Introduction

Research status

With the deepening of oil and gas exploration, it is gradually discovered that not all parts under the regional mudstone caprock near the oil source faults have oil and gas accumulation and distribution, which is mainly controlled by the distribution of vertical sealing evolution form of oil source faults in addition to the influence whether the trap is developed1,2,3. Oil and gas can be accumulated and preserved only when it is located in the distribution position of the vertical sealing evolution form of the oil source faults that is conducive to the accumulation and preservation of oil and gas. Otherwise, there is no oil and gas accumulation and preservation. The key to oil and gas exploration under regional mudstone caprock at oil source faults is to accurately predict the distribution of vertical sealing evolution forms of oil source faults.

At present, there is no literature guidance on the distribution of vertical sealing evolution form of oil source faults, but corresponding research has been carried out, which can be summarized in the following two aspects: ① The determination of vertical sealing of current oil source faults is mainly based on the assumption that the oil source faults are rock strata inclined into surrounding rock, and the displacement pressure of fault rock is the same as that of surrounding rock, which is mainly affected by its burial depth of compaction and shale content4,5,6. The displacement pressure of fault rock is calculated by calculating the burial depth of compaction and shale content of fault rock7,8, and using the empirical relationship among the measured displacement pressure of surrounding rock and the burial depth of compaction and shale content9,10. Then use the burial depth of compaction and shale content of the underlying reservoir rock to calculate the displacement pressure of the reservoir rock based on the relationship among the measured displacement pressure of the reservoir rock and the burial depth of compaction and shale content11,12,13. Finally, the relative magnitude of displacement pressure of fault rock and reservoir rock is compared to study the vertical sealing of current oil source faults. If the displacement pressure of fault rock is greater than or equal to that of underlying reservoir rock, the oil source faults is vertically sealed. On the contrary, it is unsealed. ② The determination of the vertical sealing of the oil source faults in the oil and gas accumulation period is mainly based on the relative size of the regional mudstone caprock caprock-fault juxtaposition thickness (the regional mudstone caprock thickness minus the fault displacement of the oil source faults) and the maximum caprock-fault juxtaposition thickness required for the segmented growth of the fault in the regional mudstone caprock to connect up and down. If the caprock-fault juxtaposition thickness of regional mudstone caprock is greater than or equal to the maximum caprock-fault juxtaposition thickness required for the segmented growth of the fault in the regional mudstone caprock to connect up and down, the oil source faults are vertically sealed14. On the contrary, it is unsealed. These research results play a very important role in correctly understanding the distribution of oil and gas under the regional mudstone caprock at the oil source faults.

However, these studies were conducted separately on the vertical sealing of oil source faults at present and during the oil and gas accumulation period, and were also conducted on different oil source faults15. Lacking of the study on the sealing position of the vertical sealing evolution form of oil source faults, was not conducive to the in-depth exploration of oil and gas under the regional mudstone caprock at the oil source faults. Therefore, it is of great significance to carry out the research on the prediction method of the distribution of vertical sealing evolution form of oil source faults for correctly understanding the distribution rule of oil and gas under the regional mudstone caprock in the oil source faults of oil and gas bearing basins and guiding their oil and gas exploration.

Evolution form and distribution position of vertical sealing of oil source faults

Due to the different activity characteristics of the oil source faults itself and the development characteristics of the regional mudstone caprocks staggered by it, the vertical sealing distribution characteristics of the oil and gas accumulation period and the current oil source faults are different, resulting in the different vertical sealing evolution forms of the oil source faults. The following four forms can be summarized: the first is the vertical sealed - sealed evolution form of the oil source faults, this evolution form means that both the oil and gas accumulation period and the current oil source faults are vertically sealed, and the overlapping position of the two is the distribution position of the vertical sealed - sealed evolution form of oil source faults, which is conducive to the accumulation and preservation of oil and gas under the regional mudstone caprock at the oil source faults, as shown in Fig. 1a. The second is the vertical sealed - unsealed evolution form of oil source faults. This evolution form refers to the vertical sealing of oil source faults during the oil and gas accumulation period, but is unsealed at present. The overlapping position of the two is the distribution position of the vertical sealing evolution form of oil source faults, which is not conducive to the accumulation and preservation of oil and gas under the regional mudstone caprock at the oil source faults, as shown in Fig. 1b. The third is the vertical unsealed - sealed evolution form of oil source faults. This evolution form refers to the vertical unsealed oil source faults during the oil and gas accumulation period, but the current oil source faults are vertically sealed. The overlapping part of the two is the distribution part of the vertical sealing evolution form of oil source faults. Generally, it is not conducive to the accumulation and preservation of oil and gas under the regional mudstone caprock at the oil source faults, but it is conducive to the accumulation and preservation of oil and gas under specific geological conditions, as shown in Fig. 1c. The fourth is the vertical unsealed - unsealed evolution form of oil source faults. This evolution form refers to that both the oil and gas accumulation period and the current oil source faults are unsealed vertically. The overlapping position of the two is the vertical unsealed - unsealed evolution form distribution position of oil source faults, which is not conducive to the accumulation and preservation of oil and gas under the regional mudstone caprock at the oil source faults, as shown in Fig. 1d.

Fig. 1
figure 1

Determination of vertical sealing evolution form and location of oil source faults.

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Geological background

Geographical location

The Qikou Sag is located in the central - northern part of the Huanghua Depression in the Bohai Bay Basin. It is a continental rift that developed in the middle Eocene, with a total area of approximately 9000 km[216. The northern boundary of the sag is the Hangu Fault, connecting it to the Yanshan Fold Belt. To the south, it is adjacent to the Chengning Uplift, and the strata gradually overlap onto it. The western boundary is the Cangdong Fault, separating it from the Cangxian Uplift. In the east, across the Shaleitian Uplift and the Shanan Sag, it faces the Bozhong Depression (Fig. 2).

Fig. 2
figure 2

Structural units division of Qikou Sag.

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Stratigraphic characteristics

The Qikou Sag is a fault—depression with long-term inheritance development. It has mainly experienced the sedimentary filling of the Paleogene rifting stage and the Neogene—Quaternary depression stage. The sedimentary filling of the former is characterized by the development of a lake basin, with the thickness of the formed sediments ranging from 4000 to 6000 m. The developed strata, from bottom to top, are the Kongdian Formation, the Shahejie Formation, and the Dongying Formation. The sedimentary filling of the latter is characterized by continental fluvial deposition, with the thickness of the formed sediments being approximately 4000 m. The developed strata, from bottom to top, are the Guantao Formation, the Minghuazhen Formation, and the Pingyuan Formation (Fig. 3)17.

The thickness of the Shahejie Formation (Es) ranges from 200 to 4,250 m, with the center located on the down - thrown side of the boundary faults on the northern and western margins of the sag. Two incomplete sedimentary cycles are developed. According to the lithological and electrical characteristics of the section, the Shahejie Formation can be subdivided into Es3 (the 3rd member of the Shahejie Formation), Es2 (the 2nd member of the Shahejie Formation), and Es1 (the 1st member of the Shahejie Formation) from bottom to top (Fig. 3).

Es3: The thickness of this member ranges from 100 to 2400 m, and it is widely distributed. It is the first set of source rock strata developed in the sag, and it has an angular unconformable contact with the underlying Kongdian Formation. Generally, its lithology is composed of interbeds of dark gray and gray - brown mudstones with light gray sandstones and glutenites in varying thicknesses. Gray - brown oil shale can be seen in the middle and lower parts. It belongs to deep - lake, semi - deep - lake, shore - shallow - lake, and fan - delta deposits.

Es2: The thickness of this member ranges from 0 to 450 m, and its distribution range is relatively small. It is absent north of the Cangdong Fault and the Beitang Sub - sag, and south of the Yang’erzhuang - Koucun Fault. Usually, two incomplete positive sedimentary cycles are developed. It has a conformable contact with the underlying Es3 in the middle of the sag, and has a disconformable or unconformable contact in the slope area and the uplift area. It mainly develops braided - river delta, fan - delta, and shore - shallow - lake deposits.

Es1: This member is distributed throughout the region, with stable deposition. Its thickness ranges from 100 to 1,400 m, gradually thinning from the sag area to the Kongdian - Yangsanmu Uplift area. It is the second set of source rock strata developed in the sag, and belongs to gravity - flow, braided - river delta, fan - delta, and shore - shallow - lake deposits.

Fig. 3
figure 3

Comprehensive stratigraphic column of Qikou Sag.

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Data and methods

Data

In this paper, drilling and 3D seismic data are applied to obtain parameters such as the fault displacement and the thickness of the cap rock. Methods from the references are used to calculate the parameters required for the study, such as the ancient caprock-fault juxtaposition of the mudstone cap rock, providing data support for the determination of the vertically sealed and unsealed parts. This study involves a large amount of original data, and only a part of the research data is listed here. Table 1 shows the values of the paleo - fault - juxtaposed thickness of mudstone caprocks at some different survey lines.

This paper involves drilling and 3D seismic data. The manual reading of parameters such as fault displacement and caprock thickness may introduce human errors, which in turn could lead to deviations in the calculation results. Therefore, during actual operations, it is necessary for multiple personnel to separately verify the data to reduce human errors.

Table 1 The data of basic.
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Methods

  1. (1)

    Utilize drilling and 3D seismic data to determine the vertically sealed and unsealed parts of oil—source faults during the hydrocarbon accumulation period.

To predict the distribution position of vertical sealing evolution form of oil source faults, it is necessary to determine the vertical sealed and unsealed positions of oil source faults during the oil and gas accumulation period and at present.

To determine the vertical sealed and unsealed positions of oil source faults during oil and gas accumulation, drilling and 3D seismic data must be used to read the current fault displacement of oil source faults in regional mudstone caprocks and the current thickness of regional mudstone caprocks that are broken by them. Applied the methods for restoring the ancient fault displacement of faults18 and for restoring the ancient thickness of strata19, restore the ancient fault displacement of the oil source faults in the regional mudstone caprock during the oil and gas accumulation period and the ancient thickness of the regional mudstone caprock that is dislocated by it, subtract the ancient fault displacement of the oil source faults from the ancient thickness of the regional mudstone caprock, and calculate the ancient caprock-fault juxtaposition thickness of the regional mudstone caprock at different positions, as shown in Fig. 2. Using the relationship between the ancient caprock-fault juxtaposition thickness of the regional mudstone caprock in the study area and its upper and lower oil and gas distribution, determine the maximum caprock-fault juxtaposition thickness20 required for the segmented growth of upper and lower connections in the regional mudstone caprock, as shown in Fig. 2. Circle the parts where the ancient caprock-fault juxtaposition thickness of the regional mudstone caprock is greater than or equal to the maximum caprock-fault juxtaposition thickness required for the segmented growth of the upper and lower connections in the regional mudstone caprock, that is, the vertical sealed positions of the oil source faults during the oil and gas accumulation period. Circle the parts where the ancient caprock-fault juxtaposition thickness of the regional mudstone caprock is less than the maximum caprock-fault juxtaposition thickness required for the segmented growth of the upper and lower connections in the regional mudstone caprock, that is the vertical unsealed positions of the oil source faults in the oil and gas accumulation period, as shown in Fig. 4.

Fig. 4
figure 4

Determination of vertical sealed and unsealed parts of oil source faults during oil and gas accumulation.

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  1. (2)

    Determine the vertically sealed and unsealed parts of current oil—source faults by using drilling and 3D seismic data.

To determine the vertical sealed and unsealed positions of the current oil source faults, it is necessary to determine the current displacement pressure of the fault rock in the regional mudstone caprock and that of the underlying reservoir rock of the oil source faults. Use drilling and 3D seismic data to calculate the current fault displacement of the oil source faults in the regional mudstone caprock and the current thickness and shale content of the faulted strata. Calculate the shale content of the fault rock of the oil source faults in the regional mudstone caprock according to Eq. (1). The drilling and 3D seismic data are used to calculate the current burial depth, dip angle, inactive time of the oil source faults in the regional mudstone caprock, and the compaction and diagenesis time of the surrounding rock with the same burial depth, which are substituted into Eq. (2) to calculate the compaction and diagenesis burial depth of the fault rock. Substitute the shale content and compacted burial depth of fault rock of oil source faults in the regional mudstone caprock into the empirical relationship among the measured displacement pressure of surrounding rock in the study area and its compacted burial depth and shale content (Eq. 3), and then the current displacement pressure of fault rock of oil source faults in the regional mudstone caprock can be obtained, as shown in Fig. 5.

$$R_{f} = \frac{{\sum\nolimits_{{i = 1}}^{n} {H_{i} R_{i} } }}{L}$$
(1)

where Rf is shale content of fault rock (percentage), Hi is the ith faulted strata thickness (m), Ri is the ith faulted strata shale content (percentage), L is the fault displacement (m), i is the ith layer of faulted strata, and n is the number of layers of the faulted strata.

$${Z_{\text{f}}}=\frac{{{T_0}}}{T} \cdot \frac{Z}{{\cos \theta }}$$
(2)

where\({Z_{\text{f}}}\)is the compacted burial depth of fault rock (m), \({T_{{0}}}\) is the period when the fault activity stops (Ma), T is the period of rock compaction and diagenesis (Ma), Z is the current burial depth of fault rock (m), θ is the fault dip angle (°).

$${P_{\text{c}}}={\text{a}}{(\frac{{{Z_{\text{c}}}{R_{\text{c}}}}}{{100}})^{\text{b}}}$$
(3)

Where \({P_{\text{c}}}\) is displacement pressure of mudstone caprock (MPa), \({Z_{\text{c}}}\)is the compaction and diagenesis burial depth of mudstone caprock (m), \({R_{\text{c}}}\)is the shale content of mudstone caprock (decimal), a, b is the constant related to the region.

Fig. 5
figure 5

Determination of vertical sealed and unsealed parts of current oil source fault.

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Use drilling data to calculate the current burial depth of the underlying reservoir rock, and use natural gamma logging data to obtain the shale content of the underlying reservoir rock21. Substitute the burial depth and shale content of the underlying reservoir rock into the empirical relationship among the measured displacement pressure of the reservoir rock in the study area and the burial depth and shale content (Eq. 4), and then the current displacement pressure of the underlying reservoir rock can be obtained, as shown in Fig. 3.

$${P_{\text{s}}}={\text{c}}{(\frac{{{Z_{\text{s}}}{R_{\text{s}}}}}{{100}})^{\text{d}}}$$
(4)

Where Ps is displacement pressure of mudstone caprock (MPa), Zs is the compaction and diagenesis burial depth of mudstone caprock (m), Rs is the shale content of mudstone caprock (decimal), c, d is the constant related to the region.

Comparing the determined current displacement pressure of fault rock in the regional mudstone caprock of the oil source faults with that of the underlying reservoir rock, we can get the vertical sealed and unsealed positions of the current oil source faults, as shown in Fig. 5.

Superimposing the determined the vertical sealed and unsealed positions from oil and gas accumulation period and the current oil source faults, we can get the distribution positions of the vertical sealing evolution form of the oil source faults, as shown in Fig. 1.

Results and discussion

The method proposed in the study is applied to Dazhangtuo fault in Qikou sag, Bohai Bay basin, to predict the distribution position of the vertical sealing evolution form of the Dazhangtuo fault in the regional mudstone caprock of the middle sub-member of Es1. The analysis on the relationship between the prediction results and the current oil and gas distribution under the regional mudstone caprock of the middle sub-member of Es1 at the Dazhangtuo fault, proves the feasibility in predicting the distribution of vertical sealing evolution form of oil source faults.

The Dazhangtuo fault is a NE trending normal fault about 21.3 km long in the north of Qikou sag. The dip angle of the fault plane is 55°–62° and it dips to the southeast from the lower basement up to near the surface, showing the characteristics of long-term inheritance, as shown in Fig. 6. The strata developed in the northern part of Qikou sag where the Dazhangtuo fault is located are Paleogene, Neogene and a small amount of Quaternary from the bottom to the top. The Paleogene includes Kongdian Formation, Shahejie Formation and Dongying Formation, and the Neogene includes Guantao Formation and Minghuazhen Formation. At present, the oil and gas discovered at the Dazhangtuo fault are mainly distributed in the lower sub-member of Es1. The oil and gas mainly come from the underlying source rock of Es3, and the caprock is a large set of regional mudstone developed in the middle sub-member of Es1. The Dazhangtuo fault connects the source rock of underlying Es3 and the reservoir of the lower sub-member of Es1, and it should be the oil source faults of the lower sub-member of Es1 during the oil and gas accumulation period - the middle and late period of Minghuazhen Formation sedimentation. It can be seen from Fig. 6 that the discovered oil and gas in the lower sub-member of Es1 of Dazhangtuo Fault are mainly distributed in the east and a small amount in the west. This is mainly affected by the distribution of vertical sealing evolution form of Dazhangtuo Fault in the regional mudstone caprock of the middle sub-member of Es1, in addition to the influence of the development of traps and sand bodies. Therefore, it is very important for oil and gas exploration to accurately predict the distribution position of vertical sealing evolution form of Dazhangtuo fault in the regional mudstone caprock of the middle sub-member of Es1.

Fig. 6
figure 6

Relationship between Dazhangtuo fault and hydrocarbon distribution of Ban 2 reservoir group in Es1.

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By using drilling and 3D seismic data, the current fault displacement of the Dazhangtuo fault in the regional mudstone caprock of the middle sub-member of Es1 and the current thickness of the faulted regional mudstone caprock of the middle sub-member of Es1 by the Dazhangtuo fault are calculated. According to the restored methods of ancient fault displacement22 and the ancient thickness of strata23, restore the ancient fault displacement in the regional mudstone caprock of the middle sub-member of Es1 and the ancient thickness of the faulted regional mudstone caprock of the middle sub-member of Es1 by the Dazhangtuo fault. According to the ancient thickness minus the ancient fault displacement of Dazhangtuo fault, the ancient caprock-fault juxtaposition thickness of regional mudstone cap rock in the middle sub-member of Es1 at different positions of Dazhangtuo fault is calculated, as shown in Fig. 7. It can be seen from Fig. 7 that the ancient caprock-fault juxtaposition thickness of regional mudstone caprock in the middle sub-member of Es1 at Dazhangtuo fault is relatively large in the west, up to 500 m in the maximum, relatively small in the east, and only 70 m in the minimum. From Fig. 8, the maximum caprock-fault juxtaposition thickness required for segmented growth to connect up and down of the Qikou sag fault in the regional mudstone caprock the middle sub-member of Es1 is about 139 m. From the Fig. 7, The vertically sealed and unsealed positions of Dazhangtuo fault in regional mudstone caprock of the middle sub-member of Es1 during oil and gas accumulation are only distributed in the eastern part of the fault, and most of the other positions are vertical sealed positions.

Fig. 7
figure 7

Determination of vertical sealed and unsealed positions of Dazhangtuo fault in the regional mudstone caprock of the middle sub-member of Es1 during oil and gas accumulation.

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Fig. 8
figure 8

Determination of minimum breaking thickness required for regional Mudstone Caprock Sealing in the middle sub-member of Es1 of Qikou Sag.

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By using drilling and 3D seismic data, the current fault displacement of the Dazhangtuo fault in the regional mudstone caprock of the middle sub-member of Es1 and the current thickness and shale content of the faulted regional mudstone caprock are calculated. The current shale content is calculated according to Eq. 1. Calculate the burial depth, dip angle and inactive time, and compaction and diagenesis time with the same burial depth in the regional mudstone caprock of the middle sub-member of Es1. The shale content and compacted burial depth of fault rock are calculated from Eq. 2 and substituted into the empirical relationship among the measured displacement pressure of surrounding rock in Qikou sag and its burial depth and shale content (Eq. 5), then the current displacement pressure of fault rock in the regional mudstone caprock of the middle sub-member of Es1 of Dazhangtuo fault can be obtained, as shown in Fig. 9. The maximum displacement pressure of fault rock can reach 3.5 MPa, which is mainly distributed in two positions in the east of the fault. The displacement pressure gradually decreases from two relatively high value areas to the east and west of the fault rock, and decreases to below 2.5 MPa at the east end.

$${P_{\text{c}}}=0.28{\left( {\frac{{{Z_{\text{c}}}{R_{\text{c}}}}}{{100}}} \right)^{1.415}}$$
(5)

Where \({P_{\text{c}}}\) is measured displacement pressure of caprock in Qikou Sag (MPa), \({Z_{\text{c}}}\) is the burial depth of caprock (m), \({R_{\text{c}}}\) is the shale content of caprock (decimal).

The current burial depth of the lower sub-member of Es1 reservoir at Dazhangtuo fault is calculated by using drilling data, and the shale content is calculated by using natural gamma logging data according to the method for calculating shale content24. The burial depth and shale content are substituted into the empirical relationship among measured displacement pressure of reservoir rock in Qikou Sag and its burial depth and shale content (Eq. 6), then calculate the current displacement pressure of the reservoir rocks, as shown in Fig. 9. It can be seen from Fig. 9 that there are two relatively high values of the displacement pressure of the reservoir rocks, one in the west and one in the end of east. The displacement pressure decreases from two relatively high values to the east and west, and decreases to less than 1.5 MPa in the east.

Fig. 9
figure 9

Schematic of determining the vertical sealed and unsealed parts of the current Dazhangtuo fault in the regional mudstone caprock of the middle sub-member of Es1.

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$${P_{\text{s}}}=0.197{\left( {\frac{{{Z_{\text{s}}}{R_{\text{s}}}}}{{100}}} \right)^{1.312}}$$
(6)

Where Ps is measured displacement pressure of reservoir rock in Qikou Sag (MPa), Zs is the burial depth of reservoir (m), Rs is the shale content of reservoir (decimal).

Comparing the current displacement pressure of fault rock in the regional mudstone cap rock of the middle sub-member of Es1 of the Dazhangtuo fault identified above with that of reservoir rock in the lower the middle sub-member of Es1, we can get the vertical sealed and unsealed positions of the current Dazhangtuo fault in the regional mudstone caprock of the middle sub-member of Es1, as shown in Fig. 9. At present, the vertical sealing positions are mainly distributed in the west and middle east of the fault, and only the middle west and the end of east are vertical unsealed positions.

By superposing the above vertical sealed and unsealed positions of oil and gas accumulation period and that of the current Dazhangtuo fault in the regional mudstone caprock of the middle sub-member of Es1, we can get the distribution positions of the vertical sealing evolution form of the Dazhangtuo fault, as shown in Fig. 10. It can be seen from Fig. 10 that there are four vertical sealing evolution forms of the Dazhangtuo fault in the regional mudstone caprock of the middle sub-member of Es1. The vertical sealed—sealed evolution form is mainly distributed in the west, middle and east, the vertical sealed - unsealed evolution form is only distributed in the west, the vertical unsealed—sealed evolution form is mainly distributed in the two position of the east, and the vertical unsealed - unsealed evolution form is only distributed in the end of the east.

Fig. 10
figure 10

Determination of vertical sealing evolution form and location of Dazhangtuo fault in the regional mudstone caprock of the middle sub-member of Es1.

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It can be seen from Fig. 11 that at present, oil and gas in the lower sub-member of Es1 of the Dazhangtuo Fault are mainly distributed in the east and a small amount in the west, which is located in the vertical sealed - sealed and unsealed - sealed evolution forms of the Dazhangtuo Fault. This is because only the lower sub-member of Es1 located in the vertical sealed - sealed evolution form of the Dazhangtuo Fault, both the oil and gas accumulation period and the current Dazhangtuo fault are vertically sealed in the regional mudstone caprock of the middle sub-member of Es1, which is conducive to the accumulation and preservation of oil and gas in the lower sub-member of Es1obtained from the source rock of the underlying Es3 through the Dazhangtuo fault, and the oil and gas can be obtained through oil and gas drilling. However, in the lower sub-member of Es1 where the Dazhangtuo fault is vertically unsealed—sealed evolution form, although the Dazhangtuo fault was vertically unsealed in the regional mudstone caprock of the lower sub-member of Es1 during the oil and gas accumulation period, which was not conducive to the oil and gas generated from the source rock of Es3 along the Dazhangtuo fault into the lower sub-member of Es1 for accumulation and preservation, the Dazhangtuo fault disappeared upward in the regional mudstone caprock of Minghuazhen Formation overlying, and its top was sealed, resulting in the inability of oil and gas to migrate upward along the Dazhangtuo fault, and oil and gas can only be obtained by oil and gas exploration when they are accumulated and preserved in the lower sub-member of Es1 under the regional mudstone caprock of the middle sub-member of Es1.

Fig. 11
figure 11

Relationship between distribution position of vertical sealing evolution form of Dazhangtuo fault in the regional mudstone caprock of the middle sub-member of Es1 and oil and gas distribution of the lower sub-member of Es1.

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This method is applied to a case study of the evolution forms of vertical sealing of the Dazhangtuo Fault in the middle sub - member of Es1 within the regional mudstone caprock in the Qikou Sag of the Bohai Bay Basin. The results show a good correspondence with the oil and gas discovered in the lower sub - member of Es1 at the Dazhangtuo Fault at present, which verifies the feasibility of this method for studying the distribution of the evolution forms of vertical sealing of oil source faults. However, there are still two problems with this method:

  1. (1)

    This method is mainly applicable to predicting the distribution of the evolution forms of vertical sealing of extensional normal oil source faults in sand—shale petroliferous basins. It is not applicable to predicting the distribution of the evolution forms of vertical sealing of compressional and strike - slip oil source faults in the strata of sand - shale petroliferous basins, and is even less applicable to predicting the distribution of the evolution forms of vertical sealing of oil source faults in volcanic or carbonate rock strata.

  2. (2)

    When using this method to study the distribution of the evolution forms of vertical sealing of oil source faults, it is necessary to determine the displacement pressures of fault rocks and reservoir rocks. Therefore, this method can only be used in basins or sags with a large number of measured displacement pressures of surrounding rocks. Otherwise, it is difficult to apply this method.

Conclusions

  1. (1)

    The vertical sealing evolution of oil source faults mainly includes vertical sealed—sealed, vertical sealed - unsealed, vertical unsealed - sealed and vertical unsealed—unsealed evolution form.

  2. (2)

    By comparing the ancient caprock-fault juxtaposition thickness of regional mudstone caprock and the maximum such thickness for fault vertical growth, the vertical sealed-unsealed positions of oil source faults during hydrocarbon accumulation are determined. Based on the current displacement pressures of fault and underlying reservoir rocks, the current vertical sealed-unsealed positions of oil source faults are found. Combining these two, a method for predicting the distribution of vertical sealing evolution forms of oil source faults is established. Example applications prove its feasibility.

  3. (3)

    In the middle sub-member of Es1 in Dazhangtuo fault, Qikou sag, Bohai Bay basin, there are four vertical sealing evolution forms in regional mudstone caprock. Sealed-sealed forms are mainly in the west, middle and east; sealed-unsealed form is only in one west location; unsealed-sealed forms are mainly in two east locations; unsealed-unsealed form is in one location. The distribution of sealed-sealed and unsealed-sealed forms benefits hydrocarbon accumulation and preservation from Es3 source rock in the lower sub-member of Es1 at Dazhangtuo fault, consistent with the current hydrocarbon distribution in the lower sub-member of Es1.

  4. (4)

    This method is mainly applicable to the prediction of the distribution of vertical sealing evolution patterns of oil-source faults in sandstone petroliferous basins. In the future, attempts can be made to extend it to other types of petroliferous basins, such as carbonate formations. By studying the characteristics and main controlling factors of fault sealing in different formation types, the method can be improved and refined.