Method for delimiting continental lake basin shale bed series lithofacies combination boundary

Abstract

The invention discloses a continental lake basin shale bed series lithofacies combination boundary delimiting method, and belongs to the technical field of exploration and evaluation. The delimiting method comprises the following steps: carrying out rock core observation and analysis on a shale bed series rock sample to obtain lithologic characteristics of the sample; and carrying out organic carbon determination and rock pyrolysis analysis experiment on the shale strata rock sample to obtain the total organic carbon content and the free hydrocarbon content, and determining the abundance of organic matters and comprehensively naming. And determining the sedimentary environment of the sample according to the total organic carbon content and the lithologic characteristics of the sample. And obtaining a relational graph of the continuous thickness of the shale and the free hydrocarbon content and a relational graph of the thickness proportion of the land occupation layer of the shale and the free hydrocarbon content. And establishing a lithofacies combination division standard through the indexes, and carrying out longitudinal lithofacies combination division on the shale bed series rock sample. According to the method, lithofacies can be effectively distinguished through comprehensive analysis of lithology, mineral composition, organic carbon content, sedimentary structure and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of exploration and evaluation, more particularly to a method for demarcating facies association boundaries of shale series in a terrestrial lake basin. BACKGROUND

[0002] Most of the oil resources are preserved in shale. However, due to the dense nanoscale pore structure of shale, most of the generated oil is firmly adsorbed in the rock and cannot flow naturally, which is called finding oil from millstone by geologists.

[0003] As an important part of unconventional oil and gas resources, the exploration and development of shale oil depends on the accurate division of underground facies association. In the terrestrial shallow lake basin environment, the facies of shale series has complex sedimentary characteristics, and accurate division of different facies and their combinations is crucial for evaluating the exploration potential of shale oil.

[0004] The existing facies association division method often focuses on the analysis of sedimentary facies, ignoring the actual needs of shale oil exploration, especially without considering the influence of oil and gas occurrence characteristics, organic matter abundance and hydrocarbon expulsion efficiency. The traditional facies division method has problems such as inaccurate division and inability to effectively guide oil and gas resource evaluation in practical application. SUMMARY

[0005] In view of the above problems, the present application provides a method for demarcating facies association boundaries of shale series in a terrestrial lake basin. The comprehensive analysis of lithology, mineral composition, organic carbon content, and sedimentary structure can effectively distinguish facies.

[0006] The first object of the present application is to provide a method for demarcating facies association boundaries of shale series in a terrestrial lake basin, comprising the following steps: Core observation and analysis of shale series rock samples are performed to obtain the lithological characteristics of the samples.

[0007] Organic carbon determination and rock pyrolysis analysis experiments are performed on the shale series rock samples to obtain total organic carbon content and free hydrocarbon content, and determine the organic matter abundance comprehensive designation.

[0008] The sedimentary environment of the sample is determined according to the total organic carbon content and the lithological characteristics of the sample.

[0009] The relationship between the continuous thickness of mudstone and shale and the free hydrocarbon content is obtained by statistically analyzing the continuous thickness of mudstone and shale of the shale series rock samples.

[0010] The relationship between the proportion of mudstone and shale in the thickness of the stratum and the free hydrocarbon content is obtained by statistically analyzing the proportion of mudstone and shale in the thickness of the stratum of the shale series rock samples.

[0011] A lithofacies classification standard was established by comprehensively identifying the lithological characteristics and organic matter abundance of the samples, the sedimentary environment of the samples, the relationship between the continuous thickness of mudstone and shale and the free hydrocarbon content, and the relationship between the proportion of mudstone and shale in the stratigraphic thickness and the free hydrocarbon content.

[0012] Based on the lithofacies assemblage classification criteria, the rock samples of the shale strata were classified vertically according to lithofacies assemblage.

[0013] Current conventional classification schemes often rely on lithology to delineate lithofacies assemblages, neglecting the constraints imposed by sedimentary microfacies and oil-bearing variations. However, the ultimate goal of continental shale oil reservoirs is to identify oil-rich, favorable intervals, rendering existing lithofacies classification schemes inapplicable. This invention, based on continental lacustrine basin shale formations and incorporating multiple methods, can more accurately classify different lithofacies assemblages, reflecting the relationship between lithofacies assemblages and factors such as hydrocarbon occurrence characteristics, organic matter abundance, and hydrocarbon expulsion efficiency. This invention comprehensively considers petrological and oil-bearing characteristics, and the classification results can directly guide shale oil exploration and evaluation needs.

[0014] In a preferred embodiment of the present invention, the lithological characteristics of the samples are determined based on core observation and analysis, and the shale strata rock samples are divided into shale, silty mudstone, argillaceous siltstone, siltstone and fine sandstone.

[0015] In a preferred embodiment of the present invention, the classification results of the comprehensive naming of organic matter abundance are as follows: if the organic carbon content is <0.5%, the rock sample belongs to the carbon-containing type; if the organic carbon content is 0.5% ≤ organic carbon content <1.0%, the rock sample belongs to the low-carbon type; if the organic carbon content is 1.0% ≤ organic carbon content <1.5%, the rock sample belongs to the medium-carbon type; and if the organic carbon content is ≥1.5%, the rock sample belongs to the high-carbon type.

[0016] In a preferred embodiment of the present invention, the organic carbon content is ≥1.5%, the rock sample is shale, and the depositional environment is a semi-deep lacustrine facies.

[0017] The organic carbon content is 1.0% ≤ < 1.5%, the rock sample is shale or silty mudstone, and the depositional environment is shallow lacustrine.

[0018] The organic carbon content is 0.5% ≤ < 1.0%, and the rock samples are silty mudstone, argillaceous siltstone, siltstone or fine sandstone. The depositional environment is an interdistributary bay at the delta front.

[0019] The organic carbon content is <0.5%, the rock samples are siltstone and fine sandstone, and the depositional environment is a delta front distributary channel.

[0020] In a preferred embodiment of the present invention, the envelope curve equation is obtained by the relationship between the continuous thickness of mudstone and shale and the free hydrocarbon content, and the continuous thickness of mudstone and shale at the inflection point is determined by the envelope curve equation.

[0021] In a preferred embodiment of the present invention, when the continuous thickness of mudstone and shale at the inflection point is <6m, the lithological assemblage of the shale strata is interbedded, interbedded, or sandstone; when the continuous thickness of mudstone and shale at the inflection point is >6m, the lithological assemblage of the shale strata is lamellar. Specifically, when the continuous thickness of mudstone and shale at the inflection point is 2m to 6m, the lithological assemblage of the shale strata is interbedded or interbedded; when the continuous thickness of mudstone and shale at the inflection point is <2m, the lithological assemblage of the shale strata is sandstone.

[0022] In a preferred embodiment of the present invention, the envelope curve equation is obtained by using the relationship between the thickness ratio of mudstone and shale in the strata and the free hydrocarbon content, and the thickness ratio of mudstone and shale in the strata at the inflection point is determined by the envelope curve equation.

[0023] It should be noted that the distribution of scatter points obtained varies depending on the region, thus altering the envelope curve. The standard for determining the envelope in this invention is to connect the corresponding highest points on each X-axis, forming a curve, and then use Excel regression to obtain the corresponding equation.

[0024] In a preferred embodiment of the present invention, the proportion of mudstone and shale occupying the stratum thickness at the inflection point is less than 0.45%, and the shale strata are interbedded lithofacies; the proportion of mudstone and shale occupying the stratum thickness at the inflection point is between 0.45% and 0.65%, and the shale strata are interbedded lithofacies; and the proportion of mudstone and shale occupying the stratum thickness at the inflection point is greater than 0.65%, and the shale strata are laminated lithofacies.

[0025] In a preferred embodiment of the present invention, the shale strata rock sample is a terrestrial lacustrine sedimentary sample.

[0026] Compared with the prior art, the present invention has the following beneficial effects: Existing technologies often only provide qualitative classifications of lithofacies assemblages without giving precise boundaries; moreover, lithofacies changes rapidly in terrestrial lacustrine shale formations, and existing lithofacies classification methods cannot guide oil and gas exploration, thus their practicality is not obvious.

[0027] This invention proposes an improved method for classifying lithofacies assemblages in terrestrial shallow lacustrine basins through comprehensive analysis of lithology, mineral composition, organic carbon content, and sedimentary structures. This method effectively distinguishes different lithofacies assemblages and, from an exploration perspective, sets the boundary at the location of hydrocarbon-bearing inflection points, providing stronger guidance for oil and gas exploration and serving as an evaluation basis for sweet spots in shale oil exploration. This method not only overcomes the shortcomings of existing technologies but also provides scientific support for the exploration and development of shale oil in terrestrial shallow lacustrine basins. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the process of the present invention.

[0029] Figure 2The relationship between the continuous thickness of mudstone and shale and free hydrocarbons.

[0030] Figure 3 The relationship between the thickness ratio of mudstone and shale formations and free hydrocarbons.

[0031] Figure 4 The results show the vertical lithofacies assemblage classification of a single well in Example 2. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Due to the strong heterogeneity and rapid vertical variation of terrestrial and lacustrine lithologies, it is necessary to determine lithofacies classification criteria to guide the selection of favorable strata in shale oil and gas exploration. Therefore, there is an urgent need for a lithofacies classification method based on the needs of shale oil exploration and evaluation to provide more accurate resource prediction and exploration guidance.

[0034] Example 1 This embodiment provides a method for delineating the lithofacies boundaries of terrestrial lacustrine basin shale strata, as shown in the flowchart below. Figure 1 As shown, the specific steps include: Step 1: Conduct core observation and description, and name the lithology of rock samples from the shale strata.

[0035] The lithological classification is determined based on a detailed description of the macroscopic core and should be divided into shale, silty mudstone, argillaceous siltstone, siltstone, and fine sandstone.

[0036] Step 2: Conduct geochemical analysis experiments on shale strata rock samples, namely, perform total organic carbon determination and rock pyrolysis analysis experiments to obtain organic carbon content and free hydrocarbon content; organic carbon content is denoted as TOC, and free hydrocarbons are denoted as S1.

[0037] Total organic carbon was tested according to the standard "Determination of Total Organic Carbon in Sedimentary Rocks" (GB / T 19145-2022).

[0038] The content of free hydrocarbon S1 was obtained by freezing and sealing followed by low-temperature crushing and pyrolysis: After obtaining the core sample by pressure-controlled core sampling, it was immediately sealed at ultra-low temperature and then sent to the corresponding warehouse for freezing and storage for a certain period of time; the core sample was rapidly cut and crushed into 200-mesh powder under liquid nitrogen freezing conditions, and the core powder sample was placed in a rock pyrolysis instrument and kept at 300℃ for 3 minutes to obtain the content of free hydrocarbon S1.

[0039] For the comprehensive naming of organic matter abundance, the TOC measurement results should be combined: when TOC < 0.5%, the rock sample belongs to the carbon-bearing type; when 0.5% ≤ TOC < 1.0%, the rock sample belongs to the low-carbon type; when 1.0% < TOC < 1.5%, the rock sample belongs to the medium-carbon type; when TOC ≥ 1.5%, the rock sample belongs to the high-carbon type.

[0040] Step 3: Analyze the sedimentary environment by combining core observation and description with organic carbon determination.

[0041] When determining the sedimentary environment, it should be determined by combining lithology and TOC: when TOC ≥ 1.5%, the rock sample is shale and the sedimentary environment is semi-deep lake facies; when 1.0% ≤ TOC < 1.5%, the rock samples are shale and silty mudstone and the sedimentary environment is shallow lake facies; when 0.5% ≤ TOC < 1.0%, the rock samples are muddy siltstone and silty mudstone and the sedimentary environment is interdistributary bay of the front delta; when TOC < 0.5%, the rock samples are siltstone and fine sandstone and the sedimentary environment is distributary channel of the front delta.

[0042] Step 4: Statistically calculate the continuous thickness of mud shale and the proportion of mud shale in the formation thickness, establish a relationship diagram between the continuous thickness of mud shale and the content of free hydrocarbon S1, and a relationship diagram between the proportion of mud shale in the formation thickness and the content of free hydrocarbon S1, and establish a lithofacies combination division standard through the inflection points of the envelope curve.

[0043] The continuous thickness means that there is no sandstone interlayer with a thickness greater than 0.5 m in the middle of the mud shale.

[0044] The proportion of mud shale in the formation thickness refers to the ratio of the thickness of mudstone to the thickness of the whole mudstone. It should be noted that the thickness of the whole mudstone refers to the thickness of the mudstone in the whole formation where the mudstone is located. Figure 1 The oiliness in it refers to the content level of free hydrocarbon in the mud shale.

[0045] Statistically calculate the continuous thickness of mud shale and the proportion of mud shale in the formation thickness in a certain depth interval of the formation, match the depth of the interval with the actual core test results, draw a scatter plot, respectively obtain the relationship between the continuous thickness of mud shale and the content of free hydrocarbon S1, and the relationship between the proportion of mud shale in the formation thickness and the content of free hydrocarbon S1, use a curve to draw the envelope curve of the scatter plot, determine the inflection points of the continuous thickness of mud shale, the proportion of mud shale in the formation thickness and the content of free hydrocarbon S1, so as to establish a division standard for lithofacies combination.

[0046] According to Figure 2 and Figure 3The inflection point can determine the lithofacies assemblage. When the continuous thickness of mudstone and shale is <6m, the lithofacies assemblage is interbedded, interbedded, or sandstone-type; when the continuous thickness of mudstone and shale is >6m, the lithofacies assemblage is laminar. When the proportion of mudstone and shale in the strata thickness is less than 0.45%, the lithofacies assemblage is interbedded; when the proportion of mudstone and shale in the strata thickness is between 0.45% and 0.65%, the lithofacies assemblage is interbedded; and when the proportion of mudstone and shale in the strata thickness is >0.65%, the lithofacies assemblage is laminar.

[0047] Step 5: Combining the above methods for classifying lithofacies and lithofacies assemblages, establish the lithofacies assemblages classification criteria for continental lacustrine basin shale strata. See Table 1.

[0048] Table 1. Criteria for classifying lithofacies assemblages in terrestrial shallow lacustrine basins

[0049] Example 2 Step 1: Take samples of the Jurassic strata in the Sichuan Basin, conduct core observation and description, and name the rock samples of the shale strata according to their lithology. Finally, based on the lithological characteristics, determine that the lithofacies are shale, silty shale, argillaceous siltstone, siltstone and fine sandstone.

[0050] Step 2: Total organic carbon (TOC) and rock pyrolysis analysis were conducted on shale strata rock samples to obtain the TOC content and free hydrocarbon (S1) content. The results showed that the TOC content of the samples ranged from 0.18% to 6.75%, and the free hydrocarbon (S1) content ranged from 0.02 mg / g to 6.98 mg / g.

[0051] Based on lithological observations and Table 1, the lithological assemblages of the samples were determined to be lamellar, interbedded, and alternating assemblages.

[0052] Step 3: Calculate the continuous thickness of mudstone and shale and the proportion of mudstone and shale to the thickness of the strata, and establish the relationship between the continuous thickness of mudstone and shale, the proportion of mudstone and shale to the thickness of the strata, and the free hydrocarbon content S1.

[0053] Step 4: Combine organic carbon determination to analyze the sedimentation environment.

[0054] When determining the sedimentary environment, lithology and TOC should be considered. In this example, the rock samples are semi-deep lacustrine interbedded type, shallow lacustrine interbedded type, and delta front interbedded type.

[0055] Figure 2 The relationship between the continuous thickness of mudstone and shale and the content of free hydrocarbon S1 was shown. By establishing the envelope curve of scattered points, the envelope curve equation was obtained. The continuous thickness of shale at the inflection point was determined by the envelope curve equation, thereby establishing the lithofacies classification standard.

[0056] Figure 2The equation of the envelope curve in the figure is y = -0.0041x. 3 +0.0834x 2 -0.0424x+2.4047.

[0057] Figure 2 In the middle, the inflection point is located at a continuous thickness of 6m for mudstone and shale. When the continuous thickness of mudstone and shale is <6m, the sample is interlayered and interbedded, and there is hydrocarbon expulsion in this area with low residual hydrocarbons. When the continuous thickness of mudstone and shale is >6m, the sample is lamellar shale, which is conducive to the enrichment of pure shale-type oil and gas.

[0058] Figure 3 The relationship between the proportion of shale and mudstone in the strata thickness and the free hydrocarbon content S1 is shown. By establishing the envelope curve of the scattered points, the equation of the envelope curve can be obtained, which can determine the proportion of shale and mudstone in the strata thickness at the inflection point, thereby determining the lithological assemblage.

[0059] Figure 3 The equation of the envelope curve in the figure is y = -859.85x. 4 +2057.5x 3 -1784.3x 2 +672.34x-89.828.

[0060] At the inflection point, if the proportion of mudstone and shale in the strata thickness is less than 0.45%, the mudstone and shale are interbedded shale; if the proportion of mudstone and shale in the strata thickness is between 0.45% and 0.65%, the mudstone and shale are interlayered shale; and if the proportion of mudstone and shale in the strata thickness is greater than 0.65%, the mudstone and shale are laminated shale.

[0061] This clarifies the limit values ​​for the continuous thickness of mudstone and shale with different combinations.

[0062] Step 5: Perform vertical lithofacies assemblages for each well.

[0063] For the target layer of the Lianggaoshan Formation, Liang 2nd Member, the vertical lithofacies assemblage division results are as follows: Figure 4 As shown, from shallow to deep, the types are sandstone type, delta front interbedded type, sandstone type, semi-deep lacustrine interbedded type, sandstone type, shallow lacustrine interbedded type, and delta front interbedded type.

[0064] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0065] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for delineating the lithofacies boundaries of terrestrial lacustrine basin shale strata, characterized in that, Includes the following steps: The lithological characteristics of shale strata rock samples were obtained through core observation and analysis. Organic carbon and rock pyrolysis analysis experiments were conducted on shale strata rock samples to obtain total organic carbon content and free hydrocarbon content, and to determine the comprehensive name of organic matter abundance. The sedimentary environment of the sample was determined based on the total organic carbon content and the lithological characteristics of the sample. A graph showing the relationship between the continuous thickness of mudstone and shale and the free hydrocarbon content was obtained by statistically analyzing the continuous thickness of mudstone and shale in rock samples of the shale strata. The relationship between the proportion of mudstone and shale in the thickness of rock samples of shale formation and the content of free hydrocarbons was obtained by statistical analysis. The lithofacies classification criteria were established by comprehensively identifying the lithological characteristics and organic matter abundance of the samples, the sedimentary environment of the samples, the relationship between the continuous thickness of mudstone and shale and the free hydrocarbon content, and the relationship between the proportion of mudstone and shale in the stratigraphic thickness and the free hydrocarbon content. Based on the lithofacies assemblage classification criteria, the rock samples of the shale strata were classified vertically according to lithofacies assemblage.

2. The method for delineating the lithofacies boundaries of terrestrial lacustrine basin shale strata according to claim 1, characterized in that, The lithological characteristics of the samples were determined based on core observation and analysis. The shale strata rock samples were divided into shale, silty mudstone, argillaceous siltstone, siltstone and fine sandstone.

3. The method for delineating the lithofacies boundaries of terrestrial lacustrine basin shale strata according to claim 1, characterized in that, The comprehensive classification results of organic matter abundance are as follows: organic carbon content <0.5%, shale rock samples belong to carbon-containing type; 0.5%≤organic carbon content<1.0%, shale rock samples belong to low-carbon type; 1.0%≤organic carbon content<1.5%, shale rock samples belong to medium-carbon type; organic carbon content ≥1.5%, shale rock samples belong to high-carbon type.

4. The method for delineating the lithofacies boundaries of terrestrial lacustrine basin shale strata according to claim 3, characterized in that, The organic carbon content is ≥1.5%, the rock sample is shale, and the depositional environment is semi-deep lacustrine. The organic carbon content is 0% ≤ < 1.5%, the rock sample is shale or silty mudstone, and the depositional environment is shallow lacustrine facies; The organic carbon content is 5% ≤ < 1.0%, and the rock samples are silty mudstone, silty mudstone, siltstone or fine sandstone. The depositional environment is an interdistributary bay at the delta front. The organic carbon content is <0.5%, the rock samples are siltstone and fine sandstone, and the depositional environment is a delta front distributary channel.

5. The method for delineating the lithofacies boundaries of terrestrial lacustrine basin shale strata according to claim 1, characterized in that, The envelope curve equation was obtained by plotting the relationship between the continuous thickness of mudstone and shale and the free hydrocarbon content. The continuous thickness of mudstone and shale at the inflection point was then determined by the envelope curve equation.

6. The method for delineating the lithofacies boundaries of terrestrial lacustrine basin shale strata according to claim 5, characterized in that, When the continuous thickness of mudstone and shale at the inflection point is less than 6m, the lithofacies assemblage is interbedded, interbedded, or sandstone. When the continuous thickness of mudstone and shale at the inflection point is greater than 6m, the lithofacies assemblage is laminated.

7. The method for delineating the lithofacies boundaries of terrestrial lacustrine basin shale strata according to claim 1, characterized in that, The envelope curve equation was obtained by plotting the relationship between the thickness ratio of mudstone and shale in the strata and the free hydrocarbon content. The thickness ratio of mudstone and shale in the strata at the inflection point was then determined by the envelope curve equation.

8. The method for delineating the lithofacies boundaries of terrestrial lacustrine basin shale strata according to claim 7, characterized in that, At the inflection point, the proportion of mudstone and shale in the strata thickness is less than 0.45%, indicating an interbedded lithofacies assemblage; at the inflection point, the proportion of mudstone and shale in the strata thickness is between 0.45% and 0.65%, indicating an interbedded lithofacies assemblage; and at the inflection point, the proportion of mudstone and shale in the strata thickness is greater than 0.65%, indicating a laminated lithofacies assemblage.

9. The method for delineating the lithofacies boundaries of terrestrial lacustrine basin shale strata according to claim 1, characterized in that, The shale formation rock samples are terrestrial lacustrine sedimentary samples.

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