1. Source beds in Subei Basin
The main source rocks in Subei Basin developed from the upper Cretaceous to Paleogene, but also developed in other Mesozoic strata to some extent.
(1) Cenozoic and Upper Cretaceous Taizhou Formation
There are 6 sets of source rock series from Upper Cretaceous to Paleogene in Subei Basin (Figure 4- 13), which are Tai 2, Fu 1, Fu 2, Fu 3, Fu 4 and Dai 1 from bottom to top.
Fig. 4- 13 diagram of organic matter content and type frequency of oil-bearing layer in Subei basin
The second member of Tai 'er Formation was formed in the superimposed period of intense expansion of lake basin and humid climate. Because the amount of fresh water injected into the lake basin is greater than evaporation, the water body is desalinated, and a set of relatively pure deep lake-semi-deep lake mudstone strata is developed, and the carbonate content in mudstone is lower than 10%. The thickness of source rock is 80 ~150m. Organic carbon content is 1%~ 1.5%, with an average of1.2%; Chloroform asphalt "A" is 0.07%~0.2%, with an average value of 0.65438 0.02%; The total hydrocarbon content is 300 ~ 500mg/L, with an average of 360mg/L. The kerogen type is mainly Type II (Figure 54), and the maturity threshold is 2400～2600m. ..
Fu 1 member is a set of freshwater lake facies source rocks with a thickness of 100~300m, which was formed in fresh water, weak acid, weak alkali and weak oxidation-weak reduction environment. The parent material type is mainly humus, and the organic carbon content is mainly between 0.4- 1.2%, with an average value of about 1%. Chloroform asphalt "A" is between 0.02 and 0.1%,with an average of 0.08%. The average total hydrocarbon content is about 200mg/L, which is a poor source rock. The threshold of oil generation is 2600 ~ 2800 meters.
The middle and lower part of Fu-2 member is a set of high salinity semi-salinized deep-water and semi-deep-water lacustrine source rocks. The total thickness of dark mudstone, marl and carbonate rocks is 100- 150m, of which Gaoyou sag can reach 240m, and marl and carbonate rocks account for 25%~50% of the total thickness. Organic carbon content is mainly concentrated in 1.43%~2.08%, with an average of 1.6%. The highest content of chloroform asphalt "A" is 0.35%~0.47%, with an average of about 0.2%. The highest total hydrocarbon content can reach 1588~ 1983mg/L, with an average of 500 ~ 600 mg/l; The types of organic matter are good, mainly sapropelic type and mixed A type. The threshold of oil generation is 1600 ~ 1800 m in Jinhu sag, 1800 ~ 2200 m in Gaoyou sag and 2000 ~ 2400 m in other sags. This set of source rocks has the characteristics of high organic matter abundance, good parent material type and early hydrocarbon generation and expulsion, and is a good source rock. The biogeochemical facies of this set of source rocks are obviously different from the east to the west. The quality of this set of source rocks in Jinhu sag and Gaoyou sag in the east of the basin is good, with high abundance of lower aquatic organisms, high abundance of salt water algae, great hydrocarbon generation potential and small maturity threshold, while the quality of this set of source rocks in the east sag is inferior to that in the west and the maturity threshold is higher than that in the west.
A set of pure deep lake-semi-deep lake mudstone strata is developed in the upper part of the second member of Fu. This set of mudstone was formed in a brackish water-fresh water environment with weak alkalinity and reduction-strong reduction. The source rocks are 50~90m thick and the sedimentary thickness is stable. Organic carbon content is 1%~ 1.5%, with an average of1.2%; Chloroform asphalt "A" is 0.07%~0.2%, with an average value of 0.65438 0.02%; The total hydrocarbon content is 300 ~ 500 mg/L, with an average of 360 mg/L. The kerogen type is mainly type II, and the maturity threshold is 2400～2600m m.
The third member of Fu Formation is a set of freshwater lake facies source rocks with a thickness of 100~300m, which was formed in the environment of fresh water, weak acid, weak alkali and weak oxidation-weak reduction. The parent material type is mainly humus, and the organic carbon content is mainly between 0.2%- 1.0%, with an average of 0.8%. Chloroform asphalt "A" is between 0.0 1%-0.08%, with an average of 0.05%. The average total hydrocarbon content is about 50 mg/L, which belongs to a set of poor source rocks. The threshold of oil generation is 2600 ~ 2800 meters. The lower part of the fourth member of Fu is a set of micro-salt lake facies source rocks, and a set of relatively pure deep-semi-deep lake facies mudstone strata is developed. The bio-organic geochemical characteristics of this mudstone formation are basically similar to those of Fu-2 member, and it is also formed in weak alkaline and reduction-strong reduction environment. The thickness of source rocks is 150~200m, and the sedimentary thickness is stable. Organic carbon content is1-1.5%; Chloroform asphalt "A" is 0.07% ~ 0.2%; The total hydrocarbon content is 300 ~ 500 mg/L, kerogen type is mainly type II (Figure 54), and the maturity threshold is 2400~2600m m. ..
The middle and upper part of the fourth member of Fu Formation is a set of low salinity brackish water semi-deep-deep lacustrine source rocks, with dark mudstone mixed with marl, accounting for about 5%~ 15% of the total thickness and 25% locally. They are formed in the environment of reduction-strong reduction and weak alkali-alkaline brackish water and deep lake facies. The source rocks are 200-350m thick, and Gaoyou sag is the largest. The organic carbon content is 1%~2%, with an average value of 1. 19%. The high value of chloroform asphalt "A" is 0.2%~0.25%, with an average value of 0. 17%, and the total hydrocarbon content can reach 800 ~/kloc at the highest. The maturity threshold of Jinhu sag and Gaoyou sag in the central and western part of the basin is 2000 ~ 2200m, and that of other areas is 2200 ~ 2400m. ..
Dai 1 member is also a set of freshwater lake facies source rocks, with an average thickness of 100~300m, which was formed in fresh water, weak acid, weak alkali and weak oxidation-weak reduction environment. The parent material type is mainly humus, and the organic carbon content is mainly between 0.4% ~ 1.6%, with an average of about 1.0%~ 1.2%. Chloroform asphalt "A" is between 0.0 1%-0. 16%, with an average of 0. 1%. The average total hydrocarbon content is 100mg/L, which is a set of poor source rocks. The threshold of oil generation is 2600 ~ 2800 meters.
(2) Pukou Formation of Upper Cretaceous
The Pukou Formation of Upper Cretaceous in Subei Basin is the most widely distributed, mainly composed of red clastic rocks. In some depressions, shallow lake-semi-deep lake facies environment appears in stages, and dark mudstone of salt lake facies develops. Huai 'an-Funing area is the sedimentary center of Pukou Formation in northern Jiangsu Basin, in which the accumulated thickness of dark mudstone in the third and fourth layers of thick gypsum-salt rock and dark mudstone development section of Pukou Formation is 200~400m, accounting for 20%~35% of the thickness of this section, and it is formed in dry salt lake facies environment under weak oxidation-weak reduction environment. The content of organic carbon is 0.29%~ 1.23%, mostly 0.43%~0.7%, which belongs to medium-low organic matter abundance. The type of organic matter is mainly Type III, and some are Type II. Chloroform asphalt "A" content is 0.0 14%~0.04%, and total hydrocarbon content is 40 ~146 mg/L. Dark mudstone has basically reached the standard of medium-poor source rock, mainly gas-generating parent rock. The threshold of oil generation is less than1600m.
(3) Lower Cretaceous Gecun Formation
The source rocks of the Lower Cretaceous Gecun Formation were formed in shallow-water sedimentary environment with weak oxidation and weak reduction, mainly distributed in the residual basins of the Lower Cretaceous, with limited distribution and small area. The source rock of Gecun Formation in Subei Basin has an organic carbon content of 0.5%, and its chloroform pitch "A" and total hydrocarbon are extremely low, so it belongs to non-poor source rock. Generally speaking, this set of dark mudstone has poor hydrocarbon generation ability, and can only be used as gas-generating parent rock in well-preserved and buried areas.
(4) Xiangshan Formation of Middle and Lower Jurassic
Xiangshan Formation is a set of strata with interbedded dark mudstone and clastic rock in shallow lake facies, which was formed in weak oxidation-weak reduction environment. Dark mudstone is distributed in the middle and upper part of the formation and widely distributed in the residual fault depression area of Xiangshan Formation. The thickness, organic matter abundance and quality of this group of source rocks vary greatly in each residual fault depression area. The sedimentary thickness of dark mudstone of most residual faults is 100~300m, accounting for 20%~50% of the total section thickness. This set of source rocks contains 0.5%~0.65% of organic carbon, 300 ~ 600 mg/L of chloroform asphalt "A" and 200~300mg/L of total hydrocarbons, belonging to medium source rocks.
2. Subei Basin Reservoir
Small and medium-sized fault depressions and volcanic-sedimentary fault depressions in the lower Yangtze region were developed under the background of Mesozoic overall compressive uplift, and the supply of provenance was extremely rich. Coarse clastic rocks are mainly developed in fault depressions, and the lake environment only appears briefly. Therefore, the Mesozoic clastic reservoirs in Subei Basin are well developed, including Xiangshan Formation, Gecun Formation, Pukou Formation and Chishan Formation. The Cenozoic in Subei Basin is dominated by rivers, deltas, fan deltas, subaqueous fans and lakeside facies reservoirs, and 9 sets of reservoirs are developed: Taizhou Formation, Fu-1 Member, Fu-2 Middle and Lower Member, Fu-3 Member, Dai-1 Member, Dai-2 Member, Duo-1 Member, Duo-2 Member and Yan-1 Member.
Controlled by the sedimentary characteristics and diagenesis of Mesozoic strata from strong to weak, the physical properties of Mesozoic reservoirs have obvious horizon, which gradually improved from Jurassic to Cretaceous. Chishan Formation has the best physical properties, with total porosity of 15%~25% and permeability of (100~300)× 10-3μm2, which is a high porosity and high permeability reservoir. Upper Cretaceous Gecun Formation and Pukou Formation are moderately poor in physical properties, with porosity of 5%~ 12% and permeability generally less than 50× 10-3μm2, which are medium-low porosity and low permeability reservoirs. Xiangshan Formation has the worst physical properties, with porosity between 5%~ 10% and permeability generally less than 10× 10-3μm2, which belongs to low porosity and ultra-low permeability reservoir.
The reservoir of Taizhou Formation is mainly glutenite developed in the middle and lower part of Taizhou Formation, and sand bodies are widely distributed in the whole basin. Generally, the thickness of sandstone is 30~ 100m, and the maximum thickness can reach 200m·m, and the single layer thickness is 3~5m, and the maximum thickness is 50 m. The reservoirs in Taizhou Formation are mainly Ⅱ 2, Ⅲ and Ⅳ, and some of them are Ⅱ1,belonging to medium-low permeability-ultra-low permeability reservoirs.
The reservoir rocks in the first member of Fu are mainly concentrated in the lower and upper parts, and the thickness is generally 50~ 150m, and the maximum cumulative thickness can reach 450m ... The single layer thickness is generally 2.5~4m, and the maximum thickness is 15m. The lower sand bodies of Fu 1 member are widely distributed in the whole basin, and the upper sand bodies of Fu 1 member are mainly distributed in the west of the basin. Fu-1 member is mainly of Class III and IV reservoirs, and some of them are of Class II and 2 reservoirs, belonging to medium-low permeability-ultra-low permeability reservoirs.
The clastic reservoir in the middle and lower part of Fu 2 member is mainly distributed in Tianchang-Maba Delta in the west of Jinhu Depression. The cumulative thickness of sandstone is 10~30m, and the average single-layer thickness is1.5m. The reservoir in the second member of Fu is mainly of Class IV, and some of them are of Class III, belonging to low permeability and ultra-low permeability reservoirs.
The reservoirs in the third member of Fu are mainly concentrated in the upper and lower parts of this member. Sand bodies are mainly distributed in five deltas in the basin. The total thickness of sandstone is generally 30 ~ 200m, and the thickness of single layer is generally1.5 ~ 8.5m. The reservoirs in the third member of Fu Formation are mainly Class III and IV reservoirs, and some are Class II reservoirs, belonging to medium-low permeability-ultra-low permeability reservoirs.
Dai 1 reservoir is mainly concentrated in the lower part. Sand bodies are mainly developed in three deltas of the basin and underwater alluvial fans in the east of Qintong sag. The total thickness of sandstone is generally 50~200m, the thickness of single layer is generally 3~4m, and the maximum thickness can reach 1 8.5m. The reservoir in zone1section is mainly Class II, and some of them are Class I 2 or III1,which belongs to medium permeability-low permeability reservoir.
The total thickness of Dai 2 reservoir is generally 50~200m, the single layer thickness is 2.5~3.5m, and the maximum thickness can reach 20 m. The distribution of sand bodies inherits the pattern of Dai 1 section. Dai-2 member reservoir is mainly of Class II, and part of it is of Class I 2 or III1,which belongs to medium permeability-low permeability reservoir.
The reservoir rocks in the first member of Dui are widely distributed, accounting for more than 70% of the sedimentary distribution area in the first member of Dui. The total thickness of sandstone is 100~200m, and the maximum thickness can reach 320m, which is mainly concentrated in the lower part. The thickness of a single layer is generally 10~20m, and the maximum thickness can reach 54m. Sand bodies are mainly distributed in five river delta systems in the basin. The reservoirs in the first member of Dui Formation are mainly Class I and II reservoirs, and some of them are Class III reservoirs, which belong to high and medium permeability reservoirs.
The reservoir rocks in the second member of Duoer Formation are often interbedded with sandstone and mudstone. The total thickness of sandstone is 20~500m, and the thickness of single layer is generally 5~ 10m, and the maximum is 14m. The distribution of sand bodies inherits the sedimentary period model of the first member of the pile. The reservoirs in the second member of Dui Formation are mainly Class I and II reservoirs, and some are Class III reservoirs, which belong to medium and high permeability reservoirs.
The first extension is widely distributed and has a large thickness. The thickness of single layer is generally 5~ 15m, the maximum thickness is 25m, and the total thickness of sandstone is 100~300m. The first member of Yanchang Formation is mainly Class I, and part of it is Class II1,which belongs to high porosity and high permeability reservoir rocks.
3. caprock of Subei basin
The Mesozoic-Cenozoic in the inland of Subei Basin mainly includes the regional caprock of Pukou Formation of Upper Cretaceous, the regional caprock of Cenozoic (including Taizhou Formation) and the local caprock of Lower Cretaceous-Middle-Lower Jurassic.
Upper Cretaceous Pukou Formation (1) regional caprock
The middle and upper part of Pukou Formation of Upper Cretaceous is a set of extremely thick shallow-semi-deep lacustrine mudstone, gypsum-salt mudstone and gypsum-salt rock, the maximum thickness of which can reach1500m, which are widely distributed in the residual basins in the lower Yangtze region. The determination of petrophysical properties shows that this set of mudstone has undergone strong compaction and is a good regional caprock in Subei Basin.
(2) Cenozoic (including Taizhou Formation) regional caprock
There are six sets of regional caprocks in Cenozoic in Subei Basin, which are the second member of Tai, the middle member of Fu-1, the second member of Fu-2, the fourth member of Fu-4, the upper member of Fu-1 and the upper member of Fu-2. In addition, there are two sets of local caprocks in the basin, namely the upper part of Dai 1 member and the upper part of Dai 2 member (Table 4-6).
Table 4-6 Parameter Table of Micropore Structure Characteristics of argillaceous Rock in Subei Basin
(According to Wang Jinyu et al., 2000)
The caprocks of Tai-2 member, Fu 1 member, Fu-2 member and Fu-4 member are basically mudstone or mudstone with limestone, and the thickness of each mudstone section is 50-250m, 200-400m, 200-300m and100-500m respectively. The distribution of the whole basin is stable, and only a few depressions or structural belts are transformed into coarse clastic rocks.
The caprock in the middle and upper part of the first member was formed after strong extensional fault blocks and strong volcanic eruption, and the basin quickly subsided, forming an under-compensated environment. Except for coarse fluvial debris near the marginal provenance, a set of variegated shore-shallow lacustrine argillaceous deposits with a thickness of150 ~ 250 m ~ 250 m are developed in most areas of the basin. It is a set of flood plain-intermittent lake light gray-green variegated mudstone with sandstone, with stable lateral distribution and thickness of 80 ~150 m. The uplift and outer slope belt in the west of the basin are affected by Triassic movement, extrusion, uplift and erosion, and this set of regional cover is missing.
The local caprocks in the upper part of Dai-1 member and the middle and upper part of Dai-2 member were formed in the lacustrine transgression stage of squeezing residual fault depression in Wubao movement. The upper part of Dai 1 member is a set of shallow lake-semi-deep lake mudstone, mainly dark mudstone, with a thickness of 50~200m m. The middle and upper part of Dai-2 member is a set of red argillaceous rocks with a thickness of 50~ 150m, belonging to shallow lake-flood plain facies. The distribution of these two sets of local caprocks is unstable, which has obvious influence on sedimentary system.
In addition, the Yancheng Formation also developed argillaceous rocks with stable distribution and wide range. The cumulative thickness of mudstone exceeds 100 m, but the thickness of single layer is only 7 ~ 15 m ... Because there are few faults in Yancheng Formation and the scale is small, this kind of caprock can also play a good sealing role.
(3) Local caprock of Lower Cretaceous-Middle-Lower Jurassic
There are two sets of local caprocks in Subei Basin, which are the mudstone development section of Xiangshan Formation of Middle and Lower Jurassic and Gecun Formation of Lower Cretaceous. Unstable distribution, thin thickness, many sandstone interlayers and impure mudstone quality.
4. The control and influence of the Tan-Lu fault zone on the source-reservoir-cap assemblage in Subei Basin.
(1) source-reservoir-cap assemblage
There are six source-reservoir-cap rock assemblages (Song Jianguo et al.,1997) in Cenozoic in Subei Basin: ① Taizhou Formation and the lower part of Fu-1 member: Tai-2 member is source rock and cap rock, Tai-1 member and Fu-1 member are source rock and cap rock, and Fu-1 member is upper part. This source-reservoir-cap combination can capture both self-generated and self-stored oil and gas and secondary oil and gas. ② Source-reservoir-cap assemblage of Fu-1 member and Fu-2 member: the middle and lower parts of Fu-2 member and the top of Fu-1 member are source rocks and cap rocks, the upper part of Fu-2 member is also source rocks and cap rocks, and the upper and lower parts of Fu-1 member are reservoirs. In addition to clastic rocks, reservoir rocks also include biolimestone and volcanic reservoirs. Because the salt lake in the middle and lower part of Fu 2 member is rich in carbonate source rocks, it has high oil-generating ability, good kerogen type and low maturity threshold, and it directly contacts with the underlying reservoirs in the middle and lower part of Fu 2 member or the upper part of Fu 1 member. Therefore, this combination has formed a large number of shallow-buried authigenic reservoirs, which is the stratum with the highest exploration success rate. ③ Source-reservoir-cap assemblage of Fu-2, Fu-3 and Fu-4 member: Fu-2 and Fu-4 member are source rocks, mudstone in the middle of Fu-3 member is secondary source rock, mudstone in the middle of Fu-3 and Fu-4 member is caprock, reservoir rock is sandstone, and primary oil assemblage is authigenic, self-storage, upper storage and upper storage. In addition, a certain amount of oil flow has also been obtained by invading the diabase cracks in the lower part of Fu 3 and Fu 4 members. The assemblage is sandwiched between two sets of regional caprocks and main source rocks in the basin and has good reservoir-forming conditions. Most blocks in the basin are bounded by mudstone caprock in the middle of Fu-3 member, forming two secondary reservoir-cap assemblages with upper and lower sandstone as reservoirs. The lower sandstone stratum mainly captures the oil and gas of the second member of Fu, and the upper sandstone stratum mainly captures the oil and gas of the fourth member of Fu. ④ Source-reservoir-cap assemblage of Dai Nan Formation: self-generated, self-stored and secondary oil and gas reservoirs can be formed. The self-generating and self-storing oil and gas reservoir is composed of Fu-4 member and Dai-1 member, which stores oil. The upper part of Dai Nan Formation is caprock and the reservoir rock is sandstone. The secondary oil and gas reservoirs take Taizhou Formation and Funing Formation as source beds, and the oil and gas migrate along the contemporaneous faults and unconformities to Dai Nan Formation. ⑤ Source-reservoir-cap assemblage of Sanduo Formation: The source rocks are Funing Formation and Taizhou Formation, which migrated to Sanduo Formation through unconformities and faults. The caprock is the middle-upper mudstone section of the first member of Duodui Formation and the middle-upper mudstone section of the second member of Duodui Formation, and the reservoir rocks are sandstone, belonging to secondary assemblage. ⑥ Source-reservoir-cap assemblage of the lower member of Yanchang Formation: The source rocks are Taizhou Formation and Funing Formation, which move vertically along the fault to the sandstone reservoir of the lower member of Yanchang Formation, and the mudstone of Yanchang Formation is the caprock. The industrial gas flow has been tested in Zhouzhuang area.
Mesozoic source-reservoir-cap assemblage in Subei basin (Tian Zaiyi et al.,1996; Song Jianguo et al., 1997) includes Pukou Formation gypsum member/Pukou Formation dark mudstone member/Pukou Formation sandstone interlayer and glutenite member combination, middle and upper Gecun Formation dark mudstone/lower Gecun Formation sandstone, middle and upper Xiangshan Formation dark mudstone/lower Xiangshan Formation sandstone, upper Jurassic-lower Cretaceous volcanic cover/Xiangshan Formation reservoir, etc. Among them, Pukou Formation is the most widely distributed regional caprock, which is widely distributed in Subei Basin. The middle and upper part of Pukou Formation is composed of extremely thick shallow-semi-deep lacustrine mudstone, gypsum mudstone and gypsum salt rock, with the maximum thickness of1500m.
From the development of sedimentary facies in Subei basin, it can be seen that the sedimentary centers of Taizhou Formation and Fu 1 member are near east-west, which indicates that the basin deposition in this period is mainly controlled by near east-west faults. Since the deposition of the second member of Fu, the sedimentary center of the basin has changed to NE-NNE direction, indicating that the basin has changed under the influence of the Tan-Lu fault system. The sedimentary centers are all located near the NE-trending faults, so the source-reservoir-cap assemblage since Fu-2 member is mainly controlled by the Tan-Lu fault system.
5. Distribution law of oil and gas in Subei Basin
The discovered oil and gas fields in Subei basin are basically distributed in Dongtai sag in the south, and about 97% of the oil and gas reserves are distributed in Gaoyou, Jinhu and Qintong sag, among which Jinhu and Gaoyou are the most abundant. The main pay zones and hydrocarbon accumulation characteristics of different oil-bearing faults are obviously different. The oil-bearing property of each depression in the basin is comprehensively controlled by the migration of sedimentary center, the evolution of sedimentary system, and the structure and thermal effect.
The formation and evolution of Mesozoic-Cenozoic basins in northern Jiangsu experienced three stages: basin formation stage, basin depression stage and basin depression stage. The main stratigraphic deposition, oil and gas generation, migration and accumulation and trap accumulation were all formed in the basin fault depression period. At present, the regional tectonic framework of the basin also mainly reflects the characteristics of the basin during the fault depression period. Because the basement of Mesozoic and Cenozoic basins is mostly Mesozoic and Cenozoic fault sedimentary strata, rather than rigid basement, during the development of fault depression, the internal structure of the basin is very broken due to the influence of basement, and the structural traps formed are mainly complex small fault blocks. The complexity of structural traps and the diversity of reservoir-forming types in Subei basin make it very important to study the genesis, distribution and formation law of structural traps.
From Late Cretaceous to Paleogene, there was a strong fault depression in Subei Basin. Most structural traps in Subei basin were formed under this background. The causes of these structural traps can be divided into four types: fault-nose fault-block groups in depressions and slopes, fault-block traps on both sides of main faults, fault-nose fault-block traps at the turning point of main faults and complex fault-block traps between fault steps.
(1) oil and gas reservoir distribution
The distribution of main structural traps and oil and gas reservoirs in Gaoyou sag and Jinhu sag, two major depressions in Subei basin, has obvious laws.
There are two types of structural traps in Jinhu sag: fault nose fault block group in the slope area of the sag and fault block traps on both sides of the main control fault. A large number of oil and gas reservoirs have been found in fault nose fault blocks in depressions and slopes. The main oilfields are Cui Zhuang Oilfield, Nanhu-Fan Zhuang-Anle Oilfield, Wanglongzhuang Oilfield, Yangjiaba-Biandong-Lizhuang Oilfield and the first member of Funing Formation in Minqiao Oilfield. Fault block traps on both sides of the main control fault are mainly developed on both sides of the main control fault at the boundary of Sanhe sag. At present, oil and gas reservoirs have been found in the rising plate, and it is difficult to form traps in the falling plate because the faults are mainly in the downward direction of the stratum, and no oil and gas reservoirs have been found.
A large number of oil and gas reservoirs have been found in depressions and fault blocks in the slope area of Gaoyou Depression, such as the first member of Funing Formation in Shai 'an Oilfield, Matouzhuang Oilfield, Chian Oilfield and Wa-2 Oilfield in the east of the depression. The trap types of Wubao fault zone mainly include fault block traps on both sides of the main fault and fault nose fault block traps at the turning point of the main fault. Chenbao Oilfield, located at the transitional part of Wu-1 and Wu-2 faults, has the largest reserves and production in Jiangsu Oilfield at present, and its structural trap characteristics and reservoir-forming methods are very unique and representative. There are many fault-block traps on both sides of the Wu (①) fault, and the Dai Nan Formation oil and gas reservoir is formed in the footwall, belonging to the fault nose fault-block trap oil and gas reservoir that controls the torsional deformation of the fault. The traps and oil and gas reservoirs in Taizhou Formation are formed in the rising plate and belong to the fault block traps on both sides of the main control fault. There are many complex fault blocks and oil and gas reservoirs in the south terrace of the depression, such as Xuzhuang Oilfield, Xu Qian 1, Zhenz43, China-Hong Kong 1, etc., which belong to feather fault blocks on both sides of the main control fault. Sanduo Formation and Dai Nan Formation in deep depression area have Zhenwu, Fumin, Huang Jue, Cao Zhuang, Xujiazhuang and Yong 'an reservoirs, which belong to fault nose and fault block group reservoirs in depression slope area. In addition, in Anfeng Oilfield and Liangduo Oilfield discovered in Hai 'an Depression, the way of primary hydrocarbon expulsion and secondary migration and accumulation of oil and gas is consistent with the way of reservoir formation of fault nose and fault block group in the slope area of the depression, except that the horizon moves down to Taizhou Formation as a whole. Therefore, its oil and gas distribution law is similar to Jinhu sag and Gaoyou sag.
6. The influence of Tancheng-Lujiang fault zone on oil and gas distribution in Subei basin.
The strike-slip extension of the (1) Tan-Lu fault zone is conducive to the formation of an effective source-reservoir-cap assemblage.
The Tan-Lu fault zone constitutes the western boundary of the basin. Because the deep fault depression is surrounded by the large uplift of the basin edge and strong topographic difference, a large-scale provenance system is developed in this area, which is generally the most important and largest provenance in the basin, and can form an inherited large-scale river-delta-deep-water lake basin sedimentary system with large reservoir scale and multiple sets of reservoirs superimposed. Therefore, this area has good reservoir conditions, which is conducive to long-distance lateral migration of oil and gas and easy to form a large-scale transport system; At the same time, high-quality source rocks are developed in this area. Fu-2 member and Fu-4 member are two sets of efficient source rocks in Subei Basin, which are often collocated with large sandstone transport system to form efficient reservoir-forming combination.
The source rocks in the middle and lower part of Fu-2 member in Subei basin have strong hydrocarbon generation ability and small maturity threshold depth, and all blocks in the basin generally enter the maturity threshold. The whole basin in the upper part of this set of source rocks is widely covered with pure mudstone of the second member of Fu, with a thickness of 50~80m. Therefore, the middle and lower parts of the second member of Fu mainly form a source-reservoir combination with the bottom of the second member of Fu and the upper part of the first member of Fu. However, the distribution of reservoir development areas in the lower part of Fu-2 member and the upper part of Fu-1 member is controlled by the large-scale provenance system at the edge of the western basin, which determines that the favorable allocation area of this set of source rocks and the reservoirs in the lower part of Fu-2 member and the upper part of Fu-1 member are mainly distributed in Jinhu, Gaoyou and the west of Qintong sag. At present, it has been found that the reserves of primary oil and gas reservoirs formed by this structure account for more than 30% of the total reserves.
(2) The multi-stage fault activity caused by the Tan-Lu fault zone is beneficial to oil and gas migration.
In the fault depression near the Tan-Lu fault zone in the northern Jiangsu basin, the continuous activity of the syngenetic faults in the fault depression is controlled, and the syngenetic fault zone can ensure the vertical migration of oil and gas for a long time until the middle of Neogene. However, the fault depression far away from the Tan-Lu fault zone, the syngenetic faults controlling the fault depression stopped moving at the end of Paleogene, and the vertical migration of oil and gas could not be guaranteed for a long time. At the same time, near the Tan-Lu fault zone, a deep fault depression with universal inherited activities has developed. Therefore, a large number of source rocks reached the mature stage at the end of Paleogene or before the Neogene syngenetic faults stopped active, which provided a material basis for the vertical migration and accumulation of oil and gas. In addition, the fault depression near the Tan-Lu fault zone generally deposited a thick Neogene-Quaternary system on the Paleogene source rocks, which provided warm and pressure conditions for the transformation and migration of oil and gas, and played an important role in the further migration, accumulation and preservation of oil and gas to structural traps.
(3) Magmatism caused by the activity of the Tan-Lu fault zone makes the adjacent depressions rich in resources.
The strike-slip extension of the late Cretaceous-Paleogene Tanlu fault zone made the volcanic activity in Jinhu sag and Gaoyou sag in the west of Subei basin more intense. Because volcanic activity is a sign of tensile strength and heating degree of the depression, the intensity of volcanic activity is obviously related to the oil and gas abundance of the depression. The oil-bearing property of the basin is obviously zonal, first from north to south, and the abundance of oil and gas resources in Dongtai sag in the south of the basin is obviously higher than that in Yanfu sag in the north. Secondly, in Dongtai sag, the abundance of oil and gas resources and the characteristics of oil and gas enrichment in each sag obviously show the zoning of west, middle and east.
During the sedimentary period of source rocks of Taizhou Formation and Funing Formation, the basalt eruption zone in Liubao-Minqiao structural belt on the western margin of Jinhu Depression in the western part of the basin continued to move, resulting in a relatively isolated semi-salt lake sedimentary environment during the sedimentary period of Fu 2 and Fu 4 members in Jinhu Depression. This sedimentary environment is not only related to the positive landform division formed by volcanism, but also related to the local surface thermal environment formed by volcanism. In this environment, it is beneficial to biological development, mainly composed of lower algae, good preservation conditions of organic matter, and possible inorganic hydrogenation to form high-quality source rocks. This kind of source rock is characterized by high organic matter abundance, good parent material type and low maturity threshold (1500~ 1800m). The volcanic shallow platform is formed in the basalt eruption zone, and basalt, biogenic limestone and oolitic limestone reservoirs are developed, which together with high-quality source rocks form a good source-reservoir-cap combination. Basalt, biolimestone and oolitic limestone have become important reservoirs in Jinhu sag.
At the same time, magmatism reflects the heating conditions of each depression. After the deposition of source rocks, the geothermal gradient in the depression with strong magmatism is high, and the maturity threshold of source rocks is obviously reduced. For example, in Jinhu, Gaoyou and Qintong depressions with strong volcanism, the paleogeothermal gradient is about 3.7℃/ 100 m, the maturity threshold of kerogen Ⅰ and Ⅱ is1500 ~ 2,200m, and that of kerogen Ⅲ is 2500m, while in Yancheng depression with weak volcanism, the geothermal gradient is about 3./. The maturity threshold of kerogen Ⅰ and Ⅱ is 2800 m, and that of kerogen Ⅲ is 3000m m. The paleogeothermal gradient in Hai 'an Depression is between the above volcanic intensities, about 3.3℃/ 100 m, the maturity threshold of kerogen Ⅰ and Ⅱ is 2500 m, and the maturity threshold of kerogen Ⅲ is 2800 m. Due to the difference of heating in the depression, there are obvious differences in oil and gas abundance in the depression. Qintong, Gaoyou and Jinhu depressions have strong volcanism and are the main oil and gas-rich areas in Subei Basin. At present, only three small oil and gas fields have been discovered in Haian sag, with less reserves; Although the thickness of Cenozoic in Yancheng sag is more than 5000 meters, most deep exploration wells have not found oil and gas.
(4) The Tancheng-Lujiang fault zone controls local structures and its relationship with oil and gas accumulation.
Controlled by the strike-slip extension of the Tan-Lu fault zone, several rows of inherited fault noses and anticlines arranged in NE-NNE direction are developed in Jinhu sag near the Tan-Lu fault zone, while the local structure on the side far from the Tan-Lu fault zone is small and mainly fault blocks.
The NE-NNE-trending nose structural belt in Subei basin plays a strict control role in oil and gas distribution. There are many rows of NE-NNE-oriented nose structural belts in Jinhu sag, and all oil and gas fields are distributed in the nose structural belts, and 15 oilfield has been discovered. There are many rows of nose-shaped structural belts in Gaoyou sag, and all the major oil fields are distributed in these structural belts, and 2 1 oil field has been discovered, among which the oil fields near the two sides of the main deep depression of the fault zone are the largest, with the largest reserves of 2000×104t. Similarly, the structural high zone in Qintong sag is also distributed in the northeast, and 15 oilfield has been discovered.
In a word, the Subei Basin, located on the east side of the Tan-Lu fault zone, has the following characteristics near the lithospheric fault zone: high tensile strength, large scale of fault depression, high subsidence rate and good subsidence persistence, and many sets of high-quality and efficient source rocks are developed; The geothermal gradient is high and the maturity threshold of source rocks is low; There are many sets of high-quality reservoirs near the edge of the basin, with strong faulting and high oil and gas migration and accumulation efficiency. Large-scale inherited uplift structural belt and dustpan fault depression are developed, which are huge oil and gas accumulation places. Therefore, the abundance of oil and gas resources in Gaoyou, Jinhu and other western deep depressions near the Tanlu fault zone is much higher than that in other belts.