Stable isotope analysis-based stromatolite and tufa discrimination method and system

Abstract

The application discloses a laminite and tufa discrimination method and system based on stable isotope analysis. The method comprises the following steps: obtaining a standard discrimination chart, wherein the standard discrimination chart is a carbon-oxygen stable isotope crossplot of laminite and tufa; obtaining carbon-oxygen stable isotope analysis results of a rock sample to be discriminated; and discriminating the type of the rock sample based on the carbon-oxygen stable isotope analysis results of the rock sample and the standard discrimination chart. The system comprises a standard discrimination chart acquisition module, a carbon-oxygen stable isotope analysis result acquisition module and a rock sample type discrimination module which realize the above steps one by one. According to the application, the problem that laminite and tufa are difficult to distinguish in the existing microbial carbonate rock exploration and development can be solved.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas geology, and more specifically, relates to a method and system for distinguishing stromatolites and travertine based on stable isotope analysis. Background Technology

[0002] According to classical classification, microbial rocks are divided into four types: stromatolites, tuffaceous rocks, dendritic rocks, and homogeneous rocks. However, travertine is included in the subclass of stromatolites (Riding, 2006), which genetically indicates that travertine also belongs to microbial rocks. Stromatolites, as the most important oil and gas reservoirs, have been found in regions such as the Santos Basin, Campos Basin, Oman Salt Basin, Tarim Basin, Sichuan Basin, and Permian Basin, with strata including the Sinian, Cambrian, Cretaceous, and Permian systems, showing great exploration and development potential. Reports of travertine as an oil and gas reservoir are relatively rare.

[0003] In terms of both macroscopic morphology and microscopic structure, stromatolites and travertine are quite similar. Specifically, both can exhibit carbonate rock uplifts in macroscopic morphology, and both possess typical tree-like or shrub-like structures in microscopic morphology. Therefore, in practical studies, it is easy to mistakenly equate travertine with stromatolites. Because stromatolites and travertine differ significantly in their formation mechanisms, controlling factors, and distribution patterns, correctly distinguishing between them is fundamental to subsequent sedimentological and reservoir geology research, and can provide decision-making guidance for oil and gas exploration and development.

[0004] Currently, there are many research findings specifically on the identification of stromatolites and travertine. Generally speaking, the identification is mainly based on petrological characteristics and distribution patterns.

[0005] The concept of stromatolites has a long history, initially referring primarily to a class of rocks with layered structures, without any genetic significance. Various hypotheses exist regarding the origin of stromatolites, such as annual laminar ... At the microscopic scale, cyanobacteria or gracilis filaments can be identified under a regular microscope (Ahr, 2009), while the single-celled morphology of rock-forming microorganisms is mostly spherical, dumbbell-shaped, filamentous, rod-shaped, or spiral (Luo et al., 2013; Wang Hongmei, 2013). Furthermore, the identification of extracellular polymers as evidence of the presence of microbial carbonate rocks is widely accepted. According to reports, extracellular polymer morphologies include honeycomb-like, microsphere-like, needle-like, and plate-like forms (Folk 2001; Catto, 2016; Warren 2017).

[0006] Travertine refers to non-marine calcium carbonate deposits formed around springs, rivers, lakes, or caves (Liu Haisheng, 2020), corresponding to "tufa" and "travertine," but "tufa" mostly refers to freshwater deposits in karst formations at room temperature, widely distributed in canyons, hillsides, foothills, and river valleys / basins (Yang Hankui, 1986). Because cyanobacteria grow on the surface of these deposits and the sedimentary rocks exhibit cyanobacterial lamellae, freshwater travertine is of microbial origin (Tian Youping, 1998). Hot spring travertine is mainly formed by the rising of hot spring water rich in calcium and bicarbonate ions to the surface, resulting in the release of carbon dioxide and precipitation of calcium carbonate; it is mostly distributed around the hot spring vents. Its main factors include hydrochemistry, hydrodynamics, biological effects, and the sedimentary environment (Liu Haisheng, 2020). In terms of microscopic characteristics, similar to dendritic / shrub-like stromatolites, travertine also has a dendritic / shrub-like structure and is extremely heterogeneous. Previous studies have shown that the porosity of Pleistocene hydrothermal travertine carbonate rocks in Italy ranges from 4% to 30%, with permeability reaching up to several hundred millidarcy (Ronchi, 2015).

[0007] With the surge in research on microbial carbonate rocks, numerous studies on stromatolites have been reported by scholars both domestically and internationally, covering diverse stratigraphic levels. The oldest reservoir is the Sinian Dengying Formation (Chen Yana, 2017), while international studies primarily focus on the Cretaceous (Zhang Demin, 2018; Wang Ying, 2016). In contrast, research on travertine deposits is relatively limited, primarily focusing on Pleistocene sediments, with hydrothermal travertine being more prevalent. However, research distinguishing between the two is still scarce.

[0008] Given the morphological similarities between stromatolites and travertine deposits, but the differences in their formation mechanisms and distribution patterns, it is necessary to distinguish between the two from a new perspective, so as to provide new ideas and methods for the exploration and development of microbial carbonate rocks. Summary of the Invention

[0009] The purpose of this invention is to solve the problem of difficulty in distinguishing between stromatolites and travertine in the existing exploration and development of microbial carbonate rocks.

[0010] To achieve the above objectives, the present invention provides a method and system for distinguishing stromatolites from travertine based on stable isotope analysis.

[0011] According to a first aspect of the present invention, a method for distinguishing stromatolites from travertine based on stable isotope analysis is provided, the method comprising the following steps:

[0012] Obtain a standard discrimination chart, which is a carbon-oxygen stable isotope cross plot of stromatolites and travertine;

[0013] Obtain the carbon and oxygen stable isotope analysis results of the rock sample to be identified;

[0014] The type of the rock sample was determined based on the carbon and oxygen stable isotope analysis results and the standard discrimination chart.

[0015] Preferably, the acquisition of the standard discrimination chart includes:

[0016] Obtain predetermined stromatolite and travertine samples, with each stromatolite and travertine sample taken from a predetermined stratum in its respective predetermined region.

[0017] Obtain the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample;

[0018] The standard discrimination chart was obtained based on the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample.

[0019] Preferably, obtaining the standard discrimination chart based on the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample includes:

[0020] An initial map was constructed with oxygen stable isotopes as the x-axis and carbon stable isotopes as the y-axis.

[0021] The carbon and oxygen stable isotope analysis results of each sample are plotted onto the initial chart to obtain the standard discrimination chart.

[0022] Preferably, the sampling strata for each stromatolite sample and each travertine sample are from the Neogene strata with known depositional environments.

[0023] Preferably, before obtaining the carbon and oxygen stable isotope analysis results of the rock sample to be identified, the method further includes:

[0024] Preliminary sampling: Petrological observation and sampling of the target stratum rock were carried out, and the obtained samples were ground to obtain thin sections;

[0025] The sampling principle in this step is: biological shells are preferred, followed by microcrystalline limestone.

[0026] Preferably, the petrological observations of the target stratum include rock type, sedimentary structure, tectonics, paleontology, diagenetic type, and pore type.

[0027] In the preliminary sampling, the samples obtained were those with relatively low degree of diagenetic alteration and relatively few sedimentary structures, cracks and pores overall.

[0028] Preferably, after the initial sampling, the method further includes:

[0029] Fine sampling: Microscopic petrological analysis and cathodoluminescence observation are performed on the thin section to determine the area to be finely sampled, and the rock sample to be identified is obtained in the area to be finely sampled using a predetermined micro-area sampling tool;

[0030] The area to be finely sampled is a region unaffected by micropores, microcracks, multi-stage diagenesis, and diagenetic heterogeneity.

[0031] Preferably, the process includes the following after the fine sampling:

[0032] Carbon and oxygen stable isotope analysis is performed on the rock sample to be identified: the rock sample is crushed to a predetermined mesh size, and carbon and oxygen stable isotope analysis is performed on the crushed rock sample using a predetermined isotope analysis device.

[0033] Preferably, the determination of the rock sample type based on the carbon and oxygen stable isotope analysis results and the standard discrimination chart includes:

[0034] The carbon and oxygen stable isotope analysis results of the rock samples were plotted onto the standard discrimination chart;

[0035] Based on the carbon and oxygen stable isotope analysis results of the rock sample and the plotted area on the standard discrimination chart, the rock sample is determined to be either stromatolite or travertine by analogy and by comparing the present with the past.

[0036] According to a second aspect of the present invention, a system for distinguishing stromatolites from travertine based on stable isotope analysis is provided, the system comprising the following functional modules:

[0037] The standard discrimination chart acquisition module is used to acquire a standard discrimination chart, which is a carbon-oxygen stable isotope cross plot of stromatolites and travertine.

[0038] The carbon and oxygen stable isotope analysis result acquisition module is used to acquire the carbon and oxygen stable isotope analysis results of the rock sample to be identified.

[0039] The rock sample type discrimination module is used to determine the type of the rock sample based on the carbon and oxygen stable isotope analysis results and the standard discrimination chart.

[0040] The beneficial effects of this invention are as follows:

[0041] The present invention provides a method for distinguishing stromatolites from travertine based on stable isotope analysis. First, a standard discrimination chart is obtained, which is a cross-plot of carbon and oxygen stable isotopes for stromatolites and travertine. Second, the carbon and oxygen stable isotope analysis results of the rock sample to be identified are obtained. Finally, the type of the rock sample is determined based on the carbon and oxygen stable isotope analysis results and the standard discrimination chart, thereby effectively solving the problem of difficulty in distinguishing stromatolites from travertine in existing microbial carbonate rock exploration and development.

[0042] The stromatolite and travertine discrimination system based on stable isotope analysis of the present invention belongs to the same general inventive concept as the above-mentioned stromatolite and travertine discrimination method based on stable isotope analysis, and therefore has the same beneficial effects as the above-mentioned stromatolite and travertine discrimination method based on stable isotope analysis.

[0043] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0044] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments of the invention.

[0045] Figure 1 A flowchart illustrating the implementation of a method for distinguishing stromatolites from travertine based on stable isotope analysis according to Embodiment 1 of the present invention is shown.

[0046] Figure 2 A standard discrimination diagram according to Embodiment 1 of the present invention is shown;

[0047] Figure 3 A carbon and oxygen stable isotope plot of a rock sample from a subsalt study area in Brazil according to Example 1 of the present invention is shown. Detailed Implementation

[0048] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0049] Example 1: Figure 1 A flowchart illustrating the implementation of a method for distinguishing stromatolites from travertine based on stable isotope analysis, according to an embodiment of the present invention, is shown. (Refer to...) Figure 1 The method for distinguishing stromatolites from travertine based on stable isotope analysis according to embodiments of the present invention includes the following steps:

[0050] Obtain a standard discrimination chart, which is a carbon-oxygen stable isotope cross plot of stromatolites and travertine;

[0051] Obtain the carbon and oxygen stable isotope analysis results of the rock sample to be identified;

[0052] The type of the rock sample was determined based on the carbon and oxygen stable isotope analysis results and the standard discrimination chart.

[0053] Furthermore, in this embodiment of the invention, obtaining the standard discrimination chart includes:

[0054] Obtain predetermined stromatolite and travertine samples, with each stromatolite and travertine sample taken from a predetermined stratum in its respective predetermined region.

[0055] Obtain the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample;

[0056] The standard discrimination chart was obtained based on the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample.

[0057] Furthermore, in this embodiment of the invention, obtaining the standard discrimination chart based on the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample includes:

[0058] An initial map was constructed with oxygen stable isotopes as the x-axis and carbon stable isotopes as the y-axis.

[0059] The carbon and oxygen stable isotope analysis results of each sample are plotted onto the initial chart to obtain the standard discrimination chart.

[0060] Furthermore, in this embodiment of the invention, the sampling strata of each stromatolite sample and each travertine sample are all Neogenes with known depositional environments.

[0061] Specifically, in this embodiment of the invention, carbon and oxygen stable isotopes of stromatolite and travertine samples from different regions and strata around the world are analyzed, carbon and oxygen stable isotope cross plots are made, the distribution areas of stromatolite and travertine are identified, and a standard discrimination chart is formed. Figure 2 A standard discrimination diagram according to an embodiment of the present invention is shown. Figure 2 As shown, stromatolites and travertine exhibit significant regionalization. Both freshwater and arid-sea stromatolites show isotopic plots in the first quadrant. Travertine distribution is divided into two categories: freshwater travertine exhibits isotopic characteristics similar to paleosols, with isotopic plots in the third quadrant; while hydrothermal travertine shows isotopic plots in the second quadrant.

[0062] In this embodiment of the invention, when compiling and analyzing the carbon and oxygen stable isotopes of stromatolite and travertine samples from different regions and strata around the world, the focus is on the known sedimentary environments of the Neogene. For carbon and oxygen stable isotope data of samples from different geological periods and rock types, they should be used with caution after verification with Neogene samples.

[0063] In this embodiment of the invention, Figure 2 The data table corresponding to the standard discrimination chart shown is shown in Table 1:

[0064]

[0065]

[0066]

[0067]

[0068]

[0069]

[0070]

[0071]

[0072] Table 1 Standard Discrimination Chart Data Table

[0073] Furthermore, in this embodiment of the invention, before obtaining the carbon and oxygen stable isotope analysis results of the rock sample to be identified, the method further includes:

[0074] Preliminary sampling: Petrological observation and sampling of the target stratum rock were carried out, and the obtained samples were ground to obtain thin sections;

[0075] The sampling principle in this step is: biological shells are preferred, followed by microcrystalline limestone.

[0076] Specifically, in this embodiment of the invention, a larger biological shell portion is preferred. If the target layer contains less biological debris, a microcrystalline limestone portion is preferred.

[0077] Furthermore, in this embodiment of the invention, the petrological observation of the target stratum includes rock type, sedimentary structure, tectonics, paleontology, diagenetic type, and pore type.

[0078] In the preliminary sampling, the samples obtained were those with relatively low degree of diagenetic alteration and relatively few sedimentary structures, cracks and pores overall.

[0079] Specifically, in this embodiment of the invention, the purpose of conducting petrological observations on the target stratum is to select samples that have not undergone diagenetic alteration or have undergone weak diagenetic alteration, and to avoid samples containing sedimentary structures, cracks and pores. It is preferable to select larger biological shell parts, such as bivalves (mainly thick-shelled clams) and brachiopods. When biological debris is lacking, homogeneous microcrystalline calcite samples can be used instead.

[0080] Furthermore, in this embodiment of the invention, after the initial sampling, the method further includes:

[0081] Fine sampling: Microscopic petrological analysis and cathodoluminescence observation are performed on the thin section to determine the area to be finely sampled, and the rock sample to be identified is obtained in the area to be finely sampled using a predetermined micro-area sampling tool;

[0082] The area to be finely sampled is a region unaffected by micropores, microcracks, multi-stage diagenesis, and diagenetic heterogeneity.

[0083] Specifically, in this embodiment of the invention, a dental drill is used as a micro-area sampling tool to perform in-situ sampling of the area to be finely sampled.

[0084] Furthermore, in this embodiment of the invention, after the fine sampling, the method further includes:

[0085] Carbon and oxygen stable isotope analysis is performed on the rock sample to be identified: the rock sample is crushed to a predetermined mesh size, and carbon and oxygen stable isotope analysis is performed on the crushed rock sample using a predetermined isotope analysis device.

[0086] Specifically, in this embodiment of the invention, the rock sample was pulverized to 200 mesh using an agate mortar in the laboratory, and carbon and oxygen stable isotope analysis was carried out using a stable isotope mass spectrometer.

[0087] Furthermore, in this embodiment of the invention, the determination of the type of the rock sample based on the carbon and oxygen stable isotope analysis results and the standard discrimination chart includes:

[0088] The carbon and oxygen stable isotope analysis results of the rock samples were plotted onto the standard discrimination chart;

[0089] Based on the carbon and oxygen stable isotope analysis results of the rock sample and the plotted area on the standard discrimination chart, the rock sample is determined to be either stromatolite or travertine by analogy and by comparing the present with the past.

[0090] Specifically, in this embodiment of the invention, rock samples from the Brazilian pre-salt study area were selected, and the types of the selected rock samples from the Brazilian pre-salt study area were identified. The carbon and oxygen stable isotope data of the rock samples from the Brazilian pre-salt study area are shown in Table 2.

[0091]

[0092]

[0093]

[0094] Table 2. Stable carbon and oxygen isotope data of rock samples from the Brazilian subsalt study area.

[0095] Figure 3 The diagram shows carbon and oxygen stable isotope plotting of rock samples from a subsalt study area in Brazil, according to an embodiment of the present invention. Figure 3 As shown in the figure, the isotopic plotting points of the rock samples from the Brazilian subsalt study area are located in the first quadrant, which is close to that of stromatolites, and they are identified as stromatolites.

[0096] The present invention provides a method for distinguishing stromatolites and travertine based on stable isotope analysis. Addressing the problem of morphological similarity between stromatolites and travertine, particularly the difficulty in distinguishing them from core samples, this method introduces carbon and oxygen stable isotope analysis. By creating a standard discrimination chart, carefully studying rocks, selecting samples unaffected by diagenetic alteration, analyzing their carbon and oxygen stable isotope characteristics, and comparing them with the standard discrimination chart, rapid and effective differentiation between stromatolites and travertine is achieved.

[0097] Example 2: Based on the method for distinguishing stromatolites and travertines based on stable isotope analysis proposed in Example 1, this embodiment of the invention proposes a system for distinguishing stromatolites and travertines based on stable isotope analysis. This system includes the following functional modules:

[0098] The standard discrimination chart acquisition module is used to acquire a standard discrimination chart, which is a carbon-oxygen stable isotope cross plot of stromatolites and travertine.

[0099] The carbon and oxygen stable isotope analysis result acquisition module is used to acquire the carbon and oxygen stable isotope analysis results of the rock sample to be identified.

[0100] The rock sample type discrimination module is used to determine the type of the rock sample based on the carbon and oxygen stable isotope analysis results and the standard discrimination chart.

[0101] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for distinguishing stromatolites from travertine based on stable isotope analysis, characterized in that, include: Obtain a standard discrimination chart, which is a carbon-oxygen stable isotope cross plot of stromatolites and travertine; Obtain the carbon and oxygen stable isotope analysis results of the rock sample to be identified; The type of the rock sample was determined based on the carbon and oxygen stable isotope analysis results and the standard discrimination chart. The acquisition of the standard discrimination chart includes: Obtain predetermined stromatolite and travertine samples, with each stromatolite and travertine sample taken from a predetermined stratum in its respective predetermined region. Obtain the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample; The standard discrimination chart was obtained based on the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample. The acquisition of the standard discrimination chart based on the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample includes: An initial map was constructed with oxygen stable isotopes as the x-axis and carbon stable isotopes as the y-axis. The carbon and oxygen stable isotope analysis results of each sample are plotted onto the initial chart to obtain the standard discrimination chart; The determination of the rock sample type based on the carbon and oxygen stable isotope analysis results and the standard discrimination chart includes: The carbon and oxygen stable isotope analysis results of the rock samples were plotted onto the standard discrimination chart; Based on the carbon and oxygen stable isotope analysis results of the rock sample and the plotted area on the standard discrimination chart, the rock sample is determined to be either stromatolite or travertine by analogy and by comparing the present with the past.

2. The method for distinguishing stromatolites from travertine according to claim 1, characterized in that, The sampling strata for each stromatolite sample and each travertine sample were all from the Neogene strata with known depositional environments.

3. The method for distinguishing stromatolites from travertine according to claim 1, characterized in that, Before obtaining the carbon and oxygen stable isotope analysis results of the rock sample to be identified, the following steps are also included: Preliminary sampling: Petrological observation and sampling of the target stratum rock were carried out, and the obtained samples were ground to obtain thin sections; The sampling principle in this step is: biological shells are preferred, followed by microcrystalline limestone.

4. The method for distinguishing stromatolites from travertine according to claim 3, characterized in that, The petrological observations of the target strata include rock type, sedimentary structure, tectonics, paleontology, diagenetic type, and pore type. In the preliminary sampling, the samples obtained were those with relatively low degree of diagenetic alteration and relatively few sedimentary structures, cracks and pores overall.

5. The method for distinguishing stromatolites from travertine according to claim 3, characterized in that, Following the initial sampling, the process also includes: Fine sampling: Microscopic petrological analysis and cathodoluminescence observation are performed on the thin section to determine the area to be finely sampled, and the rock sample to be identified is obtained in the area to be finely sampled using a predetermined micro-area sampling tool; The area to be finely sampled is a region unaffected by micropores, microcracks, multi-stage diagenesis, and diagenetic heterogeneity.

6. The method for distinguishing stromatolites from travertine according to claim 5, characterized in that, Following the fine sampling, the following is also included: Carbon and oxygen stable isotope analysis is performed on the rock sample to be identified: the rock sample is crushed to a predetermined mesh size, and carbon and oxygen stable isotope analysis is performed on the crushed rock sample using a predetermined isotope analysis device.

7. A system for distinguishing stromatolites and travertine based on stable isotope analysis, characterized in that, include: The standard discrimination chart acquisition module is used to acquire a standard discrimination chart, which is a carbon-oxygen stable isotope cross plot of stromatolites and travertine. The carbon and oxygen stable isotope analysis result acquisition module is used to acquire the carbon and oxygen stable isotope analysis results of the rock sample to be identified. The rock sample type discrimination module is used to determine the type of the rock sample based on the carbon and oxygen stable isotope analysis results and the standard discrimination chart. The acquisition of the standard discrimination chart includes: Obtain predetermined stromatolite and travertine samples, with each stromatolite and travertine sample taken from a predetermined stratum in its respective predetermined region. Obtain the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample; The standard discrimination chart was obtained based on the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample. The acquisition of the standard discrimination chart based on the carbon and oxygen stable isotope analysis results of the stromatolite sample and the travertine sample includes: An initial map was constructed with oxygen stable isotopes as the x-axis and carbon stable isotopes as the y-axis. The carbon and oxygen stable isotope analysis results of each sample are plotted onto the initial chart to obtain the standard discrimination chart; The determination of the rock sample type based on the carbon and oxygen stable isotope analysis results and the standard discrimination chart includes: The carbon and oxygen stable isotope analysis results of the rock samples were plotted onto the standard discrimination chart; Based on the carbon and oxygen stable isotope analysis results of the rock sample and the plotted area on the standard discrimination chart, the rock sample is determined to be either stromatolite or travertine by analogy and by comparing the present with the past.

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