A method and system for evaluating atomic ratio saturation of a pulsed neutron energy spectrum logging

Abstract

The present application relates to the well logging technical field in oil and gas exploration, disclose a kind of pulsed neutron energy spectrum logging atomic ratio saturation evaluation method and system, through the element yield of energy spectrum data analysis, the atomic number ratio of each element in formation is extracted, numerical calculation model is established using MCNP simulation software, by establishing element atomic number sensitivity factor and formation element normalization factor F, the accurate calculation of formation element atomic number ratio is realized.The present application considers the contribution of other element Compton effect in the gamma count of element characteristic energy window, and solves the problem that element yield cannot reflect the atomic composition of each element in formation medium, combined with the atomic number ratio of carbon element and oxygen element in formation and element atomic number saturation calculation model, the calculation of oil saturation is realized, the sensitivity of reservoir oil-water identification is improved, and the deficiencies of conventional window method and element yield method in oil saturation evaluation are made up.

Description

Technical Field

[0001] This invention relates to the field of well logging technology in oil and gas exploration, specifically to a method and system for evaluating atomic ratio saturation in pulsed neutron energy spectrum well logging. Background Technology

[0002] As exploration and development deepen, unconventional oil and gas reservoirs, such as tight reservoirs, deep-water deep-layer oil and gas, and shale oil and gas, are increasingly accounting for a larger proportion of exploration. Quantitative evaluation of oil saturation in unconventional reservoirs is a key parameter for reservoir assessment and reserve calculation, and is of great significance to oil and gas exploration and development. Resistivity logging and nuclear magnetic resonance logging have played important roles in evaluating formation oil and gas content. However, resistivity logging and nuclear magnetic resonance logging have significant errors in low-contrast oil and gas reservoirs and tight formations. Pulsed neutron logging technology, by measuring the gamma rays generated after high-energy neutrons interact with formation materials, can identify the formation framework and fluid composition. Among these, carbon-oxygen ratio logging using pulsed neutron logging technology to measure the secondary gamma spectrum is one of the effective methods for evaluating formation oil and gas content. By selecting characteristic gamma energy windows for carbon and oxygen elements, obtaining the C / O value based on the gamma count ratio of the energy window, and combining it with the instrument's ground calibration formula, carbon-oxygen ratio energy spectrum logging plays a key role in evaluating the residual oil saturation after casing.

[0003] The complex mineral composition and low porosity / permeability of unconventional oil and gas reservoirs pose significant challenges to the accurate identification of pore fluids. Using carbon-oxygen energy window counting ratios to identify reservoir hydrocarbon content introduces uncertainties, severely impacting the quantitative monitoring of reservoir oil saturation and failing to meet measurement requirements. In particular, the Compton effect of other elements contributes significantly to the gamma counts of elemental characteristic energy windows, leading to low accuracy in oil saturation evaluation. This results in substantial errors in calculating saturation in tight, unconventional reservoirs, and the weak formation response further complicates quantitative saturation assessment. Furthermore, elemental yields represent the relative intensity of gamma rays produced by each element in the secondary gamma spectrum, failing to reflect the atomic composition of the formation medium. Therefore, there is an urgent need to propose a saturation evaluation method based on pulsed neutron spectroscopy logging technology to eliminate the high Compton background of the windowing method, reflect the true atomic composition of formation elements, and improve the sensitivity of reservoir oil-water identification. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a method and system for evaluating atomic ratio saturation in pulsed neutron energy spectrum logging. It applies formation element logging methods to pulsed neutron saturation evaluation, uses inelastic gamma spectroscopy to obtain the atomic ratio of formation elements, accurately calculates the atomic ratio of each element in the formation, eliminates the contribution of the Compton effect of other elements in the gamma count of element characteristic energy windows, realizes high-precision measurement of C / O parameters, and improves the sensitivity of reservoir oil-water identification.

[0005] This invention is achieved through the following technical solution:

[0006] A method for evaluating atomic ratio saturation in pulsed neutron energy spectrum logging, employing a pulsed neutron saturation measurement system consisting of a DT neutron source and a gamma-ray detector, includes the following steps:

[0007] Step 1: Based on the selected pulsed neutron saturation measurement system parameters, and on the basis that the MCNP simulation and the measured energy spectrum of the standard spectrum calibration well are consistent, establish the instrument formation model, change the formation medium to the elemental substances of each element, simulate the net inelastic gamma energy spectrum of each element in the detector, and use the net inelastic gamma energy spectrum as the standard spectrum of each elemental substance.

[0008] Step 2: Using the established instrument underlying model, with oxygen in each elemental substance as the benchmark, change the formation medium to a two-element mixed medium of oxygen and other formation elements, analyze the net non-elastic gamma spectrum obtained from the simulation to obtain the yield of oxygen and other formation elements, and use the atomic number sensitivity factor of different types of elements to calculate the atomic number of different types of elements using the proportion of oxygen and other formation elements in the model.

[0009] Step 3: For the net non-elastic gamma spectrum simulated under arbitrary formation conditions, the yield of each element is analyzed. Based on the fact that the sum of the atomic number proportions of all elements in the formation is 1, an atomic number normalization closed model is established to calculate the formation normalization factor F. The atomic number proportion of each element in the formation is obtained by using the element yield and the element atomic number sensitivity factor.

[0010] Step 4: Based on the selected instrument formation model, set up pure water-bearing and pure oil-bearing conditions for formations with different porosities, calculate the atomic ratio of carbon and oxygen elements, obtain the C / O oil-water line equation, and calculate the formation oil saturation by combining the carbon and oxygen atomic ratio obtained under the target formation conditions.

[0011] Preferably, the pulsed neutron saturation measurement system may have one or more detectors.

[0012] Preferably, the strata in the established instrumental stratigraphic model are divided into multiple annular grid elements.

[0013] Preferably, in step 1, the formation medium includes Si, Ca, Fe, Mg, S, C, and O elements.

[0014] Preferably, in step 2, the expression for calculating the atomic number sensitivity factor is as follows:

[0015]

[0016] Among them, S i y is the sensitivity factor for the number of atoms of element i.i Let n be the yield of element i in a bi-element medium. i y represents the percentage of atoms of element i in the model; o n represents the oxygen yield in a two-element medium. o This represents the percentage of oxygen atoms in the model.

[0017] Preferably, in step 2, the formation medium includes a bi-element mixed medium of Si and O, Ca and O, Fe and O, Mg and O, S and O, and C and O.

[0018] Preferably, in step 3, the expression for the atom number normalized closed-loop model is as follows:

[0019]

[0020] The formulas for calculating the percentage of atoms of each element in a stratum are as follows:

[0021]

[0022] Where n represents the number of element types, y i For the output of element i, S i K is the atomic number sensitivity factor for element i, F is the formation element atomic number normalization factor, and K is the atomic number sensitivity factor for element i. i This represents the percentage of atoms in element i.

[0023] Preferably, in step 4, the formula for calculating the formation oil saturation is as follows:

[0024]

[0025] Among them, S o The formation oil saturation, C / O m The carbon to oxygen atom ratio obtained under the target formation conditions, i.e., K C / K O C / O o and C / O w These represent the carbon-to-oxygen atomic ratios under conditions of pure oil-bearing and pure water-bearing formations, respectively.

[0026] Preferably, in step 4, the formation porosity is changed from 0% to 30%, increasing by 5% each time; the pore fluid is set to be pure water and pure oil, respectively.

[0027] Preferably, in step 4, the calculation formulas for the C / O oil-water line are as follows:

[0028] C / O o =A1·φ 2 +B1·φ+C1;

[0029] C / O w=A2·φ 2 +B2·φ+C2;

[0030] Among them, C / O o and C / O w These represent the carbon to oxygen atom ratios under pure oil-bearing and pure water-bearing conditions, respectively. φ represents the formation porosity, and A, B, and C are constant coefficients.

[0031] A pulsed neutron energy spectrum logging atomic ratio saturation evaluation system includes:

[0032] The data processing module is used to establish an instrument formation model based on the selected pulse neutron saturation measurement system parameters, and on the basis of the consistency between MCNP simulation and the measured energy spectrum of the standard spectral calibration well. The formation medium is changed to the elemental substance of each element, and the net inelastic gamma energy spectrum of each element in the detector is simulated. The net inelastic gamma energy spectrum is used as the standard spectrum of each elemental substance.

[0033] The first data calculation module is used to utilize the established instrument underlying model, taking oxygen element in each elemental substance as the benchmark, changing the formation medium to a two-element mixed medium of oxygen element and other formation elements, analyzing the net non-elastic gamma energy spectrum obtained from the simulation to obtain the yield of oxygen element and other formation elements, and using the atomic number ratio of oxygen element and other formation elements in the model to calculate the atomic number sensitivity factor of different types of elements.

[0034] The data acquisition module is used to simulate the net non-elastic gamma spectrum for arbitrary formation conditions, analyze the yield of each element, establish a closed-loop atomic number normalization model based on the fact that the sum of the atomic number percentages of all elements in the formation is 1, calculate the formation normalization factor F, and obtain the atomic number percentage of each element in the formation using the element yield and the element atomic number sensitivity factor.

[0035] The second data calculation module is used to set up pure water-bearing and pure oil-bearing conditions for formations with different porosities based on the selected instrument formation model, calculate the proportion of carbon and oxygen atoms, obtain the C / O oil-water line equation, and calculate the formation oil saturation by combining the carbon and oxygen atom ratio obtained under the target formation conditions.

[0036] Compared with the prior art, the present invention has the following beneficial technical effects:

[0037] This invention provides a method for evaluating the atomic ratio saturation of well logging using pulsed neutron energy spectrum. It extracts the atomic percentage of each element in the formation from the elemental yield obtained through energy spectrum data analysis. A numerical calculation model is established using MCNP simulation software. By establishing an elemental atomic number sensitivity factor and a formation element normalization factor F, the accurate calculation of the atomic percentage of formation elements is achieved. This invention considers the contribution of the Compton effect of other elements in the gamma count of elemental characteristic energy windows and solves the problem that elemental yield cannot reflect the atomic composition of each element in the formation medium. It combines the carbon and oxygen atomic number ratio and the elemental atomic number saturation calculation model to calculate oil saturation, improving the sensitivity of reservoir oil-water identification and overcoming the shortcomings of conventional windowing methods and elemental yield methods in evaluating oil saturation.

[0038] This invention also provides a pulsed neutron energy spectrum logging atomic ratio saturation evaluation system, which intelligently realizes the accurate calculation of the atomic ratio of formation elements through a data processing module, a first data calculation module, a data acquisition module, and a second data calculation module, thereby improving the sensitivity of reservoir oil and water identification. Attached Figure Description

[0039] Figure 1 This invention uses the pulsed neutron saturation logging instrument to perform formation MCNP numerical calculations.

[0040] Figure 2 This is the non-elastic gamma standard spectrum of each element in this invention.

[0041] Figure 3 This is the sensitivity factor for the atomic number of Si, Ca, Fe, Mg, S, C, and O elements in this invention.

[0042] Figure 4 This is a comparison between the calculated atomic number ratio under a formation condition calculated using this invention and the model setting value.

[0043] Figure 5a Comparison of the oil saturation calculation model obtained in this invention under limestone strata conditions

[0044] Figure 5b Comparison of C / O oil-water sensitivity calculated by the windowing method and the atomic number ratio method of this invention under limestone formation conditions;

[0045] Figure 6 This is a schematic diagram of the saturation calculation results of well X measured by the instrument in an embodiment of the present invention.

[0046] In the diagram: 1 is the wellbore; 2 is the casing; 3 is the cement sheath; 4 is the formation; 5 is the pulsed neutron saturation logging instrument; 6 is the DT neutron source; 7 is the tungsten-nickel-iron shield; and 8 is the gamma-ray detector. Detailed Implementation

[0047] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. 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 should fall within the scope of protection of the present invention.

[0048] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0049] The present invention will now be described in further detail with reference to the accompanying drawings:

[0050] This invention provides a method for evaluating atomic ratio saturation in pulsed neutron energy spectrum logging. It applies formation element logging methods to pulsed neutron saturation evaluation, uses inelastic gamma spectroscopy to obtain the atomic ratio of formation elements, accurately calculates the atomic ratio of each element in the formation, eliminates the contribution of the Compton effect of other elements in the gamma count of element characteristic energy windows, realizes high-precision measurement of C / O parameters, and improves the sensitivity of reservoir oil-water identification.

[0051] Specifically, this pulsed neutron energy spectrum logging atomic ratio saturation evaluation method employs a pulsed neutron saturation measurement system consisting of a DT neutron source and a gamma-ray detector, characterized by the following steps:

[0052] Step 1: Based on the selected pulsed neutron saturation logging instrument parameters, and on the basis of the consistency between MCNP simulation and measured well energy spectra calibrated according to standard spectra, establish an instrument formation model, change the formation medium to elemental substances, and simulate the net inelastic gamma spectrum of each element in the detector, which is then regarded as the standard spectrum of each element. For example... Figure 1As shown, the numerical calculation model includes wellbore 1, casing 2, cement sheath 3, formation 4, and pulsed neutron saturation logging instrument 5. The pulsed neutron saturation logging instrument 5 is a pulsed neutron saturation measurement system consisting of a DT neutron source 6, a tungsten-nickel-iron shield 7, and a gamma-ray detector 8. In the instrument formation numerical calculation model, the formation is divided into multiple annular grid elements with a size of 0.5cm × 0.5cm. By changing the formation medium to elemental substances such as Si, Ca, Fe, Mg, S, C, and O, the net nonelastic gamma spectrum was simulated, as shown below. Figure 2 As shown.

[0053] Step 2: Using the established instrumental formation model, the formation is divided into multiple annular grid elements with a size of 0.5cm × 0.5cm. The formation media are set as two-element mixed media such as Si and O, Ca and O, Fe and O, Mg and O, S and O, and C and O. The net inelastic gamma spectrum obtained by analytical simulation using the non-negative least squares method is used to obtain the yields of oxygen and other formation elements. The atomic number sensitivity factors of different types of elements are calculated using the proportion of oxygen and other formation elements in the model. Figure 3 As shown.

[0054] The expression for calculating the atomic number sensitivity factor is shown in formula (1):

[0055]

[0056] In the formula, S i y is the sensitivity factor for the number of atoms of element i. i Let n be the yield of element i in a bi-element medium. i y represents the percentage of atoms of element i in the model. o n represents the oxygen yield in a two-element medium. o This represents the percentage of oxygen atoms in the model.

[0057] Step 3: For the net non-elastic gamma spectrum simulated under arbitrary formation conditions, the yield of each element is obtained analytically. Based on the idea that the sum of the atomic number percentages of all elements in the formation is 1, an atomic number normalization closed model is established to calculate the formation normalization factor F. The atomic number percentages of each element in the formation are obtained using the element yield and the element atomic number sensitivity factor.

[0058] The expression for the atom number normalized closed-loop model is shown in equation (2):

[0059]

[0060] The calculation method for the percentage of atomic numbers of each element in the strata is shown in formula (3):

[0061]

[0062] In the formula, n represents the number of element types, and y i For the output of element i, S i K is the atomic number sensitivity factor for element i, F is the formation element atomic number normalization factor, and K is the atomic number sensitivity factor for element i. i This represents the percentage of atoms in element i. Figure 4 To compare the calculated atomic number ratio under a formation condition using the present invention with the model setting value, the bar chart numbers represent the absolute error of the atomic number ratio, verifying the accuracy of the atomic number ratio calculated by the present invention.

[0063] Step 4: Based on the selected formation model, set up pure water-bearing and pure oil-bearing conditions with different porosities. The formation is divided into multiple annular grid elements with a size of 0.5cm × 0.5cm. The formation porosity is changed from 0% to 30%, increasing by 5% each time. The pore fluid is set as pure water-bearing and pure oil-bearing, respectively. The net inelastic gamma spectrum is obtained through simulation. The atomic percentages of carbon and oxygen are calculated using the above method, and the C / O oil-water line equations are obtained as follows:

[0064] C / O o =A1·φ 2 +B1·φ+C1 (4)

[0065] C / O w =A2·φ 2 +B2·φ+C2 (5)

[0066] In the formula, C / O o and C / O w These represent the carbon to oxygen atom ratios under pure oil-bearing and pure water-bearing conditions, respectively. φ represents the formation porosity, and A, B, and C are constant coefficients.

[0067] Based on the carbon-to-oxygen atomic ratio obtained under the target formation conditions, the formation oil saturation is calculated. The formula for calculating formation oil saturation is shown in formula (6):

[0068]

[0069] In the formula, S o The formation oil saturation, C / O m The carbon to oxygen atom ratio obtained under the target formation conditions, i.e., K C / K O C / O o and C / O w These represent the carbon-to-oxygen atomic ratios under conditions of pure oil-bearing and pure water-bearing formations, respectively.

[0070] according to Figure 5a and Figure 5bSpecifically, this paper compares the oil saturation calculation model obtained in this invention with the sensitivity of C / O oil-water ratio calculated by the windowing method and the atomic number ratio method under limestone formation conditions. As shown in Figure 5, the C / O oil-water line provided by this invention greatly improves the sensitivity compared with the windowing method, which is beneficial for the quantitative monitoring of reservoir oil saturation and provides technical support for the quantitative calculation of formation oil saturation.

[0071] This invention also provides a pulsed neutron energy spectrum logging atomic ratio saturation evaluation system, including...

[0072] The data processing module is used to establish an instrument formation model based on the selected pulse neutron saturation measurement system parameters, and on the basis of the consistency between MCNP simulation and the measured energy spectrum of the standard spectral calibration well. The formation medium is changed to the elemental substance of each element, and the net inelastic gamma energy spectrum of each element in the detector is simulated. The net inelastic gamma energy spectrum is used as the standard spectrum of each elemental substance.

[0073] The first data calculation module is used to utilize the established instrument underlying model, taking oxygen element in each elemental substance as the benchmark, changing the formation medium to a two-element mixed medium of oxygen element and other formation elements, analyzing the net non-elastic gamma energy spectrum obtained from the simulation to obtain the yield of oxygen element and other formation elements, and using the atomic number ratio of oxygen element and other formation elements in the model to calculate the atomic number sensitivity factor of different types of elements.

[0074] The data acquisition module is used to simulate the net non-elastic gamma spectrum for arbitrary formation conditions, analyze the yield of each element, establish a closed-loop atomic number normalization model based on the fact that the sum of the atomic number percentages of all elements in the formation is 1, calculate the formation normalization factor F, and obtain the atomic number percentage of each element in the formation using the element yield and the element atomic number sensitivity factor.

[0075] The second data calculation module is used to set up pure water-bearing and pure oil-bearing conditions for formations with different porosities based on the selected instrument formation model, calculate the proportion of carbon and oxygen atoms, obtain the C / O oil-water line equation, and calculate the formation oil saturation by combining the carbon and oxygen atom ratio obtained under the target formation conditions.

[0076] Example

[0077] Selecting well X, measured by the instrument, as an implementation example, the actual measured energy spectrum of the instrument is first preprocessed to obtain the processed net inelastic gamma energy spectrum. The least squares spectral decomposition method is used to decompose the instrument's measured inelastic gamma energy spectrum using the elemental standard spectrum obtained in step 1 to obtain the measured gamma energy spectrum yield result.

[0078] The yield results obtained after spectral analysis are used to calculate the atomic number content of formation elements by using the elemental sensitivity factors for carbon, oxygen, silicon and calcium described in step 2, combined with the processing method described in step 3.

[0079] Based on the results of instrument numerical simulation and calibration experiments, the oil-water line equation of this instrument is:

[0080] C / O0=7.92E-5·φ 2 +0.00181·φ+0.119 (7)

[0081] C / O w = -1.44E-6·φ 2 +2.95E-5·φ+0.119 (8)

[0082] In the formula, C / O o and C / O w These represent the carbon-to-oxygen atomic ratios under pure oil-bearing and pure water-bearing conditions, respectively, with φ representing the formation porosity.

[0083] Substituting into the saturation calculation formula described in step 4,

[0084]

[0085] In the formula, S o The formation oil saturation, C / O m The carbon to oxygen atom ratio obtained under the target formation conditions, i.e., K C / K O C / O o and C / O w The carbon and oxygen atomic ratios are given under pure oil-bearing and pure water-bearing conditions, respectively, as shown in formulas (7) and (8).

[0086] The saturation calculation results are as follows Figure 6 As shown in the figure, the first track is the depth track, the second track is the naked-eye data, the third track is the compensated density curve, and the fourth track is the deep and shallow resistivity curves. The fifth track is the lithological profile, the sixth track is the C / O atomic ratio and the corresponding oil-water line, and the seventh track is the interpreted oil saturation. The oil saturation interpretation results based on the atomic ratio correspond well with the resistivity curves, and oil was produced after test firing, verifying the effectiveness of the method.

[0087] In summary, this invention discloses a method and system for evaluating atomic ratio saturation in pulsed neutron spectroscopy logging. Based on the fundamental principles of formation elemental logging, this method extracts the atomic percentage of each element in the formation from the elemental yield obtained through energy spectrum data analysis, and establishes an oil saturation calculation model to achieve quantitative monitoring of saturation. First, standard spectra for each element in the formation are established by combining MCNP simulations and measured well spectra calibrated with standard spectra. Second, using oxygen as a benchmark, a mixed formation model of other elements and oxygen is established. The yield of the target element and oxygen is obtained by analyzing the net inelastic gamma spectrum, and the atomic number conversion factor for each element is calculated. Under formation conditions with arbitrary elemental composition, the elemental yield and atomic number conversion factor are combined, and the atomic percentage of each element in the formation is obtained using the formation factor F. Finally, based on the atomic ratio of carbon and oxygen in the formation, the oil saturation is calculated using the elemental atomic number saturation calculation model. This invention obtains the true atomic composition of the formation medium through element yield, eliminates the contribution of the Compton effect of other elements to the gamma count of the element characteristic energy window in the conventional windowing method, significantly improves the sensitivity of C / O evaluation of oil saturation, and provides technical support for the quantitative calculation of formation oil saturation.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for evaluating atomic ratio saturation in pulsed neutron energy spectrum logging, comprising a pulsed neutron saturation measurement system consisting of a DT neutron source and a gamma-ray detector, characterized in that... Includes the following steps: Step 1: Based on the selected pulsed neutron saturation measurement system parameters, and on the basis that the MCNP simulation and the measured energy spectrum of the standard spectrum calibration well are consistent, establish the instrument formation model, change the formation medium to the elemental substances of each element, simulate the net inelastic gamma energy spectrum of each element in the detector, and use the net inelastic gamma energy spectrum as the standard spectrum of each elemental substance. Step 2: Using the established instrument underlying model, with oxygen in each elemental substance as the benchmark, change the formation medium to a two-element mixed medium of oxygen and other formation elements, analyze the net non-elastic gamma spectrum obtained from the simulation to obtain the yield of oxygen and other formation elements, and use the atomic number sensitivity factor of different types of elements to calculate the atomic number of different types of elements using the proportion of oxygen and other formation elements in the model. The expression for calculating the atomic number sensitivity factor is as follows: ； in, For elements i The atomic number sensitivity factor, Elements in a two-element medium i Output For elements in the model i Percentage of atoms; The oxygen yield in a two-element medium. This represents the percentage of oxygen atoms in the model. Step 3: For the net non-elastic gamma spectrum simulated under arbitrary formation conditions, the yield of each element is analyzed. Based on the fact that the sum of the atomic number proportions of all elements in the formation is 1, an atomic number normalization closed model is established to calculate the formation normalization factor F. The atomic number proportion of each element in the formation is obtained by using the element yield and the element atomic number sensitivity factor. The expression for the atom number normalized closed-loop model is as follows: ； The formulas for calculating the percentage of atoms of each element in a stratum are as follows: ； in, n For element types, For elements i Output For elements i The atomic number sensitivity factor, F The normalization factor for the atomic number of elements in the formation. For elements i Percentage of atoms; Step 4: Based on the selected instrument formation model, set up pure water-bearing and pure oil-bearing conditions for formations with different porosities, calculate the atomic ratio of carbon and oxygen elements, obtain the C / O oil-water line equation, and calculate the formation oil saturation by combining the carbon and oxygen atomic ratio obtained under the target formation conditions. The formula for calculating formation oil saturation is as follows: ； in, The oil saturation of the formation. The carbon to oxygen atom ratio obtained under the target formation conditions, i.e. , and These represent the carbon-to-oxygen atomic ratios under conditions of pure oil-bearing and pure water-bearing formations, respectively.

2. The method for evaluating atomic ratio saturation in pulsed neutron energy spectrum logging according to claim 1, characterized in that, The pulsed neutron saturation measurement system may contain one or more detectors.

3. The method for evaluating atomic ratio saturation in pulsed neutron energy spectrum logging according to claim 1, characterized in that, The strata in the established instrumental stratigraphic model are divided into multiple annular grid elements.

4. The method for evaluating atomic ratio saturation in pulsed neutron energy spectrum logging according to claim 1, characterized in that, In step 1, the formation medium includes elements Si, Ca, Fe, Mg, S, C, and O.

5. The method for evaluating atomic ratio saturation in pulsed neutron energy spectrum logging according to claim 1, characterized in that, In step 2, the formation medium includes a bi-element mixed medium of Si and O, Ca and O, Fe and O, Mg and O, S and O, and C and O.

6. The method for evaluating atomic ratio saturation in pulsed neutron energy spectrum logging according to claim 1, characterized in that, In step 4, the calculation formulas for the C / O oil-water line are as follows: ； ； in, and These represent the carbon to oxygen atom ratios under conditions of pure oil-bearing and pure water-bearing formations, respectively. denoted as formation porosity, and A, B, and C as constant coefficients.

7. A pulsed neutron energy spectrum logging atomic ratio saturation evaluation system, characterized in that, A method for evaluating the atomic ratio saturation of pulsed neutron energy spectrum logging as described in any one of claims 1-6, comprising: The data processing module is used to establish an instrument formation model based on the selected pulse neutron saturation measurement system parameters, and on the basis of the consistency between MCNP simulation and the measured energy spectrum of the standard spectral calibration well. The formation medium is changed to the elemental substance of each element, and the net inelastic gamma energy spectrum of each element in the detector is simulated. The net inelastic gamma energy spectrum is used as the standard spectrum of each elemental substance. The first data calculation module is used to utilize the established instrument underlying model, taking oxygen element in each elemental substance as the benchmark, changing the formation medium to a two-element mixed medium of oxygen element and other formation elements, analyzing the net non-elastic gamma energy spectrum obtained from the simulation to obtain the yield of oxygen element and other formation elements, and using the atomic number ratio of oxygen element and other formation elements in the model to calculate the atomic number sensitivity factor of different types of elements. The data acquisition module is used to simulate the net non-elastic gamma spectrum for arbitrary formation conditions, analyze the yield of each element, establish a closed-loop atomic number normalization model based on the fact that the sum of the atomic number percentages of all elements in the formation is 1, calculate the formation normalization factor F, and obtain the atomic number percentage of each element in the formation using the element yield and the element atomic number sensitivity factor. The second data calculation module is used to set up pure water-bearing and pure oil-bearing conditions for formations with different porosities based on the selected instrument formation model, calculate the proportion of carbon and oxygen atoms, obtain the C / O oil-water line equation, and calculate the formation oil saturation by combining the carbon and oxygen atom ratio obtained under the target formation conditions.

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