A brief discussion on the geological period of reservoirs in my country's main oil and gas regions

my country has a vast territory, with 9.6 million square kilometers of land and 4.73 million square kilometers of sea. After the founding of the People's Republic of China, the exploration and development of oil and natural gas resources have been continuously strengthened. After more than 70 years of unremitting efforts, oil and gas fields of varying sizes have been discovered in the Bohai Bay, Songliao, North China, Tarim, Ordos, Junggar, Sichuan, Pearl River Estuary, South China Sea, East China Sea and other basins.

The rock formation that produces oil and gas is called a reservoir. The depth of oil burial ranges from surface outcrops to strata more than 9,000 meters deep, covering four major sections: the Cenozoic, Mesozoic, Paleozoic, and Proterozoic, and the time span can be traced back to the primitive era more than 600 million years ago.

Oil and natural gas are buried deep in the earth. However, the formation period of the rock layers containing oil and gas varies from basin to basin. Some people make subjective assumptions based on the depth of burial, and mistakenly believe that the deeper the oil and gas reservoir is, the older its geological age is. In fact, this is not the case. Due to the continuous complex and changeable geological movement of the earth's crust under the action of internal and external forces, it sometimes rises and sometimes sinks during the long evolution of tens of millions or even hundreds of millions of years. There are many phenomena of bending, deformation, fracture superposition and even partial loss. Therefore, the burial depth of oil and gas reservoirs is not fixed in the order of formation.

The geological age of the formation of reservoirs in my country's major oil and gas-bearing areas is a heavenly book that reveals the mysteries of the strata. It varies in depth and thickness, including large oil fields formed in the Mesozoic terrestrial environment and large gas fields nurtured in the Paleozoic marine environment. A simple review of the timeline of reservoirs in my country's major oil and gas areas from recent to ancient times will reveal that nature is full of fun and mystery that has yet to be scientifically explained.

After the founding of the People's Republic of China, the first step was to search for oil and gas fields in the Gobi Desert in the northwest based on the oil and gas seedlings that emerged on the surface. In the mid-1950s, major breakthroughs were made on the edge of the Junggar Basin and the hinterland of the Qaidam Basin, and the Karamay Oilfield and the Qinghai Lenghu Oilfield, which had relatively small reserves, were discovered. According to the core research obtained by drilling, the reservoirs of these two oil fields were formed relatively late, belonging to the Pliocene and Miocene of the Upper Tertiary System of the Cenozoic Era, and have only evolved for no more than 25 million years. The fossils of large mammals such as mammoths and woolly rhinoceroses buried in the strata after their extinction indicate that the earth was in a cold ice age at that time. Oil and gas took the opportunity to migrate during the orogenic movement of the Qinghai-Tibet Plateau, and were trapped in geological structures with no escape route, thus forming oil and gas reservoirs that can be mined.

The Lower Tertiary rock formations, which were formed earlier than the Early Tertiary of the Cenozoic Era, have a history of 70 to 25 million years. The fossils contained in them prove that it was a prosperous era of biodiversity. Gymnosperms and mammals filled every corner of the earth. The survival of the fittest and the elimination of the weak were the result of natural selection. The remains of a large number of dead organisms provided rich organic matter for the generation of oil and gas. The oil and gas fields formed during this period are mainly concentrated in the Bohai Bay Basin, including the North China Plain, the southern part of the Songliao Plain and the entire Bohai Sea. Since the mid-1960s, Shengli, Dagang, Liaohe, Huabei, Zhongyuan, Nanyang, Jidong and other oil and gas fields have been discovered on land one after another. In recent years, Hai 4, Chengbei, Suizhong, Jinzhou and other oil and gas fields have been discovered in the Bohai Sea, forming a cluster of oil and gas fields with huge geological reserves, connected by land and sea, and scattered all over the place. In addition, there are the Yumen Oilfield discovered in the Jiuquan Basin in the late 1930s and the Jianghan Oilfield discovered in the middle reaches of the Yangtze River in the early 1970s. The strata storing oil and gas are all products of the Tertiary System in the Cenozoic Era. Although the burial areas and depths vary greatly, in terms of the geological age of the reservoirs, they belong to the same period.

The Songliao Plain in northeast my country is a cornucopia of oil and gas resources. In the late 1950s, the Daqing and Jilin oil and gas areas were discovered. Drilling results confirmed that the main reservoirs containing oil and gas were formed in the Upper Cretaceous System of the Mesozoic Era, which is older than the Cenozoic Era. Although the burial depth is only 400 to 2,000 meters, it has an evolution history of 135 million to 70 million years. The oil and gas reservoirs in this area are due to the typical inland ecological environment. The surface was once a vast ancient Songliao Lake, which nourished a large variety of freshwater organisms, formed a complete food chain, and provided a last dinner for large animals such as dinosaurs that were about to become extinct. The continuously sinking and silting lake basin became a huge cemetery for burying the remains of animals and plants, creating a sedimentary environment similar to the ocean, providing a solid scientific basis for the establishment of the theory of terrestrial generation of oil and gas.

In the stratigraphic sequence of the Mesozoic, the section that was born earlier than the Cretaceous System is called the Jurassic System. This name is familiar to many people. A Hollywood science fiction movie called "Jurassic Park" depicts the thrilling domination of dinosaurs on Earth. It was a golden age from 225 million to 135 million years ago. The warm and humid climate prompted the flourishing of large biological populations. The dinosaur family became monsters that dominated the land, sky and ocean, interpreting the jungle law of the survival of the fittest. Therefore, this period was also the main stage of oil and gas generation. The oil and gas fields discovered in the Sichuan Basin, Junggar Basin and Shaanxi-Gansu-Ningxia region in my country have fossils from the Triassic to the Jurassic in the reservoirs. The Changqing Oilfield, located in the Loess Plateau and the Inner Mongolia Plateau, has an exploration area of ​​more than 200,000 square kilometers, spanning the three provinces of Shaanxi, Shanxi and Gansu and the two autonomous regions of Ningxia and Inner Mongolia. A large number of oil and gas fields have been discovered one after another, and the reservoirs are mainly concentrated in the Jurassic and Triassic systems. Among them, the Ordos Basin has the largest area and is known as "half basin of oil and full basin of gas". It gave birth to the Sulige large gas field with a geological reserve of over 1 trillion cubic meters, providing sufficient clean energy for the eastward transmission of Shaanxi gas. The current annual oil and gas equivalent output of Changqing Oilfield has reached more than 60 million tons, surpassing the former Daqing Oilfield and becoming the number one rising star in my country.

The miracles created by the Jurassic system of the Mesozoic Era are not limited to the Saishang Plateau. In the Tarim Basin in Xinjiang, known as the "Sea of ​​Death", the difficult oil and gas exploration lasted for more than half a century, and finally revealed the secrets hidden by the world's second largest desert. Since the 1980s, more than 30 oil and gas fields have been discovered in succession. The oil and gas reservoirs in this area broke the world record for burial depth. Drilling found that there are still good oil and gas shows in the strata below 9,000 meters. From a geological point of view, ultra-deep buried oil and gas reservoirs do not mean that they were formed at an older age. The oil and gas-rich strata in the Tarim Basin are also the result of the creation of the Jurassic and Triassic systems of the Mesozoic Era. Compared with other oil and gas areas, the biggest difference is the formation of strata deposits formed in a marine environment. There are both sandstone reservoirs with relatively high porosity and permeability, and carbonate reservoirs with well-developed cracks and cavities. Although drilling is very difficult, the oil and gas production of a single well is very considerable. Therefore, the Tarim Basin has become the main gas source and oil production base for my country's "West-East Gas Transmission". The energy magic demonstrated by the vast desert is breathtaking.

Are there any oil and gas reservoirs in my country that are older than the Mesozoic? The answer is yes. From ancient times to the present, the Sichuan Basin has discovered the most natural gas seedlings, and has always been a key area for oil and gas exploration. In recent years, science and technology have made rapid progress, and equipment manufacturing capabilities have been significantly improved. Gas fields have been discovered one after another in the Land of Abundance. In addition to conventional gas fields such as Anyue, Luojiazhai, Puguang, and Yuanba, there are also large shale gas fields discovered in southern Sichuan and Fuling. The lower biological fossils obtained by drilling clearly show that the formation of the natural gas reservoirs in the Sichuan Basin is older than other regions. There are gas-producing layers from the Permian system formed in the late Paleozoic to the more primitive Cambrian system. This stratum, which can be called the ancestor of the earth's crust, was formed approximately 250 million to 600 million years ago. It hides a suspense that has puzzled the scientific community. There was no sign of life on Earth 600 million years ago. Why did a large number of lower organisms suddenly emerge in the Cambrian system? The mysterious mystery of the origin of life is still a scientific problem that has not been solved.

Is it possible that there is no possibility of storing oil and natural gas in the Proterozoic strata that are older than the Cambrian system? The drilling results in the Sichuan Basin overturned people's predictions and found that there are still strata containing natural gas in the upper Sinian system formed 600 million years ago. The source of these natural gases is difficult to give a satisfactory explanation using the geological theory of organic generation. The cause of formation is quite puzzling and intriguing.

The amazing creation of nature, the widespread storage of oil and natural gas in the strata from the Cenozoic to the Proterozoic, shows that human beings' exploration of the deep crust is far inferior to their understanding of outer space. So far, the deepest drilling well in the world is only more than 10,000 meters deep, which is only one-thirtieth of the average thickness of the crust. Is it possible to find oil and natural gas in deeper and more ancient strata? I believe that the continuous development of science and technology will give the expected answer.

Author: Gong Ke