Mudstone cover layer evaluation method for low-exploration-degree area

Abstract

The invention belongs to the technical field of geophysical exploration, and particularly relates to a mudstone cover layer evaluation method for a low-exploration-degree area. A mudstone cover layer evaluation method for a low-exploration-degree area comprises the steps that firstly, parameters influencing the capping capacity of a cover layer are calculated and evaluated according to single well data; and then on the basis of researching the capping capability of the capping layer by utilizing single well data, solving the plane distribution of the capping layer and the capping capability of the capping layer by depending on seismic attributes and seismic interval velocity data, and finally achieving the spatial distribution of the capping layer sealing property when no well point exists in the space. According to the method, on the basis of single well lid layer evaluation, the mud-to-ground ratio is fitted by means of two-dimensional and three-dimensional seismic attributes, the breakthrough pressure is solved by means of the seismic horizon velocity, the mud-to-ground ratio and the breakthrough pressure are fully combined to evaluate and predict the mudstone cover layer plane distribution, the one-sided influence of single parameter evaluation is effectively solved, and the method is high in prediction precision, wide in application range and easy to popularize. And the mud rock cover layer evaluation requirement of a low-exploration-degree area can be met.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of geophysical exploration, and particularly relates to a mudstone caprock evaluation method for a low exploration degree area. BACKGROUND

[0002] In the process of oil and gas accumulation, the preservation condition of oil and gas is very important, and the sealing condition and sealing capacity of the overlying caprock are crucial to the preservation of oil and gas. At present, with the continuous deepening of oil and gas exploration, the research on caprock is paid more and more attention by geologists.

[0003] The foreign countries firstly started to study the sealing mechanism of caprock, and the research contents mainly include the capillary sealing capacity and abnormal pressure sealing capacity of caprock. Thomas et al. (1968) carried out constant temperature displacement pressure test on rock samples by using nitrogen, and tested the minimum pressure of displacement fluid by injecting pressure into the experimental sample. Ibrahim et al. (1970) and Schowalter (1979) tested the breakthrough pressure of rock, that is, the gas was driven into the rock on one side until the gas was discharged from the other side of the rock. Iverson et al. (1994) analyzed and obtained that the sealing capacity of rock is directly affected by porosity and permeability. Lee et al. (2000) and David Deming et al. (2002) proposed a capillary force sealing model of caprock when studying the overpressure sealing effect of caprock in Anadarko Basin in the middle and south of the United States. Heath et al. (2012) found that the pore throat development morphology in rock sample can affect the interfacial tension of internal fluid, and further affect the displacement pressure of rock when studying the relationship between pore throat development morphology and displacement pressure of rock.

[0004] In domestic research, Chinese scholars have further deepened their studies on the evaluation of caprock sealing capacity based on international understanding. Xiao Wuran et al. (1984), in their study of the microscopic sealing capacity of caprocks, found that important parameters affecting the microscopic sealing capacity of argillaceous caprocks include: caprock breakthrough pressure, specific surface area of ​​caprock samples, and median radius of caprock porosity. Hao Shisheng et al. (1991) experimentally verified the relationship between breakthrough pressure and breakthrough time, deriving a method for calculating caprock breakthrough pressure. Jiang Zhenxue et al. (1995) found a relationship between caprock breakthrough pressure and seismic layer velocity, which could be calculated. Pang Xiongqi et al. (1994), in their study of the comprehensive quantitative evaluation of caprock sealing capacity in the Songliao Basin, used seismic data for analysis. Zhu Panliang et al. (1994) used isothermal nitrogen adsorption-desorption experiments to study the significance of pore throat morphology and micro-fracture morphology in caprocks and their impact on caprock sealing capacity. Huang Haiping et al. (1995), in their study of the argillaceous caprock in the Liaohe Basin, believed that factors such as the composition and content of clay minerals in the caprock, the pore size and throat size of the caprock, pore throat pattern, porosity, permeability, and breakthrough pressure affect the caprock's sealing capacity. Fu Guang et al. (1996, 1998), in their investigation of caprock sealing capacity, summarized the analysis and evaluation of caprock sealing capacity based on parameters such as the thickness of individual argillaceous layers, argillaceous content, effective porosity, and permeability. Lü Yanfang et al. (2000) quantitatively explored the relationship between model parameters such as caprock porosity, permeability, adsorption, and breakthrough pressure that affect caprock sealing capacity. Tan Yuming et al. (2003) comprehensively evaluated the sealing capacity of argillaceous caprock based on test parameters such as breakthrough pressure, pore radius, pore permeability, and porosity.

[0005] Invention patent CN110764159A discloses a method for dynamically evaluating the effectiveness of caprocks using macroscopic indicators; evaluating the effectiveness of caprocks using microscopic indicators; and evaluating the effectiveness of caprocks using formation water chemistry and organic geochemical indicators in formation water. This invention first qualitatively studies the effectiveness of caprocks based on macroscopic factors such as lithology, burial depth, thickness, and distribution of strata in different structural locations. Then, it combines microscopic studies to quantitatively evaluate the sealing performance of the caprocks using breakthrough pressure and the content of ultra-micropores smaller than 2.5 nm. Finally, it analyzes the sealing effect of the caprocks on formation water based on formation water chemistry characteristics, ultimately achieving a comprehensive evaluation of caprock effectiveness using a three-pronged approach encompassing macroscopic, microscopic, and hydrochemical methods. However, this method requires a large amount of experimental data on formation water chemistry and organic geochemical indicators in formation water, and cannot provide an effective evaluation of mudstone caprocks in areas with low exploration levels. Invention patent CN117408417A discloses a method for comprehensively evaluating shallow gas caprock layers in the Daqing Oilfield, which relates to the field of oilfield exploration technology. This method includes: obtaining the breakthrough pressure, sonic transit time, thickness, and mud-to-soil ratio of the mudstone caprock in developed oilfields based on well logging data; comparing the actual similarity with a preset standard similarity to obtain a comparison result; determining the processing method of the spontaneous potential curve based on the comparison result to obtain the spontaneous potential amplitude difference; measuring the breakthrough pressure of the mudstone caprock in the oilfield to be developed using mercury intrusion porosimetry; establishing a second functional relationship based on a first functional relationship and the spontaneous potential amplitude difference to obtain the target breakthrough pressure of the mudstone caprock in the oilfield to be developed; and comprehensively evaluating the gas sealing capacity of the mudstone caprock in the oilfield to be developed based on the target breakthrough pressure, the thickness, and the mud-to-soil ratio. This method uses single-well caprock characteristics to represent the characteristics of the caprock in the oilfield to be developed, and is mostly applicable to the evaluation of caprock in oilfields to be developed. However, it is not suitable for basins with low exploration levels, rapid sedimentary facies changes, unclear planar distribution of mudstone caprock, or large areas. Invention patent CN117348065A discloses a method for evaluating the development degree of sandstone-mudstone reservoir-caprock assemblages. Specifically, it includes: conducting regional data analysis for the study area, obtaining core / cuttings data, logging data, and oil testing data from drilled well locations, as well as seismic data and seismic interpretation stratigraphic data for the entire study area; performing pre-stack deterministic inversion based on the Zeoppritz equation to obtain the P-wave and S-wave velocity ratio data volume for the study area; extracting the P-wave and S-wave velocity ratio values ​​along the seismically interpreted sandstone layers to obtain a planar map of the sandstone reservoir; extracting the P-wave and S-wave velocity ratio values ​​along the seismically interpreted mudstone layers to obtain a planar map of the mudstone caprock; quantifying the relative development degree of the reservoir-caprock assemblages using a development degree index calculation formula, constructing a sandstone-mudstone reservoir-caprock assemblage development degree index planar map, and quantitatively classifying the development degree of the reservoir-caprock assemblages. This invention can clearly define the development range of high-quality reservoir-caprock assemblages, delineate favorable exploration areas, and further guide well location deployment. This method requires P-wave and S-wave data and 3D seismic data; generally, there is no relevant data in areas with low exploration degree. Invention patent CN117130068A discloses a method and device for identifying lateral diversion and migration of oil and gas in a fracture-sand configuration.This method includes: determining the paleodisplacement pressure of the fracture filler within the mudstone caprock during the hydrocarbon accumulation period; determining the paleodisplacement pressure of the sandstone beneath the mudstone caprock during the hydrocarbon accumulation period; and determining whether lateral divergence migration of hydrocarbons occurred based on the paleodisplacement pressures of the fracture filler within the mudstone caprock and the sandstone beneath it. This invention can accurately obtain the paleodisplacement pressures of the fracture filler within the mudstone caprock and the paleodisplacement pressures of the sandstone on both sides below it, thereby accurately determining the migration direction of hydrocarbons and reducing the risks associated with hydrocarbon exploration. This method is only applicable to evaluating the displacement pressure of fault zones within mudstone caprocks. Invention patent CN115561818A discloses a method for characterizing the sealing performance of mudstone caprock considering sandstone interlayers in the Ordos Basin. The method is as follows: All sandstone interlayers in the caprock are identified on the logging curves of regional production wells, and the mudstone caprock and sandstone interlayer segments are numbered from bottom to top; the average sonic transit time and thickness hij of each sandstone interlayer segment are read, and the displacement pressure Pij of each sandstone interlayer segment is calculated; the sonic transit time curve of each mudstone caprock segment is piecewise fitted, and the average sonic transit time and thickness hck of each mudstone caprock segment are calculated, and the displacement pressure Pck of each mudstone caprock segment is calculated; the overall displacement pressure P of the caprock is calculated. This invention considers the actual geological conditions of sandstone interlayers developing in the mudstone caprock of the Ordos Basin during the evaluation of mudstone caprock sealing performance. This method only evaluates the characteristics of each mudstone caprock segment at a single well point and does not involve planar evaluation of the caprock. Chinese patent CN115220123A discloses a method and apparatus for quantitatively characterizing the leakage risk of mudstone caprock in structural traps. The method includes: obtaining the density and current effective confining pressure of the target mudstone caprock segment to be evaluated; inputting the mudstone density and current effective confining pressure into a mudstone density versus fracture strain graph to obtain the fracture strain of the mudstone; calculating the structural strain of the structural trap by measuring the length of the line segment on the main geological profile perpendicular to the structural trap axis; calculating the ratio of the fracture strain to the structural strain to obtain the strain ratio; calculating the overconsolidation ratio of the target mudstone based on its pre-consolidation stress and current effective confining pressure; and inputting the strain ratio and the overconsolidation ratio into a mudstone integrity assessment graph to obtain the leakage risk of the mudstone caprock. This method only evaluates local caprock characteristics and does not involve planar caprock evaluation.Invention patent CN113761698A discloses a method and apparatus for predicting the sealing properties of mudstone caprock. The method includes: obtaining the breakthrough pressure of a mudstone caprock sample obtained by performing a pressure test on the sample; determining the microscopic characteristic parameters of the crushed mudstone caprock sample based on nitrogen adsorption characteristic data obtained by performing a nitrogen adsorption test on the crushed sample; determining the correlation between the breakthrough pressure and the microscopic characteristic parameters based on the breakthrough pressure and the microscopic characteristic parameters; determining the breakthrough pressure of the mudstone caprock in the area to be analyzed based on the correlation between the breakthrough pressure and the microscopic characteristic parameters; and predicting the sealing properties of the mudstone caprock in the area to be analyzed based on the breakthrough pressure. This method mainly evaluates caprock characteristics based on experimental data, and has significant limitations. Invention patent CN112965108A provides a method and system for determining the vertical sealing properties of a trapped caprock. The method for determining the vertical sealing of the trap caprock includes: acquiring the post-stack seismic data volume, mudstone caprock parameters, and stratigraphic positions of the target segment; classifying the target segment into multiple types of mudstone caprock based on the mudstone caprock parameters; creating a low-frequency interpolation model based on the stratigraphic positions and reservoir inversion sensitive parameter curves corresponding to each type of mudstone caprock; generating mudstone caprock thickness data for the target segment based on the low-frequency interpolation model, the post-stack seismic data volume, and the cutoff values ​​corresponding to each type of mudstone caprock; and determining the vertical sealing of the trap caprock based on the mudstone caprock thickness data. This method only uses mudstone caprock thickness data to analyze the vertical sealing of the caprock, considering only one factor. Furthermore, parameters such as mudstone porosity, breakthrough pressure, and sand content also affect the sealing of the caprock. Summary of the Invention

[0006] To address the aforementioned problems, the purpose of this invention is to provide a method for evaluating mudstone caprock in areas with low exploration levels. This method is based on single-well caprock evaluation, relies on fitting the mudstone-to-soil ratio using two-dimensional and three-dimensional seismic attributes, and uses seismic layer velocity to determine the breakthrough pressure. It fully utilizes the combination of mudstone-to-soil ratio and breakthrough pressure to evaluate and predict the planar distribution of mudstone caprock, effectively solving the problem of the one-sided influence of single-parameter evaluation. This method has high prediction accuracy, wide application range, and can meet the evaluation needs of mudstone caprock in areas with low exploration levels.

[0007] The technical solution of this invention is: a method for evaluating mudstone caprock in areas with low exploration levels, comprising the following steps:

[0008] S1: Collect regional geological data, seismic data, and single-well data, including well logging data, well logging data, lithological data, gas logging data, and well testing data;

[0009] S2: Using the single-well data collected in step S1, a lithological profile is obtained through well logging curve processing, and the sand content, porosity, and thickness of the mudstone are determined. The porosity of the mudstone includes effective porosity and total porosity.

[0010] S3: Determine the mudstone breakthrough pressure based on the effective porosity and total porosity of the mudstone obtained in step S2;

[0011] S4: Based on the mudstone sand content, total porosity, effective porosity, thickness, and breakthrough pressure of the single well obtained in steps S2 and S3, and combined with the single well oil test data and oil, gas and water layer distribution, establish a single well mudstone caprock evaluation standard suitable for the study area, and evaluate the single well mudstone caprock.

[0012] S5: Extract seismic attributes using the regional seismic data collected in step S1. Determine the mudstone-to-soil ratio parameters based on the mudstone sand content, total porosity, effective porosity, thickness, and breakthrough pressure parameters obtained in step S4. Establish a model relating seismic attributes and mudstone-to-soil ratio parameters to predict the planar distribution of the mudstone-to-soil ratio.

[0013] S6: Based on the mudstone breakthrough pressure obtained in step S3 and the seismic attributes obtained in step S5, establish a relationship model between the mudstone breakthrough pressure and the seismic layer velocity of the corresponding layer, and then predict the plane distribution of the breakthrough pressure.

[0014] S7: Based on the evaluation of mudstone caprock layers in each well in step S4, the prediction of mudstone-to-soil ratio in step S5, and the prediction of breakthrough pressure in step S6, a comprehensive evaluation of the regional mudstone caprock is carried out.

[0015] In step S2, the sand content V of the mudstone sd The porosity φ of the mudstone was obtained by processing natural gamma curves, neutron-density cross-plots, or resistivity curves from well logging data. The mudstone content V was also obtained by processing neutron-density cross-plots. sh The relationship between the mudstone sand content and porosity is as follows:

[0016] V sd +V sh +φ=1 (1)

[0017] In the formula V sd Sand content, %; V sh φ represents clay content, in percentages; φ represents porosity, in percentages.

[0018] The specific process for determining the mudstone breakthrough pressure in step S3 is as follows:

[0019] S31: The breakthrough pressure is determined by calculating the total porosity of mudstone, as shown in the following expression:

[0020]

[0021] Where P bt To determine the breakthrough pressure for total porosity, φt Total porosity;

[0022] S32: The formula for determining the breakthrough pressure by using the effective porosity of mudstone is as follows:

[0023]

[0024] Where P be To determine the breakthrough pressure for effective porosity, φ e Effective porosity;

[0025] S33: Determine the breakthrough pressure of mudstone: Select the minimum value between the breakthrough pressure determined by total porosity and the breakthrough pressure determined by effective porosity; this value is the breakthrough pressure of mudstone.

[0026] p b =min(p bt ,p be (4)

[0027] Where P b To overcome the pressure; P bt P be The breakthrough pressure is determined by total porosity and the breakthrough pressure is determined by effective porosity, respectively.

[0028] The process of establishing a model relating seismic attributes and mud-soil ratio parameters in step S5, and then predicting the planar distribution of mud-soil ratio, is as follows:

[0029] S51: Extract the amplitude, frequency, and other related seismic attributes of the target layer within the region based on regional geological and seismic data, and statistically analyze the attribute values ​​A of the various seismic attributes at the well point locations;

[0030] S52: Calculate the mud-to-soil ratio of the target interval using lithological data from a single well. The specific formula is as follows:

[0031]

[0032] Where R sh The mud-to-soil ratio of the target formation in a single well; H sh H represents the total thickness of the mudstone in the target layer; H represents the total stratigraphic thickness of the target layer.

[0033] S53: Calculate the mud-soil ratio R of the target formation in a single well sh The correlation between the attribute values ​​A of each attribute and the well point location is analyzed, and the optimal correlation coefficient r is selected. 2 The highest attribute, the specific formula is as follows:

[0034]

[0035] Where r is the correlation coefficient; R sh, where A is the mud-to-soil ratio of the target layer; A is the attribute value of each attribute at the well point location; and n is the number of wells counted.

[0036] S54: Utilizing the preferred attribute value A and the mud-soil ratio R of the target formation in a single well. sh A model is established to relate earthquake attributes to the mud-soil ratio, predicting the planar distribution of the mud-soil ratio. The specific formula is as follows:

[0037] R sh =bA+a (7)

[0038]

[0039] In the formula, R sh The ratio of muddy soil to silt; a and b are regression coefficients; A represents the average mud-to-soil ratio; A represents the well point location attribute value. is the attribute average; n is the number of statistical wells.

[0040] In step S6, a model is established to show the relationship between the mudstone breakthrough pressure and the seismic velocity of the corresponding layer, thereby predicting the planar distribution of the breakthrough pressure. The specific formula is as follows:

[0041] p b =bv+a (9)

[0042]

[0043] Among them, P b To overcome the pressure; a and b are regression coefficients; v is the seismic layer velocity; Average speed; To break through the average pressure; n is the number of statistical wells.

[0044] In step S7, the mudstone caprock evaluation coefficient is obtained by multiplying the mudstone-to-soil ratio plane and the breakthrough pressure plane. The greater the breakthrough pressure and the greater the mudstone-to-soil ratio, the greater the weight and the better the caprock quality; conversely, the smaller the breakthrough pressure and the smaller the mudstone-to-soil ratio, the smaller the weight and the worse the caprock quality. The specific formula is as follows:

[0045] q = R sh ·p b (11)

[0046] Where q is the planar evaluation coefficient of the mudstone cap; R sh The mud-to-soil ratio is represented by Pb, which represents the breakthrough pressure.

[0047] The technical advantages of this invention are as follows: 1. Based on single-well caprock evaluation, this invention relies on fitting the mudstone-soil ratio using two-dimensional and three-dimensional seismic attributes, and uses seismic layer velocity to determine the breakthrough pressure. It fully utilizes the combination of mudstone-soil ratio and breakthrough pressure to evaluate and predict the planar distribution of mudstone caprock, effectively solving the problem of the one-sided influence of single-parameter evaluation. This method has high prediction accuracy, wide application range, and can meet the needs of mudstone caprock evaluation in low-exploration areas; 2. This invention adopts a "point-to-surface" comprehensive evaluation of caprock sealing capacity. "Point" mainly evaluates the mudstone sealing capacity revealed by a single well; "surface" mainly uses the relationship between seismic attributes and mudstone development degree, and the relationship between seismic layer velocity and mudstone breakthrough pressure, to predict the development degree and sealing capacity of mudstone caprock in a plane, resulting in high prediction accuracy; 3. This invention improves the accuracy of predicting mudstone caprock capacity using a combination of well-seismic analysis; at the same time, the required parameters are more conventional and easier to obtain, and it can be widely applied to the prediction of mudstone caprock sealing capacity in low-exploration areas.

[0048] The following will provide further explanation in conjunction with the accompanying drawings. Attached Figure Description

[0049] Figure 1 This is a flowchart of a method for evaluating mudstone caprock in areas with low exploration levels, as described in an embodiment of the present invention.

[0050] Figure 2 This is a diagram showing the division of basin tectonic units in Embodiment D of the present invention.

[0051] Figure 3 This is a diagram showing the relationship between pressure and porosity in an embodiment of the present invention.

[0052] Figure 4 This is a diagram showing the seismic attribute-mud-soil ratio relationship and mud-soil ratio distribution of the Lower Cretaceous Doba and Kedeni Formations in Basin D of the present invention.

[0053] Figure 5 This is a diagram showing the breakthrough pressure-layer velocity relationship and breakthrough pressure distribution of the Lower Cretaceous Doba and Kedeni Formations in Basin D of the present invention.

[0054] Figure 6 This is a comprehensive evaluation diagram of the Cretaceous mudstone caprock in Basin D of the present invention. Detailed Implementation

[0055] Example 1

[0056] like Figure 1 As shown, a method for evaluating mudstone caprock in areas with low exploration levels includes the following steps:

[0057] S1: Collect regional geological data, seismic data, and single-well data, including well logging data, well logging data, lithological data, gas logging data, and well testing data;

[0058] S2: Using the single-well data collected in step S1, a lithological profile is obtained through well logging curve processing, and the sand content, porosity, and thickness of the mudstone are determined. The porosity of the mudstone includes effective porosity and total porosity.

[0059] S3: Determine the mudstone breakthrough pressure based on the effective porosity and total porosity of the mudstone obtained in step S2;

[0060] S4: Based on the mudstone sand content, total porosity, effective porosity, thickness, and breakthrough pressure of the single well obtained in steps S2 and S3, and combined with the single well oil test data and oil, gas and water layer distribution, establish a single well mudstone caprock evaluation standard suitable for the study area, and evaluate the single well mudstone caprock.

[0061] S5: Extract seismic attributes using the regional seismic data collected in step S1. Determine the mudstone-to-soil ratio parameters based on the mudstone sand content, total porosity, effective porosity, thickness, and breakthrough pressure parameters obtained in step S4. Establish a model relating seismic attributes and mudstone-to-soil ratio parameters to predict the planar distribution of the mudstone-to-soil ratio.

[0062] S6: Based on the mudstone breakthrough pressure obtained in step S3 and the seismic attributes obtained in step S5, establish a relationship model between the mudstone breakthrough pressure and the seismic layer velocity of the corresponding layer, and then predict the plane distribution of the breakthrough pressure.

[0063] S7: Based on the evaluation of mudstone caprock layers in each well in step S4, the prediction of mudstone-to-soil ratio in step S5, and the prediction of breakthrough pressure in step S6, a comprehensive evaluation of the regional mudstone caprock is carried out.

[0064] In step S2, the sand content V of the mudstone sd The porosity φ of the mudstone was obtained by processing natural gamma curves, neutron-density cross-plots, or resistivity curves from well logging data. The mudstone content V was also obtained by processing neutron-density cross-plots. sh The relationship between the mudstone sand content and porosity is as follows:

[0065] V sd +V sh +φ=1 (1)

[0066] In the formula V sd Sand content, %; V sh φ represents clay content, in percentages; φ represents porosity, in percentages.

[0067] The specific process for determining the mudstone breakthrough pressure in step S3 is as follows:

[0068] S31: The breakthrough pressure is determined by calculating the total porosity of mudstone, as shown in the following expression:

[0069]

[0070] Where P bt To determine the breakthrough pressure for total porosity, φ t Total porosity;

[0071] S32: The formula for determining the breakthrough pressure by using the effective porosity of mudstone is as follows:

[0072]

[0073] Where P be To determine the breakthrough pressure for effective porosity, φ e Effective porosity;

[0074] S33: Determine the breakthrough pressure of mudstone: Select the minimum value between the breakthrough pressure determined by total porosity and the breakthrough pressure determined by effective porosity; this value is the breakthrough pressure of mudstone.

[0075] p b =min(p bt ,p be (4)

[0076] Where P b To overcome the pressure; P bt P be The breakthrough pressure is determined by total porosity and the breakthrough pressure is determined by effective porosity, respectively.

[0077] The process of establishing a model relating seismic attributes and mud-soil ratio parameters in step S5, and then predicting the planar distribution of mud-soil ratio, is as follows:

[0078] S51: Extract the amplitude, frequency, and other related seismic attributes of the target layer within the region based on regional geological and seismic data, and statistically analyze the attribute values ​​A of the various seismic attributes at the well point locations;

[0079] S52: Calculate the mud-to-soil ratio of the target interval using lithological data from a single well. The specific formula is as follows:

[0080]

[0081] Where R sh The mud-to-soil ratio of the target formation in a single well; H sh H represents the total thickness of the mudstone in the target layer; H represents the total stratigraphic thickness of the target layer.

[0082] S53: Calculate the mud-soil ratio R of the target formation in a single well sh The correlation between the attribute values ​​A of each attribute and the well point location is analyzed, and the optimal correlation coefficient r is selected. 2 The highest attribute, the specific formula is as follows:

[0083]

[0084] Where r is the correlation coefficient; R sh , where A is the mud-to-soil ratio of the target layer; A is the attribute value of each attribute at the well point location; and n is the number of wells counted.

[0085] S54: Utilizing the preferred attribute value A and the mud-soil ratio R of the target formation in a single well. sh A model is established to relate earthquake attributes to the mud-soil ratio, predicting the planar distribution of the mud-soil ratio. The specific formula is as follows:

[0086] R sh =bA+a (7)

[0087]

[0088] In the formula, R sh The ratio of muddy soil to silt; a and b are regression coefficients; A represents the average mud-to-soil ratio; A represents the well point location attribute value. is the attribute average; n is the number of statistical wells.

[0089] In step S6, a model is established to show the relationship between the mudstone breakthrough pressure and the seismic velocity of the corresponding layer, thereby predicting the planar distribution of the breakthrough pressure. The specific formula is as follows:

[0090] p b =bv+a (9)

[0091]

[0092] Among them, P b To overcome the pressure; a and b are regression coefficients; v is the seismic layer velocity; Average speed; To break through the average pressure; n is the number of statistical wells.

[0093] In step S7, the mudstone caprock evaluation coefficient is obtained by multiplying the mudstone-to-soil ratio plane and the breakthrough pressure plane. The greater the breakthrough pressure and the greater the mudstone-to-soil ratio, the greater the weight and the better the caprock quality; conversely, the smaller the breakthrough pressure and the smaller the mudstone-to-soil ratio, the smaller the weight and the worse the caprock quality. The specific formula is as follows:

[0094] q = R sh ·p b (11)

[0095] Where q is the planar evaluation coefficient of the mudstone cap; R sh The mud-to-soil ratio is represented by Pb, which represents the breakthrough pressure.

[0096] Example 2

[0097] Taking the evaluation of mudstone caprock in the low-exploration area of ​​Basin D as an example, the evaluation method for mudstone caprock in low-exploration areas described in Example 1 is adopted. The specific process is as follows:

[0098] S1: Collect regional geological data, seismic data, and single-well data; Basin D is located in central Africa, with an exploration area of ​​approximately 8000 km². 2 It is an intracontinental rift basin controlled by the Central African shear zone against the backdrop of the Mesozoic-Cenozoic breakup of Pangaea. It features five secondary tectonic units: a northern steep slope zone, a depression zone, a central low uplift zone, a southern gentle slope zone, and a transition zone. Figure 2 As shown, the basin contains Cretaceous, Paleogene, Neogene, and Quaternary strata deposited from bottom to top. Among them, the Cretaceous strata include the Lower Cretaceous and Upper Cretaceous, with the Lower Cretaceous being the main exploration target layer, with a thickness of 1700-5000m. From bottom to top, it can be subdivided into four stratigraphic groups: Mangara, Kedeni, Doba, and Koumra. Since the 1970s, several Western oil companies have successively deployed four exploration wells. Only D4 obtained low-yield oil flow in the Kedeni Formation, while D1, D2, and D5 were unsuccessful. The foreign companies' evaluation concluded that the upper strata of the basin had well-developed reservoirs but poor caprock quality, and the lower strata had undeveloped reservoirs. Overall, they believed that the basin had a poor reservoir-caprock combination and no regional caprock, making it difficult to form large-scale oil reservoirs, and thus withdrew from exploration. After Chinese companies intervened in exploration, the D3 well deployed in the northern steep slope zone obtained high-yield oil flow, while the D6 well deployed in the transition zone was unsuccessful. The analysis concluded that the lack of development of mudstone caprock was the main reason for the failure.

[0099] S2: Using well logging curves, high-precision lithological profiles and mudstone porosity are obtained: effective porosity, total porosity, and thickness.

[0100] S3: The mudstone porosity obtained through step S2, based on the relationship between mudstone porosity and mudstone breakthrough pressure, such as... Figure 3 As shown;

[0101] S4: Establish evaluation standards for single-layer mudstone caps;

[0102] Based on well logging data, evaluation parameters such as mudstone sand content, total porosity, effective porosity, thickness, and breakthrough pressure were calculated for six exploration wells in the study area. Quantitative evaluations of the two main caprocks, the Kedeni and Doba Formations, were completed. The classification criteria for mudstone caprock sealing, as shown in Table 1, were adopted by Li Guoping et al. A comprehensive evaluation of the mudstone caprock in the study area was conducted, and the results are shown in Table 2. The evaluation parameters of mudstone sand content, total porosity, effective porosity, thickness, and breakthrough pressure for six wells in the basin were statistically analyzed. The results show that the mudstone caprock evaluation parameters of wells D1, D2, D3, and D4 in the depression zone and the northern steep slope zone are better than those of well D5 in the central low uplift zone, while well D6 in the transition zone is the worst. Vertically, the breakthrough pressure of mudstone in the Kedeni Formation is generally greater than that in the Doba Formation, indicating better sealing capacity. Based on the basin's geological characteristics and oil, gas, and water distribution characteristics, a classification criteria for mudstone caprock sealing in the basin was established.

[0103] Table 1. Classification criteria for mudstone caprocks (based on Li Guoping et al.)

[0104]

[0105] Table 2. Statistical and Evaluation Results of Mudstone Cap Layer Parameters in Basin D

[0106]

[0107]

[0108] Note: Values ​​from 1.2 to 74.0 (12.6) in the table represent the minimum to the maximum (average) values.

[0109] S5: Extract seismic attributes using the regional seismic data collected in step S1. Determine the mudstone-to-soil ratio parameters based on the mudstone sand content, total porosity, effective porosity, thickness, and breakthrough pressure parameters obtained in step S4. Establish a model relating seismic attributes and mudstone-to-soil ratio parameters to predict the planar distribution of the mudstone-to-soil ratio.

[0110] Given the large area of ​​Basin D, the limited number of wells, and the predominance of 2D seismic data, the discovered oil reservoirs are mostly interbedded sand and mudstone. Single-layer mudstone caprocks are difficult to identify and trace seismically. While the mudstone-to-soil ratio does not represent the development degree of a single mudstone layer, it can indirectly reflect the overall development degree of mudstone in the strata. Therefore, the mudstone-to-soil ratio is used instead of mudstone thickness to evaluate the spatial distribution continuity of the mudstone caprock. Analysis of the relationship between seismic attributes and the mudstone-to-soil ratio shows a linear and strong correlation between the mudstone-to-soil ratio and seismic multichannel correlation attributes in Basin D. Using lithological profiles obtained from well logging, the percentage of mudstone in the Cretaceous Kedeni and Doba Formations below a single well was statistically determined. Extrapolation was performed using the mudstone-to-soil ratio at each well point combined with seismic multichannel correlation attributes to compile a planar distribution map of the mudstone percentage in each stratum, as shown below. Figure 4 As shown;

[0111] S6: Based on the mudstone breakthrough pressure obtained in step S3 and the seismic attributes obtained in step S5, a relationship model between seismic layer velocity and mudstone breakthrough pressure is established, and the planar distribution of breakthrough pressure is predicted. Breakthrough pressure has a negative correlation with porosity, while porosity has a positive correlation with logging sonic transit time. Since sonic transit time and formation layer velocity are similar, the breakthrough pressure at well points in Basin D has a good correlation with formation layer velocity. Therefore, a correlation fitting is performed between the breakthrough pressure at well points and formation layer velocity to draw a predicted distribution map of mudstone breakthrough pressure in Basin D, as shown below. Figure 5 As shown;

[0112] S7: Based on the evaluation of mudstone caprock layers in each well in step S4, the prediction of mudstone-to-soil ratio in step S5, and the prediction of breakthrough pressure in step S6, a comprehensive evaluation of the regional mudstone caprock is conducted. Overlaying the breakthrough pressure plane distribution map with the mudstone-to-soil ratio plane distribution map provides a direct reflection of the quality of the mudstone caprock. In Basin D, the product of breakthrough pressure (MPa) and mudstone-to-soil ratio (m / m) is used as the weight (one of the overlay evaluation methods) to reflect the quality of the mudstone caprock. That is, the greater the breakthrough pressure and the greater the mudstone-to-soil ratio, the greater the weight and the better the caprock quality; conversely, the smaller the breakthrough pressure and the smaller the mudstone-to-soil ratio, the smaller the weight and the worse the caprock quality. Based on the drilling and reservoir analysis in Basin D, combined with the characteristics of the weight plane distribution, the caprock is divided into four levels for comprehensive evaluation: weights greater than 6 are Class I caprocks, 2-6 are Class II caprocks, 1-2 are Class III caprocks, and less than 1 are Class IV caprocks. Figure 6 As shown;

[0113] Evaluation of the Kedeni and Doba caprocks in the Lower Cretaceous of Basin D revealed that the Kedeni mudstone caprock generally performed better than the Doba mudstone caprock in terms of key evaluation parameters, exhibiting characteristics such as low sand content, low porosity, and high breakthrough pressure. This further clarifies that the Kedeni assemblage is the main hydrocarbon accumulation assemblage in the basin, while the Doba mudstone caprock is of lower quality and can be considered a secondary hydrocarbon accumulation assemblage for further exploration. Based on the caprock evaluation results, combined with assessments of oil source, traps, sedimentary facies, and tectonic evolution, the northern steep slope zone, the central low uplift zone, and the southern gentle slope zone are selected as favorable areas for hydrocarbon accumulation, and are also areas with well-developed and high-quality caprocks. The newly deployed D7 well in the central low uplift zone yielded high-yield oil flows in both the Doba and Kedeni formations during testing. Well D8 only showed oil layers in the Kedeni formation during logging interpretation and is awaiting further testing. The post-drilling mudstone caprock evaluation results are largely consistent with the predicted results, as shown in Table 3.

[0114] Table 3. Statistical and Evaluation Results of Mudstone Caprock Parameters in Wells D7 and D8 of Basin 3D

[0115]

[0116] Note: 1.0 to 21.2 (6.4) in the table represent the minimum to the maximum value (average).

[0117] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for evaluating mudstone cap rock in a low exploration degree area, characterized in that: The method comprises the following steps: S1: collecting regional geological data, seismic data and single well data, wherein the single well data comprises logging data, drilling data, lithology data, gas logging data and oil testing data; S2: obtaining a lithology profile through logging curve processing by using the single well data collected in step S1, and determining the sand content, porosity and thickness of the mudstone, wherein the porosity of the mudstone comprises effective porosity and total porosity; S3: determining the mudstone breakthrough pressure according to the effective porosity and total porosity of the mudstone obtained in step S2; S4: combining the single well oil testing data and the distribution of oil, gas and water layers, establishing a single well mudstone caprock evaluation standard suitable for the study area, and evaluating the single well mudstone caprock by using the sand content, total porosity, effective porosity, thickness and breakthrough pressure of the single well obtained in steps S2 and S3; S5: extracting seismic attributes by using the regional seismic data collected in step S1, determining the mudstone-to-ground ratio parameter according to the sand content, total porosity, effective porosity, thickness and breakthrough pressure obtained in step S4, and establishing a relationship model between the seismic attributes and the mudstone-to-ground ratio parameter, and then predicting the plane distribution of the mudstone-to-ground ratio; S6: establishing a relationship model between the seismic interval velocity and the mudstone breakthrough pressure by using the mudstone breakthrough pressure and the seismic interval velocity of the corresponding layer obtained in steps S3 and S5, and then predicting the plane distribution of the breakthrough pressure; S7: performing regional mudstone caprock comprehensive evaluation according to the mudstone caprock evaluation of each layer in step S4, the prediction of the plane distribution of the mudstone-to-ground ratio in step S5 and the prediction of the plane distribution of the breakthrough pressure in step S6.

2. The method for evaluating mudstone cap rock in a low exploration degree area according to claim 1, characterized in that: The sand content V of the mudstone in the step S2 sd The porosity φ of the mudstone is obtained by processing the neutron-density cross curve, and the shale content V of the mudstone is obtained by processing the natural gamma curve or the resistivity curve in the logging data. sh The relationship between the sand content and the porosity of the mudstone is: V sd +V sh +φ=1 (1) where V sd is the sand fraction, %; V sh is the shale content, %; φ is the porosity, %.

3. The method for evaluating mudstone cap rock in a low exploration degree area according to claim 1, characterized in that: The specific process of determining the mudstone breakthrough pressure in step S3 is as follows: S31: determining the breakthrough pressure by using the total porosity of the mudstone, and the expression is as follows: where P bt is the breakthrough pressure, φ t is the total porosity; and, S32: determining the breakthrough pressure by using the effective porosity of the mudstone, and the expression is as follows: where P be is the breakthrough pressure, φ e is the effective porosity; S33: determining the mudstone breakthrough pressure: selecting the minimum value of the breakthrough pressure determined by the total porosity and the breakthrough pressure determined by the effective porosity, and the minimum value is the mudstone breakthrough pressure, p b = min(p bt , p be ) (4) where P b is the breakthrough pressure; P bt , P be are the breakthrough pressure determined by total porosity, the breakthrough pressure determined by effective porosity, respectively.

4. The method for evaluating mudstone cap rock in a low exploration degree area according to claim 1, characterized in that: The process of establishing the relationship model between the seismic attributes and the mudstone-to-ground ratio parameter in step S5, and then predicting the plane distribution of the mudstone-to-ground ratio is as follows: S51: extracting the amplitude, frequency and multiple seismic attributes related to the target layer in the region according to the regional geological data and seismic data, and counting the attribute values A of the multiple seismic attributes at the well point positions; S52: calculating the mudstone-to-ground ratio of the target layer through the lithology data of the single well, and the specific formula is as follows: wherein R sh is the ratio of the thickness of the mudstone to the total thickness of the formation in the target interval of the single well; H sh is the total thickness of the mudstone in the target interval; and H is the total thickness of the formation in the target interval. S53: Calculate the R value of the single well target zone sh The correlation between the attribute value A of each attribute of the well point position and the correlation coefficient r is preferably selected 2 The highest attribute, the specific formula is as follows: where r is the correlation coefficient; R sh is the mud ratio of the target interval; A is the attribute value of each attribute of the well point position, and n is the number of statistical wells. S54: Use the preferred attribute value A and the single well target interval shale ratio R sh The relationship model between the seismic attribute and the shale ratio is established, and the shale ratio plane distribution is predicted. The specific formula is as follows: R sh = bA + a (7) where R sh is the mud ratio; a, b are regression coefficients; is the average value of mud ratio; A is the attribute value of well point location; is the average value of attribute; n is the number of statistical wells.

5. The method for evaluating mudstone cap rock in a low exploration degree area according to claim 1, characterized in that: The specific formula of establishing the relationship model between the seismic interval velocity and the mudstone breakthrough pressure in step S6, and then predicting the plane distribution of the breakthrough pressure is as follows: p b = bv + a (9) Where, P b is breakthrough pressure; a, b are regression coefficients; v is seismic interval velocity; is average velocity; is average breakthrough pressure; n is the number of statistical wells.

6. The method for evaluating mudstone cap rock in a low exploration degree area according to claim 1, characterized in that: The specific formula of multiplying the mudstone caprock evaluation coefficient by the plane of the mudstone-to-ground ratio and the plane of the breakthrough pressure in step S7 is as follows: q = R sh • p b (11) wherein q is a shale cap rock plane evaluation coefficient; R s h is the mud ratio; and Pb is the breakthrough pressure.

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