A method for judging dissolution intensity of ancient karst in carbonate rock far from a source area

Abstract

The present application relates to the technical field of carbonate reservoir exploration, and particularly relates to a method for judging the dissolution intensity of carbonate rock paleokarst far from a source area. The related indexes of the prior art are not sensitive to the karstification of the carbonate rock reservoir far from the source area, which leads to the technical problem that there is no method for judging the dissolution intensity of the karst reservoir of this type. Through the total amount of rare earth elements and the classification of mineral trace elements, a new method for judging the dissolution intensity of carbonate rock paleokarst is obtained.

Description

Technical Field

[0001] This invention relates to the field of carbonate reservoir exploration technology, specifically to a method for judging the intensity of ancient karst dissolution in carbonate rocks far from the source area. Background Technology

[0002] Paleokarstification is the most important constructive process in carbonate reservoir formation, and large-scale carbonate oil and gas reservoirs associated with paleokarstification are currently a crucial area for increasing reserves and production in my country. Paleokarstification is the comprehensive result of the interaction between various dissolving fluids and carbonate water during geological history. Since dissolution is the foundation of carbonate reservoir formation, clarifying the intensity of paleokarstification in carbonate reservoirs is of great significance for evaluating the quality, scale, and distribution of karst reservoirs.

[0003] Numerous studies have been reported on the characterization and intensity assessment of paleokarst processes in carbonate rocks. However, existing methods and techniques rely heavily on petrological observations, which, influenced by differing geological perspectives, make them difficult to apply appropriately across different blocks and stratigraphic levels. Furthermore, in paleokarst reservoirs of carbonate rocks far from their source areas, due to less terrigenous clastic contamination, whole-rock geochemical analysis cannot accurately capture the impact of terrigenous clastic materials on dissolution, thus limiting the accurate assessment of paleokarst intensity. With the increasing prevalence of laser micro-area trace element testing, new technical means have been provided for assessing the intensity of paleokarst dissolution. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a method for judging the intensity of ancient karst dissolution in carbonate rocks far from the source area based on trace element analysis. This method solves the technical problem that existing technologies are not sensitive to karstification in carbonate rock reservoirs far from the source area, thus leading to a lack of methods for judging the intensity of dissolution in this type of karst reservoir.

[0005] Therefore, the technical solution of this application is: a method for judging the intensity of ancient karst dissolution in carbonate rocks, comprising the following steps:

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

[0007] Dissolution intensity judgment: Compare the samples in pairs; First, compare the total rare earth content and rare earth distribution curve of the two groups of samples: If the rare earth distribution curve of the karst system and the bedrock of the sample are inconsistent and the total rare earth content of the karst system is greater than 10, then the sample with the greater total rare earth content of the karst system has stronger dissolution intensity.

[0008] If the rare earth element distribution curves of the karst system and the bedrock of the sample change in the same way and the total rare earth element content is less than 10, then the box plots of Si and Al elements of the karst fabric of the two groups of samples are compared: Si element in the karst system is not enriched relative to its corresponding bedrock. The enrichment of Al element in the karst system is used to judge the intensity of dissolution. The more enriched Al element is, the greater the intensity of dissolution.

[0009] If both of the above elements show enrichment characteristics, then conduct enrichment characteristic analysis of Ti, Th, Sc, and Zr elements in the bedrock and corresponding karst systems of the two sets of samples. The more types of the above elements enriched in the karst system relative to the bedrock, the greater the dissolution intensity. If all four elements show obvious enrichment characteristics, then compare the enrichment characteristics of Zr, which is the most inert element. The larger the Zr enrichment coefficient, the greater the dissolution intensity.

[0010] Furthermore, the consistency of the rare earth distribution curves is determined by the following method: 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) in different compositions of each sample are calculated, and the calculated data are sorted. If the enrichment coefficients of different types of rare earth elements in the bedrock of the sample are consistent with those in the corresponding karst system, then their distribution curve characteristics are considered to be consistent. Conversely, if the sorting is inconsistent, it means that the distribution curve characteristics are different.

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

[0012] Furthermore, the element enrichment characteristics of different fabrics are compared by comparing the average, median, and upper quartile of the same element in two fabrics. If the above three factors of a certain element in the karst system are all greater than those in the bedrock, then the element is considered to be relatively enriched in the karst system compared to the bedrock.

[0013] Furthermore, the rare earth element distribution curves were obtained by standardizing and plotting rare earth element data from the post-Archaeological Australian shale.

[0014] Furthermore, the rare earth elements and trace elements of minerals such as Si, Al, Ti, Th, Sc, and Zr were obtained through in-situ micro-region geochemical testing using laser-coupled inductively coupled plasma mass spectrometry.

[0015] Furthermore, the element bins for Si, Al, Ti, Th, Sc, and Zr are obtained by drawing an element bin comparison diagram, which is based on calculating the mean, 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 test data for a single element, take the integer part of i as i', sort the data, let its arbitrary value be X(y), y is the ordinal number, the upper quartile Q1 = X(i')*(i-i')+X(i'+1)*(1-i+i');

[0017] Lower quartile: Let i = (1+n) / 4*3, where n is the number of data points. Take the integer part of i as i', sort the data, and let its arbitrary value be X(y), where y is the ordinal number. The 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 formula for calculating the enrichment factor of Zr is E. Zr = Zr content in karst system - Zr content in bedrock.

[0021] The beneficial effects of this invention are as follows: This application addresses the technical problem that existing indicators are not sensitive to karstification in carbonate reservoirs far from the source area, resulting in a lack of methods for judging the dissolution intensity of this type of karst reservoir. By using rare earth element distribution curves to judge the total amount of rare earth elements and mineral trace element classification, a new method for judging the dissolution intensity of ancient karst in carbonate rocks far from the source area is obtained. Attached Figure Description

[0022] Figure 1 These are two samples, A and B, that need to be compared in Example 1;

[0023] Figure 2 The rare earth distribution curves and total rare earth content (ΣREE) of samples A and B are shown.

[0024] Figure 3 Box plots of Al and Si elements for samples A and B with different compositions;

[0025] Figure 4 Box plots for Ti, Th, Sc, and Zr elements in different compositions of samples A and B. Detailed Implementation

[0026] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0027] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0028] Unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; other raw materials, reagents, test methods and techniques not specifically mentioned herein refer to raw materials and reagents commonly used by one of ordinary skill in the art, as well as commonly employed test methods and techniques.

[0029] Example 1: Analysis of the dissolution intensity of the Permian Maokou Formation carbonate paleokarst reservoir in the Sichuan Basin

[0030] 1. Core samples of the Maokou Formation were collected, and the karst fabric and bedrock were separated. Corresponding samples, A and B, were obtained. Laser thin sections were prepared from portions of both samples that exhibited karst fabric and bedrock. The microscopic characteristics of the thin sections are as follows: Figure 1 As shown: (A) represents the macroscopic and microscopic characteristics of sample A. Macroscopically, the bedrock is cut by a black karst system. Under a microscope, the bedrock is bioclastic limestone, and the karst system is filled with carbonate sand-sized debris. (B) represents the macroscopic and microscopic characteristics of sample B. Macroscopically, the boundary between the bedrock and the karst system is obvious. Under a microscope, the bedrock is bioclastic limestone, and the karst system is filled with carbonate sand-sized debris.

[0031] 2. In-situ laser trace element analysis was conducted on karst and bedrock fabrics under a microscope using laser-coupled inductively coupled plasma mass spectrometry (LA-ICP-MS) to obtain data on rare earth elements and other trace elements.

[0032] 3. Treatment of rare earth elements and trace elements:

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

[0034] (2) Rare earth element data (including Y element) were standardized using Post-Archaeological Australian Shale (PAAS) data, and a rare earth element distribution map (REEY) was plotted. Figure 2As shown, the enrichment coefficients of various rare earth elements were calculated. For sample A, the enrichment coefficients of light rare earth elements (LREE), medium rare earth elements (MREE), and heavy rare earth elements (HREE) in the bedrock and karst system were 0.1441, 0.4601, 0.3959 and 0.1487, 0.4716, 0.3797, respectively. For sample B, the enrichment coefficients of LREE, MREE, and HREE in the bedrock and karst system were 0.1037, 0.4732, 0.4231 and 0.1395, 0.4862, 0.3743, respectively. Both samples showed that the enrichment coefficient of MREE was greater than that of HREE, which was greater than that of LREE. Therefore, the rare earth element distribution curves of the karst system relative to its corresponding bedrock of each sample were consistent, and the total rare earth element content was less than 10. Therefore, further comparison was carried out.

[0035] (3) By calculating the parameters required for box plots of Si, Al, Ti, Th, Sc, and Zr elements, box plots of different samples and configurations are drawn, such as... Figure 3 , Figure 4 : Figure 3 To compare the Al and Si element box plots of different samples, (A) shows the Al and Si element box plots of sample A's bedrock and karst system, and (B) shows the Al and Si element box plots of sample B's bedrock and karst system. In both samples, the karst system shows relative enrichment of Al and Si elements compared to the bedrock. Therefore, a comparison of Ti, Th, Sc, and Zr element box plots was also conducted. Figure 4 To compare the box plots of Ti, Th, Sc, and Zr elements in different compositions of the samples, (A) is the box plot of Ti, Th, Sc, and Zr elements in the bedrock and karst system of sample A. The karst system is relatively enriched in all four elements compared to the bedrock. (B) is the box plot of Ti, Th, Sc, and Zr elements in the bedrock and 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. However, in sample B, only Ti is relatively enriched relative to the bedrock in the karst system. Therefore, it is concluded that sample A has undergone stronger dissolution 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, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining the intensity of ancient karst dissolution in carbonate rocks far from the source area, characterized in that, Includes the following steps: Sampling: Sampling was carried out on the karst system of the target carbonate paleokarst reservoir to be compared. The sampling range was the karst fabric and its corresponding bedrock. Dissolution intensity judgment: Compare the samples in pairs; First, compare the total rare earth content and rare earth distribution curve of the two groups of samples: If the rare earth distribution curve of the karst system and the bedrock of the sample are inconsistent and the total rare earth content of the karst system is greater than 10, then the sample with the greater total rare earth content of the karst system has stronger dissolution intensity. If the rare earth element distribution curves of the karst system and the bedrock of the sample change in the same way and the total rare earth element content is less than 10, then the box plots of Si and Al elements of the karst fabric of the two groups of samples are compared: Si element in the karst system is not enriched relative to its corresponding bedrock. The enrichment of Al element in the karst system is used to judge the intensity of dissolution. The more enriched Al element is, the greater the intensity of dissolution. If both of the above elements show enrichment characteristics, then conduct enrichment characteristic analysis of Ti, Th, Sc, and Zr elements in the bedrock and corresponding karst systems of the two sets of samples. The more types of the above elements enriched in the karst system relative to the bedrock, the greater the dissolution intensity. If all four elements show obvious enrichment characteristics, then compare the enrichment characteristics of Zr, which is the most inert element. The larger the Zr enrichment coefficient, the greater the dissolution intensity.

2. The determination method according to claim 1, characterized in that, The consistency of the rare earth distribution curves is determined 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) for different compositions of each sample, and sort the calculated data. If the enrichment coefficients of different types of rare earth elements in the bedrock of the sample are consistent with those in the corresponding karst system, then their distribution curve characteristics are considered consistent. Conversely, if the sorting is inconsistent, it indicates that the distribution curve characteristics are different.

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

4. The determination method according to claim 1, characterized in that, The element enrichment characteristics of different textures are determined by comparing the mean, median, and upper quartile of the same element in two textures. If the mean, median, and upper quartile of a certain element in the karst system are all greater than those in the bedrock, then the element is considered to be relatively enriched in the karst system compared to the bedrock.

5. The determination method according to claim 1, characterized in that, The rare earth element distribution curves were obtained by standardizing and plotting rare earth element data from the late Archean Australian shale.

6. The determination method according to claim 1, characterized in that, The rare earth elements and trace mineral elements such as Si, Al, Ti, Th, Sc, and Zr were obtained through in-situ micro-region geochemical testing using laser-coupled inductively coupled plasma mass spectrometry.

7. The determination method according to claim 1, characterized in that, The element bins for Si, Al, Ti, Th, Sc, and Zr were obtained by drawing element bin comparison diagrams, which were drawn based on the calculation of the mean, median, upper quartile, lower quartile, upper limit, and lower limit of each element; among them, Upper quartile: Let i = (1+n) / 4, n is the number of test data for a single element, take the integer part of i as i', sort the data, let its arbitrary value be X(y), y is the ordinal number, the 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 ordinal number, the 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 formula for calculating the enrichment factor of Zr is E Zr =Zr content in karst system - Zr content in bedrock.

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