Porosity inversion method for different lithofacies sandstones

Abstract

The invention discloses a porosity inversion method for different lithofacies sandstones, and belongs to the field of petroleum and natural gas exploration and development, and the porosity inversion method for different lithofacies sandstones comprises the steps: S1, determining the original porosity PO through a point counting method; s2, analyzing the structure activity history of the research area; s3, the compaction loss porosity PRCOM is calculated; s4, calculating the cement loss porosity PRCEM; s5, calculating the corrosion increase porosity PID; and S6, inverting the porosity of different lithofacies. Compared with an existing sandstone porosity inversion method, the method has the advantages that the influence degree of each diagenesis effect on the reservoir porosity can be analyzed, and meanwhile, the porosity lost by the compaction effect is distributed to each stage according to different proportions so as to solve a porosity evolution path close to the reality; and the problem of large calculation error of the original porosity of the complex lithofacies sample is solved.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas exploration and development, and in particular to a porosity inversion method for sandstones of different lithofacies. Background Art

[0002] The restoration of pore evolution history is one of the core issues in petroleum geology, especially for deep tight sandstone oil and gas reservoirs. The matching of pore evolution history and oil and gas filling history involves the reservoir formation mechanism and oil and gas enrichment law, which is related to the exploration and development deployment ideas. Therefore, the restoration of pore evolution history is crucial for deep tight sandstone oil and gas reservoirs.

[0003] The original porosity (P O ) is the primary basic data for quantitative analysis of pore evolution, which determines the compaction loss porosity (PR COM ) and cementation loss porosity (PR CEM ), thereby controlling the quality of the final porosity prediction. There are two commonly used methods for determining the original porosity of sandstone. The first is to select the average porosity of modern sediments under similar sedimentary environments as the initial porosity of all samples. Most scholars use 40% as the average original porosity of sandstone, and use this as the initial condition to restore the porosity evolution process. This approximate method is based on empirical statistics in the average sense, ignoring the differential control of the specific sedimentation in the study area on the initial porosity. Second, domestic and foreign scholars generally use the original porosity calculation formula constructed by Scherer based on Beard and Weyl wet sand filling experimental data:

[0004] φ0=20.91+22.9 / S0

[0005] As shown in the above formula, Φ o is the original porosity, S o is the Trask sorting coefficient, which is the square root of the quotient of the particle size at 25% and the particle size at 75% on the particle size cumulative curve. This method is only applicable when the particle sorting is good (S o ≤1.5). However, tight sandstone oil and gas reservoirs have strong reservoir heterogeneity, complex lithofacies, and a wide distribution range of particle sorting coefficients (1.5 to 3.5). The above two methods are not applicable to areas with complex lithofacies. Summary of the invention

[0006] In order to solve the technical problem of large errors in the calculation of the original porosity of complex lithofacies samples in the above-mentioned prior art, the present invention provides a porosity inversion method for sandstones of different lithofacies.

[0007] According to one aspect of the present invention, a porosity inversion method for sandstones of different lithofacies is provided. The porosity inversion method for sandstones of different lithofacies comprises the following steps: S1: using a point counting method to determine the original porosity P O ; S2: Calculate the compaction loss porosity PR COM ; S3: Calculate the cementation loss porosity PR CEM ; S4: Calculate the porosity increase PI by dissolution D ; S5: Inversion of porosity of different lithofacies.

[0008] Furthermore, in step S1, a rock sample is firstly taken at the test site, and then the taken rock sample is cut into rock slices with a thickness of 30 microns, and the rock slices are observed using a polarizing microscope.

[0009] Furthermore, a specific number of fields of view are selected within the observation range of the polarizing microscope, and observation points are randomly marked in each selected field of view, with a total of 300 observation points being marked.

[0010] Furthermore, the rock sample components determined by the point counting method include mineral components, structure, particle size, sorting, cement, matrix and pore content in the rock sample.

[0011] Furthermore, in step S2, the existing stratigraphic data and geothermal gradient data of the study area are combined to calculate the sedimentation rate, and a thermal evolution and burial history model of the target layer is established.

[0012] Furthermore, comparative analysis is performed through a standard plate to determine the original porosity.

[0013] Further, in step S3, the compaction loss porosity PR is calculated COM The formula used is:

[0014]

[0015] Further, in step S4, the cementation loss porosity PR is calculated CEM The formula used is:

[0016] PR CEM =(P O -PR COM )×CEM / IGV

[0017] Further, in step S5, the porosity increase PI is calculated D The formula used is:

[0018] PI D =DP×(1-PR COM )

[0019] Furthermore, in step S6, the intergranular volume percentage IGV of different lithofacies samples in the study area and the PR of different key periods are statistically analyzed. COM , PR CEM and PI D , and the porosity evolution paths of different lithofacies are obtained.

[0020] As mentioned in the background technology, the current quantitative analysis methods for pore evolution are only applicable when the particle sorting is good (So≤1.5). However, the tight sandstone oil and gas reservoirs have strong reservoir heterogeneity, complex lithofacies types, and a large distribution range of particle sorting coefficients (1.5-3.5). The current quantitative analysis methods for pore evolution are not applicable to areas with complex lithofacies development, and the errors calculated by existing methods are large. Therefore, in order to analyze the influence of each diagenetic action on the reservoir porosity, and at the same time distribute the porosity lost by compaction to each stage in different proportions, in order to approach the true porosity evolution path, and solve the problem of large errors in the calculation of the original porosity of complex lithofacies samples, the present invention provides a porosity inversion method for sandstones of different lithofacies. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0022] Figure 1 A flow chart of a porosity inversion method for sandstones of different lithofacies provided by the present invention;

[0023] Figure 2 A reservoir diagenetic evolution sequence diagram of the target horizon in the research area constructed for the present invention;

[0024] Figure 3 The porosity evolution result of calcareous cemented fine sandstone obtained by the porosity inversion method for sandstones of different lithofacies;

[0025] Figure 4 The porosity evolution result of massive medium sandstone is obtained by the porosity inversion method for sandstones of different lithofacies;

[0026] Figure 5 The porosity evolution result of massive coarse sandstone is obtained by the porosity inversion method for sandstones of different lithofacies;

[0027] Figure 6 The porosity evolution results of gravel-bearing sandstone and conglomerate obtained by the porosity inversion method for sandstones of different facies. DETAILED DESCRIPTION

[0028] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0029] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0030] Unless otherwise specifically stated, the relative arrangement of the parts and steps described in these embodiments, numerical expressions and numerical values ​​do not limit the scope of the present invention. At the same time, it should be understood that, for ease of description, the sizes of the various parts shown in the accompanying drawings are not drawn according to the actual proportional relationship. The technology, methods and equipment known to ordinary technicians in the relevant field may not be discussed in detail, but in appropriate cases, the technology, methods and equipment should be regarded as a part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once a certain item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0031] See also Figure 1 As shown, the present invention provides a porosity inversion method for sandstones of different lithofacies, and the porosity inversion method for sandstones of different lithofacies comprises the following steps:

[0032] S1: Determine the original porosity P using the point counting method O ;

[0033] S2: Analyze the history of tectonic activity in the study area;

[0034] S3: Calculate the compaction loss porosity PR COM ;

[0035] S4: Calculation of cementation loss porosity PR CEM ;

[0036] S5: Calculate the Porosity Increased by Dissolution PI D ;

[0037] S6: Inversion of porosity of different lithofacies.

[0038] In step S1, rock samples are first taken at the test site, and then the mineral composition, structure, particle size, sorting, cement, matrix and pore content in the rock samples are quantitatively analyzed by point counting method. Finally, 48 standard plates with 8 particle sizes and 6 sortings provided by Beard and Weyl are compared and analyzed, and the porosity of the plates with similar particle size and sorting is selected as the initial porosity of the sample.

[0039] The basic steps of the point counting method adopted in step S1 are as follows:

[0040] Prepare thin sections: Cut rock samples into slices about 30 microns thick to facilitate observation under a microscope.

[0041] Microscope observation: Use a polarizing microscope to observe rock slices. By adjusting the angles of polarized light and resolved light, the mineral composition and structural characteristics of the rock can be seen more clearly.

[0042] Point counting: Select a specific number of fields (usually hundreds to thousands of points) and randomly mark observation points in each field. At each point, record the type of material underneath (such as quartz, feldspar, clay minerals, pores, etc.), and use the above marking method to mark 300 observation points on each thin section.

[0043] Data Analysis:

[0044] Components: Calculate the frequency of occurrence of various mineral components in all points to obtain the percentage of each component.

[0045] Structure: Observe and record the structural characteristics of rocks, such as stratification, cracks, etc.

[0046] Grain size: Assess the particle size of the rock based on the statistical particle size distribution.

[0047] Sorting: The sorting of the rock is assessed by observing the uniformity of the particle size. The more uniform the particle size distribution, the better the sorting.

[0048] Cement and matrix: Record the type and amount of cement, and the type and amount of non-rock components (matrix).

[0049] Porosity: Calculates the frequency of pores at all points to determine the porosity.

[0050] Interpretation of results: Based on the point count results, combined with the knowledge of rock genesis and sedimentary environment, the diagenetic history and sedimentary characteristics of the rock are interpreted.

[0051] Through this method, various physical and chemical properties in rocks can be quantified relatively accurately, providing a basis for rock classification, source analysis, and reservoir evaluation.

[0052] In step S2, the tectonic activity history of the study area is analyzed, and the existing stratigraphic data and geothermal gradient data of the study area are combined to calculate the sedimentation rate and establish a thermal evolution and burial history model of the target layer.

[0053] like Figure 2 As shown, based on the X-ray diffraction data of the study area, the percentage of montmorillonite in the illite / montmorillonite mixed layer (I / S) is clarified; based on the industry standard for the division of diagenetic stages of clastic rocks, the diagenetic stage of the reservoir in the study area is clarified; based on thin section observations, the main diagenetic time sequence of the reservoir in the study area is clarified; on this basis, the reservoir diagenetic evolution sequence of the target strata in the study area can be constructed.

[0054] In step S3, the compaction loss porosity (PR COM ) is derived as follows: Assuming that the apparent volume of the rock before compaction is 1, the original pore volume is numerically equal to P O , the solid volume is 1-P O ; After compaction, the solid volume remains unchanged and is still 1-P O , let the pore volume after compaction be x. Then we get the following formula:

[0055]

[0056] In step S4, since the initial porosity remaining after compaction, PR CEM The proportion is the quotient of CEM and IGV, so:

[0057] PR CEM =(P O -PR COM )×CEM / IGV (2)

[0058] For step S5, in sandstone reservoirs where the content of dissolved secondary pores is low and does not play a significant role in controlling reservoir quality, or when the research purpose is limited to exploring the main pore reduction effect, PR COM and PR CEM The calculation can meet the research needs. However, in sandstone reservoirs where dissolution has a significant control on reservoir quality, the secondary porosity generated by dissolution must be considered when calculating pore evolution.

[0059] PI D It is numerically equal to the percentage of the secondary pore volume produced by dissolution obtained by thin section identification of the cast body to the apparent volume of the rock today (i.e. the apparent volume of the rock after compaction). When analyzing the porosity evolution, it needs to be converted to the percentage of the dissolved pore volume to the apparent volume before compaction. D for:

[0060] PI D =DP×(1-PRCOM ) (3)

[0061] For step S6, based on thin section observation and analysis, the IGV of different lithofacies samples in the study area and the PR of different key periods are statistically analyzed. COM , PR CEM and PI D , according to the calculation formula, the porosity evolution path of different lithofacies is restored.

[0062] The pore evolution parameters and their definitions used in the specification are shown in the following table:

[0063] Table 1 Pore evolution parameters and their definitions in the specification

[0064]

[0065] In order to analyze several typical porosity evolution paths in the reservoir sandstone in this area, the porosity after diagenetic changes such as compaction, early cementation, dissolution and late cementation is calculated respectively. It should be noted that due to the overlap of the duration of various diagenesis, they are cross-progressed to a certain extent and within a certain range, and there is a certain inheritance between early cementation and late cementation, which is sometimes difficult to separate. In addition, the technology of using thin section data to calculate porosity evolution is essentially a kind of action simulation, and it is difficult to simulate the changing trend of the superposition effect of various diagenesis. Therefore, in order to analyze the influence of each diagenesis on the reservoir porosity, the calculation assumes that their respective effects are relatively independent and follow one another in time, which is compaction → early cementation → dissolution → late cementation (today). At the same time, the porosity lost by compaction is distributed to each stage according to different proportions in order to approach the real porosity evolution path.

[0066] The results obtained by the above method are as follows Figure 3 As shown, taking the calcareous fine sandstone at 4288.2 m in Well Zhuang 110 as an example, the initial porosity is 35.6%, the porosity after early rapid compaction is 32.6%, the porosity after early cementation is 9.1%, the porosity after dissolution is 2.9%, and then with compaction and late calcite cementation, the porosity gradually drops to an extremely low level.

[0067] The results obtained by the above method are as follows Figure 4As shown in the figure, the initial porosity of the medium sandstone at 4271.8 m in Well Zhuang 102 is 38.9%, the porosity after early rapid compaction is 33.4%, the porosity after early cementation is 26.3%, the porosity after dissolution is 34.4%, and then the porosity gradually decreases to 25.2% with compaction and late calcite cementation. For example, the initial porosity of the medium sandstone at 4003.86 m in Well Zhuang 6 is 38.9%, the porosity after early rapid compaction is 32.7%, the porosity after early cementation is 26.3%, the porosity after dissolution is 13.1%, and then the porosity gradually decreases to 3.2% with compaction and late calcite cementation.

[0068] The results obtained by the above method are as follows Figure 5 As shown, taking the massive coarse sandstone at 4360m in Well Zhuang 104 as an example, the initial porosity is 38.9%, the porosity after early rapid compaction is 31.2%, the porosity after early cementation is 26.3%, the porosity after dissolution is 20.8%, and the porosity after late cementation to the present is 12.2%. Taking the massive coarse sandstone at 4312.15m in Well Zhuang 110 as an example, the initial porosity is 38.9%, the porosity after early rapid compaction is 29.8%, the porosity after early cementation is 25.4%, the porosity after dissolution is 16.3%, and the porosity after late cementation to the present is 5.2%.

[0069] The results obtained by the above method are as follows Figure 6 As shown, taking the gravel-bearing sandstone and conglomerate at 4244.92 m in Well Zhuang 301 as an example, the initial porosity is 37.2%, the porosity after early rapid compaction is 33.7%, the porosity after early cementation is 30.6%, the porosity after dissolution is 33.7%, and the porosity after late cementation to the present is 18.4%. Taking the gravel-bearing sandstone and conglomerate at 4328.9 m in Well Zhuang 1-2 as an example, the initial porosity is 37.2%, the porosity after early rapid compaction is 30%, the porosity after early cementation is 24.2%, the porosity after dissolution is 9%, and the porosity after late cementation to the present is 1.8%.

[0070] For ease of description, spatially relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below other devices or structures". Thus, the exemplary term "above" can include both "above" and "below". The device can also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used here are interpreted accordingly.

[0071] In addition, it should be noted that the use of terms such as "first" and "second" to limit components is only for the convenience of distinguishing the corresponding components. If not otherwise stated, the above terms have no special meaning and therefore cannot be understood as limiting the scope of protection of the present invention.

[0072] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A porosity inversion method for sandstones of different lithofacies, characterized in that: The porosity inversion method for sandstones of different lithofacies includes the following steps: S1: Determine the original porosity P using the point counting method O ; S2: Analyze the history of tectonic activity in the study area; S3: Calculate the compaction loss porosity PR COM ; S4: Calculation of cementation loss porosity PR CEM ; S5: Calculate the Porosity Increased by Dissolution PI D ; S6: Inversion of porosity of different lithofacies.

2. The porosity inversion method for sandstones of different lithofacies according to claim 1, characterized in that: In step S1, a rock sample is firstly taken at a test site, and then the taken rock sample is cut into rock slices with a thickness of 30 microns, and the rock slices are observed using a polarizing microscope.

3. The porosity inversion method for sandstones of different lithofacies according to claim 2, characterized in that: A specific number of fields of view are selected within the observation range of the polarizing microscope, and observation points are randomly marked in each selected field of view, with a total of 300 observation points marked.

4. The porosity inversion method for sandstones of different lithofacies according to claim 1, characterized in that: In step S2, the existing stratigraphic data and geothermal gradient data of the study area are combined to calculate the sedimentation rate and establish a thermal evolution and burial history model of the target layer.

5. The porosity inversion method for sandstones of different lithofacies according to any one of claims 1 to 4, characterized in that: The rock sample components determined by the point counting method include the mineral components, structure, grain size, sorting, cement, matrix and pore content in the rock sample.

6. The porosity inversion method for sandstones of different lithofacies according to claim 5, characterized in that: The original porosity is determined by comparative analysis using standard plates.

7. The porosity inversion method for sandstones of different lithofacies according to claim 6, characterized in that: In step S3, the compaction loss porosity PR is calculated COM The formula used is:

8. The porosity inversion method for sandstones of different lithofacies according to claim 7, characterized in that: In step S4, the cementation loss porosity PR is calculated CEM The formula used is: PR CEM =(P O -PR COM )×CEM / IGV 9. The porosity inversion method for sandstones of different lithofacies according to claim 8, characterized in that: In step S5, the porosity increase PI is calculated D The formula used is: P.I. D =DP×(1-PR COM , 10. The porosity inversion method for sandstones of different lithofacies according to claim 9, characterized in that: In step S6, the intergranular volume percentage IGV of different lithofacies samples in the study area and the PR of different key periods are counted. COM , PR CEM and PI D , and the porosity evolution paths of different lithofacies are obtained.

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