In the rifting stage of the basin (mainly E2), the overall pattern of central uplift (Dongsha, Panyu and Shenhu Shoal) and bilateral depressions was formed, and various types of lacustrine oil-generating depressions developed in the bilateral depressions, mainly Paleogene continental deposits. In China's inland rift basins, depressions are mostly independent sedimentary systems, and oil and gas generation, migration, storage and accumulation are mostly carried out in the same depression, and most of them are self-generated and self-stored, and self-generated and adjacent. In this way, the loss of oil source is less and the generation and storage coefficient is higher. Because the structural traps in the Pearl River Mouth Basin are not developed, the deep oil sources in the rift mostly enter the overlying marine sandstone through faults and then accumulate in high traps, and some migrate to distant traps for accumulation. Many oil fields have been found in the north of Zhu Yi sag and the south of the sag, but few in the north of the sag. It is not only related to the ancient uplift in the rift period, but also related to the difficulty in forming traps in the marine strata on the northern slope facing the mainland. At present, Liu Hua11oilfield with the largest reserves is located in the overlying reef belt of the rift period of the central uplift, and the oil source mainly comes from the northern sag and migrates to the high part of the uplift through the sandstone transport layer of Zhuhai Formation (Figure 225). From the analysis of Figure 224 and Figure 226, the hydrocarbon source conditions on the south side with deep seawater are more favorable. For example, the high oil-producing areas in the north are limited to Huizhou and other depressions, and the high oil-producing areas in the southwest, such as Baiyun sag, have a wide range, so the gas-bearing intensity is even different [1 10]. At present, the oil and gas achievements are obviously related to the deep construction conditions of seawater.
1. North side of central uplift
Many oilfields (Figure 226a) have been discovered in this area, most of which are rift lacustrine hydrocarbon sources of Wenchang Formation (e 2) and Enping Formation, and they have entered the traps of Zhuhai Formation and Marine Zhujiang Formation (N 1) through faults to form reservoirs, which are covered with regional marine mudstone caprock, and the supporting conditions for oil and gas generation, migration, storage and sealing are good. However, the sag lacks large traps and its reserves are generally small. No authigenic (adjacent) reservoir has been found in Wenchang Formation-Enping Formation sandstone in the depression, so it is considered that the physical properties of deep sandstone become worse [152]. However, when the source rocks enter the stage of oil generation window, the physical properties of adjacent or interbedded sandstone should be good, and if there are timely traps, it should be easy to form reservoirs and preserve them, so exploration in this area should be strengthened in the future. CNOOC recently announced the successful trial production of Wenchang Oilfield Group (19- 1, 15- 1,14-3,8-3), of which 19- 1. Liu Hua11Oilfield (Figure 225) formed under special conditions fully embodies the significance of land-sea transition and marine sandstone as the migration in the upper part of rift valley. Therefore, from the perspective of petroleum system, the uplift between oil-generating depressions and the adjacent regional uplift are deeply studied to find more abundant oil and gas fields. At the same time, because the Neogene coverage in this area is not very thick, the source rocks in the rift valley are still in the oil generation window, which is a favorable area for us to explore oil resources, so we should pay special attention to it.
2. South side of central uplift
The extensional β value of Zhu-2 depression in the south increased to 6 (Zhu-1 depression in the north was 1.4), which actually caused the mantle diapir, high submarine heat flow and abnormal mantle emplacement (Yao Bochu, 1994). From the analysis of hydrocarbon source conditions (Figure 226), the hydrocarbon source of Paleogene rift lacustrine facies and its upper marine argillaceous rocks in the south is better than that in the north of the central uplift, but there are not many oil and gas achievements at present. In recent years, foreign countries have frequently reported the information of deep-sea oil-rich areas, and China has also begun to strengthen this work. The discovery of Liwan gas field [50 1] has aroused great concern. Scholars are increasingly discussing the deep water area in the northern South China Sea [502 ~ 505].
At present, it is mainly a Baiyun sag with deep water and land slopes, covering an area of about 20,000 square kilometers. In different periods of Cenozoic, there are not only continental river and lake deposits, but also shallow sea and deep sea deposits. In recent years, in addition to Liwan gas field, Liu Hua 19- 1, Panyu 30- 1, 34- 1, 29- 1 and 35-/kloc have been discovered in the northern slope of Baiyun sag and its adjacent Panyu uplift and Dongsha uplift. We have a new understanding of the distribution and relationship of source rocks, reservoir rocks and caprocks, which greatly improves the oil and gas prospect evaluation in the deep water area of Baiyun sag.
Fig. 226 Oil generation intensity and oilfield distribution in Pearl River Mouth Basin [1 10]
Although Wenchang Formation (E2) has not directly drilled to obtain the material data of source rocks, according to seismic reflection and sequence analysis, it is considered that the sedimentary thickness of Wenchang Formation in Baiyun Depression has changed greatly (Figure 227), and some sub-depressions are as thick as 6,000m, with the characteristics of low frequency and strong reflection. For the adjacent area, it may be mainly deep lake high-quality source rock deposition. It is speculated that most of them have been buried deep and evolved into natural gas, but the southern uplift of the depression may have a low degree of thermal evolution and still have oil exploration conditions [502,503]. According to a large number of geochemical analysis data in adjacent areas, the average total organic carbon content of source rocks is 2. 19%, and the main type of organic matter is Ⅱ 2. In the relatively shallow buried area, some liquid oil can be generated in the early stage, mainly gas in a large range. The content of organic matter in marine sedimentary argillaceous rocks, central uplift and its north of Zhuhai Formation is generally low and the maturity is not enough. In the Baiyun sag in the deep water area, "the abundance and maturity of organic matter will become better [50 1]". It is speculated that there may be wider marine source rocks under the carbonate platform and deep-water fan of Zhuhai Formation. Therefore, the vast hydrocarbon source conditions in Baiyun sag are the most favorable in the Pearl River Mouth Basin.
Fig. 227 Profile of continental margin depression in the northern South China Sea [50 1]
There are many studies on the sedimentary facies belts of reservoirs in the deep water area of the Pearl River Mouth Basin, mainly the Zhuhai Formation and Miocene Series overlying the source rocks of Wenchang Formation and Enping Formation. There are not only reservoir deltas and deep-water fan sand bodies with good porosity and permeability [503], but also deep-sea carbonate platforms and reef reservoirs (Figure 225), as well as more extensive multi-cycle thick mudstone caprocks. Scholars emphasize various favorable facilities such as Baiyun sag from all angles. Peng Dajun predicted that "the deep-water fan system superimposed on the Paleogene potential source rocks will become the focus of exploration in the Pearl River Mouth Basin [505]"; Pang Xiong and others especially demonstrated the prospect of "gas in the north and oil in the south" in Baiyun sag, but similar prediction became the focus of the next oil and gas exploration [503, 504].
Therefore, the Pearl River Mouth Basin should not only continue to expand the reservoirs in the Central Uplift and Zhuyi and Zhusan Sags, but also focus on the deep water and continental slope areas dominated by Baiyun Sag (Zhuer), take the exploration of large gas fields as the goal, take into account the possible oil fields in the southern uplift zone, and pay special attention to the study on the formation conditions of lithologic-structural compound traps in Zhuhai Formation.
(b) Beibu Gulf Basin
1. Overview
The Cenozoic structure in the Beibu Gulf Basin is similar to the northern part of the central uplift in the Pearl River Mouth Basin, and it cracked under tensile mechanics before the oceanic crust in the South China Sea was pulled open. At the end of K period and E 1 period, small basins were filled sporadically, and the South China Sea oceanic crust was deepest when it was about to open in E2 period. Liushagang Formation is generally a deep lake-semi-deep lake facies deposit. During E3-N 1, the tension gradually eased and seawater gradually invaded. Due to the development of small faults and structures in the rift valley, and multiple sets of mudstone in E2 and E3 strata, there are certain sealing conditions in the rift valley, which is different from the reservoir-forming model of "continental marine reservoir", "lower marine reservoir" and long-distance migration in the Pearl River Mouth Basin. Instead, the authigenic reservoir in Liushagang (E2) is the main body, and the hydrocarbon-generating layer adjacent to it moves up and down and accumulates in the trap of uplift or slope in the reservoir. Locally, it migrates upward through faults to marine drape traps to form oil and gas reservoirs, or migrates to high buried hills to form oil and gas reservoirs.
Paleogene deposits in the rift valley are very thick, exceeding 5000m in the southwest, middle of the sea and Wushi sag (Figure 228a). Together with Neogene (Figure 228b), the buried depth of Paleogene often reaches 7000m. At present, most of the drilling is not in the center of the depression, but above the bulge and slope. The drilling of Liushagang Formation is only about 2000 meters, which cannot represent the physical properties of the center of the depression.
The thickness and organic matter content of source rocks in Beibu Gulf Basin are relatively high. According to the isopach map of mature source rocks of Liushagang Formation [152], there are a considerable number of source rocks in Haizhong, wei county, Wushi and Maichen depressions, and there are 50 ~ 100 m high-strength source rocks in the lower part of the second member of Liu. However, apart from active structures in southwest and Fushan areas, no decent oil and gas fields have been found. In particular, Haizhong sag [152], which has the largest sag area and the largest calculated oil production, has not been formally formed, nor has it been properly analyzed and discussed. Therefore, it can be considered that the oil and gas prospect of Beibu Gulf Basin needs further study and discussion.
2. Geological characteristics of reservoirs in Weixi South Depression.
Weixi South Depression is adjacent to the largest fault in the northern margin of the basin, with active structure and dense faults. The Cenozoic rift is 8000 meters deep and relatively narrow. There are many secondary contemporaneous faults in the depression, which are divided into several secondary depressions (Figure 229). It has been found that the oil field basically surrounds the secondary depression [496]. Most of the oil and gas are closely accumulated in Paleogene marginal faults, fault noses and other traps, a few of them enter Neogene drape traps through faults, and some of them migrate downward or laterally to low-uplift buried hills. Wei 1 1-4 oilfield is a uplift Paleogene drape anticline reservoir located between Wei southwest and Haizhong sag, and its depth is only about 1000 meters. The reservoir has good physical properties and uniform oil-water interface, but the oil saturation is not high. Wei 10-3 oilfield near the main fault near the basin also feels that the reservoir has insufficient aftereffect. These conditions reflect that although dense faults have played a certain role in oil and gas accumulation, segmentation has also led to migration and blockage, making it difficult to accumulate oil and gas in a large area.
3. Analysis of oil and gas potential in Beibu Gulf Basin
The hydrocarbon source conditions in Beibu Gulf Basin are good [152], and the source rocks gradually matured at the end of E, and a number of oil and gas reservoirs may have been formed in the traps formed in the early stage. As N+Q continues to be buried deeply, although new trap conditions are added, the deep migration and reservoir conditions become worse. The new reservoir-forming conditions are becoming more and more complicated, and the early reservoirs will also change. The problem of complexity law will be gradually recognized in new practice and research.
The marine sag with the largest area and oil production and its slopes, fault steps and bulges can be the focus of the next research and exploration. There are seven or eight sub-depressions in the sea [152], and the low uplift and fault zone between the sub-depressions are favorable for oil and gas accumulation. In particular, there are many large and medium-sized structures in the depression, which deserves great attention. The center of Paleogene rift in the sea is in the north, and the center of Neogene depression is in the south (Figure 228). Attention should be paid to the influence on early oil and gas reservoirs and the characteristics of late oil and gas reservoirs. Special attention should be paid to the bulge near the deep depression in order to find some good oil fields.
The Paleogene and Neogene in other depressions are roughly coincident, so traps can be selected and studied in secondary bulges, slopes and fault zones in the depressions. Because of poor reservoir properties and deep burial, it is difficult to find conventional oil and gas reservoirs in the center of the depression.
From the analysis of the structure and development of marine depression, it is possible to form large-scale deep basin gas, which can be specially studied from the geological conditions and feasibility.
Fig. 228 Schematic Diagram of Paleogene and Neogene Sedimentary Thickness in Beibu Gulf Basin [152]
Fig. 229 Schematic diagram of oil and gas migration and accumulation in southern Weixi sag of Beibu Gulf Basin [496]
(3) Qiongdongnan Basin
1. Overview
Qiongdongnan basin was widely transgressed in Oligocene, and a thick ne-trending central depression was formed in Neogene. Most of it is deep-sea area in modern times, covering Paleogene rift, and no Eocene sedimentary body has been seen so far. According to the adjacent data and seismic interpretation, it is considered that Eocene should also be continental strata, and the rift center is deep lake source rock deposition [1 10], which is presumed to be an important oil source layer in this area. But now it is buried deeply, and most of its early oil-generating or reservoir-forming reservoirs evolved into natural gas except the north, which mixed with its Shanghai hydrocarbon source, so it is difficult to trace the oil and gas system of lacustrine hydrocarbon source. Experts speculate that Eocene [11] is the main source rock of crude oil in the north of Ying 9 well and Yacheng1well, because the Neogene in the north is only about 2000m thick, while the central depression is six or seven kilometers thick. The Oligocene Yacheng Formation-Lingshui Formation is dominated by coastal swamp-semi-closed shallow sea facies, and most of the organic matter comes from terrestrial plants, mainly kerogen type III, which is the basis for determining that Qiongdongnan Basin is dominated by natural gas. Neogene is a thick marine deposit, with deep-sea facies to the south, which has certain oil-generating conditions, low organic matter content and insufficient maturity.
The structure of Qiongdongnan basin was active in Paleogene rifting period, and the Neogene was dominated by southward dip monocline, with undeveloped structures and faults and large sedimentary thickness, which led to deep burial of early structures. The lithology of strata thickens to the north of provenance, which is not easy to form the trend of upward dip and extinction, and needs the cooperation of faults and other structures. At present, the exploration work can only be concentrated on the upper wall and the north side of the 1 fault, so it is difficult to carry out the work in the vast central depression zone.
Fig. 230 YC 13- 1 Distribution of pressure coefficient of gas supply system in gas field and Lingshui Formation in basin [1 10]
2. Zhuoran Yacheng 13- 1 gas field.
The gas field is located in the drape structure on the high fault block connected with Yinggehai Basin in the southeast of Qionghai, and has many special advantages: first, the basement uplift anticline structure, and the combination of wedge and structure cut upward in Lingshui Formation and Sanya Formation is well trapped, which is an early structure with multiple activities at the end of Paleogene and the beginning of Neogene (Figure 230a). Sandstones of Lingshui Formation and Sanya Formation have good physical properties, forming a unified gas-water interface respectively. Secondly, the main gas layer is adjacent to Yacheng Formation, an important source rock in this area, with Yan 'an sag in the east and Yinggehai Basin rift under 1 fault in the southwest, including Eocene lacustrine gas source. Can the gas source of the gas field be imported from another basin (Yinggehai)? After a long-term debate and the comparison of various geochemical data, most scholars [1/kloc-0,508,510] think that the natural gas of Yacheng Formation in Ying Ge Basin passes through1fault and then migrates to sandstone reservoirs such as Lingshui Formation. The advantage is that two adjacent hydrocarbon-generating depressions are jointly replenished to form an atmospheric field. Third, it is in a very favorable low potential position in the ground pressure field (Figure 230b). The pressure coefficient of Lingshui Formation in Yan 'an sag and Yinggehai basin deep sag is as high as 2 ~ 2.3, while Yc 13- 1 is basically atmospheric pressure, with large pressure difference and high migration and accumulation energy. Meishan Formation covered by gas is a high-pressure caprock (Figure 230a), on which there are several sets of argillaceous regional caprocks, and the natural gas preservation conditions are ideal.
3. Discussion on oil and gas potential in Qiongdongnan basin
The goal of finding oil fields is not clear, especially in the central depression and its south area, which is difficult to consider because of its deep burial and water. There are several directions in the north of the sag: First, faults in Songnan and Bao Dao depressions in the northeast of the basin communicate with each other, and the oil source of E2 layer was formed in the edge structure of the sag during the rifting period, for example, the Song32-2 overlapping pinch-out structure obtained oil flow in Well Ying 9 [152]. Second, the Neogene marine source-reservoir assemblage is generally inclined to the south. The Neogene in Qiongdongnan basin is a gentle slope platform, and the seawater frequently advances and retreats. Most marine sand bodies are not easy to die up in the direction of provenance and can only be trapped under special conditions such as branches, steps or faults. It is very important to study and delineate marine sand bodies. Through comprehensive seismic interpretation, it is considered that Ledong (LD)30- 1 has large sand bodies with east-west distribution, with an area of over 500km2, but Well Ledong 30- 1- 1A is a high pressure potential area (2.0), and only gas logging anomalies are found. Gas reservoir was discovered in Yc35- 1 anticline (Liuhuang Formation, 4600 meters), a large sand body in the central depression. On the seismic profile between Ledong 30- 1- 1A and Yc 13- 1- 1 (Figure 23 1), it can be seen that there are obvious large-scale progradation structures in the shallow layer, which are covered with dense reflections. It is possible to find important lithologic reservoirs by studying the relationship between the sedimentary structure and distribution of Neogene marine sand bodies and marine hydrocarbon source beds. Thirdly, as long as there is a channel connecting hydrocarbon sources, the reefs and beaches on the edge of Neogene carbonate platform have their own sealing ability.
Fig. 23 1 C-427-84 sequence boundary and sedimentary facies interpretation profile [1 10]
The potential of natural gas in Qiongdongnan basin is much greater than that of oil reservoirs. Originally a source rock, it has evolved into natural gas due to its deep burial in the later period. For example, in the south and west of Yc 13- 1 gas field, the E2 formation is a deep lake facies, and the E3 formation is a marine source rock, which all existed in the form of gas in the later stage. In addition, the source rocks and coal seams of littoral marine kerogen III are mainly natural gas.
Although there is a certain potential for oil and gas in Qiongdongnan basin, most of the source rocks belong to medium-poor and good grades, especially the Neogene marine organic matter content is low. There are not many large structural traps, and the larger traps are mainly deep sand bodies, buried hills and fault blocks, and the reservoir-forming conditions and later reconstruction are complicated. The reappearance of YC 13- 1 is still unclear, and most of them are in deep water conditions. Therefore, in the near future, in-depth research is still the main task. First, in the area north of the central depression, where the sea water is shallow and the burial depth is not very deep, a number of targets are scheduled and selected for exploration, with a view to making new breakthroughs in the law of reservoir formation and gradually advancing. At the same time, organize forces to carry out research on deep-water areas and deep-buried areas in central and southern China.
(4) Yinggehai Basin
1. Overview
The structure, orientation and reservoir-forming mechanism of Yinggehai Cenozoic basin are quite different from other basins in the northern South China Sea (see Figure 223). Yinggehai Basin is not only in the mechanical range of the South China Sea oceanic crust extension, but also influenced by the relative tectonic activities of Indian zhina landmass and Chinese mainland. In the early Cenozoic, it gradually changed from extensive filling and continental rift lake basin deposition to ocean and even deep sea. It is speculated that the crust was thinned to 5km at that time (the current crust thickness is 22km- the Cenozoic sedimentary rock thickness is17 km) [110], which is close to the thickness of modern oceanic crust in the South China Sea. Obviously, the development of Yinggehai basin is directly related to the rise of mantle flow. During the transitional period of Paleogene, petroleum geological conditions may be very active; However, it is difficult to explain the generation, storage, migration and accumulation of Paleogene oil and gas due to the Neogene marine sediments of nearly 10 thousand meters. Due to the high pressure compaction caused by thick marine argillaceous rocks and high deposition rate, the ultra-high pressure and high temperature of the basin are prominent, forming a huge high pressure sealing box. The local breakthrough is that high-pressure hot fluid pierces, and deep ultra-high pressure gas escapes into shallow near-normal pressure traps to form gas reservoirs, such as Dongfang 1- 1 (which has become the largest gas field in China's sea area), Ledong 22- 1, Ledong 15- 1 and other gas fields [5/Kloc-0]
2. The hydrocarbon source conditions are good.
The structure of Yinggehai Basin is active in the Early Tertiary, and the sediments below Lingshui Formation are mainly rift lacustrine sediments (Figure 232), with high organic matter content in lacustrine argillaceous rocks. The Paleogene source rocks gradually matured in the marine depression and drape deposition period at the beginning of Neogene, and also developed rift marginal faults, traction anticlines and stratigraphic lithologic traps. It is speculated that there was a reservoir-forming peak, which may be larger than other basins in the northern South China Sea.
Neogene rifting weakened, forming a relatively unified depressed marine sedimentary basin. Shallow sea and deep sea circulate many times, developing from northwest to southeast (see Figure 223). The Neogene sedimentation rate is very high, with a total thickness of 10,000 meters. At present, it is still in the stage of rapid subsidence, and many sets of undercompacted sediments form a unique ultra-high pressure and high temperature basin.
Marine argillaceous source rocks in Yinggehai Basin developed very well after rifting (Neogene). Due to the large sedimentary thickness and high ground temperature, the maturity threshold is about 2200 m [5 12], which is mostly mature-over-mature. The organic carbon content in shallow source rocks is generally low, but the organic carbon content in the deeper part of Well Q (3750 ~ 4535 m) is high (1.52% ~ 3.03%) [513]. This sample is coring from the sidewall, and it is considered to be a good source rock after the treatment of removing drilling fluid pollution, which has great hydrocarbon generation potential.
The Neogene marine facies in Yinggehai basin is wide, and the thickness and maturity of source rocks are high, but the Neogene structure is not developed. There is no example of how to form conventional oil and gas reservoirs under ultra-high pressure in the basin, so it should be carefully explored.
Fig. 232 Vertical profile of subsidence center change in central and western Yinggehai Basin [1 10]
3. Diaphragm hot fluid fracturing secondary gas reservoir
At present, most of the known gas reservoirs in Yinggehai Basin are in the normal pressure area of shallow immature area, which is formed by the vertical migration of deep high-pressure gas to the top drape structure through diapir piercing. Early experts emphasized the role of mud diapir; Through geochemical analysis and prestack depth migration, it is considered that there is no mud diapir, but an "active thermal fluid diapir structure" [5 12, 1 10]. Or it is expounded that the tensile stress field in the basin forms a shear fracture, which triggers the diapir activity of mud-hot fluid with high temperature and high pressure. The shear fault in the upper part of mud diapir structure provides a channel for the upward migration of natural gas, and the upward release of overpressure also provides power for the centralized migration of natural gas. Dong et al. made a special discussion on this mechanism [5 12], and divided diapir thermal fluid fracturing into three stages: (1) formation of overpressure fluid sac; (2) the turtle's back is arched; (3) Fracture puncture (Figure 233). It is clearly shown in LD8- 1 seismic profile, and examples of "fuzzy zone" gas column and shallow cross-layer fracturing group can be seen.
4. Analysis of oil and gas potential in Yinggehai Basin
The overall hydrocarbon source area, thickness and abundance of different organic matter in the basin are second to none in the northern South China Sea basin. But at present, oil and gas fields and reserves have been discovered, except Dongfang 1- 1. As far as oil and gas preservation conditions are concerned, the upper sets of regional caprocks are excellent, and even the diffusion of hydrocarbon sources is minimal. The key point is that Neogene marine sediments are very thick, and the center can reach 10 thousand meters. The rapid deposition rate leads to the formation of ultra-high pressure due to undercompaction, high geothermal gradient, very strong hydrocarbon generation evolution, and high-pressure gas continues to squeeze into sandstone. Overload makes the porosity of reservoir decrease, and the phenomenon of ultra-high pressure becomes more serious, which gradually becomes a huge high-pressure sealing box. Neogene faults and local structures are undeveloped, and the relationship between gas and water in the box is very complicated, so it is very difficult to migrate horizontally and vertically.
Fig. 233 Fracture Causes of Geothermal Fluid in Ying Ge Basin and Example of Seismic Profile [5 12]
At present, the formation of shallow Neogene oil-generating window reservoir is not clear. The Neogene shallow underground organic carbon content is low (0.45% ~ 0.55%) [511], and the oil generation window of high geothermal gradient is very narrow, which is mainly gas display in drilling. However, due to the uneven distribution of drilling wells, attention should be paid to tracing and exploring oil resources as much as possible in the case of high-quality source rocks in a wide source window in the future.
5. The objective natural gas reserves in Yinggehai Basin may be very large.
Because the Cenozoic continental and marine source rocks have many combinations, large areas and high total hydrocarbon generation, and most of them exist in the form of gas, the regional sealing ability is strong and the number of gas leakage and diffusion will not be great. However, the reservoir-forming conditions are quite complicated. They are discussed as follows:
First, the source rocks in Paleogene (mainly E2) rift lake basin were deposited in Oligocene marine depression, and the lower source rocks gradually matured to generate oil. According to the rift law, they migrated from the center of the depression to the high parts or bulges on both sides of the rift, and gathered a number of early reservoirs, all of which evolved into natural gas in Neogene. When the conditions for promoting diapir were insufficient in the early stage, natural gas not only entered the overlying marine sandstone vertically through faults, but also was mostly sealed in ultra-high pressure tight sandstone layers. The diapir triggered by the late structure may spread to the deep layer (Figure 233), and the "fuzzy zone" reaches the seismic wave depth of 5.0 seconds, but most gas layers will remain in the deep seabed.
Secondly, the Oligocene and Neogene marine sediments with a thickness of nearly 10,000 meters contain multiple sets of source rocks, with unbalanced organic matter content and narrow oil generation window. Most of the gas is generated or evolved into gas, which enters the adjacent sandstone, and the upper sealing condition is particularly good. The ultra-high pressure phenomenon is partly due to under-compaction, which is mainly related to continuous hydrocarbon generation and charging. Due to the continuous load, ultra-high pressure will inevitably occur in the process of sandstone densification. Neogene structure is relatively stable, lacking faults and folds, and natural gas migration is not smooth both vertically and horizontally. The central area of the basin has become a huge high-pressure sealed box and a huge "deep basin gas". The total amount of high pressure gas filled in Oligocene and Neogene sand bodies is amazing! If the trigger conditions are met, high temperature and high pressure gas may pierce upward, but it is only a small part of this gas-bearing field. A large number of high-temperature and high-pressure gases are still stored in different degrees of tight sandstone, and high-pressure gases buried in tight and ultra-tight sandstone below 6500 meters are not easy to explore and develop at present. Can "deep basin gas" and conventional reservoir ultra-high pressure gas above 6500m be developed in the sea area? How to develop? It is necessary to study the law and feasibility of reservoir formation.
Third, in the late and recent period (about 5Ma), the structure was relatively active, and the north-south direction derived from dextral torsion was strongly compressed, or it was transformed and extended in the central depression zone, forming an echelon NS-directional shear fault [5 1 1]. It causes deep ultra-high pressure gas to pierce and migrate upward, forming atmospheric secondary gas reservoirs in shallow traps. This is the most realistic field in the near future. Of course, if there are no matching traps and caprocks, the natural gas pierced upward by the diapir will easily be lost or migrate upward along the slope in porous sandstone. Many air seedlings were seen on the west coast of Hainan Island, which was an escape phenomenon.
There are five rows of diapir structural belts [5 1 1] in the central depression, and all newly discovered gas fields are related to this. It is necessary to concentrate some scientific research forces on in-depth research and planning to form a new round of offensive in the basin and strive to find more gas fields and reserves similar to Dongfang 1- 1. At the same time, carry out exploration of ultra-high pressure and larger natural gas resources in the middle and deep layers.
(5) Southwest basin of Taiwan Province.
Southwest Taiwan basin is located at the intersection and transformation of various structures, and was driven by the South China Sea crust to dive under Luzon Island Arc in the late stage. The Cenozoic geological structure is different from other basins in the northern South China Sea, especially in Paleogene, which lacks large rift-type continental lake basins. Tainan sag has the thickest sediment and intensive drilling on the island, which does not prove the existence of early Cenozoic rifting. The basin is dominated by Neogene marine sediments, with a maximum thickness of 8500m and a bulge of about 2500m. The Miocene (N 1) in the center of the depression can reach 4000m, which is considered as the main source rock. CFC- 1 is a high-yield oil and gas well in the oil and gas fields in the south of Taiwan Province Island and the Oligocene sandstone with low uplift in the sea area, which may all originate from Oligocene [15 1].
On the whole, there is no thick and high-quality hydrocarbon-generating layer, and it may get better in the deep depression without drilling. The structural and stratigraphic lithologic traps in the basin are relatively developed, and the reservoir physical properties are also very good. From the analysis of drilling oil and gas, it seems that the hydrocarbon source is not rich enough and the stamina of oil and gas production is insufficient. The deep fault depression in the basin is not obvious, but the shallow faults are active (Figure 234), so whether large oil and gas fields can be formed and preserved is worthy of attention. Moreover, most of the sea areas are deep-water areas, so it is necessary to strengthen the analysis and research and have a new understanding before starting a new round of work.
(6) Cenozoic rift basin in northern South China Sea (southern South China)
In Cenozoic rift (mainly E2), continental rift and sea basin have the same extensional rift model, and both of them are mainly continental deep-water and semi-deep-water lacustrine hydrocarbon sources. However, the "three-layer" type (fault depression and drape) of the general rift basin is not developed on the land of South China, and it does not continue to sink after the rift, lacking depression and drape stages, and Paleogene strata and faults are directly exposed to the surface (see Figure 235). Generally speaking, there is no regional sealing system, especially the Neogene marine argillaceous caprock, which leads to serious oil and gas losses. Sporadic small reservoirs can only be formed by local caprocks, such as Sanshui basin, and it is impossible to gather into large and medium-sized oil fields in a large area. Because of shallow burial, some source rocks are immature and generally have not reached the stage of thermal evolution gas, so it is impossible to have a good gas field.
In some basins, the hydrocarbon source is close to the basement buried hill, such as Baise Basin (Figure 235b), which is covered by the whole Paleogene, and the lower T2 limestone is dissolved as a reservoir, which can form several small reservoirs [1 16]. In addition, some basins have exposed high-quality source rocks of Eogene, such as Maoming Basin, which has considerable oil shale resources [152].
Fig. 234 Schematic diagram of structure and seismic profile of southwest basin in Taiwan Province Province [15 1]
Fig. 235 Stratigraphic distribution and profile of Cenozoic fault basin in southern China [152]