Quantitative evaluation method for contribution of coal series hydrocarbon source rock exhaust to gas reservoir under lifting denudation background

Abstract

The invention provides a quantitative evaluation method for contribution of coal-series hydrocarbon source rock exhaust to a gas reservoir under a lifting denudation background, and the quantitative evaluation method comprises the following steps: measuring the methane content and the methane carbon isotope ratio of the coal-series hydrocarbon source rock exhaust; respectively determining the methane content and the methane carbon isotope ratio of the original gas reservoir and the current gas reservoir of the target gas reservoir; and according to the balance principle of methane carbon isotopes in the gas reservoir, establishing a calculation formula of the contribution of the coal hydrocarbon source rock exhaust to the gas reservoir based on the methane content and the carbon isotope mixing ratio, and calculating to obtain the contribution ratio of the hydrocarbon source rock exhaust to the gas reservoir. According to the quantitative evaluation method provided by the invention, the contribution proportion of coal series hydrocarbon source rock exhaust to the gas reservoir under the lifting denudation background is solved based on a material balance principle, quantitative evaluation of gas reservoir contribution is realized, scientific exhaust parameters are provided for large-scale gas gathering basin resource evaluation under the lifting denudation effect, and the method is suitable for large-scale gas gathering basin resource evaluation. The method has important application value in natural gas geology research and exploration.

Description

Technical Field

[0001] This invention belongs to the field of geological exploration technology and relates to an evaluation method for the contribution of coal-bearing source rocks to natural gas reservoirs, and more particularly to a quantitative evaluation method for the contribution of gas expulsion from coal-bearing source rocks to gas reservoirs under uplift and erosion background. Background Technology

[0002] Coal-bearing source rocks are a general term for coal, carbonaceous mudstone, and coal-bearing mudstone in coal-bearing strata that are capable of generating oil and gas. They mainly form in marine-continental transitional or swampy environments. Currently, natural gas in gas reservoirs is mainly formed by a mixture of two parts: gas released from the coal-bearing source rocks during subsidence and gas released after uplift and erosion. Most gas-bearing basins have undergone late-stage uplift and erosion, which has a significant impact on the formation, adjustment, and preservation of gas reservoirs. Among these factors, the release of gas from the source rocks is a crucial influence on whether a gas reservoir can be maintained.

[0003] Currently, there are two main methods for studying the gas exhaust efficiency of coal-bearing source rocks in gas-bearing basins under uplift and erosion backgrounds: one is the gas expansion force method. Under uplift and erosion, the formation pressure decreases significantly, leading to an increase in gas expansion force in the coal-bearing source rocks, which can cause gas exhaust. Li Jun et al. revealed that the volume expansion of natural gas in tight strata uplift environments triggers a series of geological effects, and believed that in the Sulige gas field of the Ordos Basin, the uplift stage triggered the gas exhaust effect of coal-bearing source rocks ("Natural Gas Expansion and Its Accumulation Effect Caused by Formation Uplift: A Case Study of the Sulige Gas Field in the Ordos Basin", Petroleum Exploration and Development, 2022, 49). 6): 1094-1106); Another method is to evaluate whether source rocks can expel gas under the background of uplift and erosion through physical simulation experiments. Zhao Wenzhi et al. proposed based on the results of physical simulation experiments that there were important desorption and hydrocarbon expulsion events in the coal-bearing source rocks of the Triassic Xujiahe Formation in the Sichuan Basin during the uplift process since the end of the Cretaceous ("Large-scale accumulation mechanism and enrichment conditions of natural gas reservoirs in Xujiahe Formation in Central Sichuan", Petroleum Exploration and Development, 2010, 37(2): 146-157). However, both of these methods are qualitative studies on the expulsion of source rocks. At present, there is a lack of reports on quantitative evaluation methods and results of expulsion of coal-bearing source rocks. Summary of the Invention

[0004] The purpose of this invention is to provide a quantitative evaluation method for the contribution of gas exhaust from coal-bearing source rocks to gas reservoirs under uplift and erosion background, solve the problem of quantitative evaluation of the contribution of gas exhaust from source rocks to gas reservoirs, and provide scientific parameters for the evaluation of natural gas resources in gas-bearing basins.

[0005] To achieve this objective, the present invention adopts the following technical solution:

[0006] This invention provides a quantitative evaluation method for the contribution of gas outgassing from coal-bearing source rocks to gas reservoirs under uplift and erosion backgrounds. The quantitative evaluation method includes the following steps:

[0007] (1) Determine the methane content and methane carbon isotope ratio of the gas discharged from the source rock, and determine the methane content and methane carbon isotope ratio of the original gas reservoir and the current gas reservoir of the target gas reservoir respectively.

[0008] (2) Based on the principle of methane carbon isotope balance in gas reservoirs, a calculation formula is established for the contribution of source rock exhaust to gas reservoir under the uplift and erosion background based on methane content and carbon isotope mixing ratio. The contribution ratio of coal-bearing source rock exhaust to gas reservoir under the uplift and erosion background is calculated.

[0009] The natural gas in current gas reservoirs is mainly formed by the mixture of two parts: gas released from the source rocks during the subsidence process and gas released from the coal-bearing source rocks after uplift and erosion. Gas reservoirs formed by gas release and filling during the subsidence process of source rocks are called primitive gas reservoirs.

[0010] The quantitative evaluation method provided by this invention has discovered a corresponding relationship between methane content and carbon isotope ratio in gas reservoirs and gas exhaust from coal-bearing source rocks under uplift and erosion background. These parameters can be used as tracer parameters for gas exhaust. Based on the principle of material balance, and according to the relationship and balance principle between methane content and carbon isotope ratio of gas exhaust from source rocks after uplift and erosion, the original gas reservoir, and the current gas reservoir, the contribution ratio of gas exhaust from coal-bearing source rocks to the gas reservoir under uplift and erosion background is calculated, thereby achieving a quantitative evaluation of the gas reservoir contribution. This provides scientific gas exhaust parameters for the resource evaluation of gas-gathering basins under large-scale uplift and erosion, improving the authenticity, reliability, and effectiveness of natural gas resource evaluation, and has important application value in natural gas geology research and exploration.

[0011] Preferably, step (1) of determining the methane content and methane carbon isotope ratio of the original gas reservoir includes:

[0012] (a) Determining source rocks using gas source comparison methods;

[0013] (b) Determine the maturity of the source rock;

[0014] (c) Determine the relationship model between the methane carbon isotope composition of the gas reservoir area and the maturity of source rock samples;

[0015] (d) Determine the methane content and methane carbon isotope ratio of the original gas reservoir based on the aforementioned relationship model.

[0016] Preferably, the gas source comparison method in step (a) includes: determining the number of source rock layers and geochemical characteristics of each set of source rocks in the gas reservoir distribution area, and determining the source rocks of the natural gas in the gas reservoir area based on the comparison of the natural gas composition and isotopes of the gas reservoir with the generated gas of each set of source rocks.

[0017] Preferably, the maturity of the source rock sample in step (b) is determined by vitrinite reflectance.

[0018] Preferably, the relationship model described in step (c) is a relationship model between the carbon isotopes or composition of methane in continuously settled coalbed methane and the reflectance of vitrinite.

[0019] Preferably, the relational model is as follows:

[0020] M = 1 - e (Ro-1.908) / 0.419

[0021] Among them, R o Where is the vitrinite reflectance, and M is the methane content;

[0022] δ 13 C1 = 14.12logR o -34.39

[0023] Where, δ 13 C1 represents the carbon isotope ratio of methane in the residual gas of the source rock.

[0024] Preferably, the method for determining the methane content and methane carbon isotope ratio of the existing gas reservoir in step (1) includes: measuring the natural gas sample of the gas reservoir using chromatography and chromatography-isotope mass spectrometry.

[0025] Preferably, the calculation formula in step (2) is as follows:

[0026]

[0027] Where X represents the proportion of source rock exhaust gas to the gas reservoir, and δ 13 C1 represents the carbon isotope ratio of methane in the residual gas from the source rock, and C1 represents the methane content in the gaseous components of the residual gas from the source rock. δ 13 C2 represents the carbon isotope ratio of methane in the original gas reservoir, C2 represents the methane content in the gaseous components of the original gas reservoir, and δ 13 C3 represents the carbon isotope ratio of methane in the current gas reservoir, and C3 also represents the methane content in the gaseous components of the current gas reservoir.

[0028] Preferably, the quantitative evaluation method further includes: geological background analysis and reservoir formation geological condition analysis of the target gas reservoir area.

[0029] In this invention, the purpose of geological background analysis and reservoir geological condition analysis is to determine whether the gas reservoir area has undergone large-scale uplift and erosion and to provide a basis for determining the relationship model between methane carbon isotopes or composition and vitrinite reflectance.

[0030] Preferably, the geological background analysis includes regional geological background, basin evolution, stratigraphic development, and tectonic features.

[0031] Preferably, the analysis of reservoir geological conditions includes the combination characteristics of source, reservoir and caprock, trap type, hydrocarbon migration and accumulation process, and the temporal and spatial configuration relationship between hydrocarbon migration and accumulation process and gas reservoir.

[0032] Preferably, the method for determining the methane content in step (1) includes chromatography.

[0033] Preferably, the method for determining the carbon isotope ratio of methane in step (1) includes chromatography-isotope mass spectrometry.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The quantitative evaluation method provided by this invention is based on the principle of material balance. It calculates the contribution ratio of gas exhaust from coal-bearing source rocks to gas reservoirs under the background of uplift and erosion, thereby achieving a quantitative evaluation of the gas reservoir contribution. This provides scientific exhaust parameters for the resource evaluation of gas-gathering basins under large-scale uplift and erosion, and improves the authenticity, reliability and effectiveness of natural gas resource evaluation. Attached Figure Description

[0036] Figure 1 This is a flowchart of a method for quantitatively evaluating the contribution of coal-bearing source rock exhaust to gas reservoirs under uplift and erosion background, as provided in Example 1. Detailed Implementation

[0037] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0038] Example 1

[0039] This embodiment provides a method such as Figure 1 The quantitative evaluation method for the contribution of source rock gas degassing to gas reservoirs under uplift and erosion backgrounds, as shown, includes the following steps:

[0040] (1) Conduct geological background analysis and reservoir formation geological condition analysis of the target gas reservoir area to be evaluated.

[0041] The purpose of the regional geological background analysis is mainly to determine whether the gas reservoir area has undergone large-scale uplift and erosion. The analysis of reservoir geological conditions is mainly to determine the number and type of source rock layers and the properties of gas reservoir fluids, so as to provide a basis for determining the relationship model between methane carbon isotopes or composition and vitrinite reflectance.

[0042] (2) Determine the methane content and methane carbon isotope ratio of the gas discharged from the coal-bearing source rock under the background of uplift and erosion.

[0043] Residual gas samples from source rocks were collected using on-site analysis. The composition of the residual gas samples was determined by chromatography to identify the relative percentage of methane in the residual gas sample components. The carbon isotope ratio of methane in the residual gas samples was determined by chromatography-isotope mass spectrometry.

[0044] (3) Determine the methane content and methane carbon isotope ratio of the original gas reservoir of the target gas reservoir.

[0045] (3.1) The source rocks were determined by gas source comparison method.

[0046] The specific process of the gas source comparison method is as follows: determine the number of source rock layers and geochemical characteristics of each set of source rocks in the gas reservoir distribution area, and determine the source of natural gas in the gas reservoir area, i.e., the source rocks, by comparing the composition and isotopes of natural gas in the gas reservoir with the generated gas of each set of source rocks.

[0047] (3.2) Determine the maturity of the source rock, wherein the maturity is measured by vitrinite reflectance.

[0048] The method for determining the vitrinite reflectance of source rocks is as follows: observe the source rock sample under a microscope to find vitrinite, measure the vitrinite reflectance using a microphotometer, and calculate the average vitrinite reflectance based on multiple measured vitrinite reflectances, which is the maturity of the source rock.

[0049] (3.3) Determine the relationship model between the methane carbon isotope composition of the gas reservoir area and the vitrinite reflectance of the source rock samples. The relationship model is a model of the relationship between the methane carbon isotope composition or composition of continuously settled coal-derived gas and the vitrinite reflectance.

[0050] The specific relational model is as follows:

[0051] M = 1 - e (Ro-1.908) / 0.419

[0052] Among them, R o Where is the vitrinite reflectance, and M is the methane content;

[0053] δ 13 C1 = 14.12logR o -34.39

[0054] Where, δ 13 C1 represents the carbon isotope ratio of methane in the residual gas of the source rock.

[0055] (3.4) Based on the relational model, determine the methane content and methane carbon isotope ratio of the original gas reservoir.

[0056] (4) Determine the methane content and methane carbon isotope ratio of the existing gas reservoir of the target gas reservoir.

[0057] Natural gas samples were collected from the target gas reservoir. The composition of the natural gas samples was determined by chromatography, the relative percentage of methane in the natural gas sample components was determined, and the carbon isotope ratio of methane in the natural gas samples was determined by chromatography-isotope mass spectrometry.

[0058] (5) Calculate the contribution of gas exhaust from coal-bearing source rocks to gas reservoirs under the background of uplift and erosion.

[0059] Based on the principle of methane carbon isotope balance in gas reservoirs, the calculation formula for the contribution of source rock exhaust to the gas reservoir is as follows:

[0060]

[0061] Where X represents the proportion of source rock exhaust gas to the gas reservoir, and δ 13 C1 represents the carbon isotope ratio of methane in the residual gas from the source rock, and C1 represents the methane content in the gaseous components of the residual gas from the source rock. δ 13 C2 represents the carbon isotope ratio of methane in the original gas reservoir, C2 represents the methane content in the gaseous components of the original gas reservoir, and δ 13 C3 represents the carbon isotope ratio of methane in the current gas reservoir, and C3 also represents the methane content in the gaseous components of the current gas reservoir.

[0062] Based on the calculation formula, by substituting the relevant methane content in the gas components and the methane carbon isotope ratio data, the contribution ratio X of the source rock exhaust to the gas reservoir under the uplift and erosion background is calculated.

[0063] Example 2

[0064] This embodiment provides a quantitative evaluation method for the contribution of coal-bearing source rock gas to gas reservoirs under uplift and erosion background. Based on the quantitative evaluation method provided in Embodiment 1, the Linxing Gas Field in the Ordos Basin is used as the research object to illustrate the specific technical solution of the present invention.

[0065] The quantitative evaluation method includes the following steps:

[0066] (1) Conduct geological background analysis and reservoir formation geological condition analysis for the Linxing gas field.

[0067] The analysis results show that the Linxing gas field area belongs to the transitional position between the Yishan slope of the Ordos Basin and the Jinxi flexural belt. It is a gentle regional westward-dipping monocline that has undergone large-scale uplift since the Early Cretaceous, with an erosion thickness of up to 1500m. Due to the pressure reduction of the coal-bearing source rocks caused by uplift and erosion, gas release occurred again. The analysis of the reservoir geological conditions shows that the source rocks in this area are mainly coal-bearing source rocks. The Ro of the source rocks in the Linxing gas field generally reaches more than 1.3%, which is in the mature to high-mature stage and can produce a large amount of gas. Therefore, the relationship model between the methane carbon isotope or composition of coal-derived gas and vitrinite reflectance was selected to calculate the natural gas composition and methane carbon isotope ratio of the original gas reservoir.

[0068] (2) Determine the methane content and methane carbon isotope ratio of the gas discharged from the source rock of Linxing Gas Field.

[0069] Coalbed desorption gas (representative source rock residual gas sample) was collected from the Linxing gas field. The methane content in the residual gas sample was determined to be 88.7%, and the methane carbon isotope ratio was -42.9‰.

[0070] (3) Determine the methane content and methane carbon isotope ratio of the original gas reservoir in the gas reservoir area.

[0071] Based on the data that the carbon isotope ratio of ethane in the gas reservoir is greater than -23‰, the gas source was compared and determined to originate from the Carboniferous-Permian coal-bearing source rocks. The vitrinite reflectance of the source rock sample was measured to be 1.3%.

[0072] Based on the gas source comparison results, the relationship model between the carbon isotope composition of methane in coalbed methane and the vitrinite reflectance of the source rock samples is determined as follows:

[0073] M = 1 - e (Ro-1.908) / 0.419

[0074] Among them, R o Where is the vitrinite reflectance, and M is the methane content;

[0075] δ 13 C1 = 14.12logR o -34.39

[0076] Where, δ 13 C1 represents the carbon isotope ratio of methane in the residual gas of the source rock.

[0077] Based on the relational model, the relative percentage of methane in the original gas reservoir was determined to be 95.74%, and the methane carbon isotope ratio was -31.8‰.

[0078] (4) Determine the methane content and methane carbon isotope ratio of the current gas reservoir in the Linxing gas field.

[0079] Natural gas samples were collected from the Linxing gas field. The relative percentage of methane in the natural gas samples was determined to be 92.5%, and the carbon isotope ratio of methane was -36.0‰.

[0080] (5) According to the calculation formula, by substituting the corresponding relative percentage of methane and the methane carbon isotope ratio data, it is found that the contribution of the source rock gas to the Linxing gas field since the Middle Cretaceous is about 33%.

[0081] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A quantitative evaluation method for the contribution of gas expulsion from coal-bearing source rocks to gas reservoirs under uplift and erosion background, characterized in that, The quantitative evaluation method includes the following steps: (1) Determine the methane content and methane carbon isotope ratio of the gas discharged from the coal-bearing source rocks, and determine the methane content and methane carbon isotope ratio of the original gas reservoir and the current gas reservoir of the target gas reservoir respectively. (2) Based on the principle of methane carbon isotope balance in the gas reservoir, a calculation formula is established for the contribution of coal-bearing source rock exhaust to the gas reservoir under the background of uplift and erosion based on methane content and carbon isotope mixing ratio. The proportion of contribution of source rock exhaust to the gas reservoir under the background of uplift and erosion is calculated.

2. The quantitative evaluation method according to claim 1, characterized in that, Step (1) of determining the methane content and methane carbon isotope ratio of the original gas reservoir includes: (a) Determining source rocks using gas source comparison methods; (b) Determine the maturity of the coal-bearing source rocks; (c) Determine the relationship model between the methane carbon isotope composition and the maturity of coal-bearing source rocks in the gas reservoir area; (d) Determine the methane content and methane carbon isotope ratio of the original gas reservoir based on the aforementioned relationship model.

3. The quantitative evaluation method according to claim 2, characterized in that, The gas source comparison method described in step (a) includes: determining the number of source rock layers and geochemical characteristics in the gas reservoir distribution area, and determining the source rock source of natural gas based on the composition and isotopes of the natural gas in the gas reservoir.

4. The quantitative evaluation method according to claim 2 or 3, characterized in that, The maturity of the source rock sample described in step (b) is determined using vitrinite reflectance.

5. The quantitative evaluation method according to any one of claims 2-4, characterized in that, The relationship model described in step (c) is a model relating the carbon isotopes or composition of methane in continuously settled coalbed methane to the reflectance of vitrinite. Preferably, the relational model is as follows: M=1-e (Ro-1.908) / 0.419 Among them, R o Where is the vitrinite reflectance, and M is the methane content; δ 13 C1=14.12logR o -34.39 Where, δ 13 C1 represents the carbon isotope ratio of methane in residual gas from coal-bearing source rocks.

6. The quantitative evaluation method according to any one of claims 1-5, characterized in that, Step (1) The method for determining the methane content and methane carbon isotope ratio of the existing gas reservoir includes: using chromatography and chromatography-isotope mass spectrometry to measure the natural gas sample of the gas reservoir.

7. The quantitative evaluation method according to any one of claims 1-6, characterized in that, The calculation formula described in step (2) is as follows: Where X represents the proportion of gas exhaust from coal-bearing source rocks to the gas reservoir, and δ 13 C1 represents the carbon isotope ratio of methane in the residual gas from the source rock, and C1 represents the methane content in the gaseous components of the residual gas from the source rock. δ 13 C2 represents the carbon isotope ratio of methane in the original gas reservoir, C2 represents the methane content in the gaseous components of the original gas reservoir, and δ 13 C3 represents the carbon isotope ratio of methane in the current gas reservoir, and C3 also represents the methane content in the gaseous components of the current gas reservoir.

8. The quantitative evaluation method according to any one of claims 1-7, characterized in that, The quantitative evaluation method also includes: geological background analysis and reservoir formation geological condition analysis of the target gas reservoir area.

9. The quantitative evaluation method according to claim 8, characterized in that, The geological background analysis includes regional geological background, basin evolution, stratigraphic development, and tectonic features; Preferably, the analysis of reservoir geological conditions includes the combination characteristics of source, reservoir and caprock, trap type, hydrocarbon migration and accumulation process, and the temporal and spatial configuration relationship between hydrocarbon migration and accumulation process and gas reservoir.

10. The quantitative evaluation method according to any one of claims 1-9, characterized in that, The method for determining methane content in step (1) includes chromatography; Preferably, the method for determining the carbon isotope ratio of methane in step (1) includes chromatography-isotope mass spectrometry.

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