Central Hubei has a small core. Craton platform type (Z2-O 1) deposits are distributed in Wuhan and Jianghan, with a thickness of less than 2,000 m, mainly carbonate rocks. The typical Cambrian Qianzhusi Stage embodies the most obvious characteristics of platform facies [99] (see Figure 28), with high content of carbonate rocks, extremely low content of clastic gravel sand and deepwater rocks, and low content of argillaceous rocks. However, the sedimentary thickness of the southern slope is slightly larger, and then the stratum thickness decreases rapidly from the south to Changsha, mainly argillaceous rocks and deep-water rocks, indicating that it has entered the deep-water basin (see Figure 18a and Figure 82). However, after the Middle Ordovician, the situation began to change, and the Silurian was more prominent. The thickness between the edge of the middle Yangtze platform and the middle and upper Yangtze suddenly increased, and the debris began to increase. The provenance obviously came from the original Nanhua block, and the deep-water basin disappeared, and the two blocks were connected, resulting in a foreland depression (see Figure 82, end of S). During this period, there was no sign of mechanical collision between the two continents. At the end of Silurian, Guangxi moved on the former South China landmass, resulting in unconformity under Devonian and metamorphism including Silurian and its lower strata. However, the thick, metamorphic Silurian argillaceous clastic rocks in the middle Yangtze region have greatly increased the abundance of hydrocarbon sources in this area and become important regional caprocks.
Fig. 85 Thickness Map of Lower Paleozoic Source Rock in Middle Yangtze Region [274]
The Lower Paleozoic oil and gas giant system in the middle Yangtze region is greatly influenced by the later geological structure development, and its mechanism is very complicated and has different discussions. But the basic course is as follows: before Indosinian and early Yanshan, the structure was dominated by regional uplift and depression, which was the most favorable stage for oil and gas accumulation; In the middle and late Yanshan period, detachment, nappe and fold were extremely strong, which made many reservoirs of Lower Paleozoic exposed or close to the surface and destroyed. Extensional rifting in late Yanshanian and Himalayan periods. The degree of tossing is much more serious than that in the upper reaches of the Yangtze River. According to the free block and the late tectonic changes, it can be discussed in three parts:
1. Jianghan area (original platform)
Mainly in the core area of central Hubei. Before Indosinian Movement, the source rocks of Lower Paleozoic were gradually or highly mature, and the oil and gas of two slopes (including S deep depression) tended to migrate to the platform area (see Figure 82), which made up for the deficiency of the source rocks of the platform itself. After Indosinian period, the platform edges on both sides rose, especially the Jiangnan uplift (see Figure 82, Indosinian period), and oil and gas were no longer supplied to the platform.
Before the structural collapse in the middle and late Yanshan period, the Lower Paleozoic oil and gas in Jianghan area were distributed according to Indosinian uplift and depression, and the more important ones were the northwest Zhongxiang-Qianjiang uplift and Huangling uplift and Honghu-Puyin uplift controlled by Dabie Mountain [27 1] (Figure 86). These regional uplifts are favorable places for timely migration and accumulation of oil and gas in Lower Paleozoic. Because many sets of caprocks such as Silurian were intact at that time, oil and gas accumulation also had a certain scale.
Subsequently, the strong tectonic stress caused by Yanshan movement in the storm warning and the revival subduction of Qinling-Dabie ancient suture zone in the north and Cathaysian ancient suture zone in the south pushed to the middle Yangtze platform, and gradually changed from thrust to fold thrust, forming a thousand-mile thrust belt along the Yangtze River (see Figure 23). Sometimes the intersection of the two forces leads to a strong arch uplift, and even the basement of Huangling uplift is exposed (Figure 86b). The upper T-J stratum of Indosinian uplift was completely denuded, and the upper wall of some faults also squeezed out the Lower Paleozoic. The Lower Paleozoic is generally preserved in the lower structural area or the footwall of the reverse slip fault. Yanshan movement in the middle Yangtze platform area is both destructive and a new oil and gas accumulation period. According to Yanshan structure, the oil and gas accumulated in Indosinian paleouplift were redistributed and accumulated. The damage of Yanshan movement to Indosinian depression (syncline) is much less. For example, in Dangyang and Mianyang (Shenhu-Tutang), the Yanshan movement is relatively weak, and most of the strata below Jurassic are preserved (Figure 86b), and there are a series of caprocks under it, which provides the basic conditions for searching for lower Paleozoic oil and gas reservoirs. Recently, it has been found that there are a series of local structures in Dangyang syncline [275] below Yanshan unconformity, and there are secondary arches in Indosinian period, and the later rifting transformation is not serious.
In the early days of various favorable ancient uplifts and traps in Jianghan area, after the Yanshan storm, new turmoil followed. Since the Late Cretaceous, due to the uplift of the mantle behind the Chinese mainland B-type subduction arc of the Pacific plate, a complex basin dominated by extensional faults has been formed. Some scholars emphasize the sealing effect of Cenozoic sedimentary rocks on deep oil and gas and the significance of secondary hydrocarbon generation in Upper Paleozoic, which is beneficial on the one hand, but the destruction on the other hand can not be ignored. Xiao et al. [27 1] believe that most of the extensional faults developed in this area destroyed the structural oil and gas reservoirs formed in the early stage of this area. Finding the weak extensional deformation zone is the key to local oil and gas exploration. Yang Yujiang [275] also thinks that Cenozoic sedimentary basins in Jianghan area were developed and formed on the basis of negative inversion of Paleozoic structures. The Paleozoic strata in the fault depression are generally buried deeply, and the degree of erosion is often serious because of the high uplift in the early stage. Jianghan drilled the Lower Paleozoic wells (He 1, Hai 1 and Zhu 4) in Yanshan syncline area, and the formation water showed desalination characteristics, and no abnormal pressure was found, reflecting that the plugging conditions were not ideal [276].
The Lower Paleozoic oil and gas system in Jianghan area has experienced a very tortuous development, and contradictions and even contrasts often appear in geological history evaluation in different periods. Due to the coverage of Cenozoic strata, the overall evaluation is difficult to improve. We must use the concept of geological historical development to examine the advantages and disadvantages of oil and gas in each period, and the key is the final result. In combination with geology and earthquake, various traps of Lower Paleozoic are searched and interpreted, and comprehensive evaluation of hydrocarbon source conditions is combined to optimize exploration. From the macro analysis of the current data, the ancient uplift in Jianghan area is not better than the ancient depression area. The preservation conditions of Dangyang syncline and Shenhu-Tutang are relatively favorable. In addition, it is easy to form a good structural trap in the deep part of the structural thrust belt near the river.
The middle Yangtze platform area should focus on the sedimentary distribution and present situation of Silurian, which is not only the best marine hydrocarbon source bed in this area, but also an important reservoir and regional cap rock. In addition to its own reservoir-forming significance, it can directly provide rich hydrocarbon sources for the Upper Paleozoic like East Sichuan, which is a part of the Lower Paleozoic oil and gas giant system and may make an important contribution to this area. In addition, due to the good sealing conditions of Cenozoic rock salt and gypsum-bearing layer, the deep lower Paleozoic natural gas or its upward migration natural gas can be protected. In addition, after the Indosinian and Yanshanian uplifts, they were rifted and buried again, and some source rocks generated hydrocarbons for the second time, which reorganized mixed-source oil and gas reservoirs in the late structural framework traps.
Fig. 86 Tectonic framework of Indosinian and early Yanshanian in Jianghan area [27 1]
2. The southern edge of the middle reaches of the Yangtze River
The hydrocarbon source abundance of Lower Paleozoic in the paleopassive continental margin on the south side of the middle Yangtze is very high, especially? The source rocks in Wufeng, Enshi 1 and S layers are 1000 m thick, and the source rocks in Xiushui and Changsha are all over 2000 m (see Figure 80 and Figure 85). Because it is close to the tectonic active zone, the variation range of each geological tectonic movement period is very large. In the early Early Paleozoic (Z2-O 1), the thickness of sedimentary rocks (see Figure 79), the content of carbonate rocks, the content of argillaceous rocks and the content of deep-water minerals (Figure 3 1) all have the dense lines of continental slope, and the argillaceous rocks favorable for oil generation have the highest value on the slope. In the late Early Paleozoic (O2-S), the deep-water basin gradually disappeared. In Late Ordovician, only turbidites from southern terrigenous sources were deposited in deep-sea facies, and there were no deep-sea basins in Silurian. The detrital grains become thinner from the SE-NW direction, indicating that Cathaysian land block is the main provenance supply area [62], and basins are formed on the south side of Yangtze land block, with a high proportion of argillaceous rocks in shallow sea. The superposition of multi-stage source rocks is an ideal oil source high abundance zone. Dolomite and marine sandstone in Group Z-O platform margin have good reservoir conditions.
After Caledonian movement, the area was mostly underwater uplift, and the hydrocarbon source of Lower Paleozoic gradually matured. The Indosinian movement was forced to form the Jiangnan (Xuefeng) uplift belt due to the resurrection and subduction of the ancient suture zone. Part of Middle and Lower Triassic and Upper Paleozoic strata were denuded. At this time, the lower Paleozoic hydrocarbon source with deep burial has matured to a high maturity stage, and the original large slope has very favorable reservoir and contemporaneous trap conditions. The huge Indosinian paleouplift is an ideal place for oil and gas migration and accumulation. Until the early Yanshanian, the huge oil and gas areas of Lower Paleozoic were still gathering.
It was after the middle Yanshanian period (early in the east and late in the west) that the ancient reservoirs in the Lower Jiangnan Uplift were really destroyed. With the revival subduction, the contradiction between deep, middle and shallow lithosphere intensifies, and huge waves separate layer by layer, exposing a large area of basement, which is called "Jiangnan ancient land". In fact, in the early Early Paleozoic, it was a slope facing the ocean, and in the later period, it has partially become a foreland basin where two continents meet and uplift. The thrust of Yanshan movement on the plane is even more amazing. From Huaying Mountain in the west to dense fold thrust-slip zone in the east, the crust is greatly shortened, which is rare since Phanerozoic. Nor is it a collision of landmasses, because South China was a unified landmass in the Late Paleozoic. For example, the Permian was quite stable. Who hit who? Of course, it is not a magical power from outer space. If it is an impact from outside the earth, the earth will be shattered. The first chapter discusses the mechanism of tectonic movement.
The most important oil and gas accumulation zone in the early stage of "Jiangnan Uplift" was catastrophically destroyed in Yanshan Movement, and all Paleozoic strata were uplifted and denuded. "No skin, but no hair!" South of the ancient uplift is the deep-sea basin, followed by the Caledonian suture zone, including the central Hunan structural group located in the arc belly of Jiangnan and Xuefeng uplift. Among them, the famous "Qiyang Arc Indosinian Structural Belt" napped to the west shows that the main force source comes from the eastern suture belt, so central Hunan still belongs to the deep water area of the shelf in the early Early Paleozoic of the Middle Yangtze. The lower Paleozoic strata and source rocks are very thin, and the deep structure is too complex, so it is of little practical significance to find oil.
Many scholars pay attention to the northwest side of Xuefeng uplift, including the southeast part of Wulingshan structure, which belongs to the upper Yangtze. The hydrocarbon source of Lower Paleozoic is the most abundant (see Figure 85), and the early reservoir-forming conditions and Xuefeng area are particularly favorable. Xuefeng Mountain Reservoir has been eroded, but there are many remains of Lower Paleozoic ancient reservoirs in this area, such as Majiang, Danzhai, Weng 'an, Tongren and Fenghuang, as well as a lot of oil and gas seedlings and asphalt. Luo Zhili thinks [266]: "There is great hope to find the Lower Paleozoic natural gas reservoirs preserved under nappe structure in the western margin of Jiangnan uplift, especially in Kaili area in eastern Guizhou". The large-scale arc structural belt from Xiushui in northern Jiangxi, Xiangbei to eastern Guizhou is the most likely area to find lower Paleozoic oil and gas reservoirs on the southern slope of the middle Yangtze. Because of the rich hydrocarbon source and the development of secondary fractures and dissolution in reservoir rocks, it is very possible to find reformed gas reservoirs in such a large area as long as the structural pattern below the reverse fault is clarified by fine earthquakes and deep traps with good sealing conditions (especially S preservation) are selected.
Near the west side of Xuefeng Uplift, liquid oil seedlings were widely seen in Lower Paleozoic, and 2.3t crude oil was fished out of huzhuang 47 well [1 16]. The reason is that the paleogeothermal gradient in eastern Guizhou and other places is low, and the area near the uplift belt is already the sedimentary margin of the red basin (T3-K 1), with a thin thickness, and it was also underwater uplift in the late Paleozoic, and the lower Paleozoic was not deeply buried. The maximum burial depth at the top of Silurian in Kaili and Majiang areas is less than 4000m, and the maximum temperature is below 120 ~ 140℃ [139]. It is also possible to find oil reservoirs in this area (including the lower part of Xuefeng nappe belt). According to the paleogeothermal conditions, it is controversial that Majiang ancient oil reservoir formed Hugary East Phase and destroyed Hugary East Phase [139]. The discoverer mainly judges the accumulation period and destruction period according to the contact between S and D strata. During the Caledonian movement, the Silurian reservoir was still on the surface or in a very shallow state, and there were no excellent caprocks such as salt and gypsum. How could it be possible to form a super-large oil field of more than 654.38+0 billion t? The reservoir-forming period should also be destroyed in the late Late Paleozoic and Yanshan period. It's basically the same as a large area.
3. The north side of the middle reaches of the Yangtze River
The main experience of Paleozoic oil and gas under the northern slope of the middle reaches of the Yangtze River is similar to that on the southern slope. There are different opinions about the extent of the early Paleozoic Qinling extensional fault. According to the seismic profile QB- 1 (see Figure 2 1), the most important feature of the South Qinling is that it pushes evenly toward the Yangtze block, and the shallow near-vertical fault gradually turns into a low-angle shovel fault in the deep [73], reflecting the state of the upper crust detachment. Chai Yuwen [46] According to the crustal balance profile, the crust shortening rate of the restored orogenic belt is as high as 6 1%, and the width of the Qinling Paleo-Tethys domain is nearly 1 000 km. At present, most scholars believe that there is a broad passive continental margin on the north side of the Yangtze River in the early Paleozoic, and there are also good conditions for hydrocarbon generation and accumulation, which are widely displayed in the lower Paleozoic ancient reservoirs in Dabashan and Beilongmen Mountains.
Based on the analysis of various data, the oil and gas experience of Lower Paleozoic in the northern margin of the Middle Yangtze has a corresponding cycle with the southern margin: from early Paleozoic to Indosinian and early Yanshan, and the reservoir-forming conditions are very favorable. During Indosinian period, the Qinling ancient suture zone revived and subducted, and the northern margin of the Yangtze River was mainly uplifted without obvious folding. At this time, the deep contradictions in the crust gradually accumulated and strengthened, and all of them were delayed and weakened from east to west when they broke out in the middle Yanshan period. Among the 338 isotope spectra in East Qinling, 200 ~ 170 Ma is the most significant peak [46], which is roughly consistent with various geological phenomena. The huge Wudang Mountain is napped and slid from the north, and the skylight in the basement section of PT2+3 is all lower Paleozoic strata (see Figure 22), which is as strong as the southern edge of the middle reaches of the Yangtze River. The structure of the northern margin of the middle Yangtze can be divided into three zones [64]: the first is the root zone, the collision between North China and the Yangtze block, that is, the deep resurrection subduction zone. Second, the middle belt is a detachment nappe belt, which is located to the north of Qingfeng-Xiangguang fault. It is difficult to preserve the oil and gas reservoirs in the above two areas, and the nappe pressure of Xiangguang fault is released in large quantities, and the structure is relatively weak. Third, the front belt has been pushed onto the platform, forming a thousand miles of Yangtze River hedge belt with the front belt from the south. North of Jianghan is Dahongshan structural area, which is mainly influenced by Dabie Mountain. It develops from Indosinian uplift to Yanshan nappe, thrusts from northeast to southwest, and can be divided into (1) Wuhan nappe segment, which overthrusts on the P+T stratum along the Silurian detachment surface. (2) In the Yunying nappe profile, the folds below the main detachment plane are gentle, and there may be large structural traps in the Lower Paleozoic, most of the Silurian are well preserved, and the later rifting has little influence on the deep part (Figure 87); (3) The section of Lexiangguan nappe is covered by P-T stratum in the east and T-J stratum under the detachment surface in the west. In a word, the Yanshanian movement front belt has advantages and disadvantages for the Lower Paleozoic oil and gas, while the Cenozoic fault depression has more disadvantages than advantages. The Paleozoic gas reservoir under the forezone has considerable potential and is not buried too deeply.
Fig. 87 Interpretation map of deep geological structure in Yunmeng and Shayang areas of Jianghan [64274]
4. Evaluation opinions on oil and gas prospects of Lower Paleozoic in the Middle Yangtze region.
Fig. 88 Schematic diagram of gas prospect zoning and prediction of Lower Paleozoic in Middle Yangtze area.
Based on the above analysis of geological history of the Middle Yangtze, the comparison of oil and gas evaluation before and after Yanshan Movement is obvious. During Yanshanian period, strong compression caused detachment and thrust, exposure of lower Paleozoic oil layer and Tiantong fault, tensile collapse in early middle Himalaya, deep surface water caused by uplift in late west Himalaya, and deep burial of lower Paleozoic oil and gas layers in some areas made the petroleum geological conditions in the middle reaches of the Yangtze River extremely complicated and confusing. The oil and gas prospects are reviewed from the following aspects:
(1) The conditions for early hydrocarbon accumulation in Lower Paleozoic on the platform margin of the middle reaches of the Yangtze River are very favorable, and the geological and historical development cycles are corresponding and similar to the prospective conditions. There are root belt, forced detachment nappe belt and fold thrust belt (front belt) on both sides of Yanshan movement (Figure 88), and the strength gradually weakens, and the preservation conditions are improved accordingly: (1) Near the root belt (belt A) where ancient suture revived subduction on both sides, the sedimentary thickness in the early Paleozoic deep sea area is thin and the reservoir rocks are poor. Complex structures and active magmatic rocks during orogeny are not conducive to the formation and preservation of oil and gas in Lower Paleozoic. (2) Early Paleozoic continental slopes on both sides have good conditions for generation, storage, transportation and accumulation. Hercynian, Indosinian and early Yanshan periods all developed into large uplift zones and local structures, and all formed important oil and gas accumulation zones. In the middle Yanshanian period, this zone was located at the head of Fuxing subduction zone, which was strongly separated from nappe (zone B). There is Wudang-Tongbai-Dabie shear zone in the north, Jiangnan-Xuefeng in the south, and a large area of basement is exposed (B zone), especially in the north. Although the existence of Lower Paleozoic has been confirmed under the nappe profile, it is difficult to consider at present. (3) After the orogenic stress crossed the detachment nappe belt, the two sides continued to develop into fold thrust structural belt (belt C), and its scale and strength were still amazing. The front belts on both sides will be close to the Yangtze River, forming a famous hedge belt. The front belts on both sides near the platform edge are rich in hydrocarbon sources and well preserved in the Lower Paleozoic, which is the most likely area to find lower Paleozoic oil and gas reservoirs in the Middle Yangtze region. According to the seismic interpretation on the north side (Figure 87), for example, in Yunmeng-Lushi area, although Yanshanian thrust faults are dense, the Lower Paleozoic below the Silurian detachment surface is well preserved with simple structure; A series of local fault block traps have been formed in Yanshan structure in Dangyang area [275]. The burial of Lower Paleozoic is generally within 6000m, and the current drilling capacity can bear it. There is no reliable seismic data in the southern front zone, and some deep wells have been drilled, including well Yu (Pize) 1 and well Li (Zixi) 2. And Z2 formation [274], the effect is not good, but it proves that there are good source reservoirs in Lower Paleozoic. We must make up our minds to choose favorable areas for tackling key problems in mountain seismic tests, such as Enshi area (with the most abundant hydrocarbon sources) and Kaili area. High-cost deep well drilling can no longer be carried out according to the surface structure, but a few purposeful scientific drilling should be carried out. Our long-term exploration principle in complex and difficult areas: "Geophysical exploration first, scientific research second, drilling fewer wells and high hit rate" is particularly important for such areas. It is speculated that there are hundreds of high points and traps in the Lower Paleozoic strata preserved underground in the southern front zone, and there will still be a certain proportion of matching in areas with rich hydrocarbon sources. Of course, it will cost more than conventional oil and gas fields, and Figure 87 can inspire people.
(2) In fact, the middle Yangtze platform area is included in the vicinity of the collision zone where the front belts on both sides meet. Due to the superposition of Cenozoic fault basins, the oil and gas coverage conditions of Lower Paleozoic were strengthened locally. However, the deep depression developed from the ancient uplift is buried too deeply; Protrusion erosion, tension is not conducive to preservation. Indosinian depression (syncline) occurs at the edge of ancient uplift such as Dangyang and Mianyang, with good preservation conditions and not too deep burial.
(3) Due to the development of faults, the oil and gas components vertically migrated to the overlying strata in the Paleogene oil and gas giant system are very large, which may exceed the oil and gas reserves of the system itself. In particular, the sealing conditions of Jianghan Salt Lake are good, but the injection of Cenozoic oil and gas into the buried hill of Lower Paleozoic in Jianghan Basin is very small. As for the secondary hydrocarbon generation in the Cenozoic fault depression in the late Lower Paleozoic, it is difficult to make a big contribution in the middle Yangtze area, and Jianghan fault depression has a small supplement.
(4) In addition to finding a large number of deep structural traps in the Lower Paleozoic in the Middle Yangtze region, the study of fractures and dissolved reservoirs should be strengthened. According to many years' experience in Sichuan, it is very important to find the facies zones that are conducive to the formation of porous reservoirs in marine strata, which is the basis of large reserves gas reservoirs. In fact, in the general dense rock stratum, the pore zone itself is a trap.
The gas-bearing problem of Silurian tight sandstone should be paid enough attention. Silurian syncline, Huaguoping syncline, Dangyang syncline and Zigui syncline are shallow buried, with well-developed interbeds and good hydrocarbon source conditions. When the red basin was deposited, the oil in the pores of sandstone cracked into gas, and the porosity and permeability decreased, becoming tight sandstone. In the post-Yanshanian syncline, natural gas is not easy to be lost, and a separated deep basin gas reservoir may be formed.