A method for evaluating helium resources

Abstract

The application discloses a helium resource quantity evaluation method, and relates to the field of helium resource exploration and evaluation. In the application, all helium source rocks in a helium resource evaluation area are divided into multiple helium source rock calculation units, and the helium generation quantity of all helium source rocks is fully considered; a helium discharge coefficient and a migration and accumulation coefficient are introduced, the loss of helium from the source rock to the gas reservoir is fully considered, and the migration and accumulation coefficient of the helium is determined by using a geological analogy method, so that the calculation result of the helium resource quantity is more scientific and accurate, and the effective evaluation of the helium resource quantity in a low exploration degree area is realized.

Description

Technical Field

[0001] The present invention relates to the field of helium resource exploration and evaluation, and in particular to a helium resource evaluation method. Background Art

[0002] Helium, as an associated gas with natural gas, has limited resources to calculate independently. Currently, there are two main methods for evaluating helium resources among domestic and international scholars. One method focuses on evaluating the helium content of natural gas in proven helium-rich natural gas reservoirs, combining reservoir parameters (including gas-bearing area, average effective thickness of gas layers, average effective porosity, average initial gas saturation, average formation temperature, surface standard temperature, average initial formation pressure, surface standard pressure, and original gas deviation coefficient) with the helium content of natural gas in evaluation wells. This method is suitable for evaluating helium resources in proven natural gas reservoirs and requires a high level of exploration, particularly the control and acquisition of accurate geological reserve parameters for exploration and evaluation wells. The other method, based on the radioactive decay of U (uranium) and Th (thorium), calculates rock-derived helium content based on parameters such as the volume of helium-bearing rocks, the abundance of U and Th elements, and the age of rock formation. This method can be broadly categorized as a genetic method. Although this method can roughly calculate the amount of helium generated by helium-producing rocks, it has three problems. First, although granite intrusions in the crust are most enriched in U and Th, which are necessary conditions for the formation of helium-rich natural gas, the helium generated in the lower crust, middle crust and sedimentary cover cannot be ignored. Although the U and Th content is low, the rocks are generally large in scale and formed early, and can still generate a large amount of helium. Calculating only the helium generation of major rocks such as granite seriously underestimates the total amount of helium generated in the crust. Second, after complex migration, only a small amount of helium generated by rocks can enter and accumulate in natural gas reservoirs. It is not scientific to use the helium generation of granite as the resource evaluation result of the basin. The helium expulsion coefficient (helium expulsion / helium generation) and migration and accumulation coefficient (helium accumulation / helium expulsion) of helium source rocks should be further clarified to evaluate helium resources. For an area with a low level of geological exploration, it is difficult to obtain the scale and helium content of underground natural gas reservoirs due to the lack of survey wells, exploration wells and evaluation wells. The evaluation of helium resources lacks a basis. Therefore, it is urgent to develop a helium resource evaluation method in low-exploration areas to provide a basis for helium exploration. Summary of the Invention

[0003] The purpose of the present invention is to provide a helium resource evaluation method, which can effectively evaluate helium resources in low-exploration areas.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] A helium resource evaluation method, comprising:

[0006] The helium source rocks in the helium resource evaluation area are divided into multiple helium source rock calculation units; the multiple helium source rock calculation units at least include lower crust, middle crust and upper crust;

[0007] The content test samples of radioactive elements of each geological body of the helium source rock calculation unit are collected, and the average abundance of uranium and thorium in each helium source rock calculation unit is obtained through laboratory testing;

[0008] The average helium generation rate of each helium source rock calculation unit is calculated according to the average abundance of uranium and thorium in each helium source rock calculation unit;

[0009] The total amount of helium generated in each helium source rock calculation unit is determined according to the volume scale, the average helium generation rate and the effective helium generation time of each helium source rock calculation unit;

[0010] The helium discharge coefficient of each helium source rock calculation unit is determined according to the burial depth of each helium source rock calculation unit;

[0011] The helium migration and accumulation coefficient is obtained by using the geological analogy method;

[0012] The total helium discharge amount is determined according to the total amount of helium generated in each helium source rock calculation unit and the helium discharge coefficient, and then the helium resource amount of the helium resource evaluation area is determined in combination with the helium migration and accumulation coefficient.

[0013] Optionally, the division of the helium source rocks in the helium resource evaluation area into multiple helium source rock calculation units specifically includes:

[0014] 1 complete helium generation, migration, enrichment and reservoir system is regarded as 1 helium resource evaluation area;

[0015] All the helium source rocks in 1 helium resource evaluation area that supply helium to natural gas reservoirs are divided into multiple calculation units according to the difference in uranium and thorium content, including lower crust, middle crust and upper crust;

[0016] The upper crust is further divided into multiple granite body calculation units, sedimentary cap rock calculation units, magnetic body calculation units and other upper crust calculation units; the other upper crust calculation units are parts of the upper crust for which the volume scale and the uranium and thorium element content can be clearly obtained.

[0017] Optionally, the calculation formula of the average helium generation rate of each helium source rock calculation unit is

[0018]

[0019] In the formula, J i represents the average helium generation rate of the i th helium source rock calculation unit, [U i ] represents the average abundance of uranium in the rock of the i th helium source rock calculation unit, and [Th irepresents the average abundance of thorium in the i th helium source rock calculation unit rock.

[0020] Optionally, the total amount of helium generation of each helium source rock calculation unit is determined according to the volume scale, the average helium generation rate and the effective helium generation time of each helium source rock calculation unit, and specifically comprises:

[0021] The effective helium generation time, the area, the average density and the average porosity of each helium source rock calculation unit are determined.

[0022] The thickness of each helium source rock calculation unit is determined; wherein the granite body is assumed to be a uniform column, and the thickness is valued according to the thickness of the upper crust.

[0023] The total amount of helium generation of each helium source rock calculation unit is calculated according to the area, the thickness, the average density, the average porosity, the average helium generation rate and the effective helium generation time of each helium source rock calculation unit, and by using the formula

[0024] In the formula, f (Q i ) represents the total amount of helium generation of the i th helium source rock calculation unit; S i represents the area of the i th helium source rock calculation unit; H i represents the thickness of the i th helium source rock calculation unit; p i represents the average density of the i th helium source rock calculation unit, represents the average porosity of the i th helium source rock calculation unit; and T represents the effective helium generation time of the i th helium source rock calculation unit.

[0025] Optionally, the helium migration and accumulation coefficient is obtained by using the geological analogy method, and specifically comprises:

[0026] A helium resource proven area with a similar helium accumulation geological background to the helium resource evaluation area is selected as a calibration area, the helium discharge amount and the helium resource amount of the calibration area are calculated, and the helium migration and accumulation coefficient c c of the calibration area is determined by using the formula c c = Q c / F k ; in the formula, Q kj represents the helium resource amount of the calibration area, and F kj represents the helium discharge amount of the calibration area.

[0027] A parameter system for risk evaluation of migration and accumulation conditions is established; the parameter system comprises source rocks of carrier gas, reservoir conditions, trap conditions, preservation conditions and migration and accumulation supporting conditions, and each condition is further divided into a plurality of sub-factors.

[0028] The migration and accumulation conditions of the calibration area and the helium resource evaluation area are respectively risk evaluated to obtain the risk evaluation values of the migration and accumulation conditions of the calibration area and the helium resource evaluation area. ​

[0029] According to the evaluation value of each condition in the risk evaluation parameter system of migration and accumulation conditions in the calibration area and helium resource evaluation area, the formula and Calculate the probability values ​​of migration and accumulation coefficients for the calibration area and the helium resource evaluation area respectively; where P represents the probability value of migration and accumulation coefficient, P k represents the probability of accumulation of the kth condition in the risk assessment parameter system of migration and accumulation conditions, q kj represents the weight of the j-th factor in the k-th condition, P kj Indicates the evaluation value of the j-th sub-item factor in the k-th condition, 0≤P kj ≤1;

[0030] According to the probability values ​​of migration and accumulation coefficients of the scale area and the helium resource evaluation area, as well as the helium migration and accumulation coefficient of the scale area, according to the formula Calculate the helium migration and accumulation coefficient c0 in the helium resource evaluation area; where P0 represents the probability value of the migration and accumulation coefficient in the helium resource evaluation area, P c Indicates the probability value of the clustering coefficient of the scale area.

[0031] Optionally, the calculation formula for the helium resource amount in the helium resource evaluation area is:

[0032]

[0033] Where Q He represents the helium resource volume in the helium resource evaluation area, α i represents the helium expulsion coefficient of the i-th helium source rock calculation unit; n represents the number of helium source rock calculation units in the helium resource evaluation area.

[0034] Optionally, the sub-factors of the carrier gas source rock include: cumulative source rock thickness, organic carbon content, organic matter type, maturity, hydrocarbon supply area coefficient, hydrocarbon supply mode, hydrocarbon generation intensity, hydrocarbon generation peak time, migration distance and dredging conditions;

[0035] The sub-factors of the reservoir conditions include: reservoir type, reservoir porosity, reservoir permeability and reservoir depth;

[0036] The sub-factors of the trap conditions include: trap type, trap area coefficient and trap amplitude;

[0037] The sub-factors of the preservation conditions include: cap rock lithology, cap rock thickness and cap rock damage degree;

[0038] The sub-factors of the migration and accumulation matching conditions include: carrier gas and reservoir configuration, helium migration pathway, groundwater dynamic field, matching of trap and carrier gas formation period, and configuration of trap and helium source rock.

[0039] According to the specific embodiments of the present application, the following technical effects are disclosed:

[0040] The present application discloses a helium resource quantity evaluation method, all helium source rocks in a helium resource evaluation area are divided into multiple helium source rock calculation units, the helium generation amount of all helium source rocks is fully considered; the discharge coefficient and the helium migration and accumulation coefficient are introduced, the loss of helium from the source rock to the gas reservoir is fully considered, and the geological analogy method is used to determine the helium migration and accumulation coefficient, so that the calculation result of the helium resource quantity is more accurate, and the helium resource quantity in the low exploration degree area is effectively evaluated. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can be obtained from these drawings without creative labor.

[0042] Figure 1 The flow chart of the helium resource quantity evaluation method provided by the present application is shown in the figure. DETAILED DESCRIPTION

[0043] The technical solutions in the embodiments of the present application will be described clearly and completely with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0044] The present application aims to provide a helium resource quantity evaluation method, which can effectively evaluate the helium resources in the low exploration degree area.

[0045] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application will be further described in detail below with reference to the drawings and specific embodiments.

[0046] The proposed helium resource assessment method solves the following problems: First, it clarifies how to divide the calculation units. Based on the differences in U and Th content in different types of helium-generating rocks, while taking into account the convenience of the calculation method, a method for dividing the helium source rock calculation units is proposed. Second, it simplifies the calculation model and calculates the volume of the intrusive granite body according to the columnar intrusion model. The volume calculation method is the area of ​​the intrusion body multiplied by the upper crust thickness. Third, based on the research results of Zhang et al. (2020), the expulsion coefficient of helium source rocks of different types and burial depths is determined. Fourth, the helium migration and accumulation coefficient is introduced. Based on the probabilistic evaluation method of the geological parameters that constrain helium migration and enrichment, the helium migration and enrichment parameters of different regions are obtained according to the geological analogy method. This forms a basin- and zone-level helium resource assessment method. Compared with previous methods, this method has the following advantages: first, the calculation system is more complete, fully considering the helium production of all helium-producing rocks; second, the calculation model method is further simplified, simplifying the complex intrusion body into a columnar model, which facilitates rapid resource evaluation; third, the introduction of expulsion coefficient and migration and accumulation coefficient fully considers the loss of helium at each link from source rock to gas reservoir, and the calculation results are more scientific and reliable; fourth, the introduction of analogy method and multiple sub-factors to evaluate the migration and accumulation coefficient of helium makes the value of the migration and accumulation coefficient more scientific.

[0047] like Figure 1 As shown, an embodiment of the present invention provides a helium resource evaluation method, comprising:

[0048] Step S1: Divide all helium source rocks in the helium resource evaluation area into a plurality of helium source rock calculation units; the plurality of helium source rock calculation units at least include the lower crust, the middle crust and the upper crust.

[0049] The scope of the evaluation area is determined based on the objectives and tasks of the resource assessment. Typically, a complete helium generation, migration, enrichment, and accumulation system is divided into an evaluation unit, which can be a basin or a zone. Once the evaluation unit is determined, all potential helium source rocks within the evaluation unit are further divided into several helium source rock calculation units. Specifically, all helium source rocks that contribute helium to natural gas reservoirs are divided into calculation units based on their U and Th contents. Although the lower and middle crust have low average U and Th contents and weak helium generation capacity, their overall thickness and significant total helium generation provide a background for helium enrichment and are therefore included in the calculations. This division scheme generally includes the lower, middle, and upper crust. The upper crust can be further divided into several calculation units, such as intrusive granites, magnetic bodies, and sedimentary cover, depending on the level of geological work. Excluding calculation units where U and Th contents are clearly known, the remaining geologically unclear portions of the upper crust are classified as a unified calculation unit, with U and Th contents calculated based on the crustal average.

[0050] Step S2: Collect radioactive element content test samples from each helium source rock calculation unit geological body, and obtain the average abundance of uranium and thorium in each helium source rock calculation unit through laboratory testing.

[0051] (1) Average abundance of radioactive uranium in helium source rocks (U i )

[0052] For granite and concealed magnetic bodies, if surface outcrop or drilled rock samples are available, the radioactive uranium content is calculated by averaging the actual measured data. If samples are difficult to obtain for concealed magnetic bodies, the average uranium abundance can be determined by testing similar rock samples collected from adjacent outcrops based on the lithologic interpretation of the concealed magnetic bodies. Uranium content data for other areas of the upper crust, middle crust, and lower crust are based on average crustal data, as shown in Table 1.

[0053] Table 1 Average abundance data of U and Th in the Earth's crust

[0054]

[0055]

[0056] (2) Average abundance of radioactive thorium in rocks (Th i )

[0057] If surface outcrop or drilled rock samples are available for granite and concealed magnetic bodies, radioactive thorium content can be calculated by averaging the actual measured data. If samples of concealed magnetic bodies are difficult to obtain, the average thorium abundance can be determined by testing similar rock samples collected from adjacent outcrops based on the lithologic interpretation of the concealed magnetic bodies. Thorium content data for other areas of the upper crust, middle crust, and lower crust are based on average crustal data (Table 1).

[0058] Step S3: Calculate the average helium generation rate of each helium source rock calculation unit based on the average abundance of uranium and thorium in each helium source rock calculation unit.

[0059] The calculation formula for the average helium generation rate of each helium source rock calculation unit is:

[0060]

[0061] Where, J i represents the average helium generation rate of the i-th helium source rock calculation unit, in cm 3 / gyear;[U i ] represents the average abundance of uranium in the rock of the i-th helium source rock calculation unit, in ppm; [Th i ] represents the average abundance of thorium in the rocks of the i-th helium source rock calculation unit, in ppm.

[0062] Step S4: determining the total helium generation amount of each helium source rock calculation unit according to the volume scale, average helium generation rate and effective helium generation time of each helium source rock calculation unit.

[0063] The calculation formula for the total amount of helium generated by each helium source rock calculation unit is:

[0064]

[0065] Where, f(Q i ) represents the total amount of helium generated by the i-th helium source rock calculation unit, in m 3 ;S i Indicates the area of ​​the i-th helium source rock calculation unit, in km 2 ;H i represents the thickness of the i-th helium source rock calculation unit, in km; ρ i Indicates the average density of the i-th helium source rock calculation unit, in g / cm 3 ; represents the average porosity of the i-th helium source rock calculation unit, and the unit is dimensionless; T represents the effective helium generation time of the i-th helium source rock calculation unit, and the unit is a (year).

[0066] The parameters in the calculation formula for the total amount of helium generated are determined as follows:

[0067] (1) Calculation unit area of ​​helium source rock (S i )

[0068] The lower and middle crustal areas are the evaluation unit areas. The calculation unit area for granite bodies or magnetic bodies is determined based on the surface outcrop area and the interpretation of regional gravity and magnetic data (Li Yuhong et al., 2018a, 2018b). Due to the differences in geological structure and rock composition in different regions, the methods and standards for interpreting gravity and magnetic data vary. Taking the Weihe Basin as an example, based on ground magnetic and aerial survey data within the region, the magnetic ΔT polarization residual anomaly was obtained. Based on the characteristics of the magnetic ΔT residual anomaly, magnetic highs with anomaly intensities above 25 nT were delineated. This means that the range of magnetic highs above 25 nT is approximately considered to represent the distribution area of ​​magnetic bodies. Different regions can obtain this range based on previous research results in the evaluation area.

[0069] (2) Calculation unit thickness of helium source rock (H i )

[0070] The thickness of the lower crust, middle crust, and upper crust are determined based on previous research results on the crust thickness in this region and globally. The thickness of granite intrusions and magnetic bodies with research results is directly used. Since the rock mass morphology is difficult to determine, in order to simplify the calculation process, the rock mass is assumed to be uniformly columnar, and the thickness is taken according to the thickness of the upper crust.

[0071] (3) Calculate the average cell density (ρi )

[0072] The density of the lower crust is between 2.8 and 2.95 g / cm 3 (Wang Qianshen et al., 2015), taking 2.9g / cm 3 The middle crust is mainly composed of gneiss, diorite, and granodiorite, with a density ranging from 2.75 to 2.85 g / cm 3 (Wang Qianshen et al., 2015), taking the average density value as 2.8g / cm 3 The rock mass and hidden magnetic bodies are mainly granite. For rock mass or magnetic bodies with surface outcrops or drill cores, laboratory measurements are carried out to determine density data. For the calculation units where samples are not available, according to previous research results, the density of the upper crust is between 2.64 and 2.78 g / cm 3 The average value of granite is 2.66g / cm 3 The average density of gneiss and schist is 2.64g / cm 3 (Yang Wencai et al., 2017). The density of sedimentary rocks in the upper crust is determined based on previous core drilling and logging data. The lower crust is composed of gneiss and intrusive rocks, with a density ranging from 2.6 to 2.7 g / cm 3 (Wang Qianshen et al., 2015; Yang Wencai et al., 2017), the average density is 2.6 g / cm 3 .

[0073] (4) Calculation of unit porosity

[0074] Except for some sedimentary rocks with an average porosity of 10%, intrusive rocks and metamorphic rocks have extremely low porosity, and sedimentary rocks only occupy a small proportion of the upper crust, so their impact on the calculation results is relatively low and can be ignored in the calculation.

[0075] (5) Calculate the effective helium generation time of the unit (T)

[0076] The formation age of major rock masses can be determined by collecting data from previous studies or performing zircon U-Pb dating. Considering that deep tectonic activity during the intrusion phase of the rock mass rapidly released helium from the source rocks, the crustal helium generation age is taken as the youngest age of the rock mass. Research has shown that helium generated in rocks diffuses from minerals into pores and fractures within a relatively short period of time. Given the relatively small size of helium molecules, their rapid escape velocity results in a relatively young age for helium release. Because most of the helium generated in the crust prior to the formation of the helium migration and accumulation system was released into the atmosphere, some background helium remains in the deep crust. Therefore, the effective helium generation time for a unit is calculated as the formation time of the helium migration and accumulation system in the region, that is, the time after the generation, reservoir, and caprock conditions were established.

[0077] Step S5, according to the burial depth of each helium source rock calculation unit, the helium discharge coefficient of each helium source rock calculation unit is determined.

[0078] According to the results of Zhang et al. (2020), 4 The He retention coefficient decreases with the increase of U and Th content in granite and minerals. The He retention coefficient of the Weihe River near-surface granite sample is 4 The He retention coefficient is distributed between 1.5% and 17.4%, which is lower than the previous study 4 The He retention coefficient in the three types of rocks is mostly less than 20%, that is, the release (discharge) coefficient of helium in the rock is as high as more than 80% (Tolstikhin and Drubetskoy 1975, 1977; Gerling et al. 1976; Martel et al. 1990; Tolstikhin et al. 1996, 2011). The average value of 90% is taken as the reference value in this calculation method. The discharge coefficient of granite and buried magnetic body is taken as 95%. The geothermal of middle and lower crust is higher than the sealing temperature of minerals, and the helium discharge coefficient of middle and lower crust is taken as 97%.

[0079] Step S6, the helium migration and accumulation coefficient is obtained by using the geological analogy method.

[0080] After the helium is discharged from the helium source rock, it undergoes a complex migration process, only a small amount of helium can migrate and accumulate in the gas reservoir. The migration and accumulation coefficient is the efficiency of helium from the helium source rock to the gas reservoir. It is the ratio of helium resources to total helium discharge of the helium source rock. The efficiency of helium migration and accumulation is controlled by five geological conditions, i.e. carrier gas source rock, reservoir condition, trap condition, preservation condition and migration and accumulation matching. The quality of each geological condition determines the efficiency of helium migration and accumulation, which is manifested as the high and low of migration distance coefficient.

[0081] This method uses the geological analogy method to obtain the helium migration and accumulation coefficient in the evaluation area. The main method is to calculate the total helium generation, helium discharge and helium resources of the helium source rock in an area with high exploration degree, and to deduce the helium migration and accumulation coefficient. This area is defined as the calibration area or known area. In the evaluation of new area (unknown area), the risk evaluation standards of five migration and accumulation conditions, i.e. carrier gas source rock, reservoir condition, trap condition, preservation condition and migration and accumulation matching, are established, and the similarity coefficient is calculated by scoring and risk evaluation of the five geological conditions in the calibration area (known area) and the evaluation area (unknown area). The similarity coefficient is used to calculate the helium migration and accumulation coefficient in the evaluation area (unknown area).

[0082] The specific calculation process is as follows:

[0083] Select a helium resource exploration area with a similar helium accumulation geological background to the helium resource evaluation area as the calibration area, calculate the helium discharge volume and helium resource volume of the calibration area, and use formula c c =Q c / F c , determine the helium transport coefficient c in the scale area c Where Q c Indicates the helium resource in the scale area, F c Indicates the amount of helium discharged in the scale area;

[0084] Establish a risk assessment parameter system for migration and accumulation conditions; the parameter system includes carrier gas source rocks, reservoir conditions, trap conditions, preservation conditions, and migration and accumulation supporting conditions, with each condition further divided into several sub-factors;

[0085] Conduct risk assessments on the migration and accumulation conditions of the calibration area and the helium resource evaluation area respectively, and obtain risk assessment values ​​of the migration and accumulation conditions of the calibration area and the helium resource evaluation area;

[0086] According to the evaluation value of each condition in the risk evaluation parameter system of migration and accumulation conditions in the calibration area and helium resource evaluation area, the formula and Calculate the probability values ​​of migration and accumulation coefficients for the calibration area and the helium resource evaluation area respectively; where P represents the probability value of migration and accumulation coefficient, P k represents the probability of accumulation of the kth condition in the risk assessment parameter system of migration and accumulation conditions, q kj represents the weight of the j-th factor in the k-th condition, P kj Indicates the evaluation value of the j-th sub-item factor in the k-th condition, 0≤P kj ≤1;

[0087] According to the probability values ​​of migration and accumulation coefficients of the scale area and the helium resource evaluation area, as well as the helium migration and accumulation coefficient of the scale area, according to the formula Calculate the helium migration and accumulation coefficient c0 in the helium resource evaluation area; where P0 represents the probability value of the migration and accumulation coefficient in the helium resource evaluation area, P c Indicates the probability value of the clustering coefficient of the scale area.

[0088] The five migration and accumulation conditions, including carrier gas source rock, reservoir conditions, trap conditions, preservation conditions, and migration and accumulation support, are further refined into 25 sub-factors. The risk assessment standards are shown in Table 2.

[0089] Table 2 Parameter system and value standard for geological evaluation of helium transport and accumulation

[0090]

[0091]

[0092] Since the level of work in the survey area is low, the evaluation criteria may be adjusted appropriately based on the situation in the survey area.

[0093] Step S7: Determine the total helium discharge amount based on the total helium generation amount and the helium discharge coefficient of each helium source rock calculation unit, and then determine the helium resource amount of the helium resource evaluation area in combination with the helium migration and accumulation coefficient.

[0094] The calculation formula for the helium resource amount in the helium resource evaluation area is:

[0095]

[0096] Where Q He Indicates the helium resource volume in the helium resource evaluation area, in m 3 ; α i It represents the helium expulsion coefficient of the i-th helium source rock calculation unit, and the unit is dimensionless; n represents the number of helium source rock calculation units in the helium resource evaluation area.

[0097] The method of the present invention is applicable to the evaluation of helium resources in various basins or zones and has good application prospects. Based on this method, helium resource evaluation was carried out in the Weihe Basin. The evaluation process is as follows:

[0098] Determination of calculation parameters:

[0099] (1) Division of helium source rock calculation units

[0100] This calculation divides the helium source rocks into 14 computational units, based on rock masses of different ages, concealed magnetic bodies in the basin basement, upper crust, middle crust, and lower crust. Rock masses of different ages on the southern margin of the basin and concealed magnetic bodies within the basin are the primary helium source rocks. The computational units for the rock masses on the southern margin of the basin are the Baoji, Taibai (Caledonian), Taibai (Indosinian), Cuihua, Muhuguan, Lantian, Laoniushan (Indosinian), Laoniushan (Yanshanian), Huashan (Indosinian), and Huashan (Yanshanian). Based on previous data, eight concealed magnetic bodies were identified within the concealed rock masses. Since radioactive element content data for these concealed magnetic bodies is not yet available, this calculation uses the average radioactive element content of the surrounding granitic bodies. The sum of their areas is then defined as one computational unit. The basin-wide area, excluding the concealed magnetic bodies, is then uniformly divided into upper crust computational units. The deep middle crust and lower crust in the basin are divided into two calculation units.

[0101] (2) Calculation unit area of ​​helium source rock (S i )

[0102] The rock mass calculation unit is determined according to the surface exposure area and the interpretation of regional gravity and magnetic data (Li Yuhong et al., 2018a, 2018b). The Baoji rock mass is 1500 km 2 , Taibai pluton (Caledonian) 700km2 , Taibai pluton (Indosinian) 700km 2 , Cuihua Mountain rock mass 600km 2 , Muhuguan rock mass 246km 2 , Lantian rock mass 154km 2 , Laoniushan rock mass (Indosinian) 338km 2 , Laoniushan rock mass (Yanshan period) 66km 2 , Huashan Rock (Indosinian) 46km 2 , Huashan Rock Massif (Yanshan Period) 84km 2 The total area of ​​the hidden rock mass is determined as one calculation unit in this calculation. The total area of ​​the hidden magnetic body is 5000km 2 The upper crust is the entire basin area excluding the hidden magnetic body area, and the unit area is calculated as 15,000 km 2 The middle and lower crusts of the basin are calculated based on the basin area, and the calculation unit area is 20,000 km. 2 .

[0103] (3) Helium source rock calculation unit Helium source rock thickness (H i )

[0104] The thickness of the rock mass calculation units was determined based on the interpretation of regional gravity and magnetic data and previous research results. Because rock mass morphology is difficult to determine, a uniform columnar shape was assumed to simplify the calculation process. The calculation unit thicknesses are: 6 km for the Baoji pluton, 8.5 km for the Taibai pluton (Caledonian), 8.5 km for the Taibai pluton (Indosinian), 7.1 km for the Cuihuashan pluton, 6 km for the Muhuguan pluton, 6 km for the Lantian pluton, 6.2 km for the Laoniushan pluton (Indosinian), 6.2 km for the Laoniushan pluton (Yanshanian), 7 km for the Huashan pluton (Indosinian), and 7 km for the Huashan pluton (Yanshanian). The average thickness of the intrusive rocks was calculated as 7 km for the buried magnetic bodies. The Moho in this region is approximately 38 km, and the upper crust is 13 km thick (Shi Yaqin et al., 2008; Wang Qianshen et al., 2015). Excluding the approximately 2000 m thickness of the Zhangjiapo Formation and above, the calculation unit thickness was assumed to be 11 km. The thickness of the middle and lower crust is about 25 km (Shi et al., 2008). The thickness of the middle and lower crust is determined according to the previous research results on the crust thickness in this region and globally, with the average thickness of the middle crust being 11 km and the average thickness of the lower crust being 14 km (Rudnick and Fountain, 1995; Wang et al., 2015).

[0105] (4) Average density of helium source rock calculation unit (ρ i )

[0106] The average density of granite in the upper crust is 2.66 g / cm 3 The average density of gneiss and schist is 2.64g / cm3 (Yang Wencai et al., 2017). The upper crust is composed of sedimentary rocks, including the Lantian-Bahe Formation, Gaoling Group, Paleogene, Carboniferous-Permian, and Lower Paleozoic residual strata. According to previous drilling core and logging data, the rock density is about 2.2-2.5 g / cm 3 The lower part of the upper crust is composed of gneiss and intrusive rocks, with a density of 2.6 to 2.7 g / cm 3 (Wang Qianshen et al., 2015; Yang Wencai et al., 2017), the average density is 2.6 g / cm 3 The middle crust is mainly composed of gneiss, diorite, and granodiorite, with a density ranging from 2.75 to 2.85 g / cm 3 (Wang Qianshen et al., 2015), taking the average density value as 2.8g / cm 3 The density of the lower crust in this area is between 2.8 and 2.95 g / cm 3 (Wang Qianshen et al., 2015), taking 2.9g / cm 3 .

[0107] (5) Average porosity of helium source rock calculation unit

[0108] Except for some sedimentary rocks with an average porosity of 10%, the intrusive rocks and metamorphic rocks in the area have extremely low porosity. Sedimentary rocks only occupy a small proportion of the upper crust, so their impact on the calculation results is relatively low and can be ignored in this calculation.

[0109] (6) Calculate the effective helium generation time of the unit (T)

[0110] The main rock masses formed between 431 and 149 Ma. Previous stratigraphic studies indicate that rifting in the Weihe Basin began at 47.8 Ma, that sedimentation of the Gaoling Group began in the Miocene, and that the initial depositional age is 23.03 Ma. Sedimentation of the Lantian-Bahe Formation began at 5.3 Ma and ended in the early Pliocene, at approximately 3.6 Ma. The effective helium generation age, calculated as the average stratigraphic age of the main reservoir formation, is 13.3 Ma.

[0111] (7) Helium emission coefficient (α i )

[0112] According to the results of Zhang et al. (2020), the near-surface granite samples of the Weihe River 4 The He preservation coefficient ranges from 1.5% to 17.4%. 4The helium preservation coefficient in most of the three major rock types is less than 20%, meaning that the helium release (expulsion) coefficient in rocks is as high as 80% or more (Tolstikhin and Drubetskoy 1975, 1977; Gerling et al., 1976; Martel et al., 1990; Tolstikhin et al., 1996, 2011). This calculation uses the average value of 90% as a reference. For temperatures above 250°C in the upper crust, the expulsion coefficient is 95%. For both the middle and lower crust, the expulsion coefficient is 97%.

[0113] (8) The abundance of radioactive uranium in rocks (U i )

[0114] According to the survey results of helium source rocks around the basin, the radioactive elements of the rock mass are obtained by taking the average value of the actual measurement data. The U content of the Baoji rock mass is between 1.0 and 13.6 ppm, with an average value of 4.09 ppm; the U content of the Taibai rock mass (Caledonian) is between 1.32 and 9.75 ppm, with an average value of 3.67 ppm; the U content of the Taibai rock mass (Indosinian) is between 1.04 and 7.22 ppm, with an average value of 3.25 ppm; the U content of the Cuihuashan rock mass is between 1.0 and 9.64 ppm, with an average value of 4.63 ppm; the U content of the Muhuguan rock mass is between 2.10 and 3.73 ppm, with an average value of 2.89 ppm; the Lantian rock mass U content ranges from 1.0 to 13.0 ppm, with an average of 5.71 ppm. The U content of the Laoniushan pluton (Indosinian) ranges from 0.58 to 10.9 ppm, with an average of 3.91 ppm. The U content of the Laoniushan pluton (Yanshanian) ranges from 1.17 to 3.83 ppm, with an average of 2.28 ppm. The U content of the Huashan pluton (Indosinian) ranges from 1.77 to 12.8 ppm, with an average of 3.8 ppm. The U content of the Huashan pluton (Yanshanian) ranges from 1.9 to 10.2 ppm, with an average of 4.1 ppm. Since no samples of hidden magnetic bodies have been obtained, a value of 5.04 ppm is assumed based on the average data for the plutons. U content data for the upper, middle, and lower crust are based on the average data for the crust (Table 1).

[0115] (9) Abundance of radioactive thorium in rocks (Th i )

[0116] The radioactive Th element in the rock mass is obtained by taking the average value of the actual measured data. The Th content of the Baoji rock mass is between 2.5 and 42.2 ppm, with an average value of 19.9 ppm; the Th content of the Taibai rock mass (Caledonian) is between 6.98 and 32.72 ppm, with an average value of 22.7 ppm; the Th content of the Taibai rock mass (Indosinian) is between 9.22 and 33.76 ppm, with an average value of 20.9 ppm; the Th content of the Cuihuashan rock mass is between 1.24 and 60.4 ppm, with an average value of 24.3 ppm; the Th content of the Muhuguan rock mass is between 10.1 and 26.0 ppm, with an average value of 16.4 ppm; the Th content of the Lantian rock mass is between 1.24 and 60.4 ppm, with an average value of 24.3 ppm; The Th content of the Laoniushan pluton (Indosinian) ranges from 1.87 to 35.6 ppm, with an average of 19.5 ppm. The Th content of the Laoniushan pluton (Yanshanian) ranges from 8.2 to 19.0 ppm, with an average of 14.4 ppm. The Th content of the Huashan pluton (Indosinian) ranges from 10.5 to 33.6 ppm, with an average of 19.2 ppm. The Th content of the Huashan pluton (Yanshanian) ranges from 9.2 to 14.6 ppm, with an average of 11.1 ppm. No samples of the hidden magnetic body have been obtained, so the average value of 19.29 ppm is used based on the average data of the pluton. The Th content of the upper, middle, and lower crust is based on the average data of the crust (Table 1).

[0117] (10) Helium transport coefficient (c)

[0118] A. Scale area selection

[0119] This calculation uses the well-studied Panhandle-Hugoton gas field in the United States as the reference area (known area), and the helium migration and accumulation coefficient was determined by reverse calculation. Helium in the Panhandle-Hugoton gas field primarily originates from crustal helium carried by the Amarillo-Wichita granite uplift and natural gas in the Anadarko and Palo Duro basins. To simplify the calculation model, this estimate divides the Panhandle-Hugoton gas field into four calculation units: the Amarillo-Wichita granite body, the upper crust of the helium reservoir system, the upper crust of the helium reservoir system, and the lower crust of the helium reservoir system. Based on its petroleum system, the helium reservoir system area is delineated to be 7.68×10 4 km 2 The Amarillo-Wichita granite uplift covers an area of ​​approximately 1.0×10 4 km 2The thickness of the Amarillo-Wichita granite is 12 km. The upper, middle, and lower crust thicknesses are taken according to the global average crust thickness. The global average continental crust thickness is 37 km, with an upper crust thickness of 12 km, a middle crust thickness of 11 km, and a lower crust thickness of 14 km (Rudnick and Fountain, 1995). The average density of the Amarillo-Wichita granite is 2.65 g / cm 3 The average density of the upper crust is 2.6 g / cm 3 The average density of the middle crust is 2.8 g / cm 3 The average density of the lower crust is 3.0 g / cm 3 . The expulsion coefficient is determined based on the expulsion coefficients of different calculation units mentioned above. The average U abundance of the Amarillo-Wichita granite is 4.5 ppm, and the average Th abundance is 18 ppm. The average U and Th abundances of the crust are taken as reference to the average crustal values ​​mentioned above (Rudnick and Fountain, 1995; Rudnick et al., 2003). The main reservoir of the gas field is the Langying Formation of the Permian System, which was formed at about 270 Ma. The source rock matured almost at the same time, but the final formation of natural gas accumulation occurred since the Paleogene (Sorenson, 2005). The median age of the main accumulation period is about 30 Ma. It is calculated that the total amount of helium expelled from the helium source rock since 270 Ma is 6931.99×10 8 m 3 Since the main accumulation period (30 Ma), the helium resources discharged from the source rocks are 770.22×10 8 m 3 (Table 3).

[0120] Table 3 Calculation of helium expulsion from helium source rocks in the Panhandle-Hugoton gas field in the United States

[0121]

[0122]

[0123] According to previous calculations of the proven natural gas and helium resources in the Panhandle-Panhand Hugoton gas field, the proven natural gas resources in the field are 2.3×10 12 m 3 Calculated based on an average helium content of 0.6%, the helium resource is 138×10 8 m 3 (Zhou and Ballentine, 2006).

[0124] Comparing the proven helium resources with the crustal helium discharge, the helium migration and accumulation coefficient during the main accumulation period was 17.92%.

[0125] B. Calculation of the migration coefficient of the evaluation area

[0126] Based on the geological conditions of the evaluation area and the calibration area, the scoring results are shown in Table 4. According to formulas 4, 5, and 6, P0 is 0.178, P c is 0.791, c c It is 17.92%, and the helium migration and accumulation coefficient c0 since the formation of the Weihe Basin reservoir system is calculated to be 4.029%.

[0127] Table 4 Evaluation area scale area scoring table

[0128]

[0129]

[0130] (11) Calculation of the migration coefficient of the evaluation area

[0131] Based on the optimization results of the above parameters (Table 5), this calculation further optimized the resource evaluation method: First, the background helium generation from other rocks in the upper and lower crust was fully considered; second, the helium expulsion coefficient in rocks was optimized based on the research results of Zhang et al. (2020); third, based on the helium migration and accumulation rules, the helium migration and accumulation coefficient was introduced to evaluate the helium resources in the basin. The evaluation results show that since the formation of the helium reservoir system in the Weihe Basin, the helium source rock expulsion capacity is 93.32×10 8 m 3 The migration and accumulation coefficient is 4.029%. According to formulas 1, 2, and 3, the helium resources in the Weihe Basin are estimated to be 3.759×10 8 m 3 (Table 5), the data are relatively close, and this calculation takes 3.76×10 8 m 3 .

[0132] Table 5 Helium generation potential, helium generation and helium release data of potential helium source rocks in the Weihe Basin

[0133]

[0134]

[0135]

[0136] Description: S i is the calculation unit area; H i is the thickness of the helium source rock calculation unit; J i is the radioactive decay of rock per unit volume 4 He generation rate; T is the rock mass formation time; [U i ] is the abundance of radioactive uranium in the rock; [Th i] is the abundance of radioactive thorium in the rock; the average density of the calculation unit is taken as 2.7 g / cm 3 ; the porosity of the granite is taken as 0; the age of the basin faulting is taken as 47.8 Ma, and the formation age of the main reservoirs Lantian-Bahe Formation is taken as 5.3 Ma; the U and Th abundances of the concealed magnetic body are estimated according to the data of the adjacent granite, and the rest of the data are measured. The data of the belt are the data of the whole rock geochemical analysis, and the others are the data of the radioactivity analysis.

[0137] The various embodiments are described in a progressive manner in the specification, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be understood by referring to each other. For the system disclosed by the embodiments, since it corresponds to the method disclosed by the embodiments, the description is relatively simple, and the relevant parts can be understood by referring to the method part.

[0138] The principles and implementation manners of the present application are described by using specific examples in the present application. The above description of the embodiments is only used to help understand the method of the present application and its core idea. Meanwhile, for the general skilled in the art, the specific implementation manners and application ranges can be changed according to the idea of the present application. In conclusion, the content of the specification should not be understood as the limitation of the present application.

Claims

1. A helium resource evaluation method, characterized in that: include: Divide all helium source rocks in the helium resource evaluation area into multiple helium source rock calculation units; The plurality of helium source rock calculation units include at least the lower crust, the middle crust and the upper crust; Collect radioactive element content test samples from each helium source rock calculation unit geological body, and obtain the average abundance of uranium and thorium in each helium source rock calculation unit through laboratory testing; Calculate the average helium generation rate of each helium source rock calculation unit based on the average abundance of uranium and thorium in each helium source rock calculation unit; Determine the total helium generation amount of each helium source rock calculation unit based on its volume scale, average helium generation rate and effective helium generation time; Determine the helium expulsion coefficient of each helium source rock calculation unit according to the burial depth of each helium source rock calculation unit; The helium migration and accumulation coefficient was obtained using the geological analogy method; The total helium discharge volume is determined based on the total helium generation volume and helium discharge coefficient of each helium source rock calculation unit, and then the helium resource volume in the helium resource evaluation area is determined in combination with the helium migration and accumulation coefficient.

2. The helium resource evaluation method according to claim 1, characterized in that: The method of dividing all helium source rocks in the helium resource evaluation area into multiple helium source rock calculation units specifically includes: Let a complete helium generation, migration, enrichment and accumulation system be a helium resource evaluation area; All helium source rocks that supply helium to natural gas reservoirs in a helium resource evaluation area are divided into multiple calculation units according to the differences in uranium and thorium content, including the lower crust, middle crust and upper crust; The upper crust is further divided into multiple granite body calculation units, sedimentary cover calculation units, magnetic body calculation units and other upper crust calculation units; the other upper crust calculation units are the parts of the upper crust other than those for which the volume scale, uranium and thorium content can be clearly obtained.

3. The helium resource evaluation method according to claim 2, characterized in that: The calculation formula for the average helium generation rate of each helium source rock calculation unit is: Where, J i represents the average helium generation rate of the i-th helium source rock calculation unit, [U i ] represents the average abundance of uranium in the rocks of the i-th helium source rock calculation unit, [Th i ] represents the average abundance of thorium in the rocks of the i-th helium source rock calculation unit.

4. The helium resource evaluation method according to claim 3, characterized in that: The method of determining the total helium generation amount of each helium source rock calculation unit according to the volume scale, average helium generation rate and effective helium generation time of each helium source rock calculation unit specifically includes: Determine the effective helium generation time, area, average density and average porosity of each helium source rock calculation unit; Determine the thickness of each helium source rock calculation unit; assuming the granite body is uniformly columnar, the thickness is determined according to the thickness of the upper crust; According to the area, thickness, average density, average porosity, average helium generation rate and effective helium generation time of each helium source rock unit, the formula Calculate the total amount of helium generated by each helium source rock calculation unit; Where, f(Q i ) represents the total amount of helium generated by the i-th helium source rock calculation unit; S i represents the area of ​​the i-th helium source rock calculation unit; H i represents the thickness of the i-th helium source rock calculation unit; ρ i represents the average density of the i-th helium source rock calculation unit, represents the average porosity of the i-th helium source rock calculation unit; T i Represents the effective helium generation time of the i-th helium source rock calculation unit.

5. The helium resource evaluation method according to claim 4, characterized in that: The helium migration and accumulation coefficient is obtained by adopting the geological analogy method, specifically including: Select a helium resource exploration area with a similar helium accumulation geological background to the helium resource evaluation area as the calibration area, calculate the helium discharge volume and helium resource volume of the calibration area, and use formula c c =Q c / F c , determine the helium transport coefficient c in the scale area c Where Q c Indicates the helium resource in the scale area, F c Indicates the amount of helium discharged in the scale area; Establish a risk assessment parameter system for migration and accumulation conditions; the parameter system includes carrier gas source rocks, reservoir conditions, trap conditions, preservation conditions, and migration and accumulation supporting conditions, with each condition further divided into several sub-factors; Conduct risk assessments on the migration and accumulation conditions of the calibration area and the helium resource evaluation area respectively, and obtain risk assessment values ​​of the migration and accumulation conditions of the calibration area and the helium resource evaluation area; According to the evaluation value of each condition in the risk evaluation parameter system of migration and accumulation conditions in the calibration area and helium resource evaluation area, the formula and Calculate the probability values ​​of migration and accumulation coefficients for the calibration area and the helium resource evaluation area respectively; where P represents the probability value of migration and accumulation coefficient, P k represents the probability of accumulation of the kth condition in the risk assessment parameter system of migration and accumulation conditions, q kj represents the weight of the j-th factor in the k-th condition, P kj Indicates the evaluation value of the j-th sub-item factor in the k-th condition, 0≤P kj ≤1; m represents the number of sub-factors included in the k-th condition; According to the probability values ​​of migration and accumulation coefficients of the scale area and the helium resource evaluation area, as well as the helium migration and accumulation coefficient of the scale area, according to the formula Calculate the helium migration and accumulation coefficient c0 in the helium resource evaluation area; where P0 represents the probability value of the migration and accumulation coefficient in the helium resource evaluation area, P c Indicates the probability value of the clustering coefficient of the scale area.

6. The helium resource evaluation method according to claim 5, characterized in that: The calculation formula for the helium resource amount in the helium resource evaluation area is: Where Q He represents the helium resource volume in the helium resource evaluation area, α i represents the helium expulsion coefficient of the i-th helium source rock calculation unit; n represents the number of helium source rock calculation units in the helium resource evaluation area.

7. The helium resource evaluation method according to claim 5, characterized in that: The sub-factors of the carrier gas source rock include: cumulative source rock thickness, organic carbon content, organic matter type, maturity, hydrocarbon supply area coefficient, hydrocarbon supply mode, hydrocarbon generation intensity, hydrocarbon generation peak time, migration distance and drainage conditions; The sub-factors of the reservoir conditions include: reservoir type, reservoir porosity, reservoir permeability and reservoir depth; The sub-factors of the trap conditions include: trap type, trap area coefficient and trap amplitude; The sub-factors of the preservation conditions include: cover rock lithology, cover rock thickness and cover rock damage degree; The sub-factors of the migration and accumulation matching conditions include: carrier gas and reservoir configuration, helium migration pathway, groundwater dynamic field, matching of trap and carrier gas formation period, and configuration of trap and helium source rock.

Images