Popular Science Illustrations | Looking back at a hundred years of geophysical exploration

Modern oil exploration began in the mid-19th century. In the early days, oil was mostly found through oil seepage, ground geological surveys and drilling. Geophysical exploration methods were not introduced into oil exploration until the 20th century. The emergence of exploration methods such as gravity, magnetic, electrical (electromagnetic), and seismic exploration has made it possible to identify underground geological structures in covered areas and find traps and oil and gas reservoirs based on them. This has significantly expanded the scope of oil and gas exploration, from the ground to the underground, from the surface and shallow layers to the deep layers.

Gravity, magneto-electric exploration enters the oil industry

The earliest geophysical method introduced into oil exploration was gravity exploration. The study of gravity phenomena originated in the era of Galileo (1564-1642). In order to commemorate him, later generations deliberately set the original unit of gravity acceleration as "Gal". In the 17th century, Newton quantified the concept of gravity with the famous law of universal gravitation. Today, almost all modern analysis methods on the earth's gravity come from the classical field theory created by Newton and other famous scientists in the 17th and 18th centuries, such as French mathematicians Laplace, Poisson, Legendre, Clairaut, and Bouguard, based on the law of universal gravitation. In order to measure the anomaly of the earth's gravity, scientists have created a precision instrument based on the principle of force balance - a torsion balance, which can "weigh" the slight changes in gravity acceleration on the ground.

The history of the torsion balance can be traced back to the gravity measuring instrument invented by Henry Cavendish, a British chemist and physicist in 1791. Because of this invention, Cavendish was called the first person in history to weigh the earth. In the late 19th century and early 20th century, Hungarian physicist R. Von. Eotvos improved Cavendish's torsion balance and developed a torsion balance suitable for field operations. He conducted gravity observations in Hungary for 10 years, and the results showed that the torsion balance can measure the changes in rock density in different parts of the earth's crust. In 1901, Eotvos used the instrument he invented to measure the depth of the lake bottom. In 1922, the torsion balance was used to successfully detect oil reservoirs related to salt dome structures on the coast of the Gulf of Mexico, making the torsion balance the earliest geophysical detection instrument used in oil exploration.

Since it takes about 5 hours to measure each measuring point with a torsion balance, the efficiency of gravity exploration is very low. In view of the low efficiency of the torsion balance, in 1934, American geophysicist JB Lacoste invented the metal spring gravimeter, and American geophysicist Sam.P. Worden invented the quartz spring gravimeter. Since this type of gravimeter significantly improves the accuracy of gravity measurement compared to the torsion balance, and the average observation time of a measuring point can be shortened to 10-30 minutes, it has completely replaced the torsion balance and has been widely used by the end of the 1930s, marking the practical application of gravity exploration. Oil exploration workers use gravimeters to observe one by one in the field according to the designed measuring points, and record the gravity values, and then return to the room for processing and analysis. Geologists can use it to study the underground geological structure and infer the lithology and depth of the formations based on the location of the captured gravity anomalies.

Magnetic exploration, as a geophysical exploration method, began with the universal magnetometer made by the Swedes Thalen and Tiberg in 1870, which made magnetic exploration practical. In 1915, German geophysicist Schmidt made a quartz blade magnetometer, which greatly improved the accuracy of magnetic measurement, making magnetic exploration not only used to find iron ore, but also to study geological structures, find salt domes and oil and gas fields. In 1936, Soviet geophysicist A.A. Rogachev developed an induction aeromagnetometer, which greatly expanded the application scope of magnetic exploration and improved efficiency.

Electrical and electromagnetic exploration began experimental research in the early 19th century. In 1815, the natural potential generated by the ore body was first observed at the Cowar Copper Mine in the UK. From 1919 to 1922, Swedish scientist RW Fox established the natural electric field exploration method through further research, and used the natural electric field method to find the first sulfide ore in 1835. From the mid-19th century to the early 20th century, a variety of electrical and electromagnetic exploration methods were proposed, such as the resistivity method in 1883, the alternating current method in 1900, the electromagnetic induction method in 1917, the transient electromagnetic method in 1933, the earth current method in 1934, and the magnetotelluric method in 1950. Among them, the electromagnetic induction method achieved the first prospecting effect in 1925. In 1929, Schlumberger of France realized the measurement of natural potential and resistivity in the well, forming the electrical logging technology. In 1924, the Soviet Union formed the world's first electrical prospecting team and used it for oil exploration for the first time. Thanks to the efforts of many scientists, electrical prospecting has developed rapidly and has become popular by the 1940s and is widely used in metal and oil exploration.

Seismic exploration becomes the main force in oil exploration

Seismic exploration technology originated in the mid-19th century. In 1845, Irish geophysicist Robert Mallet used artificially excited seismic waves to measure the propagation speed of elastic waves in the earth's crust, marking the beginning of seismic exploration methods. In 1913 during the First World War, German geophysicist L. Mintrop and his collaborators B. McCollum, JC Karcher and others developed a method and instrument for determining the position of a gun group using seismic waves generated by the recoil of artillery fire. It receives seismic waves generated by the recoil of a cannon and determines the position of the gun based on the arrival time of the seismic waves. This is the first time in human history that seismic waves have been linked to human production and life.

After the end of World War I, Mintrop, who invented the method and instrument for determining the position of artillery groups using seismic waves, applied his technology to the detection of underground geological structures. Based on the recorded information, he calculated the propagation speed and penetration depth of seismic waves in the strata, thereby accurately calculating the thickness and spatial form of the strata. In 1919, Mintrop applied for a patent for seismic exploration using the refraction wave method in Germany. In the same year, he and Lehmann improved the seismograph he invented and successfully conducted a coal seam depth measurement test. In October 1920, Mintrop conducted seismic refraction wave measurements in a well near Hamburg, Germany. In April 1921, he established a seismic exploration company, and seismic exploration officially began to be industrialized. In June 1924, Mintrop's seismic exploration company discovered a salt dome in Orchard, Texas Gulf region, for Gulf Oil Company. In 1926, oil was found in the well, which was 1,150 meters deep and produced 400 tons of oil per day. This was the first oil field in the world discovered through seismic exploration, and it pioneered the use of seismic exploration technology to find oil. By 1930, Mintrop's seismic exploration company had served 9 oil companies in the Gulf Coast area of ​​Mexico and found a total of 22 salt dome structures. In 1930, Soviet geophysicist Gamburtsev and others absorbed the advantages of reflection wave seismic exploration and made corresponding improvements to refraction wave seismic exploration. The early refraction wave method could only record the first direct wave, while the improved refraction seismic method could also record the subsequent refraction waves, which could study the propagation characteristics of seismic waveforms in more detail and contributed to the maturity of refraction wave seismic exploration technology.

Reflection wave seismic exploration technology appeared later than refraction wave seismic exploration technology. While refraction wave seismic exploration technology was gradually developing and maturing, John Clarence Karcher and others conducted field tests of reflection wave seismic exploration in 1921, and clearly recorded the reflection waves generated by artificial earthquakes in Oklahoma for the first time, proving that the images of the geological structures under the earth can be obtained by using reflected seismic waves. In 1929, they applied for a patent for reflection wave seismic imaging.

In 1926, Kacher and others founded the Geological Engineering Company and used the reflection wave seismograph prototype they produced to conduct experimental exploration of the known Tennessee salt dome, with good results. In 1928, they successfully developed a full set of reflection wave seismic exploration equipment, established the world's first reflection wave seismic team, and began the commercial application of reflection wave seismic exploration in the United States. In 1930, Kacher and others founded the famous GSI (Geophysical Service Inc.) geophysical service company, and discovered three oil and gas reservoirs in Seminole, Oklahoma using reflection wave seismic exploration technology, establishing its position as the most effective and practical oil exploration technology. Although the old refraction wave seismic exploration can still be effectively used to find salt domes, the reflection wave seismic exploration technology is more accurate in detecting complex geological structures, which further expanded the popularity of seismic exploration technology and brought seismic exploration into a stage of rapid promotion and application. Later, some well-known international geophysical service companies, such as the French Geophysical Company (CGG) and Western Geophysical Company (W ester nGeophysical), were established at that time. By 1937, there were 250 seismic teams in the United States, and seismic exploration had fully entered the stage of large-scale industrial application.

Development History of Geophysical Exploration Technology in China

Compared with Western countries, my country started to formally carry out geophysical exploration technology research nearly half a century later. According to relevant historical records, the earliest introduction of modern geophysical methods into China's oil exploration can be traced back to 1939. In that year, after returning to China with a doctorate from the University of London, the famous geophysicist Weng Wenbo taught at the Department of Physics of the National Central University (now Nanjing University and Southeast University). He was the first to open a geophysical exploration course in China, trained and absorbed a group of teachers and students who were interested in geophysics and geology, and organized several trips to the Yumen Oilfield to conduct experiments on gravity, magnetism and other exploration methods. This event marked the beginning of my country's geophysical exploration career.

my country's first geophysical exploration team was formally established in the Yumen Oilfield in September 1945. It was called the gravity exploration team at that time. It carried out gravity and magnetic exploration at the same time and conducted a gravity and magnetic survey with a scale of 1:100,000 between Yumen and Zhangye in Gansu. In June 1946, two new gravity exploration teams were established in Shanghai. They first carried out exploration in the Taihu Lake area in southern Jiangsu, and then went to Yanchang area in Shaanxi to carry out geological survey in May 1950.

The first seismic exploration team of New China began preparations in 1949 and was officially established in March 1951. In September 1951, it went from Shanghai to the Silangmiao area in northern Shaanxi to conduct seismic exploration tests on the loess plateau. In the spring of 1952, it went to Qingcaowan in the Jiuquan Basin of Gansu to conduct exploration. In the spring of 1952, the Exploration Department of the Northwest Petroleum Administration Bureau prepared to establish the second seismic exploration team and carried out construction in the area of ​​Hujia Village in Yanchang, Shaanxi. After the summer, it moved to Dahongquan and Wenshu Mountain in the Jiuquan Basin for construction. At the end of 1952, the third and fourth seismic teams were established, respectively working in the Jiaoshui area of ​​the Chaoshui Basin and near the Laojun Temple in the Jiuquan Basin. In addition, the Sino-Soviet Petroleum Company jointly established by China and the Soviet Union also established a geological survey office in Xinjiang in 1952, under which two gravity exploration teams, two magnetic exploration teams, one electrical exploration team and one seismic exploration team were established, which carried out operations in the Hutubi area and the Usu-Dushanzi area in northern Xinjiang and the Bachu-Kashgar area and Yingjisha area in southern Xinjiang.

Before the 1950s, China's oil industry was very backward due to various factors. In 1950, China had only three small oil fields, namely, Laojunmiao in Yumen, Yanchang in northern Shaanxi, and Dushanzi in Xinjiang, as well as several small gas fields, such as Ziliujing in Sichuan, Shengdengshan, and Shiyougou, with an annual output of only 300,000 tons at most. The establishment of the above-mentioned gravity, magnetic, electrical, and seismic exploration teams laid the foundation for large-scale oil exploration in Northwest China in the 1950s. In 1952, using new gravimeters, 51-type seismographs, and new drilling technologies imported from the Soviet Union and Hungary, Shiyougou, Baiyanghe, and Yaerxia oil fields were discovered in Yumen Oilfield. In 1955, gravity, magnetic, electrical, and seismic exploration were carried out on the northwest edge of the Junggar Basin in Xinjiang. First, based on the geophysical data obtained, the Heiyushan buried structure was discovered, Heiyushan No. 1 well was determined, and industrial oil flow was seen. After that, gravity and electrical exploration were further intensified in the area, and the Kewu fault zone and five nose-shaped structures developed near the fault zone were discovered. In 1958, based on gravity, magnetic and electrical exploration data, the Lenghu structural zone was discovered in the Qaidam Basin, and seven oil fields including Nanchong and Guihua were discovered in the Sichuan Basin. By 1959, the national crude oil production reached 3.73 million tons and natural gas production reached 250 million cubic meters.

While focusing on the exploration in the west, the oil exploration strategy was implemented to move eastward from 1955, and comprehensive geological surveys were carried out in the Songliao Basin and the Bohai Bay Basin. Since the main body of the Bohai Bay Basin and the Songliao Basin is the North China and Northeast China Plains, the fertile land is vast and flat, and there are almost no geological outcrops on the ground, and oil and gas seeps are rarely found, so geophysical methods have an absolute dominant position in oil and gas geological surveys. From 1956 to 1957, large-scale ground gravity and aeromagnetic exploration were carried out in the Songliao Basin, and three large electrical depth sections across the entire basin were completed. A small amount of two-dimensional seismic exploration and drilling were carried out, which preliminarily confirmed that the Songliao Basin was a large sedimentary basin in the Mesozoic and Cenozoic zones, and a large anticline structural belt was found in the center of the basin, namely the famous Daqing Changyuan. In September 1958, the location of Songji 3 Well was initially planned on the Gaotaizi anticline structural uplift discovered based on the electrical anomaly. Then, the planned well location was fine-tuned based on the Gaotaizi structural map determined by two-dimensional seismic data. Finally, Songji 3 Well was located next to Yongyue Village in Gaotaizi Town, and the drilling was officially started on April 11, 1959. On September 26, 1959, Songji 3 Well gushed out oil, marking the official discovery of Daqing Oilfield. Daqing Oilfield is the first large oilfield in the history of China's petroleum exploration that was successfully discovered using geophysical exploration methods.

After the successful exploration of Daqing Oilfield, the oil exploration team marched southward and carried out exploration in the Bohai Bay Basin based on the same exploration ideas. In 1961, the Huaba Well in the Dongying Depression produced 8.1 tons of industrial oil per day, which kicked off the large-scale oil exploration campaign in North China. The Huaba Well was the first discovery well in the entire North China Plain, achieving a new breakthrough in early oil exploration in the Bohai Bay Basin, laying the foundation for the birth of Shengli Oilfield and the construction of the subsequent large oilfields such as Huabei, Dagang, Jidong, Nanyang, and Zhongyuan.

Geophysical Exploration Technology and China's Major Oil and Gas Discoveries

After a series of breakthroughs and discoveries in oil exploration based on geophysical exploration technology from the 1960s to the mid-1980s, my country's crude oil production increased rapidly. In 1963, the national crude oil production reached 6.48 million tons, in 1978 it reached 104 million tons (of which the annual output of Daqing Oilfield exceeded 50 million tons), and in 1985 it reached 125 million tons.

Since the 1980s, China's oil reserves have been growing slowly due to the fact that large oil fields with good geographical conditions and easy exploration have been basically exhausted. In addition, old oil fields have gradually entered a period of decline, and China's oil exploration has encountered a serious bottleneck period. To this end, Chinese oil explorers have proposed a new idea of ​​"stabilizing the east, developing the west, developing oil and gas simultaneously, and vigorously developing marine exploration". While continuing to carry out exploration in the eastern region using 3D seismic exploration technology, they have focused on strengthening oil exploration in the western region, especially in the Tarim, Junggar, Tuha, Qaidam, Ordos and Sichuan basins, and discovered a large number of new oil and gas fields. While vigorously carrying out onshore oil exploration, significant progress has also been made in offshore oil and natural gas exploration. At sea, oil fields such as Penglai 19-3, Suizhong 36-1, Qinhuangdao 32-6, Bozhong 25-1, Jinxian 1-1, and Jinzhou 25-1 in the Bohai Bay, and Weizhou 11-4 and Yacheng 13-1 in the South China Sea were discovered one after another; a large number of natural gas fields were discovered in Jingbian and Surigu in the Ordos Basin, Tazhong, Lunnan, and Kela 2 in the Tarim Basin, and Dachuanzhong, Puguang, Guang'an, Datianchi, and Hechuan in the Sichuan Basin, forming a new situation of exploration and development involving both land and sea, and both oil and gas, which has ensured the steady growth of my country's annual oil and natural gas output.

Entering the 21st century, most of the main oil and gas fields in the world have entered the maturity and decline period, while new exploration target areas are becoming complex and hidden, the target strata are getting deeper and deeper, and it is becoming more and more difficult to discover large oil and gas fields on land. Conventional oil and gas discoveries have a declining trend. Under this exploration situation, new oil and gas exploration targets are becoming more and more difficult to discover. Geological conditions such as high-steep structures, complex structures, lithologic traps, compactness, deep and ultra-deep layers have put forward higher demands on geophysical exploration technology. In addition, improving exploration efficiency has also become an urgent requirement. In response to the above difficulties, while continuing to develop conventional geophysical exploration technology, China's geophysical exploration workers have also innovatively proposed epoch-making time-frequency electromagnetic exploration technology, multi-wave and multi-component seismic exploration technology, and broadband, wide-azimuth, high-density "two wide and one high" seismic exploration technology, realizing the upgrading of my country's land and sea oil and gas exploration technology, significantly improving the results of oil and gas exploration, and discovering a number of new oil and gas fields, so that my country's oil and gas production has reached and maintained at 200 million tons, effectively supporting the high-quality development of the oil industry.

Conclusion

One hundred years have passed since Mintrop applied for a patent for the seismic refraction wave method in 1919, John Clarence Kacher and others conducted field tests of seismic exploration using the reflection wave method in 1921, and gravity exploration technology was successfully used for the first time to detect oil reservoirs related to salt dome structures along the coast of the Gulf of Mexico in 1922. Over the past 100 years, more than 80% of the oil and gas fields in the world and more than 90% of the oil and gas fields in my country have been discovered through geophysical exploration, demonstrating the tremendous power and historic achievements of geophysical exploration.

Author: Chen Maoshan, Song Qianggong, Wang Chengxiang (China National Petroleum Corporation Oriental Geophysical Exploration Co., Ltd.)