Method for evaluating organic carbon content of marine hydrocarbon source rock in Yan'an gas field based on well logging curve

Abstract

The present application belongs to the technical field of oil and gas logging, especially to a method for evaluating marine hydrocarbon source rock organic carbon content of Yan'an gas field based on logging curve. The method for evaluating marine hydrocarbon source rock organic carbon content of Yan'an gas field based on logging curve obtains conventional logging curve and measured TOC value of rock sample; constructs fusion parameter; and fits measured TOC value of rock sample of the same well and the same depth with the fusion parameter to obtain fitting relationship, that is, TOC prediction model. The present application does not directly fit with conventional logging curve, but constructs fusion parameter, the fusion parameter is selected reasonably and conforms to organic matter enrichment law; the present application has the characteristics of low cost, high efficiency and strong practicability, can accurately finely depict the whole hydrocarbon source rock of the work area, and provides data support for oil and gas exploration and development and evaluation.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas logging technology, and in particular, it is a method for evaluating the organic carbon content of marine source rocks in the Yan'an gas field based on logging curves. Background Technology

[0002] Source rocks are organic-rich rocks capable of generating petroleum or natural gas, typically described quantitatively using the organic matter abundance index—total organic carbon (TOC). Compared to terrestrial source rocks, marine source rocks are predominantly carbonate rocks, deposited in deeper waters, and subjected to greater burial depths, resulting in more pronounced compaction. The marine source rocks in the Yan'an gas field are characterized by low organic matter abundance and high thermal evolution. Organic geochemical methods are commonly used to analyze samples to evaluate the entire block. While this method is highly accurate, its limitations in sample availability and the relatively high manpower, material, and financial costs of analysis make it difficult to meet exploration and development deployment requirements when evaluating the development of source rocks across the entire Yan'an gas field due to experimental constraints. Well logging data, compared to other data, offers significant advantages such as continuous measurement data, large quantity, and easier accessibility. Furthermore, some logging curves are highly sensitive to source rock responses. Therefore, using well logging curves to identify source rocks and calculate their TOC content provides a more realistic basis for well location deployment in oil and gas exploration.

[0003] Currently, the main methods for quantitatively interpreting TOC using well logging data include single-parameter well logging interpretation, the sonic transit time and resistivity curve overlay method (ΔlgR method), and multiple regression fitting method. Single-parameter well logging interpretation typically establishes a fitting relationship between a specific curve in the well logging data and the TOC content, such as establishing a fitting formula between the natural gamma ray (GR) curve or the compensated density (DEN) curve and TOC. However, due to the complex geological conditions of the Ordos Basin, a single curve is usually influenced by multiple factors, making this method unsuitable for this work area. The ΔlgR method, proposed in the 1990s, uses sonic transit time (Δt), resistivity (R), and source rock maturity (LOM) to establish a TOC prediction formula. This formula places the resistivity curve and sonic transit time curve at the fine-grained non-source rock position, i.e., the baseline (R). 基 Δt 基 If the two curves coincide at a certain point, the distance between them is the amplitude difference (ΔlgR), which is calculated using the following formula: ΔlgR=lg(R / R 基 )+0.02(Δt-Δt 基 (1) TOC=10 (2.297-0.1688LOM) ΔlgR+ΔTOC (2) In the above formula: ΔlgR is the amplitude difference; R is the resistivity; Δt is the sound wave time difference; R 基 Δt 基These represent the baseline values ​​of the resistivity curve and the sonic transit time curve, respectively; LOM is the thermal variation index; and ΔTOC is the background value of organic carbon content. The traditional ΔlgR technique, based on normally compacted strata, is not suitable for logging evaluation of source rocks with high electrical conductivity, especially deep, strongly compacted source rocks. Furthermore, the LOM value in the fitting formula ranges from 7 to 12, meaning it is only suitable for source rocks in the mature to highly mature range. However, most source rocks in this work area have values ​​above 12, indicating they are over-mature source rocks.

[0004] Multiple regression refers to using several logging curves most sensitive to TOC changes and then establishing a fitting relationship through multiple regression. This method overcomes the limitations of single logging parameters and improves the accuracy of the fitting formula by combining multiple parameters. However, the fitting results are significantly affected by regional characteristics, and the parameters used in different regions and strata vary considerably, resulting in diverse and complex fitting relationships. Therefore, there is an urgent need to find a logging curve fitting relationship more suitable for the geological, structural, and sedimentary evolution characteristics of this work area. Summary of the Invention

[0005] To address the aforementioned issues, this invention establishes a method suitable for identifying and evaluating the organic carbon (TOC) content of marine source rocks in the Yan'an gas field using well logging. This method employs multiple well logging curves, including uranium-removed gamma KTH curves, natural gamma ray GR curves, natural gamma ray energy spectrum uranium U curves, natural gamma ray energy spectrum thorium TH curves, compensated sonic transit time AC curves, compensated density DEN curves, compensated neutron CNL curves, and deep resistivity RT curves, to identify source rocks and calculate TOC content. This solves the problem of unclear responses in traditional well logging fitting formulas caused by low organic matter abundance, high thermal evolution, and significant compaction in the marine source rocks of the Yan'an gas field, thus improving prediction accuracy and providing necessary basic data support for deep oil and gas exploration in the Lower Paleozoic.

[0006] The technical solution of this invention is as follows: A method for evaluating the organic carbon content of marine source rocks in the Yan'an gas field based on well logging curves, using the following formula: TOC=a×(1-KTH / GR)+b×(U / TH)+c×(AC-150) / (200×(DEN-2))+d×CNL / (60×(DEN-2)) (1) In the formula: TOC is the organic carbon content; KTH is the uranium-depleted gamma; GR is the natural gamma; U is the natural gamma spectrum of uranium; TH is the natural gamma spectrum of thorium; AC is the compensated acoustic time difference; DEN is the compensated density; CNL is the compensated neutron; a, b, c, and d are constants.

[0007] A method for evaluating the organic carbon content of marine source rocks in the Yan'an gas field based on well logging curves is as follows: Obtain conventional logging curves and measured TOC values ​​from rock samples; Conventional logging curves include: uranium-reduced gamma KTH curve, natural gamma GR curve, natural gamma spectral uranium U curve, natural gamma spectral thorium TH curve, compensated sonic transit time AC curve, compensated density DEN curve, and compensated neutron CNL curve. Construct fusion parameters, specifically including: 1-KTH / GR, U / TH, (AC-150) / (200(DEN-2)) and CNL / (60×(DEN-2)); Since the TOC value is closely related to the development characteristics of source rocks and the sedimentary environment in the work area, and the natural gamma ray GR curve is greatly affected by radioactive elements, the (1-KTH / GR) parameter is used to eliminate some of the influence of radioactive elements, so that it is more closely related to the organic matter content in the source rocks and can reflect the organic matter enrichment characteristics. It is usually positively correlated with the TOC value. (U / TH) usually reflects the redox environment of water bodies and is positively correlated with TOC; The compensated acoustic time difference (AC) curve is affected by organic matter, and its value increases with the TOC value. Conversely, the compensated density (DEN) curve is also affected by organic matter, and its value decreases with the TOC value. Therefore, both curves are normalized separately and then combined to determine the TOC value. Similarly, the compensated neutron (CNL) curve is affected by organic matter, and the increase in hydrogen atoms results in a positive correlation between its value and TOC. It is also normalized and combined with the compensated density (DEN) curve to determine the TOC value.

[0008] The measured TOC values ​​of rock samples from the same well and at the same depth are fitted with the above-mentioned fusion parameters to obtain the fitting relationship (1), which is the TOC prediction model.

[0009] Before obtaining the conventional logging curves of the rock sample, the rock sample is cored and repositioned.

[0010] The technical advantages of this invention are as follows: 1. This invention does not directly fit the curve to conventional logging curves, but instead constructs fusion parameters. The KTH / GR ratio is used to eliminate the influence of clay content on the fitting results, while the U / TH ratio takes into account the influence of the sedimentary environment (oxidative or reducing environment) on the fitting results. The (AC-150) / (200(DEN-2)) and CNL / (60(DEN-2)) are normalized and combined for comprehensive identification, improving the accuracy of the curve's fitting results for organic carbon content. These fusion parameters are rationally selected and conform to the laws of organic matter enrichment. 2. Compared with traditional organic geochemical experimental methods, this invention can comprehensively and completely reflect the development and distribution characteristics of the entire continuous source rock of a certain well. 3. Compared with traditional methods, this invention is more suitable for source rocks with low exploration depth, low organic matter abundance, and high thermal evolution. It also features low cost, high efficiency, and strong practicality. It can accurately and finely characterize the source rocks in the work area, providing data support for oil and gas exploration, development, and evaluation. Attached Figure Description

[0011] Figure 1 These are plots showing the intersection of measured TOC values ​​of rock samples with GR and 1-KTH / GR curves in specific application cases.

[0012] Figure 2 These are plots showing the intersection of measured TOC values ​​of rock samples with U and U / TH curves in specific application cases.

[0013] Figure 3 These are plots showing the intersection of the measured TOC values ​​of rock samples with the AC and (AC-150) / (200*(DEN-2)) curves in specific application cases.

[0014] Figure 4 These are plots showing the intersection of the measured TOC values ​​of rock samples with CNL and CNL / (60*(DEN-2)) curves in specific application cases.

[0015] Figure 5 These are plots showing the intersection of measured TOC values ​​of rock samples with DEN and RT curves in specific application cases.

[0016] Figure 6 This is a comparison chart of the predicted TOC content of rock samples and the measured TOC content of some rock samples in specific application cases. Detailed Implementation

[0017] Specific application case - taking the Y2251 well in the Yan'an Gas Field of the Ordos Basin as an example A method for evaluating the organic carbon content of marine source rocks in the Yan'an gas field based on well logging curves is as follows: Step 1: Core alignment and correction of source rock samples with logging depth, as shown in Table 1; Table 1 Rock Sample Relocation Table .

[0018] Step 2: Obtain the measured TOC values ​​of source rock samples at different depths and their corresponding natural gamma ray GR, natural gamma ray energy spectrum uranium U, compensated acoustic transit time AC, deep probe resistivity RT, compensated density DEN, and compensated neutron CNL curve values, as shown in Table 2. Table 2. Statistical Table of Rock Samples and Corresponding Well Logging Curve Values .

[0019] Step 3: The statistical table of rock samples and corresponding fusion parameters is shown in Table 3; Table 3. Statistical table of rock samples and corresponding fusion parameters Correlation analysis was performed on the measured TOC values ​​of rock samples at various well depths and their corresponding GR, 1-KTH / GR, U, U / TH, AC, (AC-150) / (200×(DEN-2)), CNL, CNL / (60×(DEN-2)), DEN, and RT curves. Figures 1-5 As shown, the correlation between the measured TOC value and the fusion parameter is better than that between the single parameter and the RT curve, and there is no significant linear relationship between the measured TOC value and the RT curve.

[0020] Step 4: Based on the data in Table 3, the organic carbon content of the source rock was predicted and fitted. The fitting relationship is shown below: TOC=0.185×(1-KTH / GR)+0.098×(U / TH)+0.032×(AC-150) / (200×(DEN-2))+0.215×CNL / (60×(DEN-2)).

[0021] Step 5: Correlation analysis was performed between the predicted TOC content obtained by the method provided in this invention and the measured TOC content values ​​of some rock samples. The results showed a strong correlation between the two, such as... Figure 6 As shown, the predicted TOC content obtained by the method provided by this invention is relatively reasonable and reliable.

[0022] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for evaluating the organic carbon content of marine source rocks in the Yan'an gas field based on well logging curves, characterized in that, The organic carbon content of the marine source rocks in the Yan'an gas field was calculated using the following formula: TOC=a×(1-KTH / GR)+b×(U / TH)+c×(AC-150) / (200×(DEN-2))+d×CNL / (60×(DEN-2)) (1) In the formula: TOC is the predicted value of organic carbon content; KTH is uranium-depleted gamma; GR is natural gamma; U is uranium in natural gamma spectrum; TH is thorium in natural gamma spectrum; AC is the compensated acoustic transit time; DEN is the compensated density; CNL is the compensated neutron; a, b, c and d are all constants.

2. The method for evaluating the organic carbon content of marine source rocks in the Yan'an gas field based on well logging curves according to claim 1, characterized in that, The method is as follows: Obtain conventional logging curves and measured TOC values ​​from rock samples; Conventional logging curves include: uranium-reduced gamma KTH curve, natural gamma GR curve, natural gamma spectral uranium U curve, natural gamma spectral thorium TH curve, compensated sonic transit time AC curve, compensated density DEN curve, and compensated neutron CNL curve. Construct fusion parameters, specifically including: 1-KTH / GR, U / TH, (AC-150) / (200(DEN-2)) and CNL / (60×(DEN-2)); The measured TOC values ​​of rock samples from the same well and at the same depth are fitted with the above-mentioned fusion parameters to obtain the fitting relationship (1), which is the TOC prediction model.

3. The method for evaluating the organic carbon content of marine source rocks in the Yan'an gas field based on well logging curves according to claim 2, characterized in that, Before obtaining the conventional logging curves of the rock sample, the rock sample is cored and repositioned.

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