A method for demarcating lithofacies assemblage boundary of continental lake basin shale layer system

Abstract

The application discloses a method for demarcating lithofacies assemblage limits of shale series in a terrestrial lake basin, and belongs to the technical field of exploration and evaluation. The method comprises the following steps: obtaining the lithological characteristics of samples by core observation and analysis on the shale series rock samples; obtaining the total organic carbon content and free hydrocarbon content by organic carbon determination and rock pyrolysis analysis experiments on the shale series rock samples, and determining the organic matter abundance comprehensive designation; determining the sedimentary environment of the samples according to the total organic carbon content and the lithological characteristics of the samples; obtaining a relationship diagram of the shale continuous thickness and the free hydrocarbon content and a relationship diagram of the shale thickness proportion in the formation thickness and the free hydrocarbon content; and establishing the lithofacies assemblage division standard through the above indexes, and performing longitudinal lithofacies assemblage division on the shale series rock samples. The method can effectively distinguish the lithofacies through comprehensive analysis on the lithology, mineral composition, organic carbon content and sedimentary structure.

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 environment of a terrestrial shallow lake basin, 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] Existing facies association division methods often focus on the analysis of sedimentary facies, ignoring the actual needs of shale oil exploration, especially not considering the influence of oil and gas occurrence characteristics, organic matter abundance and hydrocarbon expulsion efficiency. In practical application, the traditional facies division method has the problems of inaccurate division and inability to effectively guide oil and gas resource evaluation. 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:

[0007] Obtaining the lithological characteristics of the sample by core observation and analysis of the shale series rock sample.

[0008] Obtaining the total organic carbon content and free hydrocarbon content by organic carbon determination and rock pyrolysis analysis experiment of the shale series rock sample, and determining the organic matter abundance comprehensive designation.

[0009] Determining the sedimentary environment of the sample according to the total organic carbon content and the lithological characteristics of the sample.

[0010] Obtaining the relationship graph of shale series rock sample shale continuous thickness and free hydrocarbon content by counting the shale continuous thickness.

[0011] Obtaining the relationship graph of shale series rock sample shale thickness proportion and free hydrocarbon content by counting the shale thickness proportion in the formation thickness.

[0012] The division standard of the lithofacies assemblage is established according to the lithological characteristics of the sample, the comprehensive naming of the organic matter abundance, the relationship diagram of the sedimentary environment of the sample, the continuous thickness of the shale and the free hydrocarbon content, and the relationship diagram of the proportion of the shale in the thickness of the stratum and the free hydrocarbon content.

[0013] According to the division standard of the lithofacies assemblage, the longitudinal lithofacies assemblage of the shale series rock sample is divided.

[0014] At present, the common planning scheme is usually based on the lithology to divide the lithofacies assemblage, and the constraint of the change of the sedimentary microfacies and the oil-bearing property on the lithofacies assemblage is not considered, but the final purpose of the continental shale oil reservoir is to find the favorable layer section rich in oil, and the existing lithofacies division scheme is not applicable. Based on the continental lake basin shale series, the present application can more accurately divide the characteristics of different lithofacies assemblages in combination with multiple methods, so as to reflect the relationship between the oil and gas occurrence characteristics, the organic matter abundance and the hydrocarbon expulsion efficiency and the lithofacies assemblage. The present application comprehensively considers the petrological characteristics and the oil-bearing property characteristics, and the division result can directly guide the shale oil exploration and evaluation demand.

[0015] In one preferred embodiment of the present application, the lithological characteristics of the sample are determined based on core observation analysis, and the shale series rock sample is divided into shale, silty mudstone, argillaceous siltstone, siltstone and fine sandstone.

[0016] In one preferred embodiment of the present application, the division result of the comprehensive naming of the organic matter abundance is that: the rock sample belongs to the carbon-containing type when the organic carbon content is less than 0.5%; the rock sample belongs to the low-carbon type when the organic carbon content is 0.5% to less than 1.0%; the rock sample belongs to the medium-carbon type when the organic carbon content is 1.0% to less than 1.5%; and the rock sample belongs to the high-carbon type when the organic carbon content is greater than or equal to 1.5%.

[0017] In one preferred embodiment of the present application, the rock sample is shale when the organic carbon content is greater than or equal to 1.5%, and the sedimentary environment is semi-deep lake facies.

[0018] The rock sample is shale or silty mudstone when the organic carbon content is 1.0% to less than 1.5%, and the sedimentary environment is shallow lake facies.

[0019] The rock sample is silty mudstone, argillaceous siltstone, siltstone or fine sandstone when the organic carbon content is 0.5% to less than 1.0%, and the sedimentary environment is delta front interdistributary bay.

[0020] The rock sample is siltstone and fine sandstone when the organic carbon content is less than 0.5%, and the sedimentary environment is delta front distributary channel.

[0021] In one preferred embodiment of the present application, the envelope surface curve equation is obtained through the relationship diagram of the continuous thickness of the shale and the free hydrocarbon content, and the inflection point of the continuous thickness of the shale is determined through the envelope surface curve equation.

[0022] In a preferred embodiment of the present application, when the continuous thickness of shale at the inflection point is less than 6m, the lithofacies combination of the shale series is interbedded type, interbedded type or sandstone type, and when the continuous thickness of shale at the inflection point is greater than 6m, the lithofacies combination of the shale series is laminated type. Specifically, when the continuous thickness of shale at the inflection point is 2m-6m, the lithofacies combination of the shale series is interbedded type or interbedded type, and when the continuous thickness of shale at the inflection point is less than 2m, the lithofacies combination of the shale series is sandstone type.

[0023] In a preferred embodiment of the present application, the envelope surface curve equation is obtained through the relationship diagram of the proportion of shale in the thickness of the stratum and the content of free hydrocarbon, and the proportion of shale in the thickness of the stratum at the inflection point is determined through the envelope surface curve equation.

[0024] It should be noted that the obtained scatter distribution is different according to different regions, and therefore the envelope surface curve changes. The standard for determining the envelope surface in the present application is to connect the corresponding highest points on each X axis to form a curve, and the corresponding equation can be obtained through excel regression.

[0025] In a preferred embodiment of the present application, the proportion of shale in the thickness of the stratum at the inflection point is less than 0.45%, the shale series is interbedded type lithofacies combination, the proportion of shale in the thickness of the stratum at the inflection point is between 0.45% and 0.65%, the shale series is interbedded type lithofacies combination, and the proportion of shale in the thickness of the stratum at the inflection point is greater than 0.65%, the shale series is laminated type lithofacies combination.

[0026] In a preferred embodiment of the present application, the rock sample of the shale series is a continental lacustrine sediment sample.

[0027] Compared with the prior art, the present application has the following beneficial effects:

[0028] The prior art often only qualitatively divides the lithofacies combination, and does not give accurate division limits. Moreover, the lithofacies of the continental lacustrine shale series changes rapidly, and the existing lithofacies combination division method cannot guide oil and gas exploration, and the practicality is not obvious.

[0029] The present application proposes an improved continental shallow lacustrine basin lithofacies combination division method through comprehensive analysis of lithology, mineral composition, organic carbon content, sedimentary structure and the like, can effectively distinguish different lithofacies combination types, and from the perspective of exploration, the limits are set at the position of the inflection point of oil and gas content, has stronger guiding significance for oil and gas exploration, and can become an evaluation basis for sweet spot sections in shale oil exploration. This method not only overcomes the shortcomings of the prior art, but also provides scientific support for the exploration and development of continental shallow lacustrine basin shale oil. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The figure is a flowchart of the present application.

[0031] Figure 2 For the relationship between the continuous thickness of shale and free hydrocarbon.

[0032] Figure 3 For the relationship between the proportion of shale in the thickness of the formation and free hydrocarbon.

[0033] Figure 4 For the longitudinal lithofacies combination division result of the single well in Example 2. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the present application.

[0035] Because the lithologic heterogeneity of terrestrial lacustrine facies is strong and changes rapidly in the vertical direction, it is necessary to determine the lithofacies combination division standard to provide guidance for the optimization of favorable intervals for shale oil and gas exploration. Therefore, there is an urgent need for a lithofacies combination division method based on the exploration and evaluation requirements of shale oil to provide more accurate resource prediction and exploration guidance.

[0036] Example 1

[0037] The present embodiment provides a method for delineating the lithofacies combination boundary of a terrestrial lacustrine basin shale series, and the specific flow chart is as shown in Figure 1 The specific steps include the following steps:

[0038] Step 1, carry out core observation and description, and name the lithology of the shale series rock sample.

[0039] The lithology naming is determined by fine description based on macroscopic core, which should be divided into shale, silty mudstone, argillaceous siltstone, siltstone and fine sandstone.

[0040] Step 2, carry out geochemical analysis experiment on the shale series rock sample, i.e. total organic carbon determination and rock pyrolysis analysis experiment, to obtain the organic carbon content and free hydrocarbon content; the organic carbon content is denoted as TOC, and the free hydrocarbon is denoted as S1.

[0041] The total organic carbon is tested according to the “Determination of Total Organic Carbon in Sedimentary Rocks” (GB / T 19145-2022).

[0042] The content of free hydrocarbon S1 is obtained by pyrolysis after cryogenic comminution following cryogenic storage: After obtaining the core sample by pressure coring, it is immediately stored at ultra-low temperature and then sent to the corresponding repository for cryogenic storage for a certain period of time; under the condition of liquid nitrogen freezing, the core sample is quickly sectioned and tightly crushed into a powder sample with a mesh size of 200. The core powder sample is placed in a rock pyrolyzer, and the content of free hydrocarbon S1 is obtained by maintaining a constant temperature of 300°C for 3 minutes.

[0043] 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-containing 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.

[0044] Step 3: Analyze the sedimentary environment by combining the core observation description with the organic carbon measurement.

[0045] When determining the sedimentary environment, it should be determined by combining the 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 sample is shale and siltstone mudstone, and the sedimentary environment is shallow lake facies; when 0.5% ≤ TOC < 1.0%, the rock sample is muddy siltstone and siltstone mudstone, and the sedimentary environment is the interdistributary bay of the front delta; when TOC < 0.5%, the rock sample is siltstone and fine sandstone, and the sedimentary environment is the distributary channel of the front delta.

[0046] 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. Establish the lithofacies combination division standard through the inflection point of the envelope curve.

[0047] 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.

[0048] The proportion of mud shale in the formation thickness refers to the ratio of the mudstone thickness to the total mudstone thickness of the entire section. It should be noted that the total mudstone thickness of the entire section refers to the mudstone thickness of the entire formation where the mudstone is located. Figure 1 The oiliness in it refers to the content level of free hydrocarbon in the mud shale.

[0049] 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 diagram, and 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 points, 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 the division standard of lithofacies combination.

[0050] According to Figure 2 and Figure 3 the inflection point position, the lithofacies association can be determined. When the continuous thickness of shale is < 6m, the lithofacies association is interbedded type, interbedded type and sandstone type, when the continuous thickness of shale is > 6m, the lithofacies association is laminated type. When the proportion of shale in the thickness of the formation is less than 0.45%, the lithofacies association is interbedded type, when the proportion of shale in the thickness of the formation is between 0.45% and 0.65%, the lithofacies association is interbedded type, and when the proportion of shale in the thickness of the formation is > 0.65%, the lithofacies association is laminated type.

[0051] Step 5, combined with the above-mentioned division method of lithofacies and lithofacies association, the division standard of lithofacies association of continental lake basin shale series is established. As shown in Table 1.

[0052] Table 1 Division standard of lithofacies association of continental shallow lake basin

[0053]

[0054] Example 2

[0055] Step 1, take the sample of Jurassic formation in Sichuan Basin, carry out core observation and description, and determine the lithology of shale series rock sample, and finally determine the lithofacies of shale, silty shale, argillaceous siltstone, siltstone and fine sandstone according to the lithological characteristics.

[0056] Step 2, total organic carbon determination and rock pyrolysis analysis experiments are carried out for shale series rock samples, and organic carbon content TOC and free hydrocarbon S1 content are obtained; the determination results show that the TOC content of the sample is 0.18% to 6.75%, and the free hydrocarbon content S1 is 0.02mg / g to 6.98mg / g.

[0057] According to the lithology observation and Table 1, the lithology association of the sample is determined to be laminated type association, interbedded type association and interbedded type association.

[0058] Step 3, the continuous thickness of shale and the proportion of shale in the thickness of the formation are counted, and the relationship between the continuous thickness of shale, the proportion of shale in the thickness of the formation and the free hydrocarbon content S1 is established.

[0059] Step 4, combined with the determination of organic carbon, the sedimentary environment is analyzed.

[0060] When determining the sedimentary environment, lithology and TOC should be combined to determine the sedimentary environment. The rock samples in this embodiment are semi-deep lake interbedded type, shallow lake interbedded type and delta front interbedded type.

[0061] Figure 2The relationship between shale continuous thickness and free hydrocarbon S1 content is shown, the envelope surface curve equation is obtained by establishing the envelope surface curve of the scatter points, the shale continuous thickness at the inflection point is determined through the envelope surface curve equation, and the lithofacies association division standard is established.

[0062] Figure 2 The envelope surface curve equation in the formula (1) is y=-0.0041x 3 +0.0834x 2 -0.0424x+2.4047.

[0063] Figure 2 In the formula (1), the inflection point is located at the shale continuous thickness of 6 m, when the shale continuous thickness is < 6 m, the sample is interbedded type and interbedded type, the region exists hydrocarbon expulsion, and the residual hydrocarbon is low; when the shale continuous thickness is > 6 m, the sample is laminated shale, which is beneficial to pure shale type oil and gas enrichment.

[0064] Figure 3 The relationship between the proportion of shale in the formation thickness and the free hydrocarbon content S1 is shown. The envelope surface curve equation is obtained by establishing the envelope surface curve of the scatter points, the proportion of shale in the formation thickness at the inflection point is determined, and the lithology combination is determined.

[0065] Figure 3 The envelope surface curve equation in the formula (2) is y=-859.85x 4 +2057.5x 3 -1784.3x 2 +672.34x-89.828.

[0066] The proportion of shale in the formation thickness at the inflection point is less than 0.45%, the shale is interbedded shale, the proportion of shale in the formation thickness at the inflection point is between 0.45% and 0.65%, the shale is interbedded shale, and the proportion of shale in the formation thickness at the inflection point is > 0.65%, the shale is laminated shale.

[0067] The shale continuous thickness limit value of different combinations is determined.

[0068] Step 5, the longitudinal lithofacies association division is carried out for a single well.

[0069] For the target layer of Lianggao Formation Liang 2 Member, the longitudinal lithofacies association division result is shown in the formula (3), from shallow to deep, it is sandstone type, delta front interbedded type, sandstone type, semi-deep lake interbedded type, sandstone type, shallow lake interbedded type and delta front interbedded type. Figure 4

[0070] ​While the preferred embodiments of the application have been described, additional variations and modifications can be made to these embodiments by those skilled in the art once they have the benefit of the present disclosure without departing from the spirit and scope of the application. Accordingly, it is intended that the appended claims include all such modifications and variations as fall within the scope of the present application.

[0071] It is apparent that those skilled in the art can make various changes and modifications to the application without departing from the spirit and scope of the application. It is therefore intended that the present application cover all such changes and modifications that are within its scope.

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 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. 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 shale formations. 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 into lithofacies assemblages. Among them, the envelope curve equation is obtained by 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 is determined by the envelope curve equation. When the continuous thickness of mudstone and shale at the inflection point is <6m, the lithofacies assemblage is interbedded, interbedded or sandstone. When the continuous thickness of mudstone and shale at the inflection point is >6m, the lithofacies assemblage is lamellar. The envelope curve equation is 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 is determined by the envelope curve equation. 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.

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 1.0% ≤ < 1.5%, the rock sample is shale or silty mudstone, and the depositional environment is shallow lacustrine facies; 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. 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 shale formation rock samples are terrestrial lacustrine sedimentary samples.

Images