedimentary rock formations. With modern physical and chemical analysis methods, it is now entirely possible to extract the various information left behind during the formation of oil and gas, to study and understand it, and to continuously explore the conditions under which oil and gas are formed. Years of experience in oil exploration have shown that wherever oil and gas fields exist, they are located right next to the sources of these hydrocarbons. Therefore, studying the formation of oil and gas is not merely about exploring their formation conditions; it is an important task in searching for oil and gas. To trace the conditions for oil and gas formation, five aspects generally need to be considered: First, it is necessary to look at the amount of organic matter in the oil-bearing strata, which is commonly referred to as \"organic matter abundance\". The main task is to determine the content of residual organic carbon, in order to identify whether there are oil-forming layers in a given area and how many such layers exist. These layers are then classified into good, medium, and poor quality based on certain standards for evaluation. Second, it is necessary to consider the quality of the organic matter in the oil-bearing layer, which is commonly referred to as the “organic matter type.” Based on their biological origin, kerogen in source rocks is divided into three main categories. The first type is humus-type organic matter; its biological origin is mainly planktonic plants and animals in water. It belongs to Type I kerogen and is the highest quality organic matter. The second category is humic organic matter, whose biological source is mainly higher plants; it belongs to type III kerogen and is considered organic matter of poor quality. The third category is a mixed type of organic matter that lies between the two, namely Type II kerogen, which is organic matter of better quality. Through the analysis of samples, the laboratory determines the type of organic matter in a region based on certain standards, thereby assessing the quality of that organic matter. Third, it is necessary to determine whether the organic matter has already produced oil and gas; this is commonly referred to as the \"maturity of organic matter.\" When a sufficient amount and quality of organic matter is transformed into oil under certain conditions, it’s as if \"raw rice has been turned into cooked rice.\" In other words, without the necessary conditions to produce oil, organic matter, no matter how abundant, has at most little practical value. Therefore, studying the maturity of organic matter is a very important aspect. By analyzing the information provided by the data, it is possible to indirectly reflect the temperature history experienced by organic matter underground. Based on these data, maturity is classified into immature, oil generation peak, wet gas, and dry gas (dry gas refers to combustible natural gas whose origin is related to petroleum, contains very little heavy hydrocarbons other than ethane, and has a methane content of over 95%). As for biogas and coal gas, their compositions are similar, but their origins are different, which is a separate matter. ) and four other stages. Evaluating the maturity stage of organic matter can tell people whether it is favorable for finding oil or gas in a particular area. Fourth, it is necessary to determine which oil reservoir the produced oil or gas originates from, a process commonly referred to as \"oil source correlation\". That is, after oil and gas are discovered, \"fingerprint compounds\" (also known as biomarker compounds) are used to compare the oil and gas with the source rock, just as one compares the blood types of parents and children, in order to explore the genetic relationship between the source rock and the oil and gas, and to determine which source layer the produced oil and gas originate from. It is easy to imagine that the more oil-bearing strata available to supply oil, the richer the oil source will be, which is more favorable for the formation of oil and gas fields. Fifth, it is necessary to estimate the resource volume. Based on the relevant parameters, the amount of oil and gas generated is estimated, providing a basis for us to make plans and further expand our exploration results.
Minerals that can be transported – the migration of oil and gas. As we all know, ordinary solid minerals such as iron ore or coal can be found where they were formed; oil and gas, however, are different. If the birthplace of oil is called its “hometown,” then the location where oil fields are found is its “final destination.” The characteristic of oil, which leaves its \"hometown\" to settle in its \"destiny place,\" is the migration of oil and gas. The reason for this characteristic is that oil and gas can flow underground. (1) Migration of oil and gas: In a broad sense, any movement of oil and gas within strata can be referred to as the migration of oil and gas. Oil and gas in a dispersed state formed in oil-bearing layers can be concentrated through migration to form oil and gas reservoirs of industrial value; existing oil and gas reservoirs can be disrupted by tectonic movements, causing the concentrated oil to become dispersed again. Some of this oil and gas reaches the surface or becomes observable as \"oil and gas seeps,\" while some can migrate to other locations and concentrate there, forming so-called \"secondary reservoirs.\" In some cases, it even comes completely to the surface and escapes entirely. However, the term oil and gas migration usually refers to the movement from a dispersed state to a concentrated state, including primary migration and secondary migration. Primary migration refers to the movement of the generated oil from the oil-bearing layer to adjacent strata that contain pores, cavities, or fractures; such strata are known as \"carrier layers\". The direction of migration is multi-directional; it can be upward, downward, or in all four directions. This helps to gather the scattered drops of oil together, just as small streams flow into larger rivers. Secondary migration is further divided into early and late stages. In the early stage, oil migrates within the carrier layer; in the later stage, it moves toward its \"destination,\" stopping and accumulating where migration can be halted to form an oil reservoir. The direction of migration is generally directional; that is, the oil in the carrier layer flows toward the higher parts of the formation where there are obstacles to its migration, resulting in the maximum concentration of oil and gas in those areas, just as rivers all flow into the sea. (II) The driving forces, mechanisms, pathways, and timing of oil and gas migration. Oil and gas tend to flow, and this is an inherent property of them; without external forces pushing them, they will not migrate. Such external forces are what drive the migration of oil and gas. The main driving force for primary migration is pressure. This pressure comes from the weight of the sediment itself. As the strata are deposited, they gradually become thicker, and their weight increases accordingly; the pressure generated by this increased weight also grows. Under this pressure, the volume of the sediment decreases; the oil and gas that have been formed are then forced out of the oil-bearing layer together with water, into the carrier layer. Once it is understood that peanut oil is extracted from peanuts using the pressure applied by an oil press, it becomes easy to comprehend the migration of oil in the oil-bearing layer under pressure. The main driving force for secondary migration is buoyancy. Buoyancy comes from the oil and gas themselves; since their density is lower than that of water, they experience buoyant force in water. When oil and gas enter a water-containing carrier layer, they move and accumulate toward higher areas due to this buoyant force. The mode of migration refers to the state of oil and gas during the migration process. Since oil and gas do not dissolve easily in water, aside from a small portion that can migrate in solution within water, most of it moves in the form of \"oil droplets\" and \"bubbles\" within the water-containing carrier layer; therefore, water serves as the \"carrier\" for the migration of oil and gas. A large number of “oil droplets” and “bubbles” move through tiny underground channels. For primary migration, the pathways are mainly the adjacent transport layers. For secondary migration, the pathways are mainly pores, fractures, unconformities, and faults. They take winding paths, overcoming many obstacles as they move forward with difficulty. Therefore, the rate of migration is quite slow. Never imagine them as light boats drifting downstream. As for when the initial migration of oil and gas begins, it depends on how much oil is generated in the oil-bearing layer and the level of pressure it is under. When the oil produced becomes sufficiently rich, or when the pressure applied is enough to \"push\" the oil out, it moves from the oil-bearing layer to the carrier layer. The spaces in which oil flows and is stored underground – oil reservoirs. We often hear terms like “oil lakes” and “oil seas,” which can lead one to think that oil is distributed underground in the same way as lakes and oceans; some people even worry that the oil from the oil fields along our borders might flow into foreign countries. In fact, this is an illusion created by an exaggerated metaphor; the reality is not like that. Oil is stored in the pores, cavities, and cracks of rocks. Any rock that has pores, holes, or cracks through which liquid can flow is called a reservoir. Oil is stored and flows in reservoirs. Professionals primarily use porosity and permeability as two factors to evaluate the quality of reservoirs. A high porosity value indicates that there is more space for storing oil, allowing it to hold a larger amount of petroleum. A high value of permeability indicates good connectivity between pores and fractures, allowing oil to flow easily and facilitating its extraction, thus enabling higher production levels. There are many types of reservoirs, which can be roughly divided into three categories: pore-type reservoirs formed between particles, dissolved cave-type reservoirs, and fractured fracture-type reservoirs. Some of these storage spaces are large enough to be seen with the naked eye, while others are so tiny that they can only be detected under a microscope. The reservoirs discovered in our country are diverse, but they do not go beyond the three types mentioned above. Reservoirs represented by the Daqing Oil Field are of the pore-type in sandstone particles; those represented by the Renqiu Oil Field are of the dissolution-cave and fracture-type in carbonate rocks; while those represented by the Sichuan Gas Field are of the fracture-type in carbonate rocks. There are also some special reservoirs, such as the volcanic rock reservoirs (porous type) found in the Liaohe Oil Field ； The metamorphic rock reservoirs (fracture-type) in the Yumen Ya’erxia Oil Field, as well as the shale reservoirs (fracture-type) in the Qinghai Youquanzi Oil Field, and so on. Oil is protected underground by a barrier layer – the cap rock that covers it. To prevent the oil and gas in the reservoir from escaping, there needs to be a dense, impermeable stratum above the reservoir to safeguard the oil and gas contained within it. This dense, impermeable protective layer is known as a cap rock; rocks suitable for use as cap rocks include shale, mudstone, salt rock, gypsum, and others. Dense marl and limestone can sometimes also serve as cap rocks. The cap rock must have a certain thickness; if it is too thin, it will not be able to withstand the pressure exerted by oil and gas, and it will fail to prevent the escape of oil and gas, thus failing to serve its protective role. The distribution of the cap layer should be stable. That is, the variation in thickness should not be too significant; there should be no areas with a cover layer and areas without one. Otherwise, a “hole” will appear above the reservoir. Oil and gas will escape through the \"leaks\". The cap rock also needs to remain intact despite tectonic movements; if a complete cap rock is fragmented by such movements, it loses its function as a cap rock. Many things in nature are interconnected and interact with one another, often forming ingenious combinations. Take the oil-bearing layers and reservoir layers as an example; they often occur in an interbedded configuration. As a result, the oil and gas generated in the oil-bearing layer, once transported to the reservoir, become a protective layer that safeguards the oil within the reservoir. Ultimately, once an oil reservoir is formed, it naturally becomes the cap rock for that reservoir. Since oil-bearing layers are generally dark-colored shales, they become very dense and impermeable rocks after being compressed by stratigraphic pressure, and they serve as excellent seal layers in most actual oil reservoirs. The underground reservoirs of oil – geological traps. Before explaining what a geological trap is, let’s conduct a simple physical experiment: Place a glass cup filled with water upside down in a basin also containing water. Insert a rubber tube at the bottom of the cup, and then blow gas or inject some oil through this tube into the cup. The oil or gas will rise to the highest point in the cup, pushing the water out of it. This experiment shows that the cup acts as a geological trap, concentrating the oil and gas and preventing it from escaping outward. Beneath the ground, any geological structure that can prevent the migration of oil and gas and concentrate the dispersed \"oil droplets\" and \"bubbles\" is called a geological trap, which is also simply referred to as a trap. It is like an underground oil storage facility that can store oil and gas in bulk. Therefore, a geological trap is the endpoint of oil and gas migration and a site where they accumulate; it is the location of the oil and gas reservoirs, and of course, it is the main target that petroleum workers seek. There are many types of geological traps, but they can be roughly divided into three categories: uplifts formed by tectonic movements, which resemble an inverted pot and are generally referred to as \"anticline structures\" or structural traps; when two sets of sedimentary layers that are not continuous form an intersecting pattern, this is known as \"stratigraphic unconformity\". If an impermeable layer acts as a barrier at the site of such an unconformity, it is called a stratigraphic trap or unconformity trap ； When a reservoir is surrounded on all sides by impermeable formations that act as a barrier, it is called a lithological trap. After oil and gas enter the trap, due to the different densities of oil, gas, and water, they further separate into three layers within the trap. The lightest gas is at the top, the heavier oil is in the middle, and the heaviest water is at the bottom, forming a complete oil and gas reservoir. A large oil and gas reservoir can form an oil and gas field on its own, or multiple reservoirs can combine to form an oil field. Thus, from its formation and migration to its accumulation and differentiation within traps, the entire process of oil field formation is completed. The Daqing and Renqiu oil fields in our country are typical structural traps, the Shengli Oil Field is a structural trap that has been complicated by faults, while the Karamay Oil Field is primarily a stratigraphic trap. Where can oil be found in this vast land? China has a land area of 9.6 million square kilometers, and within such a huge expanse, oil and gas fields account for only an extremely small fraction. Finding an oil or gas field by drilling is like looking for a needle in a haystack. One can’t help but wonder where oil can be found in this vast world. We already know how oil fields are formed, including how they are distributed. Petroleum geologists have established regular patterns of understanding and use scientific insight to explore unknown areas; by following the approaches outlined below, they can gradually discover new oil and gas fields step by step. Searching for basins → oil-generating depressions → oil and gas accumulation areas → traps → exploration wells → oil fields. In the early days of oil exploration, efforts started with observing oil or gas that came to the surface (referred to as “oil and gas signs”). Explorers paid close attention in the field to looking for and investigating whether there was oil or bubbling springs in the area under investigation; this was the most straightforward method of finding oil, and it has been the same throughout history and across different countries. China’s Karamay Oil Field attracted attention due to the presence of the “Black Oil Mountain” in its vicinity; it was discovered after drilling operations were carried out there ； The Dushanzi Oil Field is famous for its \"mud volcanoes\" formed by the long-term outflow of oil- and gas-bearing mud ； Next to the Yumen Oil Field is the “Oil Gully”” ； There are several oil seeps within the area of the Yanchang oil field; Zigong in Sichuan, which was the first place to make use of gas wells, also has many such oil seeps that can be ignited, as recorded in ancient texts. In Qinghai, there are some place names related to “oil” such as “Yousha Mountain” and “Youquanzi”; these names were given by modern petroleum explorers based on the oil seeps they found there. How is it possible to find oil and gas fields in areas where there is no visible oil or gas on the surface? For example, since the late 1950s, oil fields such as Daqing, Jilin, Liaohe, Dagang, Huabei, Shengli, as well as those in the Central Plains and Henan provinces in China have been discovered beneath grasslands, beaches, farmlands, and swamps. How did the explorers know where to start? This was made possible by the guidance of scientific theories and the use of advanced technical methods. In the early 1950s, the renowned geologist Li Siguang used his theory of geological mechanics to guide the exploration for various mineral resources. Regarding the locations where oil could be found, he believed that the plain areas in eastern China contained a series of nearly north-south oriented hidden basins (which he referred to as the “second sedimentation zone”), which represented promising sites for oil exploration. From the Songliao Basin in the northeast to the areas along the Bohai Sea in Beijing, Tianjin, Hebei, and Shandong, all of these regions should be subject to exploration efforts... Not long after the discovery of the Karamay oil field, and while efforts were being made to develop it, the oil exploration teams had already prepared for a \"strategic shift eastward\"; by 1958 they had moved on to the Songliao and North China regions. Extensive seismic surveys identified the outlines of the basins hidden beneath the plains, providing a starting point for drilling operations. So, overall, the sequence for searching for oil is roughly as follows: First, one must identify where the basins are located, because only sedimentary basins have thick layers of sediment and abundant organic matter that can generate oil. 373 such small basins have been discovered on land and at sea in our country, representing vast areas for us to search for oil. Second, oil-generating traps need to be sought in the basin, as the oil-bearing strata are most developed in these traps and they serve as the source of oil and gas. Third, the main oil and gas accumulation areas should be sought near the oil-generating depressions, as this area is closest to the source of oil, making it an ideal location. Fourth, it is necessary to search for traps in the main oil and gas accumulation areas, as these are the locations where oil and gas reservoirs may exist. Fifth, the first oil exploration well is drilled at the highest point of the trap; it is called a pre-exploration well, and its purpose is to determine whether oil or gas is present. Sixth, after oil and gas are discovered in exploratory wells, the drilling area must be expanded by drilling additional detailed exploration wells, with the aim of determining the boundaries of the oil and gas reservoirs, that is, to determine the size of the oil field. Once the scope of the oil field is determined, production wells (also known as development wells) must be installed at regular intervals in order to put the oil field into production and development. That’s how an oil field is located: by starting with a broad approach and then narrowing down the scope step by step, until it is finally found. Due to the complexity of underground conditions, coupled with people’s limited understanding, it is not possible to be 100% certain of success. A success rate of over 30% for exploratory wells can be considered quite good. Readers may ask: How are basins, depressions, oil and gas accumulation zones, and traps identified? And how is oil extracted? The following sections will answer these questions.
I’ve learned it.* Could you talk a bit about natural gas processing? I wonder if the original poster is aware of condensate oil and its formation mechanism.