(1) exploration stage
Oil and gas exploration in Qiongdongnan Basin began in the late 1950s, and has gone through three stages: (1965), self-operated exploration (1965 ~ 1979), cooperative exploration and self-operated exploration (1979). At present, the basin is still dominated by structural trap exploration, which is in the initial stage of exploration. By the end of 2005, 37 pre-exploration wells and appraisal wells had been drilled, and gas fields 1, Oketa 1, oil-bearing structures 1 1, accumulated proven geological reserves 1037.9× 108 m3 were discovered, and accumulated natural gas was produced.
(2) expanding the field of exploration
1. Cliff 13- 1 gas field exploration
Using new technology, new method and new theory to evaluate early basins. Seismic data acquisition in Qiongdongnan basin began at 1979, and the best seismic data was obtained by using the advanced data acquisition and processing technology at that time. At the same time, seismic stratigraphy method is used to comprehensively interpret and evaluate seismic data. Therefore, Yacheng 13- 1 structure is selected as the first batch of pre-exploration structures.
In the early stage of basin exploration, well Yacheng 13- 1- 1 was located on the uplift-covered anticline structure of Yacheng 13- 1 basement and on the west side of hydrocarbon-rich depression. Through the detailed description of seismic data, the thickness and distribution of sandstone in the third member of Lingshui Formation are accurately determined, and the well location is adjusted to ensure the success of drilling. After the gas reservoir was discovered in the pre-exploration well, the gas reservoir evaluation, engineering evaluation and economic evaluation were immediately carried out, and the commercial development of the gas field was quickly realized.
2. There are no significant findings in three main areas.
After Ya 13- 1 gas field was discovered in 1983, no major discoveries were made in the other three gas fields. First, rolling anticline structure (Yacheng 8-2), large broken nose structure (Yacheng 8- 1, Songtao 3 1-2) and drape anticline structure (Yacheng 14- 1, 7-4, Lingshui) were drilled around the northern depression belt. Second, Miocene Yinggehai Formation-Liuhuang Formation turbidite sandstone body and high-temperature overpressure compression anticline structure, and no gas field has been found; Third, there is no major breakthrough in oil and gas exploration in the drape structural belt of Yan' an sag.
3. New progress has been made
After many years of silence, in 2002, Ya 13-4 gas field and Ya 13-6 gas-bearing structure were discovered in Yacheng Uplift, and the exploration in the eastern part of the basin also made some progress, which brought some vitality to the basin exploration. At the same time, some new exploration fields put forward newer research topics (such as high CO2 content in some gas-bearing structures). ), which requires exploring new fields and making continuous efforts.
Second, basin characteristics and trap types
(A) Basin characteristics
1. Basin evolution
Influenced by the tectonic movements of the three plates, the Cenozoic basin in the southeast of Qionghai developed in extensional tectonic environment and experienced two stages of evolution: extension and post-cracking, with a double-layer structure. The faults in the upper structural layer are undeveloped, showing a unified depression, while the faults in the lower structural layer are developed and the basin is divided.
2. Geological conditions of oil and gas
Three sets of source rocks are developed in Qiongdongnan Basin: Eocene lacustrine mudstone, Yacheng Formation semi-closed marine mudstone and Meishan-Sanya Formation shallow-semi deep-sea mudstone. Only the coal measures and semi-closed marine mudstone of Yacheng Formation have been proved to be effective source rocks by exploration.
The two stages of tectonic evolution in Qiongdongnan basin controlled the development of sediments, formed a multi-layer system and various types of reservoirs, and formed four sets of reservoir-cap assemblages. The first set of reservoirs is beach sand, shoal sand and fan delta sand of Yacheng Formation, and the caprock is shallow sea mudstone in the second member of Yacheng Formation or its upper part. The second set is composed of fan delta, coastal sand, shoal sand and reef (beach) limestone in the third member of Ling, and mudstone (local Meishan Formation) caprock in the second member of Ling. The third set is composed of beach sand and shoal sand of Sanya Formation, and the shallow mudstone of Sanya Formation and Meishan Formation is the caprock. The fourth set of reservoirs is Neogene low tide fan, channel turbidite sand and erosion valley, and the caprock is upper semi-deep marine mudstone.
The geothermal gradient in the west of the basin is 4 ~ 4.5℃/hm, with overpressure-strong overpressure (pressure coefficient 1.2 ~ 2. 1).
(2) Trap type
In several structural belts of Huanya South Depression, structural traps such as drape anticline, fault anticline and fault nose of Lingshui Formation, Sanya Formation and Yacheng Formation and stratigraphic traps related to Neogene are mainly developed. Fault anticlines and fault noses related to boundary faults, as well as fault-related fault blocks and stratigraphic overlapping traps in slope belts are mainly developed in fault structural belt V. Fault structural belt ⅱ mainly includes fault anticline, fault block and stratum super-complex pinch-out traps, as well as fault nose and fault block traps in the descending plate of fault ⅱ. There are many planetary belt traps in the northern reef uplift zone in the deep water area of the southern depression. Various types of turbidite sand bodies have also developed in the Neogene Yinggehai Formation.
Three. Distribution characteristics of oil and gas resources
(1) Total oil and gas resources
The petroleum geological resources in Qiongdongnan Basin are 2.72× 108t, and the natural gas geological resources are1142×108m3. By the end of 2005, the accumulated proven petroleum geological reserves were 0.03× 108t, and the proven petroleum geological resources were 2.69× 108t. Proven geological reserves of natural gas 1038× 108m3, geological resources to be proven 104× 108m3, and the proven degree of natural gas is 8. 12%.
(2) Distribution characteristics
There are few oil resources in Qiongdongnan basin, and the commercial value of exploration and development in deep water area is limited. The potential petroleum resources are mainly distributed in Song Dong sag and Songxi sag in the northern depression, and may also exist in the southern North Reef low uplift in the deep water area. Among them, the former is located in shallow water, accounting for 4 1.3% of the resources, but the scale of the depression is small, and it is expected to find some medium and small oil fields. Beijiao-Nanwa (that is, Beijiao Depression) is located in deep water area, and the calculated petroleum geological resources are 1.0× 108t, which has limited commercial value for deep water exploration and development. 80% of oil resources are buried below 3500 meters, mainly conventional oil. There is no commercial discovery yet.
Natural gas resources are concentrated and have great exploration potential. Natural gas resources are mainly distributed in Lingshui and Yan 'an sag, accounting for 36.4% of the basin area. 4 1.2% of natural gas resources are distributed in shallow water, and the rest are in deep water. It is predicted that the depth distribution of natural gas is mainly shallow and middle layers, accounting for 54.5%, and deep and ultra-deep layers account for 45.5%. According to the characteristics of gas reservoir seepage, natural gas resources are mainly conventional gas, but formation overpressure generally exists in the deep layers of Ledong sag, Lingshui sag and Yan 'an sag in the southern depression zone. In some areas, high-content carbon dioxide gas reservoirs connected with the deep have been discovered. At present, the commercial discovery of the whole basin is limited to Yan 'an area, and there is no breakthrough exploration progress in the eastern part of the basin. The deepwater area is still under evaluation and drilling has not yet been carried out.
Resource potential
The potential petroleum resources in Qiongdongnan Basin include Song Dong sag and Songxi sag in the northern depression, and may also exist in the low uplift of the southern northern reef in the deep water area. Song Dong sag and Songxi sag are small in scale, and some small and medium-sized oil fields are expected to be discovered. At present, the calculated geological resources are 0.8× 108 ~ 2.7× 108 t ... Beijiao-Nanwa (i.e. Beijiao sag) is located in deep water area, and Paleogene has a clear dustpan-shaped fault depression structure, so the thermal evolution degree may be moderate. The calculated petroleum geological resources are 0.3×108 ~1.6×108t, and the estimated value is 1.0× 108t.
The natural gas geological resources of eight depressions in Qiongdongnan Basin, namely Yan 'an, Yabei, Ledong, Lingshui, Songnan, Bao Dao, Changchang and Beijiao, are calculated. At present, only Yan 'an sag has been found to have commercial value. The natural gas geological resources to be discovered in Yan 'an sag are 686× 108m3. At present, 7 depressions have not been discovered commercially, and the geological resources calculated by analogy method are 94 18× 108m3. Because the overall exploration degree of Qiongdongnan basin is not high, especially a considerable part of the exploration potential is located in the unexplored deep water area, the natural gas exploration potential is still large.
Four, oil and gas reserves, production growth trend forecast
(1) Prediction of natural gas reserves and production growth trend
1. Prediction result of comprehensive method
According to the long-term development plan of Yinggehai Basin of China Offshore Oil Corporation, and referring to the prediction results of expert evaluation method, the multi-cycle harbert model is used to predict the growth of natural gas reserves and production.
Qiongdongnan basin 1990 reported that the proven geological reserves of natural gas in Ya 13- 1 08m3, and the discovered reserves in 2003 153.0× 108m3. At present, the cumulative proven degree has reached 9.3%.
It is estimated that more natural gas reserves will be discovered in Qiongdongnan Basin in 20 10 years, with the average annual reserves of 55× 108, 142× 108 and195×/kloc-respectively every five years from 2006 to 2030. From 2015 to 2025, there was a peak of reserves discovery, and by 2030, the accumulated proven reserves were 4268× 108m3, with a proven degree of 38.3% (Table 7-12-1; Figure 7- 12- 1).
Table 7- 12- 1 Comprehensive Forecast Table of Natural Gas Reserves and Production in Qiongdongnan Basin
Fig. 7- 12- 1 comprehensive trend forecast of proven geological reserves of natural gas in Qiongdongnan basin
Fig. 7- 12-2 comprehensive trend forecast of natural gas production in Qiongdongnan basin
Since Ya 1995 was put into production, the cumulative output of Qiongdongnan Basin has reached 3 16.6× 108m3. According to CNOOC's plan, the level of 30× 108 ~ 40× 108m3 will be maintained before 2020, and the natural gas output will increase rapidly after 2020, reaching 80× 108m3 by 2030. The average annual output every five years from 2006 to 2030 is 32× 104, 34× 104, 38× 104, 45× 104 and 67× 104t, respectively, to 2033. Figure 7- 12-2).
2. Compared with the results predicted by experts.
Experts believe that Qiongdongnan basin is a gas-rich basin, and natural gas should be developed in the next 25 years. The southern part of the basin has a broad prospect, and important discoveries will be made in the future (Table 7- 12-2). The comprehensive forecast result of natural gas reserves is close to the low value of expert forecast, and the production forecast value is close to the median value of expert forecast. Generally speaking, it is in the middle and low value area predicted by experts.
Table 7- 12-2 Expert Forecast Results of Natural Gas Reserves and Output Growth Trend in Qiongdongnan Basin Table Unit: 108m3
(2) Analysis of forecast results
1. The resource potential is great, but the exploration process is complicated.
The proven petroleum geological resources in Qiongdongnan Basin are 2.69× 108t, and the proven natural gas geological resources are 104× 108m3, which has great potential for oil and gas resources. On the other hand, the controlling factors of oil and gas accumulation in Qiongdongnan basin are still unclear, especially the understanding of source rocks is different. Neogene traps lack good oil and gas diversion system and are not easy to form reservoirs. In addition, the formation conditions of high temperature and overpressure increase the difficulty of drilling. So far, oil and gas exploration in Qiongdongnan Basin has gone through nearly 50 years, and 37 pre-exploration wells and evaluation wells have been drilled, only gas fields 1 and Oketa 1 have been discovered. The exploration process can be described as complicated.
2. Small and medium-sized gas reservoirs are the main reserves of natural gas in the future.
The prediction results show that the accumulated natural gas reserves in Qiongdongnan Basin from 2006 to 2030 can reach 3230× 108m3, and it is possible to find several gas fields of 200×108 ~ 300×108m3, such as Ya13-/kloc-. In addition, it is possible to find large gas reservoirs in deep-water lithologic traps.
3. Natural gas production will increase slowly before 2020, and then increase rapidly.
Since Ya 13- 1 was put into production in 1995, the annual natural gas production in Qiongdongnan Basin has been maintained at about 30× 108m3, and it rose slowly before 2020, and rapidly after 2020, reaching 80×1030.
4. Three areas of natural gas exploration
South Asia sag is the most realistic exploration area. The exploration degree of Huanya 'nan sag is not very high, and some fault nose structural traps are large in scale, even exceeding 20km2, which are in normal pressure zone and have good reservoir-forming conditions. There is no proven oil and gas reserves in Lingshui low uplift -2 fault zone at present, but it may become a replacement field for natural gas reserves discovery in shallow water area of Qiongdongnan Basin. The potential geological resources of Lingshui sag, Songnan sag and Baodao sag reach 5000× 108m3. Neogene non-structural lithologic traps, such as low-level sand bodies and lithologic anomalies, are developed in the modern continental slope of Qiongdongnan Basin and the deep water area south of it. Multi-layer low-water turbidite sand bodies, mainly slope fans and basin floor fans, have large sand body area (mostly between 50 and 50~500km2) and shallow burial, which is expected to become one of the potential exploration fields in the future.