Method for judging corrosion strength of ancient karst of carbonate rock away from material source area

Abstract

The invention relates to the technical field of carbonate reservoir exploration, in particular to a method for judging the corrosion strength of ancient karst of carbonate rock away from a source area. In order to solve the technical problem that related indexes in the prior art are insensitive to karst action of a carbonate reservoir far away from a provenance area, so that a method for judging the corrosion strength of the karst type reservoir is lacked, a rare earth element distribution curve is used for judging the total amount of rare earth elements and judging mineral trace elements in a grading manner; and a new means for judging the corrosion strength of the ancient karst of the carbonate rock is obtained.

Description

Technical Field

[0001] The invention relates to the technical field of carbonate reservoir exploration, and in particular to a method for judging the paleokarst dissolution intensity of carbonate rocks far away from a source area. Background Art

[0002] Paleokarstification is the most important constructive role in carbonate reservoir formation. Large-scale carbonate oil and gas reservoirs related to paleokarstification are currently an important area for increasing reserves and production in my country. Paleokarstification is the comprehensive result of the reaction between various dissolving fluids and carbonate water-rock in the geological history period, and dissolution is the basis of carbonate reservoir formation. Therefore, clarifying the intensity of paleokarstification in carbonate reservoirs is of great significance for evaluating the quality, scale and distribution of karst reservoirs.

[0003] At present, there have been many reports on the characterization and intensity identification of carbonate paleokarst. However, the existing methods and technologies are based on the observation of petrological characteristics. Due to the differences in geological viewpoints, it is difficult to use them reasonably in different blocks and different layers. At the same time, for carbonate paleokarst reservoirs far away from the source area, due to the less contamination of terrigenous debris, the whole-rock geochemical analysis method cannot accurately obtain the influence of terrigenous debris on dissolution, thus limiting the accurate judgment of paleokarst intensity. With the gradual popularization of laser micro-area trace element testing methods, new technical means are provided for the judgment of paleokarst dissolution intensity. Summary of the invention

[0004] Based on the above problems, the present invention provides a method for judging the dissolution intensity of paleokarst carbonate rocks far away from the provenance area based on trace element analysis, so as to solve the technical problem that the relevant indicators in the prior art are insensitive to the karst effect of carbonate rock reservoirs far away from the provenance area, which leads to the lack of a method for judging the dissolution intensity of this type of karst reservoir.

[0005] To this end, the technical solution of the present application is: a method for determining the intensity of paleokarst dissolution of carbonate rocks, comprising the following steps:

[0006] Sampling: Sampling is carried out on the karst system of the target carbonate paleokarst reservoir to be compared, and the sampling range is the karst fabric and its corresponding bedrock;

[0007] Dissolution intensity judgment: the samples were compared in pairs; first, the total rare earth content and rare earth distribution curve of the two groups of samples were compared: if the changes of the rare earth distribution curves of the karst system and the bedrock of the sample were inconsistent and the total rare earth content of the karst system was greater than 10, the sample with a greater total rare earth content in the karst system had a stronger dissolution intensity;

[0008] If the rare earth distribution curves of the karst system and bedrock of the sample are consistent and the total rare earth content is less than 10, the Si and Al element box diagrams of the karst structures of the two groups of samples are compared: the Si element of the karst system is not enriched relative to its corresponding bedrock, and the enrichment of the Al element in the karst system is used to judge that the richer the Al element, the greater the dissolution intensity;

[0009] If both of the above two elements show enrichment characteristics, then an analysis of the enrichment characteristics of Ti, Th, Sc and Zr elements in the bedrock of the two groups of samples and their corresponding karst systems will be carried out. The more types of the above elements are enriched in the karst system relative to the bedrock, the greater the dissolution intensity. If obvious enrichment characteristics are shown for all four elements, then the enrichment characteristics of the most inert Zr element will be compared. The greater the Zr element enrichment coefficient, the greater the dissolution intensity.

[0010] Furthermore, the consistency of the change of the rare earth distribution curve is judged by the following method: calculate the enrichment coefficients of light rare earth elements (La, Ce, Pr, Nd), medium rare earth elements (Sm, Eu, Gd, Tb, Dy, Y), and heavy rare earth elements (Ho, Er, Tm, Yb, Lu) of different compositions of each sample, and sort the calculated data. If the enrichment coefficients of different types of rare earth elements of the bedrock of the sample and the corresponding karst system are ranked consistently, it is considered that their distribution curve characteristics are consistent. Otherwise, if the rankings are inconsistent, it means that the distribution curve characteristics are different.

[0011] Furthermore, the formula for the enrichment coefficient of different types of rare earth elements is as follows: enrichment coefficient E = rare earth content of this type / total rare earth content.

[0012] Furthermore, the element enrichment characteristics of different fabrics are analyzed by comparing the mean, median and upper quartile of the same element in the two fabrics. If the above three factors of an element in the karst system are greater than those in the bedrock, it is considered that the element is enriched relative to the bedrock in the karst system.

[0013] Furthermore, the rare earth element distribution curve is obtained by standardizing and plotting the rare earth element data of the Post-Archaean Australian shale.

[0014] Furthermore, the rare earth elements and Si, Al, Ti, Th, Sc, and Zr mineral trace elements are obtained by in-situ micro-area geochemical testing using a laser-coupled inductively coupled plasma mass spectrometer.

[0015] Furthermore, the Si, Al, Ti, Th, Sc, and Zr element boxes are obtained by drawing element box comparison diagrams, and the drawing is based on respectively calculating the average value, median, upper quartile, lower quartile, upper limit, and lower limit of each element; wherein,

[0016] Upper quartile: Let i = (1+n) / 4, n is the number of single element test data, take the integer part of i as i', sort the data, let its arbitrary value be X(y), y is the sequence number, upper quartile Q1 = X(i')*(i-i')+X(i'+1)*(1-i+i');

[0017] Lower quartile: Let i = (1+n) / 4*3, n is the number of data, take the integer part of i as i', sort the data, let its arbitrary value be X(y), y is the sequence number, lower quartile Q3 = X(i')*(i-i')+X(i'+1)*(1-i+i');

[0018] Upper limit=Q3+(Q3-Q1)*1.5;

[0019] Lower limit=Q1-(Q3-Q1)*1.5.

[0020] Furthermore, the calculation formula of Zr element enrichment coefficient is E Zr = Zr element content in karst system - Zr element content in bedrock.

[0021] The beneficial effects of the present invention are as follows: This application aims to solve the technical problem that the relevant indicators of the prior art are insensitive to the karst effect of carbonate reservoirs far away from the provenance area, resulting in a lack of a method for judging the dissolution intensity of this type of karst reservoir. By judging the total amount of rare earth elements based on the rare earth element distribution curve and judging by the graded mineral trace elements, a new means for judging the dissolution intensity of paleokarst carbonate rocks far away from the provenance area is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The two samples A and B to be compared in Example 1;

[0023] Figure 2 is the rare earth distribution curve and total rare earth content (ΣREE) of sample A and sample B;

[0024] Figure 3 The box diagrams of Al and Si elements of different structures of sample A and sample B;

[0025] Figure 4 Box plots of Ti, Th, Sc and Zr elements in different compositions of sample A and sample B. DETAILED DESCRIPTION

[0026] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0027] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0028] Unless otherwise specified, the technical and scientific terms used herein have the same meanings as commonly understood by ordinary technicians in the technical field to which the invention belongs; other raw materials, reagents, experimental methods and technical means not specifically specified in the present invention refer to the raw materials and reagents commonly used by ordinary technicians in the field, as well as the commonly used experimental methods and technical means.

[0029] Example 1 Analysis of the Dissolution Strength of Carbonate Paleokarst Reservoirs in the Permian Maokou Formation of the Sichuan Basin

[0030] 1. Collect core samples of the Maokou Formation, divide the karst structure and bedrock, and obtain corresponding samples, sample A and sample B. Select the parts of the two samples with karst structure and bedrock for laser thin section grinding. The characteristics under the thin section microscope are as follows: Figure 1 As shown: (A) is the macroscopic and microscopic features of sample A. Macroscopically, the bedrock is cut by the black karst system. Under a microscope, it can be seen that the bedrock is bioclastic limestone, and the karst system is filled with carbonate sand-grade debris. (B) is the macroscopic and microscopic features of sample B. Macroscopically, there is a clear boundary between the bedrock and the karst system. Under a microscope, it can be seen that the bedrock is bioclastic limestone, and the karst system is filled with carbonate sand-grade debris.

[0031] 2. Carry out in-situ laser trace element testing on the karst and bedrock structures under the microscope through laser-inductively coupled plasma mass spectrometry (LA-ICP-MS) to obtain data on rare earth elements and other trace elements.

[0032] 3. Rare earth elements and trace elements treatment:

[0033] (1) Obtain ΣREEY data by calculating the total amount of rare earth elements;

[0034] (2) The rare earth element data (including Y element) were standardized by Post-Archaean Australian Shale (PAAS) and a rare earth element distribution diagram (REEY) was drawn. Figure 2As shown, the enrichment coefficients of various types of rare earth elements were calculated. The enrichment coefficients of light rare earth, medium rare earth and heavy rare earth in the bedrock and karst system of sample A were 0.1441, 0.4601, 0.3959 and 0.1487, 0.4716 and 0.3797 respectively. The enrichment coefficients of light rare earth, medium rare earth and heavy rare earth in the bedrock and karst system of sample B were 0.1037, 0.4732, 0.4231 and 0.1395, 0.4862 and 0.3743 respectively. The four structures of the two samples all have the characteristics that the enrichment coefficient of medium rare earth is greater than that of heavy rare earth, which is greater than that of light rare earth. Therefore, the rare earth distribution curve characteristics of the karst system of each sample relative to its corresponding bedrock are consistent, and the total rare earth content is less than 10, so the next step of comparison is carried out.

[0035] (3) By calculating the parameters required for the box plots of Si, Al, Ti, Th, Sc, and Zr elements, box plots of different samples and different structures are drawn, such as Figure 3 , Figure 4 : Figure 3 To compare the Al and Si element box diagrams of different structures of samples, (A) is the Al and Si element box diagram of bedrock and karst system of sample A, and (B) is the Al and Si element box diagram of bedrock and karst system of sample B. The Al and Si elements in the karst system of the two samples are enriched relative to the bedrock; therefore, the box diagrams of Ti, Th, Sc, and Zr are compared. Figure 4 To compare the box plots of Ti, Th, Sc and Zr elements in different compositions of samples, (A) is the box plot of Ti, Th, Sc and Zr elements between the bedrock and the karst system of sample A. The karst system is relatively enriched in all four elements relative to the bedrock. (B) is the box plot of Ti, Th, Sc and Zr elements between the bedrock and the karst system of sample B. In the karst system of sample A, Ti, Th, Sc and Zr elements are all enriched relative to the bedrock, while in sample B, only Ti in the karst system is enriched relative to the bedrock. Therefore, it is concluded that sample A is more strongly dissolve than sample B.

[0036] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for judging the dissolution intensity of carbonate rocks far from the provenance area, characterized in that: The steps include: Sampling: Sampling is carried out on the karst system of the target carbonate paleokarst reservoir to be compared, and the sampling range is the karst fabric and its corresponding bedrock; Dissolution intensity judgment: the samples were compared in pairs; first, the total rare earth content and rare earth distribution curve of the two groups of samples were compared: if the changes of the rare earth distribution curves of the karst system and the bedrock of the sample were inconsistent and the total rare earth content of the karst system was greater than 10, the sample with a greater total rare earth content in the karst system had a stronger dissolution intensity; If the rare earth distribution curves of the karst system and bedrock of the sample are consistent and the total rare earth content is less than 10, the Si and Al element box diagrams of the karst structures of the two groups of samples are compared: the Si element of the karst system is not enriched relative to its corresponding bedrock, and the enrichment of the Al element in the karst system is used to judge that the richer the Al element, the greater the dissolution intensity; If both of the above two elements show enrichment characteristics, then an analysis of the enrichment characteristics of Ti, Th, Sc and Zr elements in the bedrock of the two groups of samples and their corresponding karst systems will be carried out. The more types of the above elements are enriched in the karst system relative to the bedrock, the greater the dissolution intensity. If obvious enrichment characteristics are shown for all four elements, then the enrichment characteristics of the most inert Zr element will be compared. The greater the Zr element enrichment coefficient, the greater the dissolution intensity.

2. The judgment method according to claim 1, characterized in that: The consistency of the change of the rare earth distribution curve is judged by the following method: calculate the enrichment coefficients of light rare earth elements (La, Ce, Pr, Nd), medium rare earth elements (Sm, Eu, Gd, Tb, Dy, Y), and heavy rare earth elements (Ho, Er, Tm, Yb, Lu) of different compositions of each sample, and sort the calculated data. If the enrichment coefficients of different types of rare earth elements of the bedrock of the sample and the corresponding karst system are ranked consistently, it is considered that their distribution curve characteristics are consistent. Otherwise, if the rankings are inconsistent, it means that the distribution curve characteristics are different.

3. The judgment method according to claim 1, characterized in that: The formula for the enrichment coefficient of different types of rare earth elements is as follows: Enrichment coefficient E = rare earth content of this type / total rare earth content.

4. It is characterized in that The element enrichment characteristics of different fabrics are analyzed by comparing the mean, median and upper quartile of the same elements in two fabrics. If the above three factors of an element in the karst system are greater than those in the bedrock, it is considered that the element is enriched relative to the bedrock in the karst system.

5. The judgment method according to claim 1, characterized in that: The rare earth element distribution curve is obtained by standardizing and drawing rare earth element data of the Post-Archaean Australian shale.

6. The determination method according to claim 1, characterized in that: The rare earth elements and Si, Al, Ti, Th, Sc, and Zr mineral trace elements are obtained by in-situ micro-area geochemical testing using a laser-coupled inductively coupled plasma mass spectrometer.

7. The determination method according to claim 1, characterized in that: The Si, Al, Ti, Th, Sc, and Zr element boxes are obtained by drawing element box comparison diagrams, and the drawing is based on respectively calculating the average value, median, upper quartile, lower quartile, upper limit, and lower limit of each element; wherein, Upper quartile: Let i = (1+n) / 4, n is the number of single element test data, take the integer part of i as i', sort the data, let its arbitrary value be X(y), y is the sequence number, upper quartile Q1 = X(i')*(i-i')+X(i'+1)*(1-i+i'); Lower quartile: Let i = (1+n) / 4*3, n is the number of data, take the integer part of i as i', sort the data, let its arbitrary value be X(y), y is the sequence number, lower quartile Q3 = X(i')*(i-i')+X(i'+1)*(1-i+i'); Upper limit=Q3+(Q3-Q1)*1.5; Lower limit=Q1-(Q3-Q1)*1.

5.

8. The determination method according to claim 1, characterized in that: The calculation formula of Zr element enrichment coefficient is E Zr = Zr element content in karst system - Zr element content in bedrock.

Images