Evaluation method of structural fractures in continental shale oil reservoirs based on lithofacies constraints

Abstract

The evaluation method for structural fractures in continental shale oil reservoirs based on lithofacies constraints belongs to the field of petroleum geology and includes the following steps: (1) classifying the lithofacies types of continental shale in the study area; (2) based on the lithofacies type classification in step (1), extracting characteristic parameters affecting the development of structural fractures: continental shale brittleness index I, rock structure index S, and rock mechanical layer thickness H; (3) analyzing the sensitivity of different characteristic parameters in step (2) to the intensity response of structural fracture development; (4) analyzing the combination patterns of brittleness index, rock structure index, and rock mechanical layer thickness index of different lithofacies; (5) establishing a quantitative evaluation index for structural fractures in continental shale oil reservoirs based on lithofacies, and calculating the fracture development index F of different lithofacies. j This invention can be applied to the evaluation of structural fractures in continental shale oil reservoirs, providing technical support for the rational development of continental shale oil reservoirs in my country.

Description

Technical Field

[0001] This invention belongs to the field of petroleum geology, specifically a method for evaluating structural fractures in continental shale oil reservoirs based on lithofacies constraints. Background Technology

[0002] In recent years, with the successive high production of continental shale oil wells, shale oil has become a realistic exploration breakthrough target and resource replacement field for major oilfields in China.

[0003] Compared to conventional and tight sandstone reservoirs, shale reservoirs have poor matrix properties, and the oil and gas permeability within the reservoir mainly depends on the development of natural fractures. However, at present, in-depth research and the establishment of accurate evaluation methods are still needed to quantitatively evaluate the natural fractures in shale oil reservoirs.

[0004] Structural fractures are the dominant type of natural fractures in shale oil reservoirs. However, due to limitations in the quantity of core data and the accuracy of well logging data, the identification and evaluation of structural fractures in shale oil reservoirs are very difficult. Therefore, there is an urgent need for a new parameter system and method that can be applied to the quantitative evaluation of structural fractures in continental shale oil reservoirs.

[0005] The lithofacies of continental shale are an intrinsic factor in the development of tectonic fractures. Differences in rock structure, mineral composition, and rock mechanical layer thickness among different lithofacies lead to significant variations in mechanical properties and failure mechanisms, resulting in differences in tectonic fracture development patterns across different lithofacies. Therefore, quantitatively predicting and evaluating the degree and distribution of tectonic fractures based on the rock structure, mineral composition, rock mechanical layer thickness, and their combination patterns of different lithofacies in continental shale oil reservoirs can provide reliable geological basis and reference for the exploration and development of continental shale oil reservoirs. Summary of the Invention

[0006] The purpose of this invention is to provide a method for evaluating structural fractures in continental shale oil reservoirs based on lithofacies constraints. This method can quantitatively predict and evaluate the development degree and distribution pattern of structural fractures, and can provide reliable geological basis and reference for the exploration and development of continental shale oil reservoirs.

[0007] The technical solution to achieve the above objective is: a method for evaluating structural fractures in continental shale oil reservoirs based on lithofacies constraints, characterized by the following steps:

[0008] (1) Classify the terrestrial shale facies types in the study area;

[0009] (2) Based on the classification of lithofacies types in the study area in step (1), the characteristic parameters affecting the development of tectonic fractures are extracted: brittleness index I of continental shale, rock structure index S, and rock mechanical layer thickness H;

[0010] (3) Analyze the sensitivity of different characteristic parameters in step (2) to the intensity response of structural crack development;

[0011] (4) Based on the lithological types and characteristic parameters identified in steps (1) and (2), further analyze the combination patterns of brittleness index, rock structure index, and rock mechanical layer thickness index for different types of rock facies.

[0012] (5) Establish a quantitative evaluation index for structural fractures in continental shale oil reservoirs based on lithofacies, and calculate the fracture development index F for different lithofacies types. j .

[0013] Furthermore, in step (1), the classification of terrestrial shale facies types is based on regional geological background, diagenetic evolution characteristics, and core data.

[0014] Furthermore, the method for determining the lithofacies type in step (1) is based on core observation and XRD whole-rock mineral diffraction data. According to the brittleness of the minerals, quartz, feldspar, and pyrite are determined to be strongly brittle minerals, dolomite and calcite are weakly brittle minerals, and clay minerals are plastic minerals. Based on this, a three-end-member lithofacies classification plate is established to determine the lithofacies.

[0015] Meanwhile, when classifying lithofacies in step (1), the complex sedimentary structure in terrestrial shale is also considered. The primary sedimentary structure of the rock is classified into lamellar, layered, and massive structures according to the degree of bedding development.

[0016] Furthermore, the thickness of a single layer in a lamellar structure is <1 mm, the thickness of a single layer in a lamellar structure is greater than 1 mm and less than <1 cm, and the blocky layering structure is not well developed.

[0017] Furthermore, in step (2), the rock brittleness index I of a certain sample point i It is calculated using XRD whole-rock mineral composition measurement data, and the calculation formula is:

[0018] Where, I i This represents the brittleness index of the i-th sample point; This represents the weight of the quartz mineral in the sample, in grams. This represents the weight of feldspar minerals in the sample, in grams. This represents the weight of carbonate minerals at this sample point, in grams; A i This represents the total weight of the minerals in the sample point, in grams.

[0019] Rock structure index S of a certain sample point i This refers to the number of weak points in the mechanical properties of a unit length of rock, calculated using the following formula:

[0020] S i =N i / T i

[0021] In the formula, S i N represents the rock structure index of the i-th sample point; i N represents the weak surface of the mechanical properties of the i-th sample point; i The thickness of the rock at the i-th sample point is represented in meters.

[0022] The rock mechanical layer thickness index H at a certain sample point i Obtained through actual measurement, the unit is meters (m).

[0023] Furthermore, the sensitivity of different characteristic parameters to the intensity response of tectonic fracture development in step (3) is specifically determined by plotting the brittleness index I, rock structure index S, and rock mechanical layer thickness H against the tectonic fracture development density to determine the sensitivity coefficient R of the brittleness index I, rock structure index S, and rock mechanical layer thickness H of continental shale oil reservoirs to the intensity response of tectonic fracture development. I R S R H .

[0024] Furthermore, the determination of the characteristic parameter combination mode in step (4) specifically involves normalizing the brittleness index, rock structure index, and rock mechanical layer thickness of different types of rock facies after determination.

[0025]

[0026] In the formula, It is the brittleness index of the j-th lithofacies, I j It is the average brittleness index of the j-th lithofacies, I min It is the minimum brittleness index, I max It is the maximum fragility index.

[0027]

[0028] In the formula, S is the rock structure index of the j-th lithofacies. j S is the average rock structure index of the j-th lithofacies. min It is the minimum rock structure index, S max It is the maximum rock structure index.

[0029]

[0030] In the formula, H is the petrological layer thickness index of the j-th lithofacies. j H is the average petrological layer thickness index (cm) of the j-th lithofacies.min H is the minimum rock mechanical layer thickness index (cm). max It is the maximum rock mechanical layer thickness index (cm).

[0031] Furthermore, in step (5), the fracture development index F of different types of lithofacies... j Calculated according to formula (1);

[0032]

[0033] The beneficial effects of this invention are:

[0034] This invention innovatively proposes a quantitative evaluation method for structural fractures in continental shale oil reservoirs based on lithofacies constraints. Compared with traditional methods, this method improves the evaluation accuracy of structural fractures in continental shale reservoirs and provides a feasible new approach for evaluating structural fractures in continental shale. This method can be applied to the evaluation of structural fractures in continental shale oil reservoirs, providing technical support for the rational development of continental shale oil reservoirs in my country. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the process of the present invention;

[0036] Figure 2 This is a diagram showing the intersection of the brittleness index I and the linear density of structural cracks.

[0037] Figure 3 This is a plot showing the intersection of the rock structure index S and the linear density of tectonic fractures.

[0038] Figure 4 This is a diagram showing the intersection of the rock mechanical layer thickness index H and the linear density of tectonic fractures.

[0039] Figure 5 The normalized results of the brittleness index, rock structure index, and rock mechanical layer thickness are illustrated in the figure.

[0040] Figure 6 The crack development index F j The calculation results are illustrated in the figure. Detailed Implementation

[0041] A method for evaluating structural fractures in continental shale oil reservoirs based on lithofacies constraints, characterized by the following steps:

[0042] (1) Based on the regional geological background, diagenetic evolution characteristics and core data, the terrestrial shale facies types in the study area are classified;

[0043] (2) Based on the classification of lithofacies types in the study area in step (1), the characteristic parameters affecting the development of tectonic fractures are extracted: brittleness index I of continental shale, rock structure index S, and rock mechanical layer thickness H;

[0044] (3) Analyze the sensitivity of different characteristic parameters in step (2) to the intensity response of structural crack development;

[0045] (4) Based on the lithological type and characteristic parameters identified in steps (1) and (2), further analyze the combination patterns of brittleness index, rock structure index and rock mechanical layer thickness index for different types of rock facies;

[0046] (5) Establish a quantitative evaluation index for structural fractures in continental shale oil reservoirs based on lithofacies, and calculate the fracture development index F for different lithofacies types. j .

[0047] The method for determining lithofacies type in step (1) is based on core observation and XRD whole-rock mineral diffraction data. According to the brittleness of minerals, quartz, feldspar, and pyrite are determined to be strongly brittle minerals, dolomite and calcite are weakly brittle minerals, and clay minerals are plastic minerals. Based on this, a three-component lithofacies classification plate is established to determine the lithofacies. This classification method takes into account the mineral composition in the rock and is related to the brittleness of the rock.

[0048] Meanwhile, when classifying lithofacies in step (1), the complex sedimentary structure in terrestrial shale was also considered. The primary sedimentary structure of the rock was classified into lamellar (single bedding thickness <1mm), layered (<1mm single bedding thickness <1cm), and massive (bedding structure not developed) according to the degree of bedding development.

[0049] In step (2), the rock brittleness index I of a certain sample point i It is calculated using XRD whole-rock mineral composition measurement data, and the calculation formula is:

[0050]

[0051] Where, I i This represents the brittleness index of the i-th sample point; This represents the weight of the quartz mineral in the sample, in grams. This represents the weight of feldspar minerals in the sample, in grams. This represents the weight of carbonate minerals at this sample point, in grams; A i This represents the total weight of the minerals in the sample point, in grams.

[0052] Rock structure index S of a certain sample point i This refers to the number of weak points in the mechanical properties of a unit length of rock, calculated using the following formula:

[0053] S i =N i / T i

[0054] In the formula, S i N represents the rock structure index of the i-th sample point; i N represents the weak surface of the mechanical properties of the i-th sample point; i The thickness of the rock at the i-th sample point is represented in meters.

[0055] The rock mechanical layer thickness index H at a certain sample point i Obtained through actual measurement, the unit is meters (m).

[0056] In step (3), the sensitivity of different characteristic parameters to the intensity response of tectonic fracture development is specifically determined by plotting the brittleness index I, rock structure index S, and rock mechanical layer thickness H against the tectonic fracture development density to determine the sensitivity coefficient R of the brittleness index I, rock structure index S, and rock mechanical layer thickness H of continental shale oil reservoirs to the intensity response of tectonic fracture development. I R S R H .

[0057] The determination of the characteristic parameter combination mode in step (4) specifically involves normalization after determining the brittleness index, rock structure index, and rock mechanical layer thickness for different types of rock facies.

[0058]

[0059] In the formula, It is the brittleness index of the j-th lithofacies, I j It is the average brittleness index of the j-th lithofacies, I min It is the minimum brittleness index, I max It is the maximum fragility index.

[0060]

[0061] In the formula, S is the rock structure index of the j-th lithofacies. j S is the average rock structure index of the j-th lithofacies. min It is the minimum rock structure index, S max It is the maximum rock structure index.

[0062]

[0063] In the formula, H is the petrological layer thickness index of the j-th lithofacies. j H is the average petrological layer thickness index (cm) of the j-th lithofacies. min H is the minimum rock mechanical layer thickness index (cm). maxIt is the maximum rock mechanical layer thickness index (cm).

[0064] In step (5), the fracture development index F of different types of lithofacies j Calculated according to formula (1);

[0065]

[0066] The embodiments of the present invention will be described in detail below through specific examples.

[0067] In a major shale oil formation in eastern my country, this method was used to evaluate tectonic fractures. Based on the regional geological background, diagenetic evolution characteristics, and core data, the main lithofacies types in the area can be classified into five types: dolomitic shale, siliceous shale, calcareous shale, mixed shale, and mudstone.

[0068] The rock structure index S for each data point is calculated based on different lithofacies types; the brittleness index I for different lithofacies is calculated using whole-rock mineral composition; and the rock thickness H for different lithofacies is determined.

[0069] After determining the above parameters, cross-plots were plotted with the tectonic fracture development density to determine the sensitivity index of these parameters, as shown in the attached diagram. Figure 2 Appendix Figure 3 Appendix Figure 4 As shown in the figure, their sensitivity coefficients are R0 and R1 respectively. I =0.4520, R S =0.3910, R H =0.5212.

[0070] After the sensitivity parameters are determined, the corresponding parameters are normalized (see appendix). Figure 5 ), and calculate the crack development index F. j The calculation results are as shown in the appendix. Figure 6 As shown in the figure. Furthermore, the calculated parameters were compared with the measured fracture density in the core sample, and the results showed a good correlation between their distributions.

Claims

1. A method for evaluating structural fractures in continental shale oil reservoirs based on lithofacies constraints, characterized in that, Includes the following steps: (1) Classify the terrestrial shale facies types in the study area; (2) Based on the classification of lithofacies types in the study area in step (1), the characteristic parameters affecting the development of tectonic fractures are extracted: brittleness index I of continental shale, rock structure index S, and rock mechanical layer thickness H; (3) Analyze the sensitivity of different characteristic parameters in step (2) to the intensity response of structural crack development; (4) Based on the lithological types and characteristic parameters identified in steps (1) and (2), further analyze the combination patterns of brittleness index, rock structure index, and rock mechanical layer thickness index for different types of rock facies. (5) Establish a quantitative evaluation index for structural fractures in continental shale oil reservoirs based on lithofacies, and calculate the fracture development index F for different lithofacies types. j ; In step (3), the sensitivity of different characteristic parameters to the intensity response of tectonic fracture development is specifically determined by plotting the brittleness index I, rock structure index S, and rock mechanical layer thickness H against the tectonic fracture development density to determine the sensitivity coefficient R of the brittleness index I, rock structure index S, and rock mechanical layer thickness H of continental shale oil reservoirs to the intensity response of tectonic fracture development. I R S R H ; In step (5), the fracture development index F of different types of lithofacies j Calculated according to formula (1); In the formula, It is the brittleness index of the j-th lithofacies. It is the rock structure index of the j-th lithofacies. It is the rock mechanical layer thickness index of the j-th rock facies.

2. The method for evaluating structural fractures in continental shale oil reservoirs based on lithofacies constraints according to claim 1, characterized in that: Step (1) is based on the regional geological background, diagenetic evolution characteristics and core data to classify the facies types of continental shale.

3. The method for evaluating structural fractures in continental shale oil reservoirs based on lithofacies constraints according to claim 2, characterized in that: The method for determining lithofacies type in step (1) is based on core observation and XRD whole-rock mineral diffraction data. According to the brittleness of minerals, quartz, feldspar, and pyrite are determined to be strongly brittle minerals, dolomite and calcite are weakly brittle minerals, and clay minerals are plastic minerals. Based on this, a three-component lithofacies classification plate is established to determine the lithofacies. Meanwhile, when classifying lithofacies in step (1), the complex sedimentary structure in terrestrial shale is also considered. The primary sedimentary structure of the rock is classified into lamellar, layered, and massive structures according to the degree of bedding development.

4. The method for evaluating structural fractures in continental shale oil reservoirs based on lithofacies constraints according to claim 3, characterized in that: The thickness of a single layer in a lamellar form is <1 mm, the thickness of a single layer in a lamellar form is greater than 1 mm and less than 1 cm, and the blocky layering structure is not well developed.

5. The method for evaluating structural fractures in continental shale oil reservoirs based on lithofacies constraints according to claim 2, characterized in that: In step (2), the rock brittleness index I of a certain sample point i It is calculated using XRD whole-rock mineral composition measurement data, and the calculation formula is: Where, I i This represents the brittleness index of the i-th sample point; This represents the weight of the quartz mineral in the sample, in grams. This represents the weight of feldspar minerals in the sample, in grams. This represents the weight of carbonate minerals at this sample point, in grams; A i This represents the total weight of the minerals in the sample point, in grams. Rock structure index S of a certain sample point i This refers to the number of weak points in the mechanical properties of a unit length of rock, calculated using the following formula: S i =N i / T i In the formula, S i N represents the rock structure index of the i-th sample point; i T represents the weak surface of the mechanical properties of the i-th sample point; i The thickness of the rock at the i-th sample point is represented in meters. The rock mechanical layer thickness index H at a certain sample point i Obtained through actual measurement, the unit is meters (m).

6. The method for evaluating structural fractures in continental shale oil reservoirs based on lithofacies constraints according to claim 1, characterized in that: The determination of the characteristic parameter combination mode in step (4) specifically involves normalization after determining the brittleness index, rock structure index, and rock mechanical layer thickness for different types of rock facies. In the formula, It is the brittleness index of the j-th lithofacies, I j It is the average brittleness index of the j-th lithofacies, I min It is the minimum brittleness index, I max It is the maximum fragility index; In the formula, S is the rock structure index of the j-th lithofacies. j S is the average rock structure index of the j-th lithofacies. min It is the minimum rock structure index, S max It is the maximum rock structure index; In the formula, H is the petrological layer thickness index of the j-th lithofacies. j H is the average petrological layer thickness index (cm) of the j-th lithofacies. min H is the minimum rock mechanical layer thickness index (cm). max It is the maximum rock mechanical layer thickness index (cm).

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