Introduction of oil and gas field exploration methods?

At present, there are four methods to explore oil and gas fields: geological method, geophysical exploration method, geochemical method and drilling method.

I. Geological law

Geological method is a basic working method in oil and gas field exploration, which mainly includes observing and studying paleostrata, rocks and oil and gas displays exposed on the ground, obtaining relevant geological data, analyzing and interpreting them, judging whether an area has geological conditions for generating and storing oil and gas, evaluating the underground oil and gas prospect in this area and determining favorable oil and gas-bearing areas. In areas where rocks are exposed, this method may directly find underground oil and gas reservoirs. This method also includes geological logging and laboratory analysis using drilling data such as underground cores and cuttings, and geological interpretation of a large number of indirect data provided by geochemical and geophysical methods.

Geological methods not only study basic geological problems such as underground rocks, strata, geological structures and the history of the earth's development, but also focus on the formation conditions of oil and gas reservoirs in underground areas and parts, such as oil generation conditions, oil storage conditions, migration conditions, traps and preservation conditions, so as to determine whether oil and gas reservoirs exist and evaluate oil and gas prospects.

Second, geophysical exploration methods

Geophysical exploration method is an indirect method to find oil and natural gas based on the principles of geology and physics and using new technologies in the fields of electronics and information theory. It uses various physical instruments to observe various physical phenomena on the earth's crust surface on the ground or in the air, infers the characteristics of underground geological structures according to the changes of physical phenomena, and looks for possible oil and gas storage structures.

Geophysical exploration methods are mainly used in modern sedimentary coverage areas and sea and lake areas. There are no strata and rocks exposed in these areas, and the geological methods are greatly limited. It is not only costly but also inefficient to understand the underground geological conditions by drilling a large number of cores.

Geophysical exploration methods mainly include gravity exploration, electrical exploration, magnetic exploration and seismic exploration. At present, the most widely used and effective method is seismic exploration.

Seismic exploration method

When explosives are placed underground or in shallow water, the impact force generated by the explosion of explosives will produce great vibration. Under the action of pressure, underground rocks will compress and expand, thus generating the vibration of rock particles and forming seismic waves. When local seismic waves meet the interface of rocks with different densities, there will be three phenomena: the first is that some seismic waves are reflected from the interface, and the reflected waves are called reflected waves. The second is that some seismic waves propagate downward through the interface, which is called transmitted wave; When the transmitted wave meets the interface again, it will be reflected. The third is that some seismic waves pass through the interface, slide along the rock interface for a period of time and then refract back. Refracted waves are called refracted waves. According to the types of waves received and studied, seismic exploration can be divided into reflection method and refraction method. At present, the reflection method is the most widely used.

The propagation speed of seismic wave is related to rock properties. Generally, the seismic wave propagation speed of dense and hard rocks is fast, but that of loose rocks is slow (see Table 3- 1).

Table 3- 1 Relationship between seismic wave velocity and lithology

According to the seismic wave propagation time recorded by seismograph and the seismic wave velocity data obtained by other methods in this area, the interface depth of underground layers can be calculated, so as to understand the ups and downs of underground strata and find favorable traps (see Figure 3- 1 and Figure 3-2).

Figure 3- 1 Schematic diagram of offshore seismic exploration principle

(2) Gravity exploration

According to the density difference of different underground rocks, the method of measuring the gravity change caused by the ground density difference and then inferring the underground geological structure and mineral deposits is called gravity exploration. Gravity exploration obtains gravity exploration data through field gravity measurement and indoor data collation. Data interpretation can be used to study the deep strata of the crust and regional geological conditions, and local geological structures can be studied in areas with conditions.

The gravity variation value obtained by gravity measurement is very small, which is called gravity anomaly, accounting for only one millionth of the total gravity value, so the measurement accuracy is very high. Due to the permanent deformation of the spring in the gravimeter, the zero change of the instrument is inevitable. Therefore, repeated observation and various corrections should be made in the field gravity survey to eliminate the gravity variation caused by non-geological factors.

The corrected gravity anomaly value mainly reflects the change of surface gravity caused by the change of underground rock density, and can be drawn into gravity anomaly plan and profile for analyzing and judging underground geological structure and mineral resources.

Figure 3-2 Schematic diagram of onshore seismic exploration principle

X- seismic line; T- seismic wave propagation time

(3) electrical prospecting

The conductivity, permeability and dielectric constant of different underground rocks are different. The method of measuring the electric field changes caused by these differences on the ground and then inferring the underground geological structure and mineral deposits is called electrical exploration. According to the causes of electric field, electrical exploration can be divided into natural field method and artificial field method. Natural field method includes magnetotelluric method and audio frequency electromagnetic method, and artificial field method includes resistivity method, artificial electromagnetic method and excitation activation method.

Electrical prospecting is the most widely used in the field of metal exploration, followed by engineering geology and hydrogeological exploration. Petroleum exploration mainly uses resistivity method, magnetotelluric method and artificial electromagnetic method to measure the interface depth of underground strata, which can study regional geological conditions and local geological structures.

(4) magnetic prospecting

There are magnetic differences between different underground rocks. The method of measuring the change of ground magnetic field (magnetic anomaly) caused by underground magnetic difference on the ground and then inferring underground geological structure and mineral deposits is called magnetic exploration. Magnetic exploration can study structural units, basement structures and sedimentary caprocks. This method can be carried out on the ground and in the air, which are called ground magnetic survey and airborne magnetic survey respectively.

Magnetic anomalies are observed by magnetometers. Magnetometers are divided into vertical magnetometers and horizontal magnetometers. There are two kinds of measurement methods: relative measurement and absolute measurement. Absolute measurement is mainly used for normal magnetic field measurement, and relative measurement is mainly used for oil and gas exploration.

The interpretation method of magnetic anomalies includes three aspects: first, the study of positive problems, that is, knowing the shape of underground geological bodies, analyzing the characteristics of magnetic anomalies formed on the ground, finding out the relationship between magnetic anomalies and the occurrence of underground geological bodies, and guiding the geological interpretation of magnetic anomalies. The second is to deal with the measured magnetic anomalies, eliminate the interference magnetic anomalies and highlight the magnetic anomalies caused by underground geological factors. The third is the inverse problem study, that is, the geological analysis of the measured magnetic anomalies is carried out to find out the corresponding underground geological characteristics and minerals.

Three. Geochemical method

Geochemical method is to use chemical analysis method to analyze various components in rocks, soil, gas and water samples, determine various chemical, physical and biochemical changes caused by underground oil and gas diffusion, and analyze the existence and distribution of underground oil and gas. Geochemical methods, also known as geochemical exploration methods, mainly include gas logging method, asphalt method, hydrochemical method, bacterial method and other specific methods.

(1) gas measurement method

Gas measurement is to determine the content of hydrocarbon gas in soil, surface rocks or water with a sensitive gas analyzer. The principle is that when underground oil and gas reservoirs exist, oil and gas will spread to the surface, resulting in abnormal gas on the upper surface, and the hydrocarbon gas content is higher than that in other areas.

At present, gas logging's method is still in the development stage, and its theory or practice is not perfect, and its effect is not ideal. The gas logging method of geophysical logging is an effective method to judge oil and gas reservoirs in drilling.

(2) Asphalt method

The asphalt method includes the determination of luminescent asphalt and chloroform asphalt "A". When the anomalies such as "A" of luminescent asphalt and chloroform asphalt are measured on the ground and underground by various methods, it shows that there are processes of oil and gas generation, migration, diffusion and oxidation in this area, which can be used to evaluate the oil and gas prospect in this area and this layer.

(3) hydrochemical methods

Hydrochemical methods mainly study the types and distribution of salts, trace elements and water on the surface of water, so as to judge the possibility of oil and gas.

(4) Bacteriological methods

Bacterial method is an indirect geochemical method. Some hydrocarbons (such as methane, ethane and propane) migrate from underground to the surface, forming a relatively rich area above the reservoir, while some bacteria have a special hobby for some hydrocarbons and often breed in these areas. Bacterial culture by sampling can reflect hydrocarbon abnormal areas and serve as an important index for finding oil and gas reservoirs and evaluating oil and gas prospects.

Fourth, the drilling method

Drilling is an indispensable means in oil and gas field exploration. Geological methods, geophysical exploration methods and geochemical methods are all indirect methods to determine favorable oil-bearing structures or reservoirs underground. Only by drilling can we finally determine whether oil and gas reservoirs exist and whether there is industrial oil and gas flow. But compared with other methods, drilling is the slowest and the most expensive. It must be carried out on the basis of comprehensive exploration by geological, geophysical and geochemical methods.