The argillaceous caprock has high displacement pressure and can seal oil and gas. Another important factor is the abnormal high pressure of undercompacted mudstone. Therefore, the displacement pressure of mudstone and the abnormal pressure of undercompacted mudstone are important parameters to evaluate the sealing performance of caprock.
5.2.4. 1 physical properties, displacement pressure and sealing performance of caprock
The physical properties of mudstone are important parameters to characterize the microstructure of mudstone. It reflects the degree of mudstone pore development and its ability to pass through fluid. The smaller the physical parameters, the smaller the reservoir space, the weaker the fluid permeability and the stronger the sealing ability. Physical properties analysis of shale in Chashen 1 Jingbayin Gobi Formation and breakthrough pressure experiments under different media saturation conditions show that Chagan sag has high breakthrough pressure, belongs to mudstone caprock and is well sealed (Table 5- 1).
Fig. 5- 16 evaluation chart of K 1 s 1 source rocks in Chagan sag
Table 5- 1 Evaluation and Classification Criteria of Source Rocks
The displacement pressure index of argillaceous caprock reflects the minimum pressure of fluid breakthrough pore seepage. The larger the numerical value, the more difficult it is for the fluid to penetrate into the pores and the higher the sealing ability. The caprocks with the same pore structure have different sealing abilities for fluids with different properties. Table 5-2 shows that when different media are saturated, the breakthrough pressure of gas changes greatly, and the breakthrough pressure of saturated water rock sample is higher than that of saturated oil rock sample, with a difference of 4 ~ 6 MPa. It shows that the sealing ability of rock samples is greatly enhanced after they are saturated with formation water under formation conditions.
Table 5-2 is the analysis table of rock sealing parameters of 1 well.
Abnormal pressure and sealing performance of 5.2.4.2
Abnormal pressure is closely related to the existence and settlement of large mudstone. During the evolution of the early Cretaceous lake basin, the sedimentary center of Chagan sag was always in the front depression and its surrounding area, and the lacustrine plastic mudstone with a thickness of nearly 1000 m was deposited. The late Bayin Gobi period to the early slaughter period is the main period of lake basin development. At this time, the deposited mudstone is thick and widely distributed, and the abnormal pressure is distributed on the sedimentary profile of this period. As we all know, under-compacted mudstone has the characteristics of high abnormal pore fluid pressure, high porosity, low density and low mechanical properties because it contains more pore fluid than normal compaction. This is helpful for us to detect overpressure development zone by using rock acoustic time difference data.
Acoustic time difference is closely related to porosity. Under normal compaction, mudstone porosity or acoustic time difference has an exponential relationship with buried depth. If there is abnormal pressure, abnormal characteristics will appear on the relationship curve between depth and acoustic time difference (Figure 5- 17). As can be seen from the figure, the anomaly in the interval below the reference well 1 2800m obviously reflects the existence of high pressure anomaly.
The characteristics of single well compaction curve can be divided into two sections: ① the compaction degree of the upper section (0 ~ 3000 m) increases with the increase of buried depth; ① In the lower member (3000~4300 m), due to the increase of overlying load, the porosity of mudstone failed to decrease with the decrease of depth, so it deviated from the normal compaction trend line and appeared under-compaction, that is, high pressure anomaly. The abnormal high pressure section can be divided into two abnormal sections: the upper section (3000~3250 m) belongs to the Yi Tu section of Su Hong and the top of Bayin Gobi Formation; The lower member (3250 ~ 4300m) belongs to Bayin Gobi Formation. The lithology of the upper high-pressure abnormal section is interbedded with sand and mudstone, and the lithology of the lower high-pressure abnormal section is mainly dolomite mudstone and argillaceous dolomite mixed with sand.
Fig. 5- 17 shows the mudstone compaction curve of 1 well.
The compaction characteristics and sealing characteristics of different mudstone compaction sections are quite different. (1) The buried depth in the normal compaction stage is 0 ~ 3000 m ... The mudstone compaction in this section is normal mechanical compaction caused by overlying sediment load. Its formation pressure basically belongs to hydrostatic pressure. Clay minerals in mudstone are mainly characterized by mixed layers of kaolinite, montmorillonite and montmorillonite, and the content of montmorillonite in montmorillonite gradually decreases from 65% to 16%. At this stage, the mudstone sealing is mainly based on capillary pressure sealing, and the sealing ability is enhanced due to the existence of a large number of expansive clay minerals. (2) The buried depth of mixed compaction stage is 3000 ~ 4300 m ... In this stage, mudstone compaction is mainly mechanical compaction, cementation and mineral transformation. Its formation pressure distribution is characterized by abnormal overpressure, and mudstone is characterized by large porosity, large acoustic time difference and low mudstone density. Clay minerals are rapidly transformed, kaolinite disappears, the mixed layer of Iraq/Mongolia decreases sharply, and the montmorillonite content in the mixed layer of Iraq/Mongolia decreases from 16% to below 10%. Among them, the characteristics of abnormal high pressure in the upper member are not obvious, corresponding to K 1s 1 the first set of source rocks characterized by interbedded sand and mud. The characteristics of abnormal high pressure in the lower member are very obvious, corresponding to the second set of source rocks of thick dolomite mudstone and argillaceous dolomite in K 1b2. This phenomenon shows that the generation of oil and gas has a positive impact on abnormal high pressure. Because the abnormal high pressure greatly enhances the sealing ability of the caprock, this stage is dominated by "pressure sealing", and the sealing conditions are the best. K 1s 1 low-production oil layer and chashen 1 well 3938 m blowout interval are located in these two high-pressure abnormal zones respectively, indicating that the formation of oil and gas reservoirs is closely related to this. (3) Buried depth in compaction stage > 4300m. At this time, the porosity of mudstone is less than 2%. Mudstone enters an "incompressible" degree, forming a solid framework of minerals to support the load of overlying sediments. According to Hao's research (1990), the fluid flow in the rock is difficult at this stage, and the formation pressure is still characterized by overpressure. However, at this stage, the abnormal pressure began to release, and obvious vertical microcracks appeared, which reduced the sealing performance and its sealing performance was relatively poor.
To sum up, the sealing mechanism of mudstone in Chagan Depression includes capillary pressure sealing and abnormal pressure sealing. Capillary pressure sealing exists in argillaceous rocks of any interval, and its sealing ability is related to the purity of mudstone, clay mineral content and diagenesis. When mudstone is pure, the content of swelling minerals in clay is high, the compaction degree of mudstone is high, and its sealing ability is improved. Abnormal pressure sealing may play an important role in the distribution area of the first and second sets of source rocks. The thickness of source rocks is consistent with the width of undercompacted zone, so the source rocks are thick and undercompacted. This section is mainly pressure sealing, and the abnormal high pressure increases with the increase of depth, and the sealing ability also increases.
Evaluation of caprock in 5.2.4.3
In the quantitative grading evaluation of oil and gas reservoir caprock, scholars at home and abroad put forward many indicators, mainly based on permeability parameters to evaluate the sealing ability of caprock. Combined with the actual situation of Chagan sag, it is meaningful to learn from the evaluation standard of caprock classification (Table 5-3) proposed by Deng Zuyou et al. (2000) combining permeability, displacement pressure and lithology. This standard is based on the quality of caprock itself, and it is difficult to evaluate caprock for depressions with low drilling degree. However, on the basis of sequence stratigraphic interpretation, according to the distribution characteristics of tight intervals and sedimentary systems, we can use the evaluation criteria of this table to evaluate the intervals that can be used as regional caprocks.
There are many sets of caprocks in Chagan sag. The evaluation of main regional caprocks in Chagan Depression is as follows (Figure 5- 18), with caprock quality as the main factor in the vertical direction and macro factor in the horizontal direction.
Sequence 2 of the second member of Chashen 1 Jingbayin Gobi Formation is semi-deep lacustrine and deep lacustrine gray-black turbidite mudstone and shale, and it is rated as I ~ II. It is widely distributed on the plane. Except the Wuhua monocline zone is thin and the Maodun intrusion zone is partially missing, all other structural zones in the depression are 500 ~ 1000 m, and the thickness of the depression is 1000 ~ 1800 m (Figures 5-4 and 5-5). It is not only the main source rock of the depression, but also the main cover of K 1 b oil reservoir and buried hill.
Mudstones of shore-shallow lake facies and flood plain facies in sequence 5 and sequence 6 of Tu-2 member are well developed and evaluated as secondary caprocks. Widely distributed in the plane and distributed in all secondary structural zones. It is the first set of regional caprock located above the main hydrocarbon source, and it is comprehensively evaluated as the main caprock. According to the macroscopic evaluation criteria of mudstone thickness, mudstone percentage and sedimentation. According to the comprehensive evaluation, the western part of the depression is a Class I caprock area, including the central and western Baron fault nose belt, the western Maodun intrusion belt, the central and western Le Hu sub-sag, and the front sub-sag. The middle part of the depression is a secondary caprock area, including the eastern part of Le Hu sag, the eastern part of Barun fault nose belt, the central part of Maodun intrusion belt, Hailisu anticline belt and the eastern part of Hantamiao sag. The eastern and southeastern parts of the depression are Class III and IV caprocks.
Table 5-3 Evaluation Grade Table of Mudstone Sealing Capacity
Fig. 5- 18 caprock evaluation map of K 1 s 2 area in Chagan sag