Due to the extremely complex geological and structural conditions in the deep part of the basin, deep exploration is still a worldwide problem. In order to break through the situation of deep exploration in Shengli oil province as soon as possible, the foreign deep oil and gas exploration methods are investigated, and new theories and technologies from abroad are provided and introduced. Especially, according to the practice of deep exploration in Shengli oil province, the CDA technology, comprehensive exploration technology and gravity and magnetic exploration method of the former Soviet Union are introduced, which has great and practical reference significance for future deep exploration.
Keywords: deep exploration methods, gravity and magnetic exploration, comprehensive exploration, CDA technology exploration examples.
I. Introduction
In recent ten years, deep oil and gas exploration has been paid more and more attention by countries all over the world. Because deep exploration is a complex and huge system engineering, involving geological research, exploration technology, drilling and post-drilling engineering. For deep exploration technology, seismic exploration is still the main exploration method. However, because the geological conditions of deep exploration are much more complicated than those of middle and shallow layers, countries with good deep exploration results in the world make full use of various exploration methods for comprehensive exploration. Therefore, how to combine gravity, magnetism, electricity and geochemical exploration with seismic exploration is a problem that needs in-depth research and experimental exploration. This paper mainly introduces the deep exploration techniques and methods currently adopted by major deep exploration countries in the world and some successful exploration examples, and carries out information research on deep exploration abroad in order to provide reference and valuable information for Shengli oil region to break through the obstacles of deep exploration as soon as possible.
Second, seismic exploration technology
1. Deep comprehensive seismic exploration
The main factors that affect the quality of seismic data in an area are: wave impedance analysis of main underground target layers, the problem of seismic downward transmission energy, static correction, full-range and interlayer multiples, reflected signal-to-noise ratio and resolution, etc. On this basis, the quality of deep seismic data can be effectively improved by improving the accuracy of field acquisition and indoor data processing methods.
In the acquisition and processing of deep seismic data, the former Soviet Union's "Common Depth Area Stacking" (CDA for short) has obvious effect on improving the resolution of seismic data. This technology can simulate the record of 24 times covering in the field and process the profile of up to 360 times covering indoors. Its basic idea is to superimpose all the information reflecting the depth points of a certain area on the ground in the same phase, so as to improve the signal-to-noise ratio and broaden the frequency band to improve the resolution. Figure 1 is an example of the West Huste-Baler Sike oilfield. The profile is only 100 ms in the longitudinal direction. Figure 1a is a horizontal superimposed profile at 24, and the frequency band width is 12 ~ 65hz. The mudstone caprock is in a white trough, and the underlying oil layer is not reflected. Figure 1b is the result of CDA technology simulating the same road section 180 times coverage. There is oil layer reflection under mudstone caprock (oil layer thickness is 5ms), and its lower frequency band is widened to 15 ~ 125 Hz, and the main frequency is 100Hz[ 1].
Figure 1 Example Diagram of Russian CDA Technology in Oilfield
Take the sandy layer of Dryopteris chrysoptera in Louisiana, North America as an example. The exploration target layer is Cib jeff sand layer with a thickness of about 15m. The spontaneous potential and apparent resistivity curves show that the sand layer is sandwiched between thick shale layers, and the depth is 4069 ~ 4084 meters. Three-dimensional acquisition, processing and interpretation were successfully carried out in this area by using vibroseis. These data are used to image and map the deep thin sand-pressed layer, and the data with high vertical and horizontal resolution are used to explain the reservoir structure that can not be explained by conventional data, and finally satisfactory results are obtained [2].
2. Refracted wave multiple coverage seismic exploration method
Refracted wave method records both refracted wave and reflected wave. It not only picks up the first break of the refracted wave, but also tracks the refracted wave by using the first break, and uses the refraction interface to identify the horizon that produces the reflected multiple waves. This method is often used in research areas with deep buried target layer, complex structure, unfavorable surface conditions and small observation area.
Besides GRM method and delay time method (or time term), the third method includes ray tracing and recursive velocity model. This method is really effective for two-dimensional complex data volume, and can be further applied to three-dimensional deep refraction data volume. Three-dimensional ray tracing is the best method to image the depth and velocity of refractor on the observation time profile. GRM method and delay time method can also be combined to image formation. The newly developed reflection parameter processing system can use reflection and scattering energy, which is helpful to image deep reflection and substrate reflection [3].
3. Three-dimensional exploration method-time gradient method
In the former Soviet Union, time gradient method, a rapid three-dimensional exploration method for deep structures in sedimentary basins, has been widely developed. This method is flexible, and recorders and seismic sources can be arranged at will, which makes the exploration work convenient and economical.
Using portable turtle seismograph to complete time gradient exploration can be automatically recorded on magnetic tape. The frequency characteristic of the whole "Turtle" seismograph (when the amplitude frequency is 0.9) is 2.5 ~ 14 Hz. At the same time, the earthquake was recorded at 12. Under the observation condition with an average distance of 6km, the research area of 1000km2 can be covered by moving seismograph twice [4].
Fig. 2 is a structural map made in the coastal area of the Black Sea according to the seismic standard layer. The standard layer corresponds to the top surface of the basement (VR = 6.2 ~ 6.5 km/s). On the structural map, small but large-scale bulges and depressions with obvious nearly north-south strike are divided, and a nearly east-west strike fault cutting the basement and the whole sedimentary cover is divided, which separates the deep structure of Gorheitsky basin from the uplift block on the south slope of the Great Caucasus [4].
Fig. 2 Structure diagram of the basement top surface of the time gradient method test area near the Black Sea.
Third, electrical prospecting.
1. Differential calibration method (differential normalization method, differential electric field method)
The differential calibration method of instantaneous electric field with active adjustable frequency (днм for short) has achieved some successful examples in Irkutsk exploration area with complex geological structure in the former Soviet Union and Caspian basin with deep target layer.
The functional feature of this method is that three different orders of P(t) parameters can be selected according to the different electrical characteristics of underground media, that is, P 1(t) means that the P 1(t) function can be used when the oil and gas reservoir as the exploration target is in high resistivity media and the total conductivity of the media profile does not exceed 100S (Siemens). P2(t) is a kind of low resistivity medium, and its overburden is several kilometers thick, so it will be more beneficial to use P2(t) function to find and delineate oil and gas reservoirs. P3(t) means that the P3(t) function can be used to find and delineate oil and gas reservoirs under the condition that the medium is covered by both high-resistivity strata and low-resistivity strata [5].
The differential calibration method has the following advantages: the error of observation parameters is small, which improves the reliability of data; The lateral resolution is high, which can eliminate the interference of vertical and horizontal anomalies; The sensitivity of detecting polarization anomalies is high and the longitudinal resolution is good. It has a more sensitive and reliable direct oil and gas exploration function [5].
Chaikins reservoir is located in orenburg area in the north of Caspian Basin, with a depth of over 4,000m. The overlying medium is thick mudstone with low resistivity (ρ = 2ω m, h = 3000m) and thick rock salt (ρ > 1000ω m, h = 2000m). In this area, the P3(t) parameter of differential calibration method is used to delineate the reservoir, and a successful example is obtained. According to seismic data, a series of complex structures were found in the depth range of 4000 ~ 5000 m, which can be divided into three types according to the shape of P3(t) curve: ① negative gradient type, which is the characteristic of deep oil-gas-free layer; (2) positive gradient, which is the feature above the oil and gas reservoir; ③ The twisted type is characterized by vertical anomalies under salt, such as faults with a depth of 4800 ~ 5200m under salt dome and small faults with a depth of 4460 ~ 4480m under salt, which have been confirmed by seismic exploration and drilling [5].
2. Magnetotelluric sounding method
As an important supplementary means of seismic exploration, magnetotelluric sounding, especially magnetotelluric sounding based on multiple coverage of regional or wide lines, has great potential in solving deep and crystalline basement and improving vertical and horizontal resolution. In the 1980s, this method was used to divide the Carboniferous carbonate reservoirs in the North Caspian Basin, which are 5 kilometers deep and only a few meters thick.
Take magnetotelluric sounding exploration [6] in the sedimentary basin of South Ontario as an example. The stratigraphic sequence of the basin is composed of carbonate rocks and shale sequences, with a small amount of evaporated salt rocks and sandstone. Devonian and Silurian moved to the northeast edge and gradually disappeared, and Ordovician basically constituted a single stratigraphic section. A group of controlled source magnetotelluric sounding data in this basin are interpreted, and the results are compared with the known geological profile. The results show that the derived electrical model is well compared with the known geological profile. Determine the location of the sounding site in order to take advantage of the inclined sedimentary layer. The final model is obtained by interpreting data from shallow to deep in the basin. This interpretation greatly reduces the inherent ambiguity in the multi-layer interpretation of single-position sounding data.
3. Transient electromagnetic sounding
Transient electromagnetic sounding (TEM) is developed on the basis of magnetotelluric sounding, and its functions in exploration accuracy, resolution, anti-interference and prediction of lithologic exploration depth have been greatly improved. Its characteristics are: the vertical resolution is obviously better than other electrical methods (it can be distinguished as long as the electrical conductivity jump of deep strata is greater than 10%), the static distortion is small, and it is less affected by the surface roughness, so it is not necessary to carry out static correction, and it is suitable for areas where the surface static correction is difficult, such as volcanic rock coverage area, carbonate rock exposure area and loess source area. The lateral influence is small, which is beneficial to detect the location of faults and find out the oil-water boundary in reservoirs related to faults; It is suitable for finding the oil-water interface in the reservoir where the high resistivity profile is located; Suitable for detecting low resistivity rock series in high resistivity profile or high resistivity basement in deep basin covered by good conductor deposition; Because the recording instrument is light, it is suitable for flexible layout and construction in complex terrain areas. This method has been included in the necessary data of drilling demonstration in some important exploration areas in Russia.
4. Electromagnetic array profiling method
Electromagnetic array profiling (EMAP) is a resistance-depth profiling method, which is based on the electromagnetic response measured by linear survey lines on the ground. This method adopts spatial arrangement data acquisition and processing technology, which can effectively deal with complex three-dimensional underground structure display. Most EMAP signal acquisition and processing techniques are the same as those of conventional magnetotelluric method, but its advantages mainly lie in intensive data sampling and effective processing of unfavorable three-dimensional structural effects, which can make reliable estimation of resistivity profile.
Due to the improvement of the field acquisition system, that is, the time domain acquisition, processing and interpretation methods of simulated earthquakes, the accuracy has been greatly improved. Due to the dense collection points, the static displacement of the surface layer is overcome, and the electromagnetic method itself has the ability to penetrate the high-resistivity layer, which can clearly distinguish the low-resistivity layer below 3 ~ 5 km from the thickness within1000 mm. Due to the improvement of resolution, it has been used to explore areas where seismic methods are difficult, such as finding the internal structure of limestone and tracking oil and gas layers under igneous rocks. Study on petroleum geological characteristics of foreign basins similar to Bohai Bay Basin in China. China petroleum and natural gas group company information research institute.
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Four, gravity and magnetic exploration
Deep faults usually present strong magnetic anomaly zones and high gravity anomaly zones. Therefore, aeromagnetic and gravity data should be fully used in the study of fault development exploration areas and deep central fault block structures.
In gravity inversion, gravity "characteristic point" method and total normalized gradient method are used to solve density profile. This method has been used to distinguish horizontal density inhomogeneity or reveal vertical deep faults. The method is to use gravity observation data for inversion calculation to obtain density profile, and then superimpose seismic and electrical data to further divide strata and distinguish possible lithology, and on this basis, establish density geological model. Taking this as the initial model, the gravity value of the model is calculated by forward method, so that the forward gravity value is fitted with the observed gravity value, and the error is within the required range. Study on petroleum geological characteristics of foreign basins similar to Bohai Bay Basin in China. China petroleum and natural gas group company information research institute.
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In Russia, a giant reef with a depth of 6 kilometers and a thickness of 2 kilometers was fitted on the density profile of northwest western Siberia by this method, which caused a sensation. Study on petroleum geological characteristics of foreign basins similar to Bohai Bay Basin in China. China petroleum and natural gas group company information research institute.
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The study on the characteristics of gravity and magnetic fields near oil and gas reservoirs in western Siberia shows that there is a certain spatial relationship between gravity and magnetic fields and oil and gas reservoirs. Firstly, the amplitude and frequency are studied by two-dimensional Fourier spectrum analysis. Then transform, filter and "moving window" analysis are carried out, and regional and local anomaly maps and potential field derivative maps are compiled to study the parameter distribution of known oil and gas reservoir areas [7].
Most oil and gas reservoirs are located on the slopes of regional gravity and magnetic anomalies, which is interpreted as related to deep rift structures. At the same time, it is also confirmed that the location of oil and gas reservoirs is usually consistent with the local minimum of heavy magnetic poles, which is caused by the low density and magnetization of the basement. All known oil and gas reservoirs in the northern part of western Siberia are located in gravity anomaly areas with a wavelength of about 90 ~ 100 km and a large gradient. This newly revealed relationship between oil and gas reservoirs and potential field parameters can be used to predict new oil and gas reservoirs in land and ocean with low exploration degree [7].
Verb (abbreviation of verb) seismic acoustic method of permeable medium
Seismic acoustics of permeable medium is a new geophysical exploration method, which is characterized by: (1) treating oil and gas reservoir model as heterogeneous medium; The fluid in the pore is a heterogeneous conductor of active dynamics, which can accumulate and convert (simulate) the wave process; Reservoir frame is a static heterogeneous conductor, which controls the motion of dynamic heterogeneous conductor [8].
This method can be directly solved by the internal parameter relationship or the volume flow of fluid relative to its rock shelf. The inverse solution is obtained by exciting, recording and analyzing a group of similar fluid waves, and its kinematics and dynamics parameters are determined by fluid flow. Comprehensive analysis of the results of acoustic logging fluid method, vertical seismic sounding method, seismic exploration and experimental observation can ensure the reliability of the obtained solution [8].
Through calculation and program comprehensive analysis, the characteristics of effective porosity, pore size, permeability, production and saturation along the depth of production profile can be obtained. This method has been successfully applied to astrakhan Dome and Eastern Siberia [8].
FMI logging technology of intransitive verb
FMI is the latest generation of resistivity imaging logging tool developed on the basis of formation inclinometer, which is called full borehole formation micro-resistivity imager. It uses high-resolution micro-resistivity to generate electrical images, studies rock bedding, structure, pore changes, fractures and sedimentary equivalence, and provides a basis for accurately judging oil and gas reservoirs. On the basis of establishing the relationship between rock images suitable for exploration area, rational application of FMI technology is an effective way to improve exploration efficiency, especially in deep exploration [9].
Seven, geochemical exploration technology
Shallow geochemical markers can be used to predict deep basin oil and gas accumulation, and the former Soviet Union has made great progress and good results in this respect.
The Puri Casbi Basin is located in the southeast of Russian platform, and the reservoir is located in the Permian salt bed, which is buried deeply (4000 ~ 5500 m), and the oil field is close to the outer edge of the basin. The study shows that the geochemical characteristics and composition of hydrocarbon fluids in the upper salt bed are similar to those in the lower salt bed. Through the analysis of geochemical characteristics of salt beds and continental sediments above salt, the location of oil reservoirs in sub-salt reservoirs can be determined [10].
The research object mainly focuses on the most prominent feature of subsalt fluid-the high concentration of ——H2S. This active component reveals the migration path from the subsalt reservoir to different overlying salt layers and strata above salt. The distribution of H2S channels along the basin can indicate the distribution of deep subsalt reservoirs without drilling through the middle of the basin [10].
The deep structure of the basin can be determined by geochemical data. Subsalt carbonate reservoir with abnormal formation pressure and fluid composition is the source of geochemical indicators of upper salt layer. H2S in terrigenous rocks is not primary, so the trace of H2S in terrigenous rocks is a reliable sign of migration. This method can also be used to predict undiscovered oil and gas resources in other basins. Through the detailed study of formation water and secondary minerals in the upper salt bed of the basin, the environmental causes and migration causes of geochemical parameters can be distinguished [10].
Eight, comprehensive exploration technology
For deep oil and gas exploration, it tends to develop in the direction of multidisciplinary combination and comprehensive application. The combination of seismic exploration with gravity and magnetic exploration, or seismic exploration with magnetotelluric exploration, non-seismic three-dimensional geophysical exploration with three-dimensional seismic exploration technology, and comprehensive application of near-surface geochemical exploration and seismic data will greatly promote deep oil and gas exploration. The principle of gravity, magnetism, electricity and chemistry comprehensive interpretation method is shown in Figure 3. Study on petroleum geological characteristics of foreign basins similar to Bohai Bay Basin in China. China petroleum and natural gas group company information research institute.
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At present, the most remarkable achievement is the combination of seismic and magnetotelluric data, which has become an effective method for deep oil and gas exploration. Study on petroleum geological characteristics of foreign basins similar to Bohai Bay Basin in China. China petroleum and natural gas group company information research institute.
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In the middle Miocene, the tectonic activity in Pannong Basin in Hungary was strong, accompanied by volcanic eruption. Magma covered the bedrock and gradually formed a rather thick volcanic stratum. Volcanic rocks can shield and scatter seismic signals, which often leads to poor quality of seismic data. In this case, MT survey can get better information under volcanic rocks than earthquake survey. Comparing MT (Bostic) and logging resistivity map, MT and logging resistivity at Miocene volcanic rocks about 2km correspond to high resistivity, and the strata above volcanic rocks are low resistivity. This phenomenon shows that the measurement results of two different methods are similar. The MT measurement results are displayed in the form of vertical pseudo-profile of Bostic resistivity distribution (Figure 4), which can clearly delineate the low-resistivity strata under high-resistivity igneous rocks. At MT 6 station (fig. 4), the resistivity value of the low resistivity layer with a depth of 4 ~ 5 km is close to the logging resistivity value of the same depth of KH well about 3 ~ 4~5km away from the station, and the low resistivity layer of MT was written by Hu Qiuping, a Cretaceous stratum. Study on petroleum geological characteristics of foreign basins similar to Bohai Bay Basin in China. China petroleum and natural gas group company information research institute. 36860.88868888686
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Fig. 3 flow chart of the combined interpretation method of gravity, magnetism, electricity and chemistry.
Fig. 4 cross-sectional view of austenite resistivity distribution.
From this example, it can be seen that according to the underground structure shape shown in MT profile and the distribution characteristics of underground resistivity (or conductivity) obtained from it, combined with seismic data, the underground lithology can be determined and its oil and gas potential can be judged. This research has opened up a broad road for deep oil and gas exploration.
Nine. Concluding remarks
The complexity of deep geological conditions determines that exploration should avoid using a single method and technology. Making full use of various exploration techniques for comprehensive exploration is undoubtedly an important means to accurately obtain deep geological information.
In the process of exploration in Caspian basin, the former Soviet Union systematically carried out a large number of * * * deep point methods and refraction wave profile comparison methods on the basis of remote sensing, gravity, magnetic and electrical exploration, and combined with deep parameter wells and general survey drilling, conducted comprehensive exploration, comprehensively understood the deep geological structure, provided an important basis for target evaluation and exploration decision-making, and achieved good results.
The deep exploration degree in Shengli oil province is low. In addition to strengthening seismic work and improving seismic reflection effect, comprehensive exploration of deep targets should also be considered by selectively combining gravity, magnetic force, electrical method and other means, which is expected to make new discoveries in the deep.
Acknowledgement: In the process of completion, this paper was guided and helped by Song, chief geologist of the Academy of Geological Sciences, and Cai, deputy chief geologist. Many problems encountered in the research process have been enthusiastically guided by Yang Pinrong, Zhao Hongbo, Chen Jie of the Academy of Geological Sciences and Guo Liangchuan, a senior engineer of Geophysical Exploration Company. I would like to express my deep gratitude here.
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