A method for identifying a diagenetic limestone-marl rhythm

Abstract

This invention discloses a method for identifying the rhythm of diagenetic limestone-marl, characterized by the following steps: Step 1: Obtain a limestone-marl rhythm sample to be tested, and then prepare a first flaky sample, a second flaky sample, and a powder sample that meet the requirements; Step 2: Perform petrological analysis on the first flaky sample and observe whether it meets the first standard requirement; perform mineralogical analysis on the powder sample and observe whether it meets the second standard requirement; perform elemental geochemical analysis on the second flaky sample and observe whether it meets the third standard requirement; When the limestone-marl rhythm sample meets the first, second, and third standard requirements simultaneously, the limestone-marl rhythm sample is a diagenetic limestone-marl rhythm. This method can effectively identify the rhythm of diagenetic limestone-marl, and is simple to operate with high accuracy.

Description

Technical Field

[0001] This invention relates to the technical field of limestone-marl rhythm type identification, specifically to a method for identifying the rhythm of diagenetic limestone-marl. Background Technology

[0002] Limestone-marl rhythms are a common type of inter-depositional sedimentary rocks in geological history, attracting widespread attention due to their regular lithological assemblages. It is generally believed that the differences in composition between limestone and marl reflect changes in the original sedimentary types and environment caused by climate change. Based on this, continuous rhythmic sedimentary records can serve as a good vehicle for orbit-driven, high-precision cyclic stratigraphy studies. However, due to the complex and variable nature of carbonate sedimentary and diagenetic processes, a crucial prerequisite for studying limestone-marl rhythms is the identification of sedimentary and diagenetic alteration signals; otherwise, it will be impossible to accurately explore the sedimentary and environmental information they carry.

[0003] Furthermore, limestone-marl rhythms can also serve as an important type of tight hydrocarbon accumulation system. In recent years, multiple wells in the Sichuan Basin have yielded industrial gas flows in the first section of the Middle Permian Maokou Formation, indicating that it serves not only as a source rock but also as an effective reservoir, belonging to the self-generating and self-storing gas reservoir type, revealing its significant oil and gas exploration potential. Existing research indicates that the Middle Permian limestone-marl rhythms in South China were primarily formed by diagenesis.

[0004] Therefore, accurate identification of the rhythm of diagenetic limestone-marl is of great significance for both sedimentology and oil and gas exploration. However, to date, no technical method has been reported that can accurately identify the rhythm of diagenetic limestone-marl, and traditional techniques are no longer sufficient to meet the needs of modern oil and gas exploration and development. Summary of the Invention

[0005] The purpose of this invention is to address the lack of an effective method for identifying the rhythm of diagenetic limestone-marl in existing technologies, and to provide a method for identifying the rhythm of diagenetic limestone-marl. The identification method disclosed in this application can effectively identify the rhythm of diagenetic limestone-marl, providing strong support for the study of the sedimentary environment and reservoir formation mechanism of limestone-marl rhythm, thereby improving exploration understanding, reducing exploration risks, and the method is simple to operate, highly accurate, and easy to promote and apply.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for identifying the rhythm of diagenetic limestone-marl, comprising the following steps:

[0008] Step 1: Obtain rhythmic samples of limestone-marl to be tested, and then prepare a first sheet sample that meets the requirements, a second sheet sample that meets the requirements, and a powder sample that meets the requirements.

[0009] Step 2

[0010] The first sheet-like sample was subjected to petrological analysis and testing, and it was observed whether it met the first standard requirements.

[0011] Mineralogical analysis was performed on the powder samples to determine whether they met the requirements of the second standard.

[0012] The second sheet-like sample was subjected to elemental geochemical analysis and testing to determine whether it met the requirements of the third standard.

[0013] When the limestone-marl rhythm sample to be tested simultaneously meets the first, second, and third standard requirements, the limestone-marl rhythm sample is a diagenetic limestone-marl rhythm.

[0014] This invention discloses a method for identifying the rhythm of diagenetic limestone-marl. First, flaky and powder samples meeting the required testing standards are prepared. Then, analysis is performed at three scales: petrology, mineralogy, and elemental geochemistry. If the standard requirements of all three scales are met simultaneously, it can be determined whether the rhythm of the limestone-marl is diagenetic. This method is simple to operate and highly accurate, providing strong support for the study of the sedimentary environment and reservoir formation mechanism of limestone-marl rhythms, thereby improving exploration understanding and reducing exploration risks.

[0015] Furthermore, in step 1, obtaining the rhythmic sample of the limestone-marl to be tested involves selecting hand specimens of limestone-marl rhythm that have sedimentary rhythmic interbedded characteristics and have not been significantly altered by later tectonic activities, collected from field outcrop profiles or borehole profiles in the inland area.

[0016] Furthermore, in step 1, the first flaky sample meeting the requirements is mainly prepared by the following method: First, a flaky sample containing both limestone and marl layers is obtained, wherein the thickness of the flaky sample is 0.02 mm to 0.05 mm; then, the flaky sample is ground, polished, cleaned, and dried to obtain the first flaky sample. Preferably, the thickness of the flaky sample is 0.25 mm to 0.04 mm.

[0017] Furthermore, in step 1, the qualified powder sample is mainly prepared by the following method: adjacent limestone and marl layers are ground into powder of 200 mesh or finer. Further, a vibratory mill is used for grinding.

[0018] Furthermore, in step 1, the second sheet-like sample meeting the requirements is mainly prepared by the following method: First, a sheet-like sample containing both limestone and marl layers is obtained, wherein the thickness of the sheet-like sample is 0.03 mm to 0.05 mm; then, the sheet-like sample is ground, polished, cleaned, and dried to obtain the second sheet-like sample. Further, the sample is cleaned with ultrapure water using an ultrasonic cleaner for 3 minutes to remove contaminants before drying.

[0019] Furthermore, in step 2, the petrological analysis of the first sheet sample mainly includes: observing whether the limestone layer and the adjacent marl layer have a gradual transitional contact relationship, determining whether they have the same biological fossil assemblage, and observing whether the proportion of clay minerals from the limestone layer to the adjacent marl layer has a gradually increasing trend.

[0020] Furthermore, by examining hand specimens and thin sections, we can observe whether there is a gradual change in color and the state of fossils (if any) from the limestone layer to the adjacent marl layer (degree of fragmentation, orientation, etc.). We can also use backscattered electron microscopy (SEM) to analyze backscattered electron images obtained from electron probe microanalysis or scanning electron microscopy to observe whether the proportion of clay minerals gradually increases from the limestone layer to the adjacent marl layer. Furthermore, we can use microscopic identification to statistically analyze the differences in the types of fossils between the limestone layer and the adjacent marl layer. Even further, if the marl layer is only missing aragonite fossils such as feet and bivalves compared to the limestone layer, then it is considered that the limestone layer and the adjacent marl layer have the same fossil assemblages.

[0021] Furthermore, the first standard requires that the following three test results be met simultaneously: a gradual transition in the contact relationship between the limestone layer and the adjacent marl layer; the presence of the same fossil assemblage; and a gradually increasing trend in the proportion of clay minerals from the limestone layer to the adjacent marl layer. Additionally, a gradual transition in color and / or the state of fossil occurrence between the limestone layer and the adjacent marl layer is required.

[0022] Furthermore, in step 2, the mineralogical analysis of the powder sample mainly includes: whether the clay mineral content of the limestone layer is lower than that of the adjacent marl layer, and whether the clay mineral types of the limestone layer and the adjacent marl layer are diagenetic clay minerals.

[0023] Furthermore, XRD (X-ray diffraction) was used to analyze the clay mineral content of the powder samples. When the clay mineral content is low, the powder samples need to be processed before XRD analysis. The specific processing procedure involves extracting the clay minerals using suspension extraction, followed by XRD testing of the clay minerals under natural air-drying, ethylene glycol saturation, and a high temperature of 550℃. Furthermore, diagenetic clay minerals typically refer to sepiolite, talc, and alicite, among others.

[0024] Furthermore, the second standard requires that the following two test results be met simultaneously: the clay mineral content of the limestone layer is lower than that of the adjacent marl layer, and the clay mineral type of both the limestone layer and the adjacent marl layer is diagenetic clay mineral.

[0025] Furthermore, in step 2, the elemental geochemical analysis of the second sheet sample mainly includes: whether the slopes of the Al2O3 and TiO2 plotting trend lines in the limestone layer, marl layer, and the lithological transition zone between the two are consistent; and whether the REE+Y rare earth matching is consistent.

[0026] Furthermore, in step 2, the main steps for elemental geochemical analysis of the second sheet sample are as follows: (1) The second sheet sample is tested using a laser ablation inductively coupled plasma mass spectrometer on the limestone layer, marl layer, and the lithological transition zone between the two, with at least 5 test points for each composition. Further, the laser beam diameter and frequency are 30μm~60μm and 5Hz, respectively. In the trace element content processing, glass standard materials BHVO-2G, BCR-2G, and BIR-1G are used for multi-external standard correction without internal standard correction, and the relative deviation of rare earth elements is controlled by monitoring the carbonate standard MACS-3 used in each set of standard samples.

[0027] (2) Plot the Al2O3 and TiO2 contents of the test samples from the limestone layer, marl layer and the lithological transition zone between the two. If the Al2O3 and TiO2 plotting trend lines of each composition have a consistent slope, perform post-Archaic Australian shale normalization (PAAS normalization) on the rare earth element contents of the test samples from the limestone layer, marl layer and the lithological transition zone between the two to determine whether the three have consistent REE+Y matching characteristics.

[0028] Furthermore, the third standard requires that the following two test results be met simultaneously: the Al2O3 and TiO2 projection trend lines of the limestone layer, marl layer and the lithological transition zone of the two have a consistent slope, and the REE+Y rare earth matching is consistent.

[0029] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0030] 1. This invention discloses a method for identifying the rhythm of diagenetic limestone-marl. First, flaky and powder samples meeting the required testing standards are prepared. Then, through petrological, mineralogical, and elemental geochemical analyses, if the standard requirements of all three scales are met simultaneously, it can be determined whether the rhythm of the limestone-marl to be tested is diagenetic. This method is simple to operate and highly accurate, providing strong support for the study of the sedimentary environment and reservoir formation mechanism of limestone-marl rhythms, thereby improving exploration understanding and reducing exploration risks.

[0031] 2. The method of the present invention can accurately identify the rhythm of diagenetic limestone-marl under multi-scale conditions, which can not only enhance the understanding of its genetic mechanism and the detailed interpretation of the sedimentary environment, but also provide a basis for the formation mechanism of limestone-marl rhythmic tight carbonate reservoirs, and thus provide reference and guidance for oil and gas exploration of similar reservoirs. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the steps of the identification method of the present invention.

[0033] Figure 2 Image showing the rhythmic outcrops, core samples, and thin sections of limestone-marl.

[0034] Figure 3 This is a characteristic image of limestone-marl under a polarizing microscope.

[0035] Figure 4 This is a rhythmic backscattered electron image of limestone-marl.

[0036] Figure 5 This is a rhythmic X-ray clay mineral analysis spectrum of limestone-marl.

[0037] Figure 6 This is a cross-sectional diagram showing the Al2O3 and TiO2 contents of different textures in limestone-marl.

[0038] Figure 7 Diagrams showing rare earth element matching patterns for different textures of limestone-marl. Detailed Implementation

[0039] The present invention will now be described in detail with reference to the accompanying drawings.

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0041] Example

[0042] A method for identifying the rhythm of diagenetic limestone-marl, including as shown in the appendix. Figure 1 The process shown is as follows:

[0043] S1. Based on the analysis of regional geological background data, rhythmic hand specimens of limestone-marl from the Middle Permian Maokou Formation (Member 1) in the Sichuan Basin, exhibiting sedimentary rhythmic interbedded characteristics and unaffected by later tectonic activity, were collected from field outcrops or borehole sections. See attached... Figure 2 As shown, this ensures that a sample simultaneously possesses both limestone and marl layers.

[0044] S2. Obtain the samples to be analyzed by slicing and grinding to meet the requirements of petrological, mineralogy and elemental geochemical analysis.

[0045] The petrological analysis thin section samples were prepared by progressively grinding the samples through coarse grinding, fine grinding, and precision grinding to achieve a smooth surface without scratches. Based on the fact that limestone-marl is mainly composed of transparent minerals, the thickness of the thin section was determined to be 0.03 mm. The mineralogy analysis samples were prepared by grinding adjacent limestone and marl layers into powders of 200 mesh or finer using a vibratory mill, achieving a smooth, flour-like feel. For the elemental geochemical analysis samples, based on the hardness of carbonate minerals and their resistance to laser ablation, to avoid the possibility of penetrating the rock thin sections, LA-ICP-MS analysis samples were preferably thin sections with a thickness of 0.04 mm. In order to minimize the contamination of the original rock sections during the grinding process, the sections were cleaned with ultrapure water using an ultrasonic cleaner for 3 minutes to remove contamination before drying and storage.

[0046] S3. Scan the thin section samples for petrological analysis and perform macroscopic observation and analysis in conjunction with the hand specimen samples, as shown in the attached document. Figure 2 As shown, microscopic analysis and backscattered electron imaging analysis were performed using thin sections, as illustrated in the attached figure. Figure 3 and attached Figure 4 As shown, the contact relationship between the limestone layer and the adjacent marl layer, as well as the characteristics of biological fossils, are determined.

[0047] Among them, using hand specimens and thin sections, it was observed that the limestone layer and the adjacent marl layer have a gradual transitional contact relationship, and the thickness of the gradual transition zone is several millimeters. More specifically, thin sections and microscopic images show a gradual change in color and fossil occurrence from the limestone layer to the adjacent marl layer, that is, the color gradually deepens and the degree of fragmentation of fossils becomes more severe. Backscattered electron imaging features show a trend of gradually increasing clay mineral content from the limestone layer to the adjacent marl layer.

[0048] More specifically, the fossil characteristics are that the limestone layer and the adjacent marl layer have the same fossil assemblage; more specifically, the marl layer is only missing aragonite fossils such as gastropods and bivalves compared to the limestone layer, and it is also believed that the two have the same fossil assemblage.

[0049] S4. The powder samples used for mineralogical analysis were subjected to whole-rock and clay mineral analysis using X-ray diffraction, as shown in the attached diagram. Figure 5 As shown, the mineral composition of the limestone and marl layers was obtained.

[0050] More specifically, powder samples of limestone and marl layers with a mesh size of 200 or higher are subjected to whole-rock analysis using an X-ray diffractometer (such as PANalytical X'Pert MPD PRO) to obtain the contents of calcite, dolomite, quartz, clay minerals, etc. For samples with low clay mineral content that cannot be directly identified by whole-rock analysis, the clay minerals are extracted using a suspension extraction method, and XRD clay mineral tests are performed under natural air-drying, ethylene glycol saturation, and high temperature conditions of 550℃.

[0051] More specifically, the diffraction pattern characteristics of the tested mineral samples were compared with the JCPDS cards in the Joint Committee on Powder Diffraction Standards database, and the results were normalized by quality. The results showed that the limestone layer had a lower clay mineral content than the adjacent marl layer, but both layers contained diagenetic clay minerals such as sepiolite, talc, and alicite.

[0052] S5. In-situ trace and rare earth element analysis was performed on the limestone layer, marl layer, and the transition zone between the two using laser ablation inductively coupled plasma mass spectrometry (ICP-MS). The trace and rare earth element compositions of different textures were obtained, as shown in the attached figure. Figure 6 and attached Figure 7 As shown.

[0053] More specifically, the pre-treated laser thin section samples were tested using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on the limestone layer, marl layer, and the lithological transition zone between the two, with at least five test points for each composition. More specifically, the laser beam spot and frequency were 44 μm and 5 Hz, respectively. For trace element content processing, glassy standards BHVO-2G, BCR-2G, and BIR-1G were used for multi-external standard correction without internal standards, and the relative deviation of rare earth elements was controlled by monitoring the carbonate standard MACS-3 used in each set of standards.

[0054] More specifically, the Al2O3 and TiO2 contents of test samples from the limestone layer and the adjacent marl layer were plotted, and the Al2O3 and TiO2 plotting trend lines of each composition had a consistent slope. Further rare earth element analysis was performed, and the rare earth element contents of test samples from the limestone layer, marl layer, and the lithological transition zone between the two were normalized to Post-Archaeological Australian Shale (PAAS normalization). The REE+Y matching characteristics showed that the limestone layer, marl layer, and the lithological transition zone between the two had consistent REE+Y matching characteristics, thus determining that the analyzed limestone-marl rhythm was a diagenetic limestone-marl rhythm.

[0055] S6. Based on the contact relationship and fossil characteristics of the limestone layer and the adjacent marl layer, combined with the mineral composition of the two, especially the clay mineral type, and matching their respective trace and rare earth element characteristics, the analysis shows the mutual compatibility of petrology, mineralogy and element geochemistry, thus determining that the analyzed limestone-marl rhythm is a diagenetic limestone-marl rhythm.

[0056] More specifically, in terms of petrological characteristics, the limestone layer and the adjacent marl layer have a gradual transitional contact relationship and the same biological fossil assemblage. Backscattered electron imaging shows that the proportion of clay minerals gradually increases from the limestone layer to the adjacent marl layer. In terms of mineralogical characteristics, the limestone layer has a lower clay mineral content than the adjacent marl layer, but the clay mineral types of both are diagenetic clay minerals, such as sepiolite, talc, and alicite. In terms of elemental geochemical characteristics, the Al2O3 and TiO2 projection trend lines of the limestone layer and the adjacent marl layer have the same slope, and the REE+Y rare earth element matching of the limestone layer, the marl layer, and the lithological transition zone between the two is consistent. Therefore, it can be comprehensively judged that the analyzed limestone-marl rhythm is a diagenetic limestone-marl rhythm.

[0057] After analysis and research by multiple experts, it was confirmed that the rhythm of the limestone-marl to be tested selected in step S1 is the diagenetic rhythm, which corresponds uniformly with the detection results of the identification method of the present invention.

[0058] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A diagenetic limestone The method for identifying a marlstone rhythm is characterized by, The method comprises the following steps: Step 1, obtaining the limestone to be tested The mudstone rhythmites sample, then, prepare the required first sheet sample, the required second sheet sample and the required powder sample; The first slice sample meeting the requirements is mainly prepared by the following method: first, a slice sample with both limestone layer and marl layer is obtained, wherein the thickness of the slice sample is 0.02mm-0.05mm; then, the slice sample is ground, polished, washed and dried to obtain the first slice sample; The powder sample meeting the requirements is mainly prepared by the following method: the adjacent limestone layer and marl layer are ground into powders with a mesh size of more than 200; The second slice sample meeting the requirements is mainly prepared by the following method: first, a slice sample with both limestone layer and marl layer is obtained, wherein the thickness of the slice sample is 0.03mm-0.05mm; then, the slice sample is ground, polished, washed and dried to obtain the second slice sample; Step 2, The first slice sample is subjected to petrological analysis and detection, and whether the first standard requirement is met is observed; The powder sample is subjected to mineralogical analysis and detection, and whether the second standard requirement is met is observed; The second slice sample is subjected to elemental geochemical analysis and detection, and whether the third standard requirement is met is observed; When the limestone to be measured The limestone to be measured is a limestone of the first type if the limestone to be measured meets the first criterion and the second criterion and the third criterion. The limestone to be measured is a limestone of the second type if the limestone to be measured meets the first criterion and the second criterion and the third criterion. The limestone to be measured is a limestone of the third type if the limestone to be measured meets the first criterion and the second criterion and the third criterion. The first standard requirement is that the following three detection results are met simultaneously: the detection result that the limestone layer and the adjacent marl layer have a gradual contact relationship, the detection result that the same biological fossil assemblage is possessed, and the detection result that the content ratio of clay minerals gradually increases from the limestone layer to the adjacent marl layer; The second standard requirement is that the following two detection results are met simultaneously: the detection result that the content of clay minerals in the limestone layer is lower than that in the adjacent marl layer, and the detection result that the types of clay minerals in the limestone layer and the adjacent marl layer are both diagenetic clay minerals; The third standard requirement is that the following two test results are met simultaneously: the test result that the trend lines of Al2O3 and TiO2 in the limestone layer, the marl layer and the lithology transition zone have consistent slopes, and the test result that the REE+Y rare earth pattern is consistent.

2. The rock according to claim 1, which is a diagenetic limestone The method for identifying a marl rhythm is characterized by, In step 1, the limestone to be tested is obtained The mudstone-limestone rhythm sample is selected from an outcrop profile or a borehole profile in the earth's interior, and has the characteristics of sedimentary rhythm interbedding and is not obviously reformed by later tectonic action The mudstone-limestone rhythm hand specimen sample.

3. The diagenic limestone according to claim 1 The method for identifying a marl rhythm is characterized by, The thickness of the slice sample is 0.25mm-0.04mm.

4. The diagenic limestone of claim 1 The method for identifying a marl rhythm is characterized by, The powder sample meeting the requirements is ground by a vibration mill.

5. The diagenic limestone according to claim 1 The method for identifying a marl rhythm is characterized by, In step 2, the petrological analysis and detection of the first slice sample mainly include: observing whether the limestone layer and the adjacent marl layer have a gradual transition contact relationship, judging whether the same biological fossil assemblage is possessed, and observing whether the content ratio of clay minerals gradually increases from the limestone layer to the adjacent marl layer.

6. The petrographical limestone according to claim 5 The method for identifying a marstone rhythm is characterized by, The color and biological fossil occurrence state from the limestone layer to the adjacent marl layer are observed by hand specimens and thin sections; whether the content ratio of clay minerals gradually increases from the limestone layer to the adjacent marl layer is observed by backscattered electron images obtained by electron probe or scanning electron microscopy.

7. The rock according to claim 1, which is diagenetic limestone The method for identifying a marstone rhythm is characterized by, In step 2, the mineralogical analysis and detection of the powder sample mainly include: whether the content of clay minerals in the limestone layer is lower than that in the adjacent marl layer, and whether the types of clay minerals in the limestone layer and the adjacent marl layer are diagenetic clay minerals.

8. The petrographical limestone according to claim 7 The method for identifying a marstone rhythm is characterized by, The content of clay minerals in the powder sample is analyzed by XRD.

9. The diagenic limestone according to claim 1 The method for identifying a marl rhythm is characterized by, The step 2 includes: whether the slope of the trend line of Al2O3 and TiO2 in the limestone layer, the marl layer and the lithologic transition zone is consistent; and whether the REE+Y rare earth pattern is consistent.

10. The diagenic limestone of claim 1 The method for identifying a marl rhythm is characterized by, In the step 2, the laser ablation inductively coupled plasma mass spectrometer is used to test the in-situ trace elements of the limestone layer, the marl layer and the lithologic transition zone of the second sheet sample, and the number of test points is not less than 5.

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