A method for evaluating favorable zones of karst reservoirs

Abstract

This invention provides a method for evaluating favorable areas of karst reservoirs, belonging to the field of geological technology for oil and gas field exploration and development in the petroleum industry. This method first classifies the factors controlling karst reservoirs into at least three categories: residual stratigraphic thickness, karst drainage system distribution, and paleofault planar distribution. Then, based on the development characteristics of the corresponding factors in the target area, an assignment table for each parameter evaluation factor is established and substituted into the calculation method for the evaluation coefficient of favorable karst reservoir areas. Through formula calculation, areas with large residual stratigraphic thickness, paleogeomorphic features of karst drainage systems, and well-developed paleofaults are ultimately selected as favorable karst reservoir areas. This invention employs a combination of geological and geophysical methods, solving the uncertainty of seismic attributes' response characteristics to favorable karst reservoirs, and achieving accurate evaluation of favorable karst reservoir areas.

Description

Technical Field

[0001] This invention relates to a method for evaluating favorable areas of karst reservoirs, belonging to the field of geological technology for oil and gas field exploration and development in the petroleum industry. Background Technology

[0002] Karst reservoirs refer to oil and gas reservoirs associated with ancient karst formations in carbonate rocks and unconformities. They are an important type of large marine oil and gas field worldwide and possess promising exploration prospects. The reservoir space in karst reservoirs is primarily composed of caves, solution pores, and fractures. Regionally, they are significantly controlled by the thickness of residual strata, ancient karst landforms, and faults. Conducting karst reservoir evaluation and selecting favorable karst reservoir areas provides direction for discovering Ordovician oil and gas reservoirs and is a key condition for achieving efficient development of karst reservoirs.

[0003] Current research on favorable areas of karst reservoirs largely utilizes geophysical methods. This involves extracting seismic attributes to match actual drilling conditions, determining the seismic response characteristics of karst reservoirs, and then reconstructing seismic data to predict and evaluate karst reservoirs. However, the ambiguity of seismic attributes introduces uncertainty into their response characteristics to favorable karst reservoirs. Consequently, relying solely on geophysical techniques to evaluate favorable areas of karst reservoirs is challenging, resulting in large errors and low accuracy. Summary of the Invention

[0004] The purpose of this invention is to provide a method for evaluating favorable areas of karst reservoirs, in order to solve the problems of large errors and low accuracy in the evaluation of karst reservoirs using seismic data.

[0005] To achieve the above objectives, the present invention provides a method for evaluating favorable zones in karst reservoirs, comprising the following steps:

[0006] 1) Perform paleogeographic restoration of the target area to obtain the residual stratigraphic thickness of the karst reservoir and the thickness of the marker layer above the karst reservoir; 2) Based on the residual stratigraphic thickness and marker layer thickness of the karst reservoir, identify the secondary geomorphic units and the tertiary geomorphic units included in the secondary geomorphic units in the target area. The secondary geomorphic units include karst slopes, and the tertiary geomorphic units include karst hills, karst depressions, and karst gullies; 3) Determine the evaluation factor assignments related to the evaluation of favorable areas of karst reservoirs. The evaluation factor assignments include at least the assignments for residual stratigraphic thickness, karst drainage system distribution, and paleofault plane distribution. The assignments for residual stratigraphic thickness, karst drainage system distribution, and paleofault plane distribution are determined using the following methods: The residual stratigraphic thickness corresponding to each tertiary geomorphic unit is determined based on the residual stratigraphic thickness of the karst reservoir. The degree of evaluation is assigned, and the residual thickness of the karst reservoir is positively correlated with the residual thickness of the strata; the ratio of the area of ​​each third-level geomorphic unit to the area of ​​the karst slope in the target area is calculated and used as the first ratio. Based on the first ratio, the distribution of karst drainage system corresponding to each third-level geomorphic unit is determined, and the first ratio is positively correlated with the distribution of karst drainage system; the ratio of the number of faults included in each third-level geomorphic unit to the area of ​​each corresponding third-level geomorphic unit is calculated and used as the second ratio. Based on the second ratio, the distribution of paleofault plane corresponding to each third-level geomorphic unit is determined, and the second ratio is positively correlated with the distribution of paleofault plane; 4) the evaluation factors assigned to each third-level geomorphic unit are multiplied together to obtain the evaluation coefficient corresponding to each third-level geomorphic unit, and the favorable reservoir area is determined based on the evaluation coefficient.

[0007] The beneficial effects of this invention are as follows: Starting from the controlling factors of karst reservoirs, this invention classifies the factors controlling karst reservoirs into at least three categories, including residual stratigraphic thickness, karst drainage pattern distribution, and paleofault plane distribution. Then, based on the development characteristics of the above-mentioned factor parameters in the target area, an assignment table for each parameter evaluation factor is established, which is then substituted into the calculation formula for the favorable coefficient of karst reservoirs, i.e., all the assigned values ​​are multiplied together. Through the formula calculation, the favorable area for karst reservoirs is ultimately determined by the following criteria: large residual stratigraphic thickness, paleogeography of karst drainage patterns, and well-developed paleofaults. This invention can evaluate favorable areas of karst reservoirs from multiple aspects. Compared with existing single-parameter judgment methods and methods that solely use seismic data to evaluate favorable areas of karst reservoirs, the judgment results are more accurate and the judgment precision is higher.

[0008] Furthermore, in the above method for evaluating favorable areas of karst reservoirs, the relationship between the residual thickness of the formation in step 3) and the assigned value of the residual thickness of the formation is as follows: a range of residual thickness of the formation in a karst reservoir corresponds to a value of the residual thickness of the formation; the larger the maximum value of the range of residual thickness of the formation in a karst reservoir, the larger the corresponding value.

[0009] Furthermore, in the above method for evaluating favorable areas of karst reservoirs, the relationship between the first ratio and the karst drainage distribution value in step 3) is as follows: one first ratio interval corresponds to one karst drainage distribution value; the larger the maximum value of the first ratio interval, the larger the corresponding karst drainage distribution value.

[0010] Furthermore, in the above method for evaluating favorable areas of karst reservoirs, the relationship between the second ratio and the ancient fault plane distribution assignment in step 3) is as follows: one second ratio interval corresponds to one ancient fault plane distribution assignment; the larger the maximum value of the second ratio interval, the larger the corresponding ancient fault plane distribution assignment.

[0011] This invention uses the product of various evaluation factors as evaluation coefficients. Therefore, the more an evaluation factor meets our expectations, the higher the value assigned to the corresponding evaluation factor, and thus the higher the final evaluation coefficient.

[0012] Furthermore, in the above-mentioned methods for evaluating favorable areas of karst reservoirs, step 1) employs sedimentary methods, sequence stratigraphy, residual thickness methods, or impression methods for paleogeographic reconstruction.

[0013] Furthermore, in the above-mentioned method for evaluating favorable areas of karst reservoirs, the residual thickness interval of the karst reservoir is divided using the following method: obtain the residual thickness data of historical karst reservoirs, use the value of the residual thickness data of historical karst reservoirs as the abscissa, and use the number of occurrences of the residual thickness of historical karst reservoirs as the ordinate to perform probability statistics, and divide the residual thickness interval of karst reservoirs according to the statistical results.

[0014] Furthermore, in the above method for evaluating favorable areas of karst reservoirs, the first ratio interval is obtained by the following method: obtaining historical first ratio data, using the value of the first ratio data as the abscissa and the number of times the first ratio appears as the ordinate for probability statistics, and dividing the first ratio interval according to the statistical results.

[0015] Furthermore, in the above method for evaluating favorable areas of karst reservoirs, the second ratio interval is obtained by the following method: obtaining historical second ratio data, using the value of the second ratio as the abscissa and the number of times the second ratio occurs as the ordinate for probability statistics, and dividing the second ratio interval according to the statistical results.

[0016] The intervals of each evaluation factor are divided according to actual statistical data. Probability is simulated using a large amount of actual data. Since each evaluation factor is an independent event, the product of each evaluation factor can represent the probability of a karst reservoir satisfying all evaluation factors.

[0017] Furthermore, in the above method for evaluating favorable areas of karst reservoirs, the first ratio interval includes three intervals.

[0018] Furthermore, in the above method for evaluating favorable areas of karst reservoirs, the second ratio interval includes three intervals. Attached Figure Description

[0019] Figure 1 This is a flowchart of the method for evaluating favorable areas of karst reservoirs according to the present invention;

[0020] Figure 2 This is an interpretation map of the karst landform in the target area of ​​this invention;

[0021] Figure 3 This is a distribution map of karst reservoirs in the target area of ​​this invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0023] The evaluation model used in this invention is as follows:

[0024]

[0025] In the formula, P is the evaluation coefficient, that is, the probability of the existence of a favorable area in the karst reservoir. i These are the parameter evaluation factors, specifically the probability that residual formation thickness is favorable to karst reservoirs, the probability that the planar distribution of karst drainage systems is favorable to karst reservoirs, and the probability that the distribution of ancient faults is favorable to karst reservoirs. Since each of these parameter evaluation factors is an independent event, the product of all these factors can be considered as the probability of the existence of a favorable karst reservoir area.

[0026] Method Implementation Examples:

[0027] The flowchart of this invention is as follows Figure 1 As shown, applying this invention to the evaluation of favorable areas of the Ma4 Member of the Lower Ordovician Majiagou Formation in a target region includes the following steps:

[0028] 1) Paleogeomorphological reconstruction of the target area. Commonly used paleogeomorphological methods include sedimentary methods, sequence stratigraphy, residual thickness methods, and imprinting methods. The paleogeomorphological units in the target area are identified based on the thickness and distribution of the overlying strata—the Carboniferous system—on the Ordovician weathering crust, as well as the stratigraphic positions of strata exposed before the Carboniferous system. Imprinting methods are used for paleogeomorphological reconstruction of the target area. Due to the relatively limited distribution of the Taiyuan Formation strata in the target area, the top surface of the Shan 1 Member of the Shanxi Formation is mainly used as the primary marker bed reference surface, and the bottom coal seam of the Taiyuan Formation is used as a secondary reference surface. Based on this, the residual thickness H1 of the Ma 4 Member strata in the target area is reconstructed, and the thickness H2 of the marker bed above the Ma 4 Member (the sum of the thicknesses of the overlying Taiyuan Formation and Shan 1 Member strata) is determined.

[0029] 2) Using the residual thickness H1 of the Ma-4 Member, and combining it with the sum of the thicknesses H2 of the overlying Taiyuan Formation and Shan-1 Member strata in the target area, secondary geomorphic units such as karst highlands, karst slopes, and karst depressions are identified. For example... Figure 2 As shown, based on data from existing drilling example A in the target area, the overlying, relatively well-developed Taiyuan Formation and Shan 1 Member strata are nearly 120m thick. Data from drilling example B indicates the Ma 4 Member strata are nearly 75m thick. When the underlying Ma 4 Member strata are completely dissolved and the overlying strata lack the Taiyuan Formation and Shan 1 Member to the area above Shan 1 Member (i.e., H1 = 0m, H2 < 0m), the underlying paleogeography is considered to be a karst highland. When the underlying Ma 4 Member strata are not completely dissolved, with remnants remaining and the overlying strata containing part of the Taiyuan Formation and Shan 1 Member (i.e., 0m), the underlying paleogeography is considered to be karst highland. Figure 3 As shown, the pinch-out line of the Ma-4 Member strata serves as the boundary. North of the boundary, the Ma-4 Member strata are underdeveloped and constitute a karst highland; south of the boundary, there is a karst slope. An evaluation of the favorable karst reservoir area of ​​the Ma-4 Member is conducted in this karst slope region.

[0030] 3) In karst slope areas (0m)

[0031] ​​4) Based on the condition that karst drainage system development is beneficial to karst reservoirs, and that the degree of karst drainage system development in the target area is directly proportional to the area of ​​the karst slope, the evaluation factor P2 is assigned a value. As shown in Table 1 below, the ratio L of the area S of the area to be evaluated to the area S' of the karst slope is used as the development index. The relationship between the karst drainage system development index and the degree of karst reservoir benefit is statistically analyzed, and the intervals of the karst drainage system development index are divided into [0, 0.1), [0.1, 0.2), and [0.2, 1], with corresponding values ​​of 0.2, 0.3, and 0.5. The karst slope area S in the target area is 851.5 km², and the area S of residual hill No. 1 is 84.6 km². 2 The ratio L1 = 84.6 / 851.5 = 0.099; the area S of residual hill No. 2 is 38.9 km². 2 The ratio L2 = 38.9 / 851.5 = 0.046; the area S of residual hill No. 3 is 210.1 km². 2 The ratio L3 = 210.1 / 851.5 = 0.247; the area of ​​the shallow depression S' is 342.3 km². 2 The ratio L3 = 342.3 / 851.5 = 0.401. Therefore, residual hill No. 1 is assigned a value of 0.2, residual hill No. 2 is assigned a value of 0.2, residual hill No. 3 is assigned a value of 0.5, and shallow depression is assigned a value of 0.5.

[0032] 5) The distribution of ancient fault planes is represented by the number of faults M per unit area. Since the fault plane distribution in the target area is uneven, the number of faults M per unit area is used as the index P3 for fault development. As shown in Table 1, residual hill No. 1 has 6 faults, M1 = 6 / 84.6 = 0.071; residual hill No. 2 has 4 faults, M2 = 4 / 38.9 = 0.103; residual hill No. 3 has 12 faults, M3 = 12 / 210.1 = 0.057; and the shallow depression has 15 faults, M' = 15 / 342.3 = 0.044. Therefore, residual hill No. 1 is assigned a value of 0.3, residual hill No. 2 is assigned a value of 0.5, residual hill No. 3 is assigned a value of 0.3, and the shallow depression is assigned a value of 0.2.

[0033] Table 1. Evaluation Factor Assignment Table for Karst Reservoirs in Application Examples

[0034]

[0035] By acquiring historical data for each evaluation factor, the historical values ​​of each factor are used as the x-axis, and the frequency of occurrence of the corresponding historical data for that factor is used as the y-axis for probability statistics. Based on the results of the probability statistics, several intervals are defined for each evaluation factor. Under the same evaluation factor, the interval more favorable to karst reservoirs is assigned a larger value for that factor.

[0036] 6) Substitute the above three types of evaluation factors into the evaluation model of this invention:

[0037]

[0038] Therefore, P = P1 * P2 * P3. Substituting the specific values ​​into the calculation: For residual hill 1, P = 0.7 * 0.2 * 0.3 = 0.042; for residual hill 2, P = 0.7 * 0.2 * 0.5 = 0.07; for residual hill 3, P = 0.7 * 0.5 * 0.3 = 0.105; for the shallow depression, P = 0.3 * 0.5 * 0.2 = 0.03. Based on the order of P values, residual hill 3 > residual hill 2 > residual hill 1 > shallow depression, indicating that residual hill 3 has the highest probability of being a favorable reservoir area, followed by residual hill 2, and the shallow depression has the lowest probability of being a favorable reservoir area.

[0039] This invention can evaluate favorable areas of karst reservoirs from multiple perspectives. Compared with existing single-parameter judgment methods and methods that rely solely on seismic data to evaluate favorable areas of karst reservoirs, the judgment results are more accurate and the accuracy is higher.

[0040] In this embodiment, three evaluation factors are used to evaluate favorable areas of karst reservoirs. This approach offers the advantage of considering more comprehensive factors and achieving higher accuracy. As another implementation method, more evaluation factors can be used depending on the actual situation, such as when the stratigraphic conditions of the evaluation area are more complex. This can further improve the accuracy and reliability of the evaluation.

[0041] In this embodiment, the evaluation factors are divided into three intervals to assign different values ​​to them. In other implementations, the evaluation factors can be divided into more intervals to increase the accuracy of the judgment. Furthermore, the assigned values ​​can be set to be positively correlated with the evaluation factors to further improve the judgment accuracy.

Claims

1. A method for evaluating favorable zones in karst reservoirs, characterized in that, Includes the following steps: 1) Perform paleogeographic restoration on the target area to obtain the residual stratigraphic thickness of the karst reservoir and the thickness of the marker layer above the karst reservoir; 2) Based on the residual stratigraphic thickness and marker layer thickness of the karst reservoir, identify the secondary geomorphic units in the target area and the tertiary geomorphic units included in the secondary geomorphic units. The secondary geomorphic units include karst slopes, and the tertiary geomorphic units include karst residual hills, karst depressions, and karst gullies. 3) Determine the evaluation factor values ​​related to the evaluation of favorable karst reservoir areas. These evaluation factor values ​​include at least the values ​​for residual stratigraphic thickness, karst drainage distribution, and paleofault planar distribution. The values ​​for residual stratigraphic thickness, karst drainage distribution, and paleofault planar distribution are determined using the following methods: The residual thickness of the karst reservoir is determined based on the residual thickness of the strata, and the residual thickness of the karst reservoir is positively correlated with the residual thickness assignment. The ratio of the area of ​​each tertiary geomorphic unit to the area of ​​the karst slope in the target area is calculated and used as the first ratio. Based on the first ratio, the distribution value of the karst water system corresponding to each tertiary geomorphic unit is determined. The first ratio is positively correlated with the distribution value of the karst water system. The ratio of the number of faults included in each tertiary geomorphic unit to the area of ​​each corresponding tertiary geomorphic unit is calculated and used as the second ratio. The paleofault plane distribution value corresponding to each tertiary geomorphic unit is determined based on the second ratio. The second ratio is positively correlated with the paleofault plane distribution value. 4) Multiply all the evaluation factors corresponding to each third-level geomorphic unit to obtain the evaluation coefficients corresponding to each third-level geomorphic unit, and determine the favorable reservoir area based on the evaluation coefficients.

2. The method for evaluating favorable zones of karst reservoirs according to claim 1, characterized in that, The relationship between the residual thickness of the karst reservoir and the assigned value of the residual thickness in step 3) is as follows: a residual thickness range of a karst reservoir corresponds to a residual thickness assignment; the larger the maximum value of the residual thickness range of the karst reservoir, the larger the corresponding assignment.

3. The method for evaluating favorable zones of karst reservoirs according to claim 1, characterized in that, In step 3), the relationship between the first ratio and the karst drainage distribution assignment is as follows: one first ratio interval corresponds to one karst drainage distribution assignment; the larger the maximum value of the first ratio interval, the larger the corresponding karst drainage distribution assignment.

4. The method for evaluating favorable zones of karst reservoirs according to claim 1, characterized in that, In step 3), the relationship between the second ratio and the ancient fault plane distribution assignment is as follows: one second ratio interval corresponds to one ancient fault plane distribution assignment; the larger the maximum value of the second ratio interval, the larger the corresponding ancient fault plane distribution assignment.

5. The method for evaluating favorable zones of karst reservoirs according to claim 1, characterized in that, In step 1), the paleogeographic method, sequence stratigraphy method, residual thickness method or imprint method is used to reconstruct the paleogeography.

6. The method for evaluating favorable zones of karst reservoirs according to claim 2, characterized in that, The residual thickness range of the karst reservoir is divided using the following method: obtain the residual thickness data of historical karst reservoirs, use the value of the residual thickness data of historical karst reservoirs as the abscissa, and use the number of times the residual thickness of historical karst reservoirs occurs as the ordinate to perform probability statistics, and divide the residual thickness range of karst reservoirs according to the statistical results.

7. The method for evaluating favorable zones of karst reservoirs according to claim 3, characterized in that, The first ratio interval is obtained by the following method: historical first ratio data is obtained, the value of the first ratio data is used as the horizontal axis, and the number of times the first ratio occurs is used as the vertical axis to perform probability statistics, and the first ratio interval is divided according to the statistical results.

8. The method for evaluating favorable zones of karst reservoirs according to claim 4, characterized in that, The second ratio interval is obtained by the following method: historical second ratio data is obtained, the value of the second ratio data is used as the horizontal axis, and the number of times the second ratio occurs is used as the vertical axis to perform probability statistics, and the second ratio interval is divided according to the statistical results.

9. The method for evaluating favorable zones of karst reservoirs according to claim 7, characterized in that, The first ratio interval includes three intervals.

10. The method for evaluating favorable zones of karst reservoirs according to claim 8, characterized in that, The second ratio interval includes three intervals.

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