(Planning and Design Institute of Shanghai Offshore Oil Bureau, Shanghai 200 120)
The East China Sea is divided into continental shelf, Okinawa Trough and Ryukyu Arc from west to east. The East China Sea Basin spans the above geomorphic units from west to east. Because it is located between the Eurasian plate and the Pacific plate, it is a special geological structure part to study the interaction between the continental plate and the Pacific plate, and its geological structure characteristics have attracted the attention of many geologists at home and abroad (including geological structure experts and oil and gas geologists). After more than 20 years of oil and gas geological exploration, Shanghai Offshore Oil Bureau has a preliminary understanding of the geological structure of the East China Sea Basin. According to the seismic and geological data, the author determines that the East China Sea Basin is a NNE rift basin dominated by Cenozoic sediments formed under the joint action of Eurasia and the Pacific Ocean. The basin is composed of several secondary structural units alternating from northeast to northeast, and each structural unit was formed at a different time. The structural characteristics of the basin are different, and the nature of the basin is also different. The west side is dominated by fault depression, which belongs to intracontinental rift basin; To the east, it is dominated by fault depression superposition, belonging to continental margin fault depression basin; Further east is the back-arc rift basin, which is stretched in modern times. The sedimentary layers, oil and gas exploration target layers, faulting, regional tectonic movement and magmatism are obviously different from east to west, so the east-west zoning is the main feature of the geological structure of the East China Sea Basin, which is also the theme of this paper.
1 Structural division of the East China Sea basin
Figure 1 indicates that the East China Sea Basin is the Zhejiang-Fujian Uplift Area to the west, the Ryukyu Uplift Area to the east, and the middle of the East China Sea Basin is NNE, in which the structural line direction of the secondary structural units is generally NNE. According to the zoning plan of east-west zoning, the East China Sea basin can be divided into three secondary structural units from west to east: the East China Sea shelf basin, the Diaoyu Islands magmatic rock belt and the Okinawa Trough basin. The East China Sea shelf basin can be divided into three tertiary structural units: western depression, middle low uplift and eastern depression. The Okinawa Trough Basin can be divided into three Tertiary structural units from west to east: the shelf front depression in the west, the Wanglong uplift in the middle, the Tulaga depression and the trough depression in the east (attached figure 1 and 2) (table 1). In fact, the structural characteristics of the western depression and the eastern depression contained in the East China Sea shelf basin are different, so this paper puts them in the East China Sea shelf basin for the time being.
Two sedimentary layers controlled by tectonic units in the East China Sea Basin
2. 1 sedimentary layer is controlled by secondary structural unit.
The East China Sea shelf basin is dominated by Cenozoic sediments, followed by Mesozoic sediments, with a thickness of 5000 ~17000 m. The Okinawa Trough basin is dominated by Cenozoic Miocene, Pliocene and Quaternary sediments, with a thickness of 5000 ~ 12000 m, and there may be Cenozoic Paleogene sediments on its east and west sides. The magmatic rocks in Diaoyu Island are mainly Pliocene and Quaternary sediments, with some Miocene sediments, and the thickness is1500 ~ 2000 m. ..
Figure 1 Structural zoning of East China Sea Shelf Basin (zoning scheme in this paper)
Fig. 2 Geological profile of East China Sea Basin (see Figure 1 for location).
Table 1 List of Structural Zoning Units of East China Sea Shelf Basin
2.2 Sedimentary layers controlled by tertiary structural units
In the western depression on the west side of the East China Sea shelf basin, there are middle-late Mesozoic-Cenozoic Paleocene deposits with a thickness of 5000 ~ 10000 m, mainly in the early Eocene, and the Paleogene deposits in the northeast are continental deposits, and the southwest gradually changes to marine deposits. Covered with1000 ~ 2000m Cenozoic Miocene, Pliocene and Quaternary sediments.
The middle-low uplift in the central part of the East China Sea shelf basin mainly covers Miocene, Pliocene and Quaternary sediments with a thickness of1000 ~ 2,000 m, and locally there are Meso-Cenozoic Paleogene sediments with a thickness of 5000 ~ 7000 m (in Minjiang sag of Wuyi low uplift L).
The eastern depression on the east side of the East China Sea shelf basin is dominated by well-developed Cenozoic sediments, and the eastern part of the northeast is continental, while the western part of the south is gradually transformed into marine. There are Mesozoic deposits on the east and west sides of the depression, with a thickness of 5000 ~17000 m. ..
The continental shelf front depression on the west side of Okinawa Trough Basin is mainly composed of marine Miocene, Pliocene and Quaternary sediments with a thickness of 5000 ~12000 m. ..
The Wanglong Uplift in the middle of Okinawa Trough Basin is covered by marine Pliocene-Quaternary sediments with a thickness of 0 ~ 2 000 m. ..
Tulaga sag and trough sag on the east side of Okinawa Trough Basin are mainly marine Pliocene and Quaternary sediments, with a thickness of over 5000m, and Miocene sediments remain on both sides of the center of the sag.
Faults and Magmatic Rocks in the East China Sea Basin
3. 1 East China Sea Shelf Basin
Among the western depressions, the Yangtze River depression in the eastern part of the northeast is dominated by extensional normal faults that descend from northeast to northeast and rise from southeast, with the northeast in the middle (Qiantang depression) and the northeast in the southwest (Oujiang, Jiulong and Jinjiang depressions) as the main ones. These extensional normal faults are mainly characterized by growth extensional normal faults. The descending plate of normal faults at all levels is the settlement center, and the ascending plate is the local structure or the high point of buried hill. Magmatic rocks from late Yanshanian to early Himalayan are locally developed in the western depression.
The middle part of the uplift is low, and the faults at the top of the uplift are generally not developed, but there are growth tensile normal faults in the northeast and northwest directions only at the edge of the uplift, which often become the dividing line between uplift, uplift and depression or depression. Late Yanshanian magmatic rocks are widely developed on it, and middle and late Himalayan magmatic rocks are found in some areas.
In the eastern depression, there were many NE-trending extensional faults before Miocene. After Oligocene, NE-trending thrust faults and compression anticlines formed by extrusion were generally developed in Xihu sag, northern Keelung sag and Hsinchu sag in southeastern Taiwan Province province, accompanied by late NW-trending normal faults.
Magmatic rocks from early Himalayan to late Himalayan are locally developed in the eastern sag.
3.2 Diaoyu Island magmatic rock belt
Most of the top faults are undeveloped, but there are tensional normal faults with small fault spacing in the southern section. NNE-NE extensional normal faults are generally developed on the east and west sides of magmatic belt, and these faults are mostly the boundary between magmatic belt and secondary depression. Magmatic rocks are relatively developed in the Upper Himalaya from the middle Himalaya to the late Himalaya (commonly found in Wudao Islands, Men's and Women's Islands, Diaoyu Islands, Huangweiyu Island, chiwei yu and other islands).
3.3 Okinawa Trough Basin
In the pre-continental shelf depression, symmetric extensional normal faults generally develop from NNE to NNE, and in the middle and late Himalayan period, they rise in the west or fall in the east. They are still in the process of development in modern times, with a small fault distance but a large number, which are accompanied by modern NW-trending extensional normal faults (Figures 3 and 4). Middle and late Himalayan magmatic rocks are locally developed in the depression.
Fig. 3 D 126-2-2 seismic interpretation profile of Okinawa Trough Basin (East China Sea Trough)
From the late Himalayan to modern times, the Wanglong Uplift is generally characterized by a NNE-trending extensional normal fault, which sinks eastward or westward to the depression. Magmatic rocks are very developed from the late Himalayan to modern times, and seamounts are formed in many places.
In Tulaga sag and trough sag, there are generally NNE-NE symmetric tensile normal faults that descend from the late Himalayan period to the center of the sag that is still active in modern times, and there are also NW-trending tensile normal faults that are still active (Figure 4). These faults are characterized by a small fault distance but a large number. Magmatic rocks that are still active from late Himalayan to modern times are locally developed in the trough depression and Tulaga depression, and sometimes escape from the seabed to form seamounts.
Fig. 4 D 126-5 seismic interpretation profile (Okinawa trough connection)
Four trap types in the East China Sea Basin
4. 1 East China Sea Shelf Basin
The western sag is dominated by buried hill, buried hill drape anticline, fault block and reverse traction anticline, followed by non-structural traps such as delta sand body, interlayer sand body and interlayer unconformity surface.
The central low uplift is dominated by shallow drape anticline at the top of the uplift, followed by non-structural traps such as weathering crust, interlayer sand body and interlayer unconformity surface at the top of the uplift.
Before Oligocene, the eastern depression was dominated by fault blocks and fault noses, and after Oligocene, it was dominated by compressive anticlines. In addition, there are non-structural traps such as interlayer sand body, interlayer unconformity surface and delta sand body.
4.2 Okinawa Trough Basin
Fault block and fault nose traps are the main types of continental shelf front depression, Tulaga depression and trough depression, followed by non-structural traps such as interlayer unconformity and interlayer sand bodies.
Wanglong uplift is mainly controlled by the anticline covered by its top.
5 Basement properties of East China Sea Basin
The basement here refers to the material on which Mesozoic and Cenozoic sedimentary basins developed.
5. 1 East China Sea Shelf Basin
With the eastern boundary of the central low uplift as the boundary, the basement material of the vast area on the west side (including the central low uplift and the western depression) is mainly a set of Jurassic or Cretaceous magmatic rocks developed in the outcrop area of the Zhejiang-Fujian uplift (confirmed by Shimentan, Mingyuefeng 1 well and most wells in the bidding area) (Tongji University, "Study on the Pre-Tertiary Geological Structure of the South Block of the Reef Uplift in the East China Sea Shelf Basin", Wang Jialin et al., 2000. Paleozoic shallow metamorphic rocks can be seen locally (Shanghai Offshore Oil Bureau, "Study on Tertiary Regional Geological Structure of Xihu Depression in the East China Sea", Zeng Jiuling, 1994). Therefore, most of the basement in the west of the East China Sea shelf basin may be the extension of Zhejiang and Fujian outcrops to the sea.
Below the Cenozoic sedimentary basin, the eastern depression is mainly magmatic rocks of late Yanshanian and local early Himalayan period, and there are Mesozoic sedimentary rocks and upper Paleozoic shallow metamorphic rocks on both sides of the depression (Study on Tertiary Geological Structure of Xihu Depression in the East China Sea by Shanghai Offshore Oil Bureau, 1994).
5.2 Diaoyu Island magmatic rock belt
It used to be called fold uplift belt, but there was no fold uplift and magmatism was obvious. Therefore, Zeng Jiuling and others of Shanghai Offshore Oil Bureau changed it to magmatic rock belt in the report of the second-level project of the "Eighth Five-Year Plan" national scientific and technological research "Tertiary geological research in Xihu sag of the East China Sea". The main materials that make up this area are magmatic rocks from the middle Himalayan to the late Himalayan, the northeast is mainly the middle Himalayan magmatic rocks, which is equivalent to Oligocene and later, and the southwest is mainly the late Himalayan magmatic rocks, which are still in the process of formation and development in modern times.
5.3 Okinawa Trough Basin
The basement of the continental shelf front depression is mainly pre-Miocene metamorphic rocks, while the basement of Tulaga and Trough Depression is mainly pre-Pliocene metamorphic rocks (with a layer velocity as high as 3,500-5,000 m) and magmatic rocks in the middle and late Himalaya.
Wanglong uplift is mainly composed of magmatic rocks from pre-Pliocene to modern times.
6 The formation and development of the East China Sea shelf basin
The East China Sea Basin is composed of several secondary depressions formed in different periods, and its basic law is that the western part is formed early, and the farther east it is, the later it is formed.
The western depression of the East China Sea shelf basin was formed in the early and middle Mesozoic, and the Paleocene was the peak of its development. In the middle and late Eocene, the area uplifted, ending the development of the depression and becoming a faulted basin.
The low uplift in the middle of the East China Sea shelf basin was formed in the late Mesozoic, consisting of a series of volcanic bulges, which gradually turned south and became Wuyi low uplift.
The eastern depression of the East China Sea shelf basin was formed in the Late Cretaceous, with the peak periods of Eocene, Oligocene and Miocene, and it is a fault-depression combined basin.
In the early Oligocene, the north-central segment of the Diaoyu Island magmatic belt was gradually formed, and its southwest segment was in the process of formation and development until modern times.
The continental shelf front depression on the west side of Okinawa Trough Basin was formed in the early Miocene, developing in Miocene, Pliocene and Quaternary, and the southwest segment gradually uplifted, reformed and shrunk.
The Wanglong Uplift in the middle of Okinawa Trough Basin was gradually formed in Miocene and is still developing in modern times.
Tulaga and Trough Depression on the east side of Okinawa Trough Basin were formed in the middle and late Miocene and are developing in modern times.
7 Regional geological tectonic movement in the East China Sea Basin
During its development, the East China Sea Basin experienced many regional tectonic movements, including Keelung movement in the early and middle Mesozoic, Yandang movement in the late Mesozoic, Oujiang movement in the middle and late Paleocene, Yuquan movement in the middle and late Eocene, Huagang movement in the middle and late Oligocene, Longjing movement in the middle and late Miocene and Okinawa Trough movement in the middle and late Pliocene. Different regional tectonic movements in different geological historical periods lead to complex geological tectonic pattern in the East China Sea Basin.
7. 1 keelung movement
Keelung movement occurred in the early and middle Mesozoic, which is the main geological tectonic movement that formed the East China Sea shelf basin. Due to this geological tectonic movement, a series of dustpan depressions with faults in the southeast and uplifts in the northwest have been formed in the East China Sea shelf basin. These depressions together constitute the present western depression, and both Mesozoic and Late Mesozoic were deposited. In the eastern part of the western depression, a middle-low bulge composed of a series of igneous bulges was formed. At the same time, in the late Mesozoic, the eastern depression had the embryonic form of an ultra-dustpan depression with east faults and west faults.
7.2 Wild Goose Swing Movement
Yandang movement took place in the late Mesozoic. This regional tectonic movement occurred in the central and western parts of the East China Sea shelf basin, which caused the early generation of the sedimentary area in the middle and late Mesozoic to rise, and the top of the Mesozoic sedimentary area was eroded, and then continued to extend along the original extensional normal fault, making the uplift longer and the depression more concave, and depositing the Paleocene. The eastern sag of the East China Sea shelf basin was basically formed and deposited in Paleocene.
7.3 Oujiang Movement
The Oujiang movement took place in the late Paleocene. The regional tectonic movement basically inherited the Yandang movement, which made the East China Sea shelf basin more depressed and uplifted, but the fluctuation range was small, and the Eocene was deposited at the same time.
7.4 Yuquan Movement
Yuquan movement occurred in the middle and late Eocene. This tectonic movement is a large-scale regional tectonic movement in the East China Sea shelf basin. Due to the occurrence and development of this regional tectonic movement, the regional tectonic framework of the East China Sea shelf basin has undergone major changes. The characteristics of this regional tectonic movement are as follows: (1) The East China Sea shelf basin generally uplifted from west to west, while the East China Sea shelf basin completely subsided from southeast to east depression; (2) Due to regional uplift, the top of Eocene in the west side of the East China Sea shelf basin was eroded in a large area due to long-term exposure to water, and only the Lower Eocene was preserved, and the Lower Oligocene and Miocene were missing because of no cover. However, the East China Sea shelf basin was underwater for a long time due to regional decline, and the Middle-Upper Eocene was developed and relatively complete, and relatively complete Oligocene and Miocene were deposited. ③ Due to Yuquan movement, the subsidence centers of the upper Miocene, Oligocene and Miocene in the East China Sea shelf basin moved to the eastern depression.
7.5 Huagang Movement
Most articles and reports have not mentioned it before, and a few reports only regard it as the first act of Longjing Movement. The author 1994 solemnly put forward the Huagang Movement in the report of the Eighth Five-Year Project "Study on Tertiary Geological Structure of Xihu Depression in the East China Sea". The Huagang movement occurred in the middle and late Oligocene, mainly at the junction of the eastern depression and the Diaoyu Islands magmatic belt, and its origin was related to the formation of the middle and northern Diaoyu Islands magmatic belt. It is characterized by compressive uplift near the west side of Diaoyu Island magmatic belt, strong erosion at the top of Oligocene and Eocene, and compressive anticline in the eastern waters of Xihu sag. Due to the influence of this tectonic movement, the Miocene subsidence center of Xihu Depression moved from southeast to northwest, and a dustpan depression was formed in the front depression of the shelf of Okinawa Trough Basin, which was deposited in Miocene.
7.6 Longjing Movement
Longjing movement mainly occurred in the middle and late Miocene. Xihu sag and Xinzhu sag in Taiwan Province Province in the eastern depression of the East China Sea shelf basin show compressional movement from east to west, accompanied by a series of compressional anticlines and thrust faults. The main reason is that Xihu sag may be related to the left-lateral translation of Jiumi fault in Yushan and a series of NW-trending faults to the north, while Hsinchu sag in Taiwan Province Province is related to the NW-trending subduction of Luzon Island to Taiwan Province Island. Meanwhile, the top of Miocene was eroded. However, the Longjing movement in Okinawa Trough basin showed a continuous expansion movement from east to west, and began to deposit Pliocene, and it showed uplift and erosion in the southwest section of Diaoyu Island magmatic belt.
7.7 Okinawa Trough Movement
The Okinawa Trough movement occurred in the middle and late Pliocene, and it showed a horizontal regional uplift in the East China Sea shelf basin, and the sedimentary thin Quaternary was a false integration with Pliocene. On the east and southwest of Diaoyu Island magmatic belt, there were regional uplifts, and the Pliocene top was eroded. However, in Okinawa Trough Basin, the shelf front depression, Tulaga depression and trough depression show continuous stretching and decline, and Quaternary sediments are thick. In the central mountain range of Taiwan Province Province, it has been rising.
8 East China Sea basin area to find oil and gas direction
The above geological structural characteristics of the East China Sea Basin fully show that the East China Sea Basin has different oil-generating target layers and different trap types in different structural parts, and regional oil and gas exploration should be treated differently.
(1) In the western depression on the west side of the East China Sea shelf basin, drilling confirmed that Paleocene was the main oil-generating target layer, and fault blocks, reverse traction anticlines, interlayer sand bodies and buried hills covered by Paleocene were the main places for oil and gas accumulation.
(2) There is no oil-generating target layer in the middle low uplift of the East China Sea shelf basin, and some secondary oil and gas reservoirs can be found in the shallow anticline at the top and the weathered crust and interlayer sand body at the top of the buried hill.
(3) Nearly 30 wells have been drilled in the east sag on the east side of the East China Sea shelf basin, which proves that Eocene in the east sag is the main oil-generating target layer, followed by Oligocene, Lower Miocene and possible Paleocene. Due to the metamorphism of Paleocene and Eocene in Xinzhu Depression, Taiwan Province, the main oil-generating target layer is in Middle and Lower Miocene. Therefore, the thrust anticline, fault block, fault nose, interlayer sand body and interlayer unconformity in the eastern sag are all favorable places for oil and gas accumulation.
(4) There is no oil-generating target layer in the Diaoyu Island magmatic rock belt, but the overlying anticline and weathering crust may be secondary oil and gas accumulation sites.
(5) In the continental shelf front depression on the west side of Okinawa Trough Basin, Miocene may be the main oil-generating target layer, in which fault blocks, fault noses, interlayer unconformity and sand bodies are favorable places for oil and gas accumulation.
(6) The Wanglong Uplift in Okinawa Trough Basin has no oil-generating target layer, but its top weathering crust may be the accumulation place of secondary oil and gas reservoirs.
(7) In Tulaga and Trough Depression on the east side of Okinawa Trough Basin, the oil-generating target layer may be in Pliocene, and fault blocks, fault noses and interlayer sand bodies may be favorable places for oil and gas accumulation.
Because the Okinawa Trough Basin is a developing basin in general, the oil and gas situation is not as good as that of the East China Sea shelf basin.
9 discussion and conclusion
9. 1 discussion
(1) The geological structural features of the western depression of the East China Sea Shelf Basin are different from those of the eastern depression of the Chongqing East China Sea Shelf Basin and the Okinawa Trough Basin, and it is likely to be an independent structural unit.
(2) The Diaoyu Island magmatic rock belt in the East China Sea Basin was gradually formed in the Oligocene and later, and its southwest was in the process of development and formation. From Oligocene to Miocene, the seawall in the north-central part of the East China Sea shelf basin was formed, but the seawall in the southwest part was not formed. Therefore, the continental sedimentary environment is dominant in the north-central part of the East China Sea shelf basin, and the marine sedimentary environment is dominant in the southwest part.
(3) Due to the late formation of the Diaoyu Island magmatic belt, the author thinks that before Oligocene, the eastern part of the northeastern part of the eastern depression on the east side of the East China Sea shelf basin may be located in the front depression of the present shelf basin, while the southern and western parts of the eastern depression are still connected with the trough depression.
(4) During the geological history from Oligocene to Pliocene, that is, before the opening of Okinawa Trough Basin, Ryukyu Uplift was the eastern boundary of the East China Sea shelf basin. The Okinawa Trough Basin began to expand in the early and middle Eocene (west of the north-central part) and expanded from the middle part to the west part at the end of Miocene. It was in the process of expansion in modern times, which led to the Ryukyu island arc constantly separating from the continental shelf of the East China Sea.
9.2 Conclusion
(1) In its geological history, the East China Sea Basin has experienced many geological tectonic movements, forming numerous NNE and NNE secondary depressions. These depressions are different in time and space. Generally speaking, the west side of the East China Sea Basin formed earlier and became new to the east.
(2) Based on the analysis of the characteristics of sediments, basement under sediments and regional fractured magmatic rocks controlled by the secondary depression in the East China Sea Basin, combined with the characteristics of gravity and magnetic fields, it is concluded that the low uplift in the middle of the central and eastern shelf basin and the western depression in the western region of the East China Sea Basin are intracontinental rift depressions, the eastern depression on the east side of the East China Sea shelf basin is continental margin rift depression, and the Okinawa Trough basin is back-arc rift basin.
(3) Due to the differences in regional structural characteristics of the depression zone from east to west in the East China Sea Basin, the oil-generating target layers and oil-gas accumulation sites are different and should be treated differently.