Hydrocarbons refer to hydrocarbons and associated or biogenic organic substances, the main components of which are carbon and hydrogen, and the secondary components are sulfur, nitrogen and oxygen.
The mineral types of hydrocarbon substances mainly include natural gas, oil, coal and oil shale.
1. Composition of natural gas
In practical research and application, natural gas refers to natural gas in gas reservoirs dominated by hydrocarbons in sedimentary rocks and industrially available carbon dioxide gas, hydrogen sulfide gas and nitrogen gas.
There are two kinds of natural gas in gas reservoirs: one is hydrocarbon-based natural gas, mainly methane gas, and the methane content is generally above 80%; Followed by heavy hydrocarbons, accounting for about10%; Trace gases include nitrogen, carbon dioxide gas and hydrogen sulfide gas. The other natural gas is mainly non-hydrocarbon gas, mainly including nitrogen, carbon dioxide gas and hydrogen sulfide gas.
According to Yakutia Qinni (1976), hydrocarbon gas reservoirs account for more than 95% of the global total gas reservoirs, while non-hydrocarbon gas reservoirs account for less than 5% of the global total gas reservoirs. Figure 1-2 is a triangular change chart constructed from about 15000 analysis data of 2000 gas reservoirs around the world, which effectively explains the distribution of main components of natural gas reservoirs.
2. Composition of petroleum
Petroleum is a kind of hydrocarbon with different structures, which exists in underground rock pores in liquid form and is a combustible organic mineral.
The elemental composition of petroleum is mainly carbon and hydrogen, followed by sulfur, nitrogen and oxygen. Different scholars have different estimates of the average elemental composition. Hunter's statistics show that carbon accounts for 84.5% by mass, hydrogen accounts for 13.0%, sulfur accounts for10.5%, nitrogen accounts for 0.5%, and oxygen accounts for 0.5%. According to Pan Zhongxiang's (1986) statistics, the contents of petroleum elements in China, the United States and the Soviet Union are listed in Figure 1-3, which is similar to Hunter's statistical results.
Petroleum compounds can be divided into hydrocarbons and non-hydrocarbons. The former includes n-alkanes, isoparaffins, cycloalkanes, aromatic hydrocarbons and cycloalkyl aromatic hydrocarbons, while the latter mainly includes nitrogen, sulfur, oxygen compounds and organometallic compounds.
Figure 1-2 Composition Diagram of World Gas Reservoir
The carbon number of n-alkanes is C 1~C60 ~ C60. According to the position and form of the carbon number of the main peak, the curve of n-alkanes (Figure 1-4) can be divided into three basic types: ① the main peak is less than C 15, and the main peak area is narrow; ② The main peak is larger than C25, and its area is wider; ③ The main peak area is between C 15 and C25, and the main peak area is wide.
The carbon number of isoparaffins is mainly less than C 10, and cycloalkanes are mainly low molecular weight cycloalkanes less than C 10. The basic types of aromatic hydrocarbons and cycloalkyl aromatic hydrocarbons are benzene, naphthalene and phenanthrene, and their molecular weights are generally large.
Figure 1-3 Composition of petroleum elements (according to Pan Zhongxiang, 1986)
Fig. 1-4 n-alkane distribution curves of different types of petroleum (according to Martin L., 1986).
Non-hydrocarbon substances in petroleum include compounds of nitrogen, sulfur and oxygen, mainly sulfuric acid, thioether, thiophene and disulfide. Low molecular weight exists in light and middle fractions of petroleum, and high molecular weight exists in colloid and asphaltene; The second is organometallic compounds, mainly mercury, lead and other compounds.
3. Composition of coal
Coal is a solid combustible organic mineral formed by underground metamorphism of plant remains in geological period through complex biological, physical and chemical actions.
The main constituent elements of coal are carbon and hydrogen, followed by oxygen, nitrogen, sulfur, phosphorus and other elements. Carbon and hydrogen account for more than 70% of the mass of organic combustible materials. The volatile matter varies with the degree of coal metamorphism, generally ranging from 5% to 55% (Figure 1-5).
Carbon and hydrogen in coal are mostly composed of cementitious components (including wood coal, vitrinite, structural vitrinite, unstructured vitrinite and cementitious matrix), silk carbonization components (including silk charcoal, vitrinite silk charcoal, vitrinite silk charcoal and silk carbonization matrix), stable components (including wood column layer, cuticle, spores and pollen, resin body) and so on. , in solid form. Inorganic substances in coal mainly include terrigenous minerals, chemical or (and) biological minerals (such as pyrite nodules, clay, siderite and so on). ) and organic matter, epigenetic minerals (such as pyrite, calcite, kaolin, etc.). ), they also exist in solid form.
Figure 1-5 Main composition of coal (according to Lu Chunyuan, 1987)
Carboniferous shale is also an important hydrocarbon organic matter. Similar to coal, carbon exists in a solid state.
Generally speaking, coal is a solid hydrocarbon with large molecular weight; Petroleum is a liquid hydrocarbon with medium molecular weight.
Natural gas is mainly gaseous hydrocarbons with small molecular weight. The formation process of these three hydrocarbons and their relationship are discussed below.
Second, the formation of hydrocarbons.
The formation process of natural gas, graphite and coal and their relationship are shown in Figure 1-6. The main source of hydrocarbons is biological organic matter existing on the surface. Biological organic matter is mainly the remains of animals and plants. Due to the biological action on the surface and near the surface, especially the active bacterial activities, a large part of animal and plant remains form biogenic natural gas through biodegradation and other processes, and a small part is deposited and buried underground.
Figure 1-6 Schematic diagram of the formation process of natural gas, graphite and coal
At a certain depth underground, biological function is weakened. When entering a greater depth, the pressure increases, the ground temperature rises, and the hot pressing effect is enhanced. After thermal catalysis and pyrolysis, the deposited organic matter began to mature. ① Humic organic matter with high abundance is transformed into coal, which forms peat, lignite, Changyang coal, gas coal, fat coal, coking coal, lean coal, lean coal and anthracite in turn, forming a large number of coal-series natural gas with methane as the main component. (2) sapropelic organic matter and some humic organic matter enter the threshold temperature of oil generation, forming petroleum and liquefied petroleum gas. If the organic matter enters the buried depth of the stratum with higher temperature and pressure, the formed coal, oil and natural gas will be converted into graphite. The formation of oil and coal is accompanied by the formation of organic hydrocarbon gas. The causes are mainly microbial action in the early stage and thermodynamic action in the later stage.
Three, the occurrence of hydrocarbon substances
Gaseous natural gas, liquid petroleum and solid coal have great differences in underground occurrence forms, especially for industrial applications. This difference often determines different exploration methods.
1. Occurrence state of natural gas
See table 1- 1 for the occurrence state of natural gas. At present, there are gas cap gas, gas reservoir gas, condensate gas and coalbed methane with wide industrial significance.
Table 1- 1 natural gas occurrence state table
There are gas cap gas and oil at the top of the oil and gas reservoir, and the content of heavy hydrocarbons above ethane is high, which belongs to oil and natural gas in genesis. Gas reservoir gas is a single natural gas, which can be liquefied petroleum gas, coal-series natural gas or other genetic types of natural gas. Condensate gas is a special gas reservoir gas, which is formed by evaporation of liquid hydrocarbons at high temperature and high pressure. Once the temperature and pressure are reduced, it will be reversely condensed to form light oil.
Coalbed methane is coal-series natural gas adsorbed in coal seam.
2. Occurrence state of oil
Oil mainly exists in liquid state in rock pores. According to whether migration occurs, it can be divided into two forms: self-generation and self-storage. The oil-generating stratum is the oil-storing stratum. The other is supergene upper reservoir or supergene lower reservoir type, that is, the source rock is not an oil reservoir, but the oil migrates and is stored in rocks or structures with large effective porosity.
3. Occurrence state of coal
Coal exists in coal measures strata in the form of natural solids. There is no destruction of special tectonic movement and no obvious coal seam migration.
Four. Migration of hydrocarbons
Compared with the parent rock strata, coal exists in "in situ" as a solid phase. Liquid oil may exist "in situ" or "in different places" after migration. The coalbed methane in natural gas may mainly exist in situ, while other natural gas mainly exists in different places. Therefore, among hydrocarbons, oil and natural gas migrate.
Petroleum migration can be divided into primary migration, secondary migration and secondary migration.
Primary migration refers to the process in which oil is discharged from fine-grained source rocks. The occurrence of primary migration may be controlled by factors such as compaction, thermal action and clay dehydration. The source rocks (mainly mudstone) are under great stress, with strong deformation and reduced volume, which will inevitably drive the flow of plastic materials (including gas, liquid and low hardness clay minerals, etc.). ). In the primary migration, oil may migrate in various phases, such as oil phase, emulsified solution, colloidal solution and gas solution, but most scholars believe that oil phase is the main phase in the primary migration. Primary migration is a process closely related to oil generation, and the migration distance is short, which is generally limited to source rock series.
Secondary migration is the continuation of primary migration and the process of oil migration from source rock to reservoir rock. The main conductive layers are coarse sandstone layers, extensional faults and unconformities. The driving forces of secondary migration are mainly buoyancy and hydrodynamic force, and migration is mostly carried out in free phase. The distance of migration ranges from tens of kilometers to hundreds of kilometers. In most cases, the result of secondary migration makes oil accumulate into oil reservoirs.
The second migration is based on the second migration and is similar to the second migration in essence.
In some areas, oil may migrate only once, while in other areas, oil may migrate twice or even again due to multi-stage tectonic activities.
According to the migration direction, oil migration can be divided into horizontal migration and vertical migration, or bedding migration or cross-layer migration. The direction of migration depends on the driving force, which may be different in different regions, but the principle followed by * * * is to migrate in the direction of least resistance.
Compared with oil migration, coal can be considered as non-migration, while natural gas has stronger migration intensity and farther migration distance.