2.7.1 GPS measurement of current crustal movement and tectonic deformation
Using the Global Positioning System (GPS) to carry out high-precision, large-scale and timely measurement of current crustal movement and tectonic deformation is a research An effective method for today's stress-strain analysis. Wang Qi et al. (2001) used observation data obtained from the China Crustal Movement GPS Detection Website to obtain the velocity vectors of 229 measuring stations in mainland China and adjacent areas, and gave the stable part of these 229 measuring stations relative to the Eurasian plate ( Siberia), among which the dominant movement direction of the five measuring points on the Qaidam Massif is N60°E, and the average movement speed is 12~14mm/a (Wang Qi et al., 2001).
Z.Chen et al. used data from the GPS detection network in mainland China in 2000 to calculate that compared with the South China block, the current crustal movement speed of the Qaidam block is about 15mm/a, and the movement direction is from NE to NNE. . The shortening rate between measuring point DQD4 in the Qaidam Basin and measuring point HCY1 on the north side of the Qilian Mountains is (10±2) mm/a.
The above-mentioned Wang Qi et al. (2001), Z.Chen et al., and Zhang Qiang and Zhu Wenyao in 2000 used the detection data of the GPS detection network in mainland China to obtain the current crustal movement rate and direction of the Qaidam Basin. , the current crustal deformation of the Qaidam Basin is shortening from NE to NNE, and the maximum compression stress direction is from NE to NNE. Table 2.2 shows the movement rates given by GPS measurements in the Qaidam Basin and its surrounding areas.
Table 2.2 Crustal movement rate table in the Qaidam Basin and surrounding areas
2.7.2 Focal mechanism solution and current stress measurement
When an earthquake occurs, the source movement process An important result is that radiated seismic waves convey earthquake source information, based on which the motion characteristics of the earthquake source can be directly understood, and the crustal stress state can be analyzed (Table 2.3). The first amplitude direction of the P wave reaching the receiver (called the initial motion direction or motion symbol) is distributed on the ground in a special image, which reflects the source motion process and the occurrence of the fault and the way the force interacts. direct relationship. The motion parameters of the seismic source can be obtained based on the single couple or double couple model, such as the source node plane, P-axis output location, orientation and inclination (i.e. the orientation and inclination of the stress release axis, also known as the orientation and inclination of the maximum principal compressive stress axis) ).
Table 2.3 In-situ stress measurement results in the surrounding areas of the Qaidam Basin
Continued table
According to internal data: Liao Chunting et al., 2002; Shi Zhaoxian et al., 1989, Discussion of in-situ stress measurement and related issues at Laxiwa Hydropower Station; Liao Chunting et al., 1989, Research summary report on the relationship between cracks in Qingtongxia Dam and the stress state of the dam body.
In the Qaidam Basin, earthquakes are mainly concentrated in the southwest of the basin, the Sanhu area and the Delingha area in the east. Among them, the 7.5-magnitude earthquake that occurred west of Delingha in 1937 was the largest earthquake. , the deformation zone that appeared after the earthquake can still be seen today, which is 300km long. Sporadic small earthquakes also occurred in the northern Qaidam area. Therefore, judging from the areas where earthquakes occur, more earthquakes occur in the adjacent parts of the Qimantage Mountains and the Altyn Mountains, the protruding parts of the South Qilian Mountains into the basin, and the Delingha area where the East Kunlun and South Qilian Mountains are nearly parallel. Today, tectonic activity is intense, but earthquakes are relatively rare within the vast basin.
According to the focal mechanism solution (Table 2.4) given by HRV and USGS, in the Qaidam Basin and adjacent areas, the orientation of the maximum principal stress axis is basically in the NNE-NE direction, and the elevation angle It is also relatively small, with about 90% of the elevation angles below 30°. Therefore, this area is currently suffering from near-horizontal compressive stress in the NNE-NE direction. The orientation of the minimum tensile stress axis is close to the EW-NW direction, and the inclination angle is scattered. Nearly 40% are below 30°, nearly 20% are between 31° and 60°, and about 40% are above 61°. Therefore, part of the minimum principal stress is vertical and the other part is nearly horizontal. The inclination angle of the intermediate stress axis reflects the type of the seismogenic fault. A smaller inclination angle indicates that dip-slip is the main force, while a large elevation angle indicates that the fault's activity mode may be mainly strike-slip.
Judging from the inclination angles of the intermediate stress axes in the Qaidam Basin and adjacent areas, the inclination angles below 30° account for about 55%, while the inclination angles above 61° account for 26%. Therefore, combined with the regional geology of the area, the main faults Mainly dip-slip, some faults are mainly strike-slip, and some faults have both strike-slip components.
Table 2.4 Solution to the focal mechanism of moderately strong earthquakes in the Qaidam Basin
Continued table
Judging from the depth range of the earthquake focal points, they are generally concentrated within 20km. It may mean that there is a large-scale detachment layer in the earth's crust, and many faults in the Qaidam Basin converge on this detachment layer at depth. Geophysical data also show that the Qaidam Massif has a significant low-velocity zone with a thickness of 510km (about 5.8km/s) near the layer corresponding to the depth of the low-velocity layer in the lower crust of the plateau (about 20km), suggesting that the Qaidam Massif The middle and lower crust of the block has low viscosity, which may be related to the existence of this detachment fault.
2.7.3 Current geostress measurement using drilling caving method
2.7.3.1 Principle of drilling caving method
Since the 1970s, many scholars have discovered that in Rock avalanches on the borehole wall often occur in deep drilling, and the major axis direction of the avalanche ellipse cross-section is often the same at different depths of the same drilling well. This caving failure phenomenon was also found in underground tunnels. Indoor experimental analysis proved the mechanical mechanism of this caving phenomenon and confirmed that the long axis direction of the caving ellipse is parallel to the direction of the minimum horizontal stress. Based on this phenomenon, people discovered the drilling caving method for determining in-situ stress.
(1) Mechanical analysis of drilling collapse
The cross-section of a vertical drilling well in the earth’s crust is usually at two horizontal principal stresses σ1 and σ2 (σ1>σ2) under compression (Wang Lianjie et al., 1996). The stress distribution near the borehole can be expressed by the following formula:
Research on the oil control effect of the Qaidam Basin structural system
In the formula: θ is the angle measured in the counterclockwise direction from the σ1 direction. ; a is the drilling radius; r is the vector diameter; σr and σθ are the radial and tangential normal stress; τrθ is the shear stress.
When r=a, the stress distribution on the wellbore wall can be obtained:
σr=0
σθ=(σ1+σ2)-2(σ1- σ2) cos 2θ
τrθ=0
When θ=π/2 and 3π/2, that is, near points M and N, the tangential stress σθ takes the maximum value, That is, σθ=3σ1-σ2
At this time, the difference between the tangential stress (σθ) and the radial stress (σr) at points M and N also takes the maximum value:
σθ－σr=3σ1－σ2
That is, the shear stress (σθ－σr)/2 takes the maximum value.
It can be seen that as the borehole approaches along the horizontal axis, the radial stress σr decreases and the tangential stress σθ increases, forming shear in the directions of θ=π/2 and 3π/2. In the stress concentration area, when the stress reaches the strength of the rock, well wall collapse occurs, and the major axis of the ellipse formed by the collapse is parallel to the minimum principal stress.
Using the Moore-Coulomb shear rupture criterion, the mechanical mechanism of caving can be explained more clearly. The Moore-Coulomb rupture criterion is expressed as follows:
The Qaidam Basin structural system controls oil Action study
In the formula: S0 is the cohesive force; μ is the internal friction coefficient.
If the right side of the equation is less than S0, the well wall is stable; if the right side of the equation is equal to or greater than S0, collapse occurs.
(2) Determination of principal stress orientation
As mentioned before, the long axis of the drilling collapse ellipse is parallel to the direction of the minimum principal stress. Therefore, determining the direction of the principal stress lies in determining the orientation of the collapse ellipse. The size and long-axis orientation of the well diameter can be measured using a four-arm or six-arm stratigraphic dip logging tool. The four-arm logging tool consists of four measuring arms, namely C1, C2, C3, and C4. Measuring arms C1 and C3 are on the same diameter, called C1-3, and measuring arms C2 and C4 are on another diameter, called C2-4.
C1-3 and C2-4 measuring arms are perpendicular to each other. When logging, the four measuring arms are in close contact with the well wall. When the well diameter changes, the four measuring arms also change, extending or shortening. When the logging tool moves upward from the bottom of the well, the logging device rotates in the well. When encountering the caving elliptical well section, one pair of measuring arms falls in the direction of expansion, while the other pair is in the direction of non-expansion. In this way, We can know the size of the well diameter from the two well diameter curves.
The formation dip angle four-arm logging tool is equipped with a corresponding positioning and orientation device, which usually gives the azimuth angle AZ of the No. 1 measuring arm.
If the C1-3 measuring arm records the long-axis well diameter, the long-axis orientation of the caving ellipse is a=AZ.
If the C2-4 measuring arm records the long-axis well diameter, the long-axis orientation of the collapsed ellipse is a=AZ+90°
Similarly, the six-arm logging tool is used in the same way Obtain the orientation of the major axis of the collapsed ellipse.
2.7.3.2 In-situ stress measurement results and analysis of borehole wall caving method in Qaidam West area
In the collected 6 well logs that can be used for borehole wall caving method in-situ stress analysis The data are from Well A3, Well Zhaxi 1, Well Yue 78, Well Hong 33, Well Dong 9 and Well Chai 6. There are both four-arm logging data and six-arm logging data. Through statistical analysis of the caving section of each drilling well, it was found that the long axis direction of the caving ellipse in each drilling well, except for the long axis direction of the caving ellipse in Well Chai 6, which is relatively discrete, the long axis direction of the caving ellipse in other drilling wells is relatively consistent (Table 2.5). On this basis, the maximum principal compressive stress directions of each drilling well were calculated to be 140° in Well A3, 30° in Well Tashi 1, 90° in Well Yue 78, 20° in Well Hong 33, 90° in Well Dong 9, and 90° in Well Chai 6. Well 50°. The above results show that the current maximum principal compressive stress direction in the western Qaidam area includes NEE-NE direction, nearly EW direction and NW-SE direction, indicating that the stress direction has local changes in different parts of the basin. However, since there are few well logging data suitable for in-situ stress analysis using the borehole wall caving method in the western Qaidam area, we cannot yet draw the regularity of local changes in in-situ stress in the western Qaidam area.
Table 2.5 In-situ stress measurement results of borehole wall caving method in western Qaidam area
2.7.4 Photoelastic experimental simulation of tectonic stress field in Qaidam Basin
Light Elastic experiment is an experimental method that combines optical and elastic theories to conduct stress analysis on research objects. It is an experimental method to use photoelastic materials to make a similar model of the research object, and use instruments to measure the changes in the optical properties of the model under similar loads, so as to analyze the stress distribution in the research object. That is, the mechanical quantities suffered by the measurement object are converted into optical interference fringes, so that small changes in physical quantities can reach the level of direct measurement and perception by people, and then the stress distribution of the research object can be analyzed.
Theoretical basic research on the photoelastic experimental method began in the mid-19th century, but due to the development of materials, it was not until the 1920s that the photoelastic experimental method began to be promoted and applied in engineering. Subsequently, due to the intuitive and full-field characteristics of the photoelastic experimental method, this method developed rapidly and became more and more widely used.
Since 1949, photoelastic experimental methods have developed rapidly in my country and have been widely used in shipbuilding, aviation, water conservancy, machinery, etc.
Using photoelastic experimental methods to simulate geological problems is mainly to simulate the tectonic stress field, and then solve various geological problems through stress field analysis.
2.7.4.1 Experimental method
During the experiment, we must first establish a geological model of the research problem, and then use photoelastic materials to create a similar model of the research object based on the geological model and similarity theory. Then it is loaded according to certain boundary conditions and the stress distribution of the model under the boundary conditions is measured to achieve the purpose of analyzing the tectonic stress field of the geological problem.
(1) Establishment of geological model
The establishment of geological model is mainly based on the geological conditions of the study area. Generally, the structural map of the study area is directly reduced to the range of the photoelastic instrument. It is sufficient to keep it within the limit, and make appropriate simplifications if necessary.
(2) Preparation of experimental model
The photoelastic material in the experiment is based on E44-6101 epoxy resin, maleic anhydride as curing agent, and phthalate. Dibutyl formate is formulated with plasticization. The mass ratio is as follows:
Epoxy resin:maleic anhydride:dibutyl phthalate=100:30:5
When making the model, first put the ring Oxygen resin, maleic anhydride and dibutyl phthalate are mixed according to the above proportions, stirred evenly, injected into a special mold, placed in an oven at a constant temperature of about 65°C, and cured for 28 hours, which is called primary curing. When the material is no longer sticky, demold it to obtain a semi-cured photoelastic sheet. Use a modified medical scalpel to mark the structural lines of the area on the plate, then put the plate into the oven, raise the temperature to 115°C at 10-15°C per hour, keep the temperature constant for 4-5 hours, and then heat it at 5-5°C per hour. Cool the temperature from 6°C to 60°C, turn off the power to the oven, and allow it to cool down to normal temperature freely for the second curing. Finally, the remaining four weeks are processed to obtain the model used in the experiment.
Due to the large initial stress in the processed model (especially the depicted faults). The model needs to be annealed to remove the initial stresses. Put the processed model into glycerin, heat it to 120°C in the oven, keep the temperature constant for 2 hours, and then cool it down at 5°C every hour to remove the initial stress in the model. If the effect of primary annealing is not satisfactory, secondary annealing is required, and the method is the same.
(3) Experimental process
During the experiment, the models were placed on the universal material testing machine, and uniform loads were added to them according to the selected boundary conditions, and then placed on the photoelastic instrument. Take the isochromatic diagrams respectively to obtain the isochromatic stripe pattern of the model, and determine the stress mode of each fracture in the model on the photoelastic instrument.
2.7.4.2 Experimental results
The most basic result obtained from the photoelastic experiment is a set of isochromatic lines with different levels of isochromatic stripes that reflect different stress states. Patterns, based on which we can perform stress analysis on experimental models and experimental objects.
According to the two-dimensional stress-optical law, the isochromatic fringe series is proportional to the principal stress difference (σ1-σ2) or the maximum shear stress (τmax=). In other words, the isochromatic structure of the photoelastic experiment The lines are equal principal stress difference lines or equal maximum shear stress lines. The isochromatic line pattern composed of different levels of isochromatic lines is the isochromatic line pattern of the principal stress difference or maximum shear stress in the model, which can be directly reflected. The size distribution of the principal stress difference or the maximum shear stress in the model. In addition, based on the isochromatic line pattern of the photoelastic experiment, we can also directly determine the stress state of each fracture in the model. There are three types of color line patterns, especially the isochromatic line patterns at the end points of each fracture.
The first type: called type I stripes, which is characterized by the distribution of isochromatic line stripes that is nearly symmetrical to the fracture direction. It can be formed by the action of compressive stress perpendicular to the direction of the fracture, or the action of tensile stress perpendicular to the direction of the fracture. To distinguish whether the fracture is subject to pressure or tensile stress, acupuncture can be used on a photoelastic instrument. Identification.
The second type is called type II stripes, which are characterized by the main axis of the isochromatic stripes being parallel to the fracture direction.
The third type: called composite stripes, is characterized by isochromatic stripes that are neither asymmetrical nor parallel to the fracture direction. They are formed because the fracture is subject to both shear stress and compressive stress or tensile stress. The result of the effect. To distinguish whether the fracture is affected by tensile shear stress or compressive shear stress, the acupuncture method is also used to identify it on the photoelastic instrument.
Therefore, according to the type of isochromatic stripes on the fracture, We can determine the stress state of each fault.
2.7.5 Photoelastic experimental simulation of the current tectonic stress field in the Qaidam West area
2.7.5.1 Experimental geological model
The geological model used in the tectonic stress field photoelastic experiment in the western Qaidam area is based on the current structural features of the western Qaidam area and takes into account the main faults in the western Qaidam area. The structural map used in the experiment is from 2000m in the western Qaidam area. , 3000m and 4000m depth structure map cut out.
2.7.5.2 Experimental boundary conditions
According to the regional tectonic stress field analysis and current in-situ stress measurement results, the maximum principal compressive stress direction in the western Qaidam area since the Quaternary is NNE-SSE Towards. Therefore, the boundary condition for the photoelastic experimental simulation of the tectonic stress field in the western Qaidam region is uniform extrusion in the NNE-SSW direction, and the specific extrusion direction is N30°E.
2.7.5.3 Experimental results
(1) The stress mode of each fracture under the action of NNE-SSW extrusion force, the stress mode of each fracture in the western Qaidam area shows different At depth, the NWW and nearly EW trending faults are mainly affected by pressure and exhibit thrust activity, manifesting as thrust faults. NW-trending faults and sections with NW-trending faults are affected by compressive stress and dextral shear stress, and exhibit compression-torsion activity, manifesting as reverse faults and dextral strike-slip components. The NNE and nearly SN-trending faults are affected by tensile stress, exhibit tensile activity, and behave as normal faults; the NEE-trending faults are affected by left-lateral shear stress, exhibit left-lateral shear activity, and behave as left-lateral strike-slip faults. The stress action mode obtained by the photoelastic experiment is consistent with the current activity mode of the faults in the western Qaidam area, indicating that the boundary conditions used in the photoelastic experiment are consistent with the geological facts in the western Qaidam area.
(2) The isochromatic line pattern obtained from the photoelastic experiment of stress magnitude directly reflects the magnitude of the maximum shear stress. The isochromatic line pattern obtained from the photoelastic experiment of the current tectonic stress field in the western Qaidam area shows:
p>a) The distribution of stress in the current tectonic stress field in the western Qaidam region is uneven. The magnitude of tectonic stress in different areas varies greatly, and there are some low-value stress areas and high-value stress areas.
b) The fault structure in the western Qaidam area has a great influence on the distribution of tectonic stress in the area. The tectonic stress on both sides of the fault is very different, and the tectonic fault has a significant separation effect on the stress distribution.
At a depth of 2000m, the low stress areas are mainly distributed in Gaskule, Youshashan, Alar, Hongliuquan, Qiquan, Beiwu, the southwest side of Younan 1 Well, and Xianshui Spring , Crescent Mountain, Jianding Mountain, near Well Qie 3 and Solkuli and other places, the isochromatic stripe series is generally from first level red to first level green, with the lowest stress value near Well Yueshen 1 and North Us, etc. The color line stripe series is below the first level green. In other areas, the level of isochromatic stripes is generally 2 to 3, and in some areas the level of isochromatic stripes can reach levels 5 to 6. In addition, the isochromatic pattern of the photoelastic experiment at a depth of 2000m also shows that the Alar fault and the Huatugou fault have a great influence on the distribution of low stress areas.
At a depth of 3000m, the low-stress areas are mainly distributed in Gaskule, Youshashan, Huatugou, Shizigou, Qiquan, southwest of Well Acan 1, Xianshui Spring, Honggouzi, In places such as Crescent Mountain, Xiaoliang Mountain, Jianding Mountain, Dafeng Mountain and Solkuli, the isochromatic stripe series is generally from first level red to first level green, with stress values ??near Gaskule, Yousha Mountain and Shizigou The lowest, isochromatic stripe progression is below level one green. Large-scale low-stress areas are mainly distributed near Gaskule, Yousha Mountain, Shizigou and Crescent Mountain. In other areas, the isochromatic stripe progression is generally 2 to 3 levels, and at the end points of partial fractures, the isochromatic stripe progression can reach 5 to 6 levels.
At a depth of 4000m, a large-scale low-stress area appears in the Huatugou-Alar-Gaskule Lake area, and the isochromatic stripe series is below the first-level green, followed by Yuejin and Qi. Gequan, Xianshuiquan, Crescent Mountain, Solkuli and other places also have a large range of low-stress areas with isochromatic stripe series below first-level green. What is different from the first two depths is that the stress values ??in different sections at the depth of 4000m have a greater difference. On the one hand, there are large-scale low-stress areas with isochromatic stripes with less than first-level green. On the other hand, isochromatic lines also appear. The fringe series is a high-value stress area above level 4, and the distribution of low-value stress areas and high-stress areas shows a NNE trend.
2.7.5.4 Experimental result analysis and oil and gas prediction
The experimental results show that the stress action mode of the current faults in the western Qaidam area is consistent with the current fault mode in the western Qaidam area. On the one hand, it shows that the experiment The selected boundary conditions and experimental methods are in line with the geological facts of the western Qaidam area. On the other hand, it shows that the NWW-trending and nearly EW-trending faults in the western Qaidam area have the best sealing properties. NW-trending faults and sections with NW-trending faults and NEE-trending faults The sealing property of the fault is second, which is a blocking factor for oil and gas migration. Only the NNE and nearly SN-trending faults are in an extensional state, have good penetration, and can become channels for oil and gas migration.
The current stress distribution in the western Qaidam area obtained from the experiment has a good correspondence with the distribution of known oil and gas fields in the western Qaidam area. The low-value stress areas often correspond to known oil and gas fields. Accordingly, based on the stress distribution characteristics of the current stress field photoelastic experimental results in the western Qaidam area, combined with other petroleum geological conditions, the following predictions are made for the oil and gas favorable areas in the western Qaidam area (known oil and gas fields will not be discussed):
(1) The area near Huatugou is a favorable area for oil and gas
Photoelastic experiment results show that the area near Huatugou is a large-scale low-stress area in the western Qaidam region where no oil fields have been found so far.
The underwater slope near Huatugou has been on the south side of Yingxiongling Depression for a long time since the Cenozoic. It has good oil generation conditions and the oil and gas migration time is long.
In long-term underwater slopes, the sediment particles are relatively coarse, the pore conditions are good, and there are better storage conditions.
(2) Favorable oil and gas accumulation area near Alar
Photoelastic experiment results show that stress is low at different depths.
This area has a long history of subsidence, good oil generation conditions, sufficient oil sources, and a long time for oil and gas migration and accumulation.
This area is located on the western slope of the depression and has dynamic and reservoir conditions for oil and gas migration.
(3) Favorable oil and gas accumulation area in the Shizigou area
This area has a large range of low stress areas in the depths. From the perspective of the relationship between stress and oil and gas, it may be an oil field distribution area .
This area has long been located on the southern edge of the Yingxiongling Depression, with thick Cenozoic sediments and good oil generation conditions.
This area has good coarse clastic sedimentation and good storage conditions.
Oil fields have been discovered in Shizigou, proving the possibility of finding oil fields in this area.
(4) The southern Yuejin area is a low-stress area
Whether this area has the conditions for oil generation and storage requires further work, and it is also a favorable area where oil fields may be found.