1. Main oil generation periods and oil and gas reservoir formation
At the end of Late Cretaceous, the northern Jiangsu Basin was in a dry and strongly oxidized sedimentary environment, which indicated that the Chishan Formation in this period was a set of typical red strata, and there could be no oil generation process.
After the deposition of Chishan Formation ended, the paleoclimate began to change and the degree of oxidation gradually weakened. Then, the Paleocene Taizhou Formation was superimposed on the Chishan Formation. According to the analysis of sporopollen fossils, it is inferred that the paleoflora and paleoclimatic sedimentary environment at that time: the early Taizhou Formation was a mixed forest of gymnosperms and angiosperms, with gymnosperms as the main species. Ferns grow under the forest, and some aquatic plants grow in the low-lying water, which belongs to the humid tropical-subtropical climate. For example, the dominant fern Pteris pteridoptera is almost all produced in the tropics. The flora of the late Taizhou Formation (equivalent to the dark mudstone section of the second member of Taizhou Formation) is coniferous forest and wet-leaved mixed forest dominated by gymnosperms. The dark mudstone stratum is rich in gymnosperms pollen, which is related to Pinus, Picea, Cedar, Podocarpus, Larix, Taxodiaceae, Cycadaceae and Ginkgo. Larch and spruce are distributed in alpine and cold regions, and angiosperm pollen is mainly composed of temperate and warm temperate molecules. It shows that the paleoclimate in this period is colder than that in the early Taizhou Formation, which is equivalent to the tropical climate in North Asia.
The change of paleoclimate and the appearance of reducing environment are beneficial to the deposition of dark mudstone and the generation of organic matter in the late Taizhou Formation. In Subei basin, this set of dark mudstone is widely distributed, with a thickness of about 40 ~ 200 meters, which is generally thick in the west and thin in the east. According to the statistics of Gaoyou area, the average content of organic carbon is 0.75%, with the maximum of 1.7 1%, and the average content of reduced sulfur is 0.6%, with the maximum of 0.8%. Therefore, the dark rocks in the second member of Tai 'er Formation are basically reduced facies deposits favorable for oil generation, which can be regarded as the earliest oil generation period of Tertiary in Subei Basin.
During the sedimentary period of Funing Formation, the basin subsided greatly. Generally speaking, except for the uplift and some high uplift, the whole Subei basin is almost in a strongly reduced sedimentary environment, even at the edge of the basin. In fact, the influence of oscillation on the sections of Funing Formation is still different. Judging from the degree of subsidence, the fourth member of Fu is the largest, followed by the second member of Fu, followed by the third member of Fu and Fu 1 member of Fu, which makes the second and fourth members of Fu become the main oil generation periods in the northern Jiangsu Basin. As far as Fu-2 member is concerned, it may be the sedimentary period of Fu-2 member, and the eastern region accepted a transgression, and seawater invaded northern Jiangsu from the South Yellow Sea. Because according to the data of Well Dong 12, Well Dong 19, Well Dong 2 1 and Well Dong 53, there is a set of tuff containing analcime in these wells. Through spectral analysis, it is found that the contents of trace elements such as strontium, barium and boron are high, and aluminum, iron, manganese, nickel, vanadium and calcium change regularly. According to foreign reports, it can be considered that this deposit has a great relationship with the influence of ancient seawater. Analcite tuff gradually decreases from Hai 'an to Gaoyou and from east to west, indicating that the influence of seawater diffusion to the west is small. The main feature of the source rocks of Fu-2 member is that they are rich in organic matter and distributed in the depression. Because of the deep burial, superior preservation and transformation conditions, especially in the sedimentary center and deep water area, the accumulation of organic matter is more abundant, so the deep depression in the dustpan fault depression is often the center of oil source.
Part of the oil generated in the second member of Fu 'er Formation has entered its own oil reservoir, especially where the marginal lithofacies belt is distributed, and the sandstone and limestone in the second member of Fu 'er Formation have become accumulation oil and gas reservoirs, such as Well Dong 60 of Liu Zhuang Structure in Jinhu Depression. Another part of oil and gas migrated to other horizons along the river.
The formation period of oil and gas reservoirs was mainly after the Wubao Movement, that is, the crust changed greatly at the end of Eocene, which created favorable structural conditions for oil and gas accumulation, and then oil and gas began to accumulate and migrate on a large scale.
The five treasures movement did not cause much influence and destructive erosion to the deep depression. After the deposition of the fourth member of Fu, the Five Fort Movement took place. Therefore, the source rocks of Fu-4 Member are well preserved in the deep part of the depression, but they are often eroded in the uplift or high slope, which is directly related to the preservation of Funing Formation oil and gas reservoirs and the supply of Dai Nan Formation oil sources, because Fu-4 Member can be used as both source rocks and caprocks. In the deep layers of Qintong sag, Gaoyou sag and Jinhu sag, the thickness of the fourth member of Fu is 400 ~ 500m, which becomes the oil and gas supply foundation of Dai Nan Formation and even Sanduo Formation, forming a relatively complete and ideal oil-forming combination, making the high porosity and high permeability sandstone of Dai Nan Formation a high-yield layer. The dissolution of the fourth member of Fu in Hai 'an Depression has greatly affected the accumulation of oil and gas in the third member of Fu. Although some structures are still in a high position, as long as the fourth member of Fu is partially preserved, it will still play a great role in the preservation of oil and gas reservoirs. For example, Wells Dong 64 and Dong 60 of Jinhu Liu Zhuang structure have tens of meters of Fu-4 formation, which is very favorable for natural gas accumulation at the high point of Liu Zhuang structure.
The formation of oil and gas reservoirs in Dai Nan Formation is closely related to a tectonic movement after the deposition of Dai Nan Formation and before the deposition of Sanduo Formation (it was suggested that it was Zhouzhuang Movement). The main sign is the unconformity and overlap between Sanduo Formation and Dai Nan Formation. According to the electrical comparison, there are some strata missing in the upper part of Dai Nan Formation and the bottom of Sanduo Formation.
In the late Oligocene, after the deposition of more than three groups, a violent "more than three movements" occurred. This movement affected the whole South Yellow Sea Basin in northern Jiangsu, resulting in the unconformity between Sanduo Formation and Yancheng Formation. Triassic movement is a regional "seesaw movement" for the basin, which caused the eastern part of the basin to sink sharply. The main way of deposition is to fill first and then overlap and cover.
Sanduo movement is the re-migration and oil-gas redistribution movement of formed oil and gas reservoirs in Subei Basin. Part of oil and gas migrated from primary oil and gas reservoirs along fault channels to newly formed structures, becoming secondary oil and gas reservoirs. However, the influence area of this movement is not consistent throughout the South Yellow Sea Basin in northern Jiangsu. In some areas, due to the influence of compressional folds, anticline structures have been formed. Most of the structures seen at present are inherited from the old structures, and the Sanduo Formation oil and gas reservoir can be formed through faults as channels. According to the recent exploration data, there are generally two types: one is shallow-buried and deep-oxidized reservoirs, which make the oil quality heavier, the specific gravity is generally above 0.9, and the oil-water relationship is complex, such as Well Rudong 3 and Well Dong 9; The other is deep-buried reservoirs with good preservation conditions, such as the oil layer seen in well Su 58 and the thick oil layer seen in well Su 63 in Fuminzhuang area, which is expected to obtain high-yield industrial oil flow. In the future, we must pay enough attention to and strengthen the study of Sanduo Formation oil and gas reservoir, because this stratum is widely distributed, good in physical properties, shallow in burial and easy to explore, which is one of the main targets of oil exploration in the South Yellow Sea in northern Jiangsu.
There is also a new tectonic movement between the upper and lower Yancheng Formation. However, during this period (Yancheng Formation deposition), there was a lack of oil-generating conditions in northern Jiangsu. However, the local structure formed after the neotectonic movement of this set of strata with a thickness of several thousand meters may form new oil and gas reservoirs under the condition of forming suitable cover protection, especially the most active natural gas in the migration process formed Yancheng Formation gas reservoir, which should be paid attention to in the general survey and exploration work.
2. The main factors of oil and gas control
At present, due to the low degree of exploration, the oil and gas control law in northern Jiangsu has not been deeply understood and fully grasped. In fact, only the existing survey and exploration data of several major depressions in Dongtai sag can be preliminarily discussed.
Since Tertiary, the late Paleocene, the middle and late Eocene are the three main oil-generating periods in Subei Basin. There may be two oil-generating periods in the early and middle Oligocene. There was a lack of oil-generating conditions in northern Jiangsu in the Middle Pliocene.
At present, there is a lack of geological data in the South Yellow Sea, but according to the eastward movement trend of Miocene sediments in northern Jiangsu, the subsidence speed from Oligocene to Pliocene is relatively high, and the eastern part should be a rapidly declining area, so it should be said that there is a great possibility of improving oil-generating conditions.
In the process of its occurrence and development, Subei basin has many sedimentary centers, especially the dustpan fault depression, which has the most favorable conditions for oil and gas generation, preservation and migration. Based on this, we put forward the following views on the main factors controlling oil and gas in northern Jiangsu:
(1) The dustpan fault depression is the main place to form oil and gas reservoirs, and the trap anticline structure (including the nose anticline blocked by faults) is the most important factor to control oil and gas.
On one side of the steep slope belt of the dustpan fault depression, due to the long-term inherited activity of the fault, the footwall of the fault caused the largest deposition and became the settlement center, which objectively has five advantages:
① The largest deposition range, the thickest deposition and the richest material base. ② The organic matter is the richest, the oil generation index is the best, and the oil source supply is sufficient. ③ The steep side of the fault depression is controlled by the first-order fault, and the periphery of the fault depression is raised and surrounded by uplift, so the regional oil and gas sealing conditions are the best. (4) Due to long-term subsidence, the deep depression is not affected by the Five Treasures Movement, with continuous sedimentation and the best preservation conditions of strata, with a complete set of multi-layer source-reservoir-cap assemblage. ⑤ The static pressure of the overlying strata is the largest, and it has conversion conditions such as temperature and pressure which are favorable for oil generation.
It is inferred that the generation and migration process of oil and gas in each dustpan fault depression in northern Jiangsu is deep depression, that is, the oil and gas generated in the sedimentary center began to migrate along the fault channel or in the form of lithologic seepage, some of which entered the high area of the deep depression zone and some entered the steep slope zone (fault step zone) or slope zone.
Taking the Fan Chuan dustpan fault depression in the south of Gaoyou as an example, oil and gas are mainly distributed in three secondary structural belts in the northeast, namely the Paleogene fault terrace belt in the south, represented by Zhenwu nose anticline, and industrial oil flow has been obtained at present. In Fumin-Huagongli high belt in central China, oil and gas shows have been obtained in Dai Nan Formation and Sanduo Formation. In the northern slope of Shabei, obvious oil and gas shows were also obtained during drilling.
Taking Qintong sag as an example, the distribution of oil and gas is also controlled by three northeast structural belts. Zhujiazhuang fault terrace belt in the south is represented by Zhujiazhuang fault block, Caoshebi fault block and Chujialou nose anticline structure, and industrial oil flows of different horizons have been obtained. The central part is a discontinuous high belt from Jiaodunzi to Dai Nan, where the industrial oil flow of Dai Nan Formation has been obtained, represented by Well Subei 20. The Shijiapu slope belt in the north is a favorable oil-bearing belt because of the discovery of oil and gas during drilling.
Taking Sanhe half graben fault depression in the north of Jinhu as an example, there are also three belts. Qiaohekou nose anticline structure was found in the southern fault step belt, which belongs to the horizon of Dai Nan Formation and is a very favorable part, that is, the part to be explored. Zhuang Lei structure was found in the middle of Zhuang Lei high belt, which also belongs to the horizon of Dai Nan Formation, and oil-bearing horizons of Dai Nan Formation and Funing Formation were found in Dong53 well near this belt. The industrial oil (gas) flow of Funing Formation was obtained from Well Dong 60 with Liu Zhuang nose anticline structure in the northern Liu Zhuang slope.
One of the three dustpan-shaped fault depressions listed above is characterized by sufficient oil sources and good sealing conditions, resulting in multi-layer oil bearing, steep slope and gentle slope, and oil bearing in deep and shallow layers.
In the fault step belt of the dustpan-shaped fault depression, the low-order anticline, that is, the high point of the third-order anticline, is the most favorable for oil and gas preservation and enrichment, because this kind of anticline is generally in the background of gravity uplift, nose-shaped and dock-shaped, with three sides descending around the deep depression and the most abundant oil supply.
In the middle-high zone or slope zone of the fault depression, in some places, except for the oil in the fault block covered by the fault, the anticline structure is the most important structure to control oil and gas accumulation, even if the anticline amplitude is only gentle, it can play a good role in oil and gas accumulation.
The controlling factors of oil and gas in the slope belt of half-graben fault depression are worth discussing, because the slope belt is generally wider than the fault step, which is a good place to accept oil and gas migration and accumulation in a large area. In order to further analyze the nature of the slope belt, it is suggested that the slope belt can be divided into inner belt and outer belt according to the distribution of strata in Dai Nan Formation. Generally, the vicinity of the deep depression of Dai Nan Formation is divided into inner zones, such as Shijiapu area in Qintong, Shanian area in Fan Chuan and the east of Sanhe Liu Zhuang. Therefore, Dai Nan Formation stratum is missing (but Funing Formation stratum is still preserved) and belongs to the outer slope zone. It is located in the high part of the structure, with shallow oil and gas burial, mainly in Funing Formation, such as Jinhu Liu Zhuang and Cui Zhuang structures.
Slope belt is quite common in northern Jiangsu. For example, recently, a semi-faulted depression was discovered in Hongze area through Zn 10 magnetic tape seismic profile, and a very obvious slope belt appeared in the north. Observed from the seismic wave group, the overlapping and pinching phenomena are very obvious. A deep earthquake has a reflection of 3 seconds and a deposition thickness of more than 3,000 meters. If there are source rocks, it will be a "small and fat" oil-bearing fault depression.
So far, the dustpan-shaped fault depressions found in Subei Basin are generally steep in the south and gentle in the north, with faults in the south and overlooking in the north, which are restricted by NE-trending faults. Compared with the outer belt of the slope belt, the thickness of the deep fault distance of the deep concave belt is quite different, generally around 2000 meters. It looks like a unilateral graben, which leads to this structural form. Regionally, this may be because the northern part of Subei Basin is controlled by the uplift of the ancient Lusulong block, resulting in a movement law in which the northern part of the depression is dominated by rising slopes and the southern part is dominated by deep faults. Look carefully, from Lusulong to Jiyang Depression in North China. Compared with northern Jiangsu, the dustpan fault depression there is just the opposite, and it is a structural form with steep north and gentle south. Although their morphological differences are different, their causes are basically the same. They are all on Yanshanian sediments. After the basin was dismembered by crustal movement, scarp faults appeared and developed on the basis of unilateral graben depression. Because it contains a huge amount of sediments, it has been steadily sinking for a long time, so it not only has favorable conditions for oil and gas generation, but also is very beneficial to preservation and migration. Because of its unique geological conditions, it has become a particularly important structural unit for petroleum exploration and petroleum geological research in northern Jiangsu.
(2) Oil control of main fracture
The near east-west fault is an early fault, which existed during the formation of the basin. It is the main structural line that divides depression and uplift, and most of them belong to first-class faults. In the past, Li Siguang speculated that "the grass-roots structure in northern Jiangsu is generally influenced by the east-west uplift ... there is a subsidence zone in the south of the uplift zone, which is also roughly east-west and extends to the sea ...". Generally speaking, the early faults of this group are not obvious in the west of the basin (Jinhu, Gaoyou, Qintong sag, etc.). ) because it was destroyed and disturbed by Himalayan movement many times in the later period, it was only in the central uplift belt and southern depression in the south of Hai 'an Depression and the South Yellow Sea. This group of faults played a major role in controlling the sedimentation of the Late Cretaceous and Funing Formation, but not in controlling oil and gas.
NE-trending fault, which is basically consistent with the strike of the east branch of the Tan-Lu fault. Some people once called it "Huaxia type", which is dominant in the whole southern Yellow Sea area of northern Jiangsu. Including primary faults, secondary faults and tertiary faults, this paper focuses on the secondary faults and tertiary faults related to oil and gas control.
Longgang fault in Jinhu sag, Wang Ying fault in Gaoyou sag, Chenbao fault, Xiaoji fault, Taizhou fault and Zhujiazhuang fault in Qintong sag all belong to secondary faults. This group of faults has dual functions. First, the preservation of Funing Formation and the deposition of Dai Nan Formation play a decisive role; The second is to block the oil and gas activities in the depression area, especially the primary oil and gas reservoirs.
According to the current exploration data, no obvious oil and gas show has been found in the upper wall of Taizhou fault and Xiaoji fault. This is because these footwall areas are often eroded and strata are missing after the Wubao movement, which is not conducive to the formation or preservation of primary oil and gas reservoirs. However, we must pay attention to the possibility of footwall oil and gas migrating to footwall along fault channels and entering structures with caprock and sealing conditions, forming secondary oil and gas reservoirs. For example, Pukou Formation of Well Su 19 on the edge of Taizhou uplift has oil-bearing marks in the strata, which is a very important mark and must be paid full attention to in the future general survey and exploration.
The most noteworthy is the control of this group of faults on oil and gas in the footwall fault step.
The fault terrace zone in Subei sag is generally composed of two groups of parallel or slightly inclined secondary faults. Therefore, it can be generally divided into three steps: upper step, middle step and lower step, which constitute a secondary structural belt in Northeast China. The upper wall of the fault belongs to the upper step (the oil and gas control situation is described above), and belongs to the middle step between the two main faults, which controls the oil content of the fault block. For example, this is the case with Zhujiazhuang fault block. The oil layer of Dong 7 Well Fu 1 section drilled on this fault block is considered as a secondary reservoir migrated laterally by low steps. Because the middle step is still in a high structural position, most of the strata of Dai Nan Formation are not completely preserved, and the Funing Formation oil layer is mainly encountered in the exploration, but there are still Dai Nan Formation strata (such as Xujiazhuang fault block in Zhenwu structure), and the depth of the low step is not much different from that of the middle step.
Main faults above low order play an extremely important role in oil and gas. Low step is the best structural position to obtain high-yield industrial oil flow in fault step zone at present. The strata of Dai Nan Formation and Sanduo Formation on this bench are well developed, while the Funing Formation below is complete, and the combination of source rocks, reservoir rocks and caprocks is ideal. At the same time, it faces a deep depression with rich oil and gas sources, and faults above low steps have played a good role in sealing oil and gas. Therefore, if there are anticlinal traps in the lower order, oil and gas-rich areas can be formed, such as Zhenwu and Chujialou.
The NE-trending faults in the depression are also well developed, which is the main factor to form fault blocks. In general survey and exploration, oil-bearing roof blocks are often formed by reverse normal faults, and most of the oil and gas are controlled at the high point of the roof, such as the oil layer seen in Dong42 well of Hekou fault block in Gaoyou Depression.
On the one hand, the main fault plays a shielding role; On the other hand, because of its large fault distance, it crosses multiple oil-bearing layers and reservoirs, and it is easy to form multiple oil-bearing, especially in the anticline structure with good oil-generating conditions and developed reservoirs, a "architectural" superimposed oil and gas reservoir can be obtained.
As for the NW-trending fault, according to the existing data in northern Jiangsu, there are roughly two situations: one is a shear fault formed at the same time as the NE-trending fault. Although most of them may be tensile, they can still be used as migration channels for oil and gas. The other is a new NW-trending fault formed by late tectonic movement. Some of these faults are inherited and developed on the basis of the old faults in the northwest direction, which have a certain impact on oil and gas reservoirs and urge the primary oil and gas reservoirs to migrate again to form secondary oil and gas reservoirs. For example, some Sanduo oil and gas reservoirs may also be related to this fault, and some new NW-trending faults that do not appear on the basis of old faults have little to do with the formation of secondary oil and gas reservoirs.
(3) Favorable lithofacies zones control oil-bearing property.
In the process of geological and historical development, Subei basin has experienced many crustal changes, forming a complex paleogeomorphology with many depressions, many bulges, many uplifts and many faults, so it cannot be regarded as a simple sedimentary basin when tracing its paleogeomorphology. Different strata in the basin formed lithofacies zones with different properties in different sedimentary periods.
Jinhu sag is the closest uplift and denudation area in the basin, and its lithologic changes and the formation of lithofacies zones are more obvious than those in the basin. According to the preliminary data of current investigation and exploration, Funing Formation in Jinhu area mainly has two lithofacies zones.
One is the carbonate facies belt of Fu 2 member formed by transgression, which is mainly composed of biolimestone, including clastic limestone, oolitic limestone, marl, some sandstone, calcareous sandstone and mudstone, and the facies belt is about 50 meters thick. It has become a good reservoir with gas in the upper part and oil in the lower part, as shown in Well Dong 60 and Well Dong 64. Biolimestone has wormhole structure and good porosity and permeability. The total porosity is 33% ~ 4 1%, the permeability is 25 ~ Darcy, the time difference of sound velocity is as high as 520 microseconds, and the single layer is 6.4m thick, so it is a good gas reservoir in Liu Zhuang. According to the preliminary test of the fourth layer of Dong 60 well, the industrial gas flow was obtained and it became the first natural gas well in northern Jiangsu. It is worth emphasizing that, similar to the biogenic limestone in Fu-2 member of Liu Zhuang, an annular favorable lithofacies belt may be formed in the slope belt around Jinhu sag, which will become a new field of oil and gas exploration in northern Jiangsu.
Another facies belt in Jinhu area is the rich 1 and 3 sandstone facies belt in the source area of southwest denudation area, which may belong to the delta system and is most developed in Tongcheng area, with a thickness of about 453 meters. Lithologically, there is one more set of sand formation (medium sand formation) than Qintong and Gaoyou areas. This lithofacies belt reaches Guantang 260m (East 6 1 well) to Sanhe168m (East 53 well) and Wanzhuang10/meter (East 66 well). The Liu Zhuang structure is only 86m thick, leaving only "three sharp points" on the electrical survey curve. If there are structural sealing conditions in this facies belt with good sand-mud ratio matching, it will be the place where oil and gas are enriched.
The sandstone facies belt of Dai Nan Formation in Qintong Depression has changed obviously. From east to west, from Mozhuang to Yedian via Dai Nan, the thickness of sandstone layers gradually decreases, and the optimal allocation of sand-mud ratio appears in the middle of the facies belt. For example, Chujialou structure and Dai Nan Formation have good reservoir and cap conditions, thus forming a good reservoir and obtaining high-yield industrial oil flow.
The change of lithofacies (Dai Nan Formation) in Qintong area can be observed not only from drilling, but also from seismic profile interpretation, and the change of wave group phase can be tracked to understand the change of lithofacies. For example, there are two strong waves on Nz34- 15 line, and the Ez line wave becomes weaker and stronger, both of which are wave group changes caused by lithologic changes. The general trend is that the westward phase transition of Dai Nan Formation in Qintong area is thin, with more mudstone and less sandstone.
The above situation fully shows that under the premise of structural trap conditions (including the shielding conditions of compressional faults), favorable lithofacies zones actually enrich the oil and gas control factors in northern Jiangsu. If we leave favorable lithofacies conditions and only have trap conditions such as anticline structure, it will be difficult to form high-yield oil and gas migration. From what has been found, the place where the structure overlaps with the favorable lithofacies belt is one of the most favorable areas for oil and gas exploration.
After the Caoshe and Chujialou structures were drilled with oil and gas, with the establishment of the East China Petroleum Geology Bureau of the Ministry of Geology, with the efforts of Zhong Teqiang, director of the bureau, and Guo Renbing, captain of the trial production brigade, they were directly converted from exploration to development, confirming that these two structures were exploitable oil fields, creating the first case for the geological department to develop oil fields, and laying the foundation for the establishment of the northern Jiangsu petroleum industrial base in the future.