A Logging Quantitative Evaluation Method for Potassium Content in Halite-Type Polyhalite

Abstract

The present invention provides a logging quantitative evaluation method for potassium content in halite-type polyhalite. By using the three sensitive parameters of low density, high potassium, and low thorium of halite-type polyhalite, a logging identification chart for halite-type polyhalite is established, and a characteristic identification curve of potassium index is constructed. The potassium index threshold values of different lithologies and halite-type polyhalite are determined through calibration with core data. At the same time, the influence of potassium content of clay minerals in halite-type polyhalite is eliminated by energy spectrum radioactivity difference correction, and a logging quantitative calculation formula for potassium content based on radioactivity difference analysis is obtained. The present invention has a reasonable concept and a simple process, can effectively solve the problem of quantitative calculation of potassium content in halite-type polyhalite, has high calculation accuracy, and the calculation result has a high degree of coincidence with the experimental determination of potassium content.

Description

Technical Field

[0001] The present invention relates to the technical field of geological exploration, and particularly relates to a logging quantitative evaluation method for the potassium content in sylvinite polyhalite. Background Art

[0002] At present, the logging lithology identification and potassium content calculation methods for sylvinite polyhalite are still blank. The exploration of deep potash resources requires a large number of deep drilling wells, and the investment for a single well is often tens of millions, with high costs and great risks. However, deep potash resources often coexist with oil and gas resources. A large number of deep drilling operations have been carried out in the target area for the oil and gas system, and a large amount of logging data has been obtained. By processing the logging data of oil and gas wells, polyhalite can be identified and the potassium content in polyhalite can be calculated. At present, the identification of polyhalite and the calculation of potassium content can mainly be carried out through natural gamma ray spectroscopy logging, acoustic logging, density logging, etc.

[0003] Natural gamma ray logging is a geophysical logging method based on the natural radioactivity of the formation, which detects the intensity of gamma rays in the formation through a gamma ray detector. In formation rocks, due to the presence of radioactive isotopes of elements such as potassium, uranium, radium, and thorium, natural gamma ray logging can detect abnormal formation radioactivity. The isotope K40 of potassium is a radioactive element, and when the formation contains potassium, the amplitude of the natural gamma ray curve increases. Therefore, through natural gamma ray logging, radioactive formations including potassium-bearing formations can be identified, thereby excluding non-radioactive formations that are unlikely to contain potassium.

[0004] The propagation speed of sound waves in the formation is related to the nature of the rock itself. Different lithologies of formations have different sound wave speeds, so the formation lithology can be distinguished through acoustic travel time logging. The sound velocity is related to the composition, elasticity, density porosity, shale content, saturation, and pore fluid phase state of the formation rock-forming minerals. When the pores of the rock formation increase, its sound velocity decreases and the acoustic travel time increases. Therefore, the acoustic travel time of dense rocks is less than that of porous rocks. In shale, due to the relatively high content of clay minerals and low content of brittle minerals such as quartz, the sound velocity is slower and the acoustic travel time is larger; at the same time, the particles of shale are relatively fine and the content of bound water is very high, and sound waves propagate slowly in fluids, so the travel time is large. Therefore, shale has a relatively high acoustic travel time compared to other lithologies and can be used as one of the markers to distinguish shale from solid potash.

[0005] Different rocks and minerals have different resistivities. Clay rocks have a lower resistivity due to their fine particles, good sorting, and developed total pores; carbonate rocks have a relatively high resistivity due to their extremely fine particles and extremely small porosity; while gypsum and rock salt have extremely low porosity and very high resistivity. Therefore, on the resistivity curve, polyhalite, sylvite, carnallite, etc. show high values, and shale shows low values.

[0006] At present, the main research content of polyhalite logging evaluation is the analysis of the response characteristics and qualitative identification of traditional polyhalite, and there is little research on quantitative evaluation methods. Some scholars summarized the geophysical logging response characteristics of minerals in salt-bearing strata and proposed to identify traditional polyhalite based on logging data by comprehensively using cross plots, BP neural networks, Fisher discriminant analysis, and support vector machine methods. However, due to the characteristics of halite-type polyhalite associated with halite, no relevant identification methods have been seen so far. Summary of the Invention

[0007] In view of the technical problems existing in the above background technology, the present invention proposes a logging quantitative evaluation method for the potassium content in halite-type polyhalite, which has a reasonable concept, a simple process, can effectively solve the problem of quantitative calculation of the potassium content in halite-type polyhalite, has high calculation accuracy, and a high degree of coincidence between the calculation result and the experimental determination of potassium content.

[0008] To solve the above technical problems, a logging quantitative evaluation method for the potassium content in halite-type polyhalite provided by the present invention uses the three sensitive parameters of low density, high potassium, and low thorium of halite-type polyhalite to establish a logging identification chart for halite-type polyhalite, construct a potassium index characteristic identification curve, determine the potassium index threshold values of different lithologies and halite-type polyhalite through calibration with core data, and at the same time eliminate the influence of the potassium content of clay minerals in halite-type polyhalite through energy spectrum radioactive difference correction, and obtain a logging quantitative calculation formula for potassium content based on radioactive difference analysis.

[0009] The logging quantitative evaluation method for the potassium content in halite-type polyhalite, wherein the calculation method mainly includes the following steps:

[0010] (1) First, calibrate the logging according to geological logging and core sample analysis and testing data, clarify the lithologies often coexisting with halite-type polyhalite and their logging curve characteristics, and establish a logging identification model for the main lithologies of the formation in the study area based on clarifying the logging response characteristics of different lithologies;

[0011] (2) Using the thorium curve, potassium curve, and density curve in the logging curve, construct a potassium index characteristic identification curve based on clarifying the three sensitive parameters of "low density, high potassium, and low thorium" of halite-type polyhalite;

[0012] (3) Calibrate the potassium index threshold values of different lithologies and halite-type polyhalite using core data to achieve high-precision logging identification of the quality of polyhalite;

[0013] (4) Calculate the shale content of the formation using the thorium curve;

[0014] (5) Inversely calculate the potassium content therein using the classical model of shale content;

[0015] (6) The influence of potassium content in argillaceous minerals is removed by gamma energy spectrum difference correction, and a quantitative calculation formula for potassium content based on radioactive difference analysis is obtained.

[0016] In the logging quantitative evaluation method for potassium content in halite-type polyhalite, in step (2), specifically, the potassium and thorium curves are integrated to enhance the sensitivity to polyhalite, and the ratio of potassium to thorium is used as a sensitive curve for identifying polyhalite; moreover, based on the first identification of the three sensitive parameters of "low density, high potassium, and low thorium" for halite-type polyhalite, a potassium index characteristic identification curve is constructed:

[0017]

[0018] In the above formulas (1)-(2), K_TH is the potassium-thorium ratio, POTA is the potassium content in the formation shown by natural gamma energy spectrum logging, THOR is the thorium content in the formation shown by natural gamma energy spectrum logging, KI is the potassium index, K_TH min is the minimum potassium-thorium ratio, K_TH max is the maximum potassium-thorium ratio, DEN is the density data in the logging data, DEN min is the minimum density data in the logging data, DEN max is the maximum density data in the logging data.

[0019] In the logging quantitative evaluation method for potassium content in halite-type polyhalite, wherein: in step (3), the KI thresholds for different lithologies and halite-type polyhalite are determined through core data calibration. When KI is between 0.1 and 0.2, it is defined as the third type of polyhalite layer; when KI is between 0.2 and 0.4, it is defined as the second type of polyhalite layer; when KI is greater than 0.4, it is defined as the first type of polyhalite layer; the KI value is used for high-precision logging identification of the quality of polyhalite.

[0020] In the logging quantitative evaluation method for potassium content in halite-type polyhalite, in step (4), the shale content of the formation is calculated using the change in the thorium curve value:

[0021]

[0022] In the above formulas (3)-(4), SH TH is the shale content index; TH max is the thorium value of the pure shale layer; TH min is the thorium value of the pure sandstone formation; V TH is the total shale amount; GCUR is the formation constant, and 2 is taken for old formations.

[0023] The logging quantitative evaluation method for potassium content in sylvinite polyhalite, wherein step 5) is to calculate the formation apparent shale content, i.e., the sum of shale content and potassium-rich minerals, using the potassium curve, and introduce the GCUR empirical constant to optimize calculation models (5) and (6):

[0024]

[0025] In the above formulas (5)-(6), SH K* is the shale content index; V K * is the apparent shale content calculated for potassium; K max is the potassium value of the pure shale layer; K min is the potassium value of the pure sandstone formation; GCUR is the formation constant, and 2 is taken for old formations.

[0026] The logging quantitative evaluation method for potassium content in sylvinite polyhalite, wherein in step 6), the process of obtaining the quantitative calculation formula for potassium content is as follows: Using the apparent shale content V K * value, calibrate the thorium curve and potassium curve with anhydrite formation as the reference lithology, and calculate the potassium-rich mineral content V K using the "gamma energy spectrum difference analysis method", and the calculation formula for the potassium-rich mineral content V K is as follows:

[0027]

[0028] In the above formula (7), V TH is the total shale amount; V K* is the potassium-rich mineral content; a is an empirical parameter, calibrated using core data, and by default, a is 1 in the absence of core data.

[0029] The logging quantitative evaluation method for potassium content in sylvinite polyhalite, wherein: The lithology and its logging curve characteristics in step 1) mainly include traditional polyhalite, sylvinite polyhalite, anhydrite, halite rock, mudstone, and mung bean rock.

[0030] Adopting the above technical solution, the present invention has the following beneficial effects:

[0031] The log quantitative evaluation method for potassium content in halite-type polyhalite of the present invention is reasonably conceived. By using the three parameters of low density, high potassium and low thorium sensitivity of halite-type polyhalite, a log identification chart for halite-type polyhalite is established, and a characteristic identification curve of potassium index is constructed. The KI thresholds of different lithologies and halite-type polyhalite are determined through calibration with core data. The identification accuracy is significantly improved compared with the conventional method, and the coincidence rate reaches more than 97%. At the same time, the influence of potassium content of clay minerals in halite-type polyhalite is eliminated through energy spectrum radioactive difference correction, and a log quantitative calculation method for potassium content based on radioactive difference analysis is formed. The calculation result is in good agreement with the experimental determination of potassium content, reaching more than 90%, effectively solving the problem of quantitative calculation of potassium content in halite-type polyhalite. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0033] Figure 1 Schematic diagram of logging response of traditional polyhalite rock involved in the log quantitative evaluation method for potassium content in halite-type polyhalite of the present invention;

[0034] Figure 2 Schematic diagram of logging response of salt crystal particle polyhalite rock involved in the log quantitative evaluation method for potassium content in halite-type polyhalite of the present invention;

[0035] Figure 3 Schematic diagram of conventional logging response characteristics of anhydrite rock involved in the log quantitative evaluation method for potassium content in halite-type polyhalite of the present invention;

[0036] Figure 4 Schematic diagram of conventional logging response characteristics of halite rock involved in the log quantitative evaluation method for potassium content in halite-type polyhalite of the present invention;

[0037] Figure 5 Schematic diagram of conventional logging response characteristics of mudstone involved in the log quantitative evaluation method for potassium content in halite-type polyhalite of the present invention;

[0038] Figure 6 Schematic diagram of logging response characteristics of mung bean rock involved in the log quantitative evaluation method for potassium content in halite-type polyhalite of the present invention;

[0039] Figure 7 Radar chart of logging responses of main lithologies involved in the log quantitative evaluation method for potassium content in halite-type polyhalite of the present invention;

[0040] Figure 8 Schematic diagram of logging identification of polyhalite in Well X involved in the logging quantitative evaluation method for potassium content in halite-type polyhalite of the present invention;

[0041] Figure 9 Schematic diagram for comparison of potassium content calculated from core analysis and logging in Well X involved in the logging quantitative evaluation method for potassium content in halite-type polyhalite of the present invention. Specific embodiments

[0042] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0043] The present invention will be further explained and described below in conjunction with specific embodiments.

[0044] The logging quantitative evaluation method for potassium content in halite-type polyhalite provided in this embodiment uses the three parameters of low density, high potassium, and low thorium sensitivity of halite-type polyhalite to establish a logging identification chart for halite-type polyhalite, construct a potassium index characteristic identification curve, and determine the KI thresholds of different lithologies and halite-type polyhalite through calibration with core data; the identification accuracy is significantly improved compared with the conventional method, and the coincidence rate reaches more than 97%; at the same time, the influence of the potassium content of clay minerals in halite-type polyhalite is eliminated through energy spectrum radioactive difference correction, forming a logging quantitative calculation method for potassium content based on radioactive difference analysis. The calculation result has a coincidence degree of more than 90% with the experimental determination of potassium content, effectively solving the problem of quantitative calculation of potassium content in halite-type polyhalite.

[0045] The present invention specifically includes the following steps:

[0046] S100. First, calibrate the logging according to geological logging and core sample analysis and test data to clarify the lithologies often coexisting with halite-type polyhalite and their logging curve characteristics. It mainly includes the logging response characteristics of lithologies such as traditional polyhalite, halite-type polyhalite, anhydrite, halite rock, mudstone, and mung bean rock.

[0047] ① Traditional polyhalite

[0048] Traditional polyhalite rock is dense and hard, occurring in massive form. It is a potassium-magnesium sulfate mineral and is insoluble in water. In the conventional logging curves, the natural gamma shows high values, generally between 171 and 180 API, with an average of 176 API; the density is relatively low, between 2.57 and 2.71 g / cm 3 3, with an average of 2.65 g / cm 3; The acoustic wave is a relatively high value, between 55 and 64 μs / ft, with an average of 60 μs / ft; the compensated neutron is a relatively high value, between 16.1 and 22.5%, with an average of 18.9%; the resistivity is overall high, mainly between 10924 and 13963 Ω·m, with an average of 12424 Ω·m; uranium is a relatively low value, between 0.204 and 1.142 ppm, with an average of 0.735 ppm; thorium is a relatively low value, between 0.78 and 1.16 ppm, with an average of 0.98 ppm; potassium is a relatively high value, between 9.94 and 11.01%, with an average of 10.46%( Figure 1 ).

[0049] ② Sylvite-type polyhalite

[0050] Sylvite-type polyhalite is a potassium polyhalite ore with salt crystal particles, distributed in the form of star dots, irregular lumps or strip-like shapes in the halite matrix. Internal clastic particles of different sizes such as polyhalite lumps and aggregates are scattered in the halite matrix. After injecting fresh water, the halite matrix as a cement quickly dissolves, and the polyhalite particles lose support and enter the brine solution, being in a random motion state and further dissolved in water, becoming soluble internal clastic particle polyhalite (such as Figure 2 ).

[0051] ③ Anhydrite rock

[0052] On the conventional logging curve, it shows: low natural gamma, generally between 11 and 19 API, with an average of 14 API; the density is a relatively high value, between 2.89 and 3.03 g / cm 3 ³, with an average of 2.96 g / cm 3 ³; the acoustic wave is a relatively medium value, between 47 and 54 μs / ft, with an average of 50 μs / ft; the compensated neutron is a relatively low value, between -1.5 and -0.2%, with an average of -0.9%; the resistivity is overall relatively high, mainly between 62000 and 99990 Ω·m, with an average of 81000 Ω·m; uranium is a relatively low value, between 1.29 and 2.34 ppm, with an average of 1.73 ppm; thorium is a relatively low value, between 1.04 and 2.32 ppm, with an average of 1.43 ppm; potassium is a relatively low value, between 0.29 and 0.49%, with an average of 0.38% (such as Figure 3 ).

[0053] ④ Halite rock

[0054] On the conventional logging curve, it shows: the natural gamma shows a low value, generally between 12 and 22 API, with an average of 17 API; the density is a relatively low value, between 1.55 and 2.48 g / cm 3 ³, with an average of 1.94 g / cm 3; The acoustic wave is a relatively high value, between 63 and 88 μs / ft, with an average of 70 μs / ft; the compensated neutron is a relatively low value, between 13 - 22%, with an average of 18.0%; the resistivity is generally high, mainly between 17,000 and 99,990 Ω.m, with an average of 92,500 Ω.m; uranium is a relatively low value, between 0.53 and 1.82 ppm, with an average of 1.23 ppm; thorium is a relatively low value, between 1.08 and 4.17 ppm, with an average of 1.76 ppm; potassium is a relatively low value, between 0.27 and 0.75%, with an average of 0.44% (such as Figure 4 ).

[0055] ⑤ Shale

[0056] On the conventional logging curves, it shows that: the natural gamma shows a high value, generally between 92.6 and 157.8 API, with an average of 138.3 API; the density is a relatively medium value, between 2.69 and 2.76 g / cm 3 3, with an average of 2.70 g / cm 3 3; the acoustic wave is a relatively high value, between 55 and 74 μs / ft, with an average of 67 μs / ft; the compensated neutron is a relatively high value, between 12 and 23%, with an average of 20%; the resistivity is generally a relatively low value, mainly between 19.5 and 73.4 Ω.m, with an average of 25 Ω.m; uranium is a relatively low value, between 2.7 and 5.5 ppm, with an average of 4.5 ppm; thorium is a relatively high value, between 5.06 and 13.54 ppm, with an average of 11.03 ppm; potassium is a relatively low value, between 2.18 and 5.54%, with an average of 4.27% (such as Figure 5 ).

[0057] ⑥ Mung bean rock

[0058] On the conventional logging curves, it shows that: the natural gamma shows a medium - high value, generally between 41 and 66 API, with an average of 56 API; the density is a medium value, between 2.71 and 2.81 g / cm 3 3, with an average of 2.75 g / cm 3 3; the acoustic wave is a medium value, between 50 and 52 μs / ft, with an average of 51 μs / ft; the compensated neutron is a relatively medium value, between 4.3 and 7.3%, with an average of 6%; the resistivity is generally a relatively high value, between 10,000 and 12,000 Ω.m, with an average of 11,000 Ω.m; uranium is a relatively low value, between 0.74 and 2.09 ppm, with an average of 1.06 ppm; thorium is a relatively low value, between 2.99 and 4.44 ppm, with an average of 4.07 ppm; potassium is a relatively low value, between 1.03 and 2.86%, with an average of 2.12% (such as Figure 6 ).

[0059] Based on the clear identification of the logging response characteristics of different lithologies, the logging identification models of the main lithologies in the Jialingjiang Formation in the study area are established (Table 1, Figure 7 ).

[0060] Table 3-1 Logging Classification and Identification Models of the Main Lithologies in the Jialingjiang Formation

[0061]

[0062]

[0063] Traditional polyhalite is a poorly soluble potassium salt, occurring in anhydrite rock in the form of bands, laminations and massive bodies. The logging curves have the characteristics of "three highs and two lows", showing high natural gamma, high potassium, high resistivity, low thorium and low uranium. There are obvious differences between salt crystal particle polyhalite and traditional polyhalite. It is scattered in the halite matrix in the form of internal clastic particles of different sizes such as spots, lumps and aggregates. The logging curves have the characteristics of "three highs, three lows and one diameter expansion", specifically showing high natural gamma, high potassium, high resistivity, low thorium, low uranium and low density. Since halite rock is an easily soluble salt, diameter expansion often occurs. "Low density and diameter expansion" are the most typical identification signs of salt crystal particle polyhalite.

[0064] S200. By using the thorium curve, potassium curve and density curve in the logging curves, on the basis of clarifying the three sensitive parameters of "low density, high potassium and low thorium" of halite-type polyhalite, a characteristic identification curve of potassium index (KI) is constructed;

[0065] The thorium element in the formation is mainly affected by the shale content and grain size changes. When the grain size of the formation rock becomes finer, the thorium value increases; when the shale content increases, the thorium value increases. In carbonate rock formations, when the thorium curve shows a low value, it can be identified as a low clay-bearing formation, excluding the influence of shale on the identification of polyhalite; the potassium element is mainly affected by potassium-rich minerals in the formation, and the potassium curve is sensitive to the response of polyhalite. When polyhalite is present in the formation, the potassium value increases significantly; at the same time, the density curve can be used as the main difference between traditional polyhalite and halite-type polyhalite. Therefore, the potassium and thorium curves can be combined to expand the sensitivity to polyhalite, and the ratio of potassium to thorium can be used as a sensitive curve for identifying polyhalite.

[0066] On the basis of first clarifying the three sensitive parameters of "low density, high potassium and low thorium" of halite-type polyhalite, the characteristic identification curve of potassium index (KI) is constructed as follows:

[0067]

[0068] In the above formulas (1)-(2), K_TH is the potassium-thorium ratio, POTA is the potassium content in the formation shown by natural gamma ray spectroscopy logging, THOR is the thorium content in the formation shown by natural gamma ray spectroscopy logging, KI is the potassium index, K_TH minis the minimum potassium-thorium ratio, K_TH max is the maximum potassium-thorium ratio, DEN is the density data in the logging data, DEN min is the minimum density data in the logging data, DEN max is the maximum density data in the logging data.

[0069] S300. Calibrate the potassium index (KI) threshold values of different lithologies and halotrichite of the rock salt type using core data to achieve high-precision logging identification of the quality of halotrichite;

[0070] Determine the KI threshold values of different lithologies and halotrichite of the rock salt type through calibration with core data. When KI is between 0.1 and 0.2, it is defined as the third type of halotrichite layer; when KI is between 0.2 and 0.4, it is defined as the second type of halotrichite layer; when KI is greater than 0.4, it is defined as the first type of halotrichite layer. Use the KI value to achieve high-precision logging identification of the quality of halotrichite (Table 2).

[0071] Table 2 Logging classification and evaluation criteria for halotrichite layers

[0072] Serial number Evaluation criteria Classification 1 KI ≥ 0.4 Type I polyhalite layer 2 0.2 ≤ KI < 0.4 Type II polyhalite layer 3 0.1 ≤ KI < 0.2 Type III polyhalite layer 4 KI < 0.1 Non-polyhalite layer

[0073] Figure 8 is the logging identification result map of the halotrichite layer in Well X. A total of 10 halotrichite-containing layers with a thickness of 99.8 m are interpreted. The single-layer thickness is between 1.5 and 30.0 m, and the average single-layer thickness is 10.0 m. The logging response characteristics are as follows: The overall natural gamma value in the halotrichite-containing layer section is medium to high, between 12.8 and 163.9 API, the non-uranium gamma value is between 7.2 and 157.2 API, the resistivity is high, between 33 and 99990 Ω·m, and the core density is between 2.22 and 3.04 g / cm 3 between.

[0074] Identify halotrichite using the characteristic identification curve of the potassium index (KI), and the accuracy is significantly improved compared with the conventional method.

[0075] S400. Calculate the formation shale content using the thorium curve

[0076] The natural radioactivity of the formation mainly comes from uranium (U), thorium (Th), and potassium (K) elements in the formation. For evaporite formations, U mainly comes from clay, kerogen, formation water, carbonate rocks, etc., which are widely present in the formation and cannot be used as an indicator element for shale; Th mainly comes from shale, and the main factors controlling its distribution in sedimentary rocks are the adsorption of Th by shale and the presence of Th in stable minerals; K mainly comes from shale and halotrichite. Since the K content in the formation is small, it can be considered that K is mainly contributed by potassium-rich minerals.

[0077] Since the presence of Th in the formation is relatively stable and has a strong correlation with the sedimentary environment and shale content, the shale content of the formation can be calculated using the change in Th value as follows:

[0078]

[0079] In the formula: SH TH , shale content index; TH max , thorium value of pure shale layer, unit PPM; TH min , thorium value of pure sandstone formation, unit PPM; V TH , total shale content, %; GCUR, formation constant, generally taken as 2 for old formations.

[0080] S500. Inverse calculation of potassium content using the classical model of shale content

[0081] When there are no potassium-rich minerals in the formation, the shale content calculated using the thorium (Th) curve and potassium (K) curve is basically the same. When there is a difference between the two, especially when the shale content calculated from the potassium curve is higher than the result calculated from the thorium curve, this difference is mainly affected by potassium-rich minerals. Therefore, after calculating the shale content using the thorium curve, the apparent shale content of the formation (the sum of shale content and potassium-rich minerals) is calculated using the potassium curve, and the GCUR empirical constant is introduced to optimize the calculation model:

[0082]

[0083] In the above formulas (5)-(6): SH K* , shale content index; V K *, apparent shale content calculated from potassium; K max , potassium value of pure shale layer, %; K min , potassium value of pure sandstone formation, unit %; GCUR, formation constant, generally taken as 2 for old formations.

[0084] S600. Using gamma energy spectrum difference correction to eliminate the influence of potassium content in shale minerals, and obtaining a quantitative calculation formula for potassium content based on radioactive difference analysis.

[0085] Using the apparent shale content V K * value, calibrate the thorium (Th) curve and potassium (K) curve with anhydrite formation as the reference lithology, and calculate the content of potassium-rich minerals V K using the "gamma energy spectrum difference analysis method" as follows:

[0086]

[0087] In the formula: V TH , total shale content, %; V K*, potassium-rich mineral content, %; a, empirical parameter, calibrated using core data, and defaulted to 1 in the absence of core data.

[0088] The practical application of the present invention is as follows:

[0089] Using the above model to calculate the potassium content of Well X ( Figure 9 ). In the research, potassium-rich minerals are used as one of the mineral components for logging calculation. The shale content is calculated using the thorium curve, and a targeted quantitative calculation model is established. The calculated lithology profile is more in line with the actual situation, effectively improving the logging evaluation accuracy of this formation.

[0090] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A logging quantitative evaluation method for potassium content in halite - type polyhalite, characterized in that: Using the three parameters of low density, high potassium and low thorium sensitivity of halite-type polyhalite, a logging identification chart of halite-type polyhalite is established, a characteristic identification curve of potassium index is constructed, the potassium index threshold values of different lithologies and halite-type polyhalite are determined through calibration with core data, and at the same time, the influence of potassium content of clay minerals in halite-type polyhalite is eliminated through energy spectrum radioactive difference correction, and a quantitative calculation formula of potassium content logging based on radioactive difference analysis is obtained; The evaluation method mainly includes the following steps: (1) First, calibrate the logging according to the geological logging and core sample analysis and test data, clarify the lithologies often coexisting with halite-type polyhalite and their logging curve characteristics, and establish the logging identification mode of the main lithologies in the study area's strata on the basis of clarifying the logging response characteristics of different lithologies; (2) Using the thorium curve, potassium curve and density curve in the logging curves, on the basis of clarifying the three sensitive parameters of "low density, high potassium and low thorium" of halite-type polyhalite, construct a characteristic identification curve of potassium index; (3) Calibrate the potassium index threshold values of different lithologies and halite-type polyhalite through core data to achieve high-precision logging identification of the quality of polyhalite; (4) Calculate the formation shale content using the thorium curve; (5) Inversely calculate the potassium content therein using the classical model of shale content; (6) Eliminate the influence of potassium content in shale minerals through gamma energy spectrum difference correction to obtain a quantitative calculation formula of potassium content based on radioactive difference analysis.

2. The logging quantitative evaluation method for potassium content in halite-type polyhalite as described in claim 1, wherein The specific content of step (2) is to comprehensively combine the potassium and thorium curves to expand the sensitivity to polyhalite, and use the ratio of potassium to thorium as the sensitive curve for identifying polyhalite; and on the basis of first clarifying the three sensitive parameters of "low density, high potassium and low thorium" of halite-type polyhalite, construct a characteristic identification curve of potassium index: ； ； In the above formulas (1)-(2), K_TH is the potassium-thorium ratio, POTA is the potassium content in the formation shown by natural gamma-ray spectroscopy logging, THOR is the thorium content in the formation shown by natural gamma-ray spectroscopy logging, KI is the potassium index, K_TH min is the minimum potassium-thorium ratio, K_TH max is the maximum potassium-thorium ratio, DEN is the density data in the logging data, DEN min is the minimum density data in the logging data, DEN max is the maximum density data in the logging data.

3. The logging quantitative evaluation method for potassium content in halite-type polyhalite as described in claim 1, characterized in that: Step (3) is to determine the KI threshold values of different lithologies and halite-type polyhalite through calibration with core data. When KI is between 0.1 and 0.2, it is defined as a third-class polyhalite layer; when KI is between 0.2 and 0.4, it is defined as a second-class polyhalite layer; when KI is greater than 0.4, it is defined as a first-class polyhalite layer; use the KI value to achieve high-precision logging identification of the quality of polyhalite.

4. The logging quantitative evaluation method for potassium content in halite-type polyhalite as described in claim 1, wherein Step (4) is to calculate the formation shale content using the change of the thorium curve value: ； ； In the above formulas (3)-(4), SH TH is the shale content index; TH max is the thorium value of the pure shale layer; TH min is the thorium value of the pure sandstone formation; V TH is the total shale content; GCUR is the formation constant, and 2 is taken for the old formation.

5. The logging quantitative evaluation method for potassium content in halite-type polyhalite as described in claim 1, characterized in that, Step (5) is to calculate the apparent shale content of the formation, that is, the sum of shale content and potassium-rich minerals, using the potassium curve, and introduce the GCUR empirical constant to optimize the calculation models (5) and (6): ； ； In the above formulas (5)-(6), SH K* is the shale content index; is the apparent shale content calculated from potassium; K max is the potassium value of the pure shale layer; K min is the potassium value of the pure sandstone formation; GCUR is the formation constant, and 2 is taken for old formations.

6. The logging quantitative evaluation method for potassium content in halite - type polyhalite as claimed in claim 1, wherein The process of obtaining the quantitative calculation formula for potassium content in step 6) is as follows: Using the shale volume content value, calibrate the thorium curve and potassium curve with anhydrite formation as the reference lithology, and calculate the potassium-rich mineral content using the "gamma energy spectrum difference analysis method" , and the calculation formula for the potassium-rich mineral content is as follows: ； In the above formula (7), V TH is the total shale content; is the content of potassium-rich minerals; a is an empirical parameter, calibrated using core data. In the absence of core data, a is defaulted to 1.

7. The logging quantitative evaluation method for potassium content in halite-type polyhalite as described in claim 1, characterized in that: The lithologies and their logging curve characteristics in step (1) mainly include traditional polyhalite, halite-type polyhalite, anhydrite, halite rock, mudstone and mung bean rock.

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