Quantitative evaluation method for deep overpressure contribution of oil and gas bearing basin

Abstract

The application discloses a quantitative evaluation method of oil and gas basin deep overpressure contribution amount, and comprises the following steps: arranging pressure data and logging data; comprehensively judging normal compaction sections and undercompaction sections according to the pressure data and the logging data; forming a density and acoustic velocity relationship chart and a density and vertical effective stress relationship chart for judging overpressure causes on the basis of the normal compaction sections of the determined well; combining the undercompaction sections determined in step 2, comprehensively judging overpressure cause types of overpressure sections, wherein the cause types include undercompaction overpressure causes and unloading overpressure causes; calculating unloading overpressure of the overpressure reservoir section; subtracting the obtained unloading overpressure from the total overpressure of the overpressure reservoir section, so as to obtain undercompaction overpressure of the overpressure reservoir section; dividing the obtained unloading overpressure of the overpressure reservoir section and the undercompaction overpressure of the overpressure reservoir section by the total overpressure, so as to obtain the contribution amount of the unloading overpressure and the undercompaction overpressure of the overpressure reservoir section to the total overpressure.

Description

Technical Field

[0001] This invention relates to the field of oil and gas exploration and development, and specifically to a quantitative evaluation method for the contribution of deep overpressure in oil and gas basins. Background Technology

[0002] Deep hydrocarbon basins, with their low exploration levels, represent a crucial area for current hydrocarbon exploration. Overpressure is prevalent in these deep layers, and its formation significantly impacts hydrocarbon accumulation. Previous studies have shown that deep overpressure often arises from a combination of multiple mechanisms, and the impact of different overpressure mechanisms on hydrocarbon accumulation varies considerably. Therefore, quantitatively evaluating the contribution of different deep overpressure mechanisms is of paramount importance. Currently, when evaluating the contribution of different overpressure mechanisms, the relationship between logging parameters such as sonic transit time and resistivity and vertical effective stress is often used (Tingay et al., 2013; Zhang et al., 2013; Liu et al., 2018; Zhang et al., 2022). However, the unloading overpressure generated by hydrocarbon generation expansion and overpressure transmission, which often develop in deep formations, can cause an increase in sonic transit time and a decrease in resistivity. In the relationship chart between sonic transit time or resistivity and vertical effective stress, hydrocarbon generation expansion or overpressure transmission in strata composed of multiple lithologies in the same area will produce different overpressure magnitudes, and the resulting increase in sonic transit time and decrease in resistivity will have different paths. However, their evolution paths are difficult to determine. Previous evaluations of unloading overpressure often assume the evolution path when sonic transit time and resistivity remain constant or assume that their evolution paths are the same to calculate the magnitude of unloading overpressure, resulting in a large error in the evaluated overpressure contribution. Therefore, there is an urgent need to discover a new method to quantitatively evaluate the magnitude of each overpressure contribution in deep multi-mechanism complex structures.

[0003] [1]Tingay MRP,Morley CK,Laird A,Limpornpipat O,Krisadasima K,Pabchanda S,Macintyre H R.Evidence for overpressure generation by kerogen-to-gas maturation in the northern Malay Basin[J].AAPG Bulletin,2013,97(4):639-672.

[0004] [2] Zhang Fengqi, Wang Zhenliang, Zhong Hongli, Yang Chao, Yang Jiangtao. Identification model and contribution of major overpressure formation mechanism in sedimentary basins [J]. Natural Gas Geoscience, 2013, 24(6): 1151-1158.

[0005] [3] Liu Tao, Liu Jingdong. Quantitative evaluation of overpressure in the cause of undercompaction and fluid expansion [J]. Acta Petrolei Sinica, 2018, 39(9): 971-979.

[0006] [4] Zhang Xuyou, Fan Caiwei, Guo Xiaowen, Wu Yunpeng, Liu Aiqun, Gao Yingbo, Huang Yahao. Quantitative evaluation of the overpressure genesis and relative contribution of the Yinggehai Formation in the Ledong area of ​​the central diapiric zone of the Yinggehai Basin [J / OL]. Earth Science: 1-16 [2023-02-17]. Summary of the Invention

[0007] The purpose of this invention is to provide a quantitative evaluation method for the contribution of deep overpressure in oil and gas basins, so as to overcome the defects of the existing technology. This invention is more practical, simple to operate and more accurate, and provides a new method for accurately evaluating the contribution of deep multi-mechanism composite overpressure in stable tectonic zones of oil and gas basins.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A quantitative evaluation method for the contribution of deep overpressure in oil and gas basins includes the following steps:

[0010] Step 1: Organize pressure data and well logging data;

[0011] Step 2: Determine the normally compacted and under-compacted sections based on pressure and well logging data;

[0012] Step 3: Based on the normally compacted section of the well determined in Step 2, generate a graph showing the relationship between density and sonic velocity and the relationship between density and vertical effective stress to identify the cause of overpressure. Then, combine this graph with the undercompacted section determined in Step 2 to comprehensively determine the type of overpressure in the overpressure section. The cause types include undercompacted overpressure and unloading overpressure.

[0013] Step 4: Based on Step 3, calculate the magnitude of the unloading overpressure of the overpressure reservoir section; subtract the calculated unloading overpressure from the total overpressure of the overpressure reservoir section to obtain the undercompacted overpressure of the overpressure reservoir section; divide the calculated unloading overpressure and undercompacted overpressure of the overpressure reservoir section by the total overpressure to obtain the contribution of the unloading overpressure and undercompacted overpressure of the overpressure reservoir section to the total overpressure.

[0014] Furthermore, the pressure data mentioned in step 1 includes measured formation pressure and drilling fluid density.

[0015] Furthermore, the logging data mentioned in step 1 includes natural gamma, sonic transit time, density, neutron porosity, and resistivity of mudstone at different burial depths.

[0016] Furthermore, in step 2, the normally compacted section and the under-compacted section are determined based on a comprehensive analysis of pressure data and well logging data, specifically as follows:

[0017] By using the data on sonic transit time, density, neutron porosity, resistivity, drilling fluid density, and measured formation pressure of mudstone at different burial depths from step 1, the normally compacted and under-compacted sections can be comprehensively determined.

[0018] Furthermore, step 3 specifically includes:

[0019] Based on the normally compacted section of the well determined in step 2, the sonic transit time and density of mudstone sections at different burial depths within the normally compacted section are used to determine the relationship models between sonic transit time and burial depth, and density and burial depth. These two models are then combined to calculate the magnitudes of sonic transit time and density for mudstone sections at different burial depths under normal compaction. Furthermore, using the reciprocal relationship between sonic transit time and sonic velocity, the sonic transit time for mudstone sections at different burial depths and the corresponding sonic velocities are calculated. These two models are then combined to establish a relationship model between sonic velocity and density in the normally compacted mudstone section, thus creating a density-sonic velocity relationship chart for identifying the cause of overpressure. Finally, density logging data from the entire well section are used to determine the formation density. A model relating density to burial depth is used to further establish a calculation model for overburden stress. This model, in turn, is used to establish a model relating vertical effective stress to burial depth in normally compacted strata. Combined with the calculated density of normally compacted mudstone strata, a model relating density to vertical effective stress in normally compacted mudstone strata is established, thus forming a density-vertical effective stress relationship chart for identifying overpressure origins. The vertical effective stress of the overpressured strata is calculated using measured formation pressure data. The vertical effective stress, sonic velocity, and density data points of this overpressured strata are projected onto the established density-sonic velocity relationship chart and density-vertical effective stress relationship chart. Combined with the undercompacted strata identified in step 2, the overpressure origin type of the overpressured strata is comprehensively determined. The overpressure origin types include undercompacted overpressure and unloading overpressure.

[0020] Furthermore, step 3 specifically includes the following steps:

[0021] Step 3.1: Establish a model showing the relationship between sonic transit time and burial depth in the normally compacted mudstone layer, as shown in the following formula:

[0022] Δt n =Δt ma +a·e -b·z

[0023] Where, Δt n The sonic transit time of the mudstone layer under normal compaction is Δt. ma denoted as , z is the burial depth, a is the fitting coefficient obtained by using the exponential relationship between the difference between well logging sonic transit time and the sonic transit time of the mudstone matrix and the burial depth, and b is the mudstone compaction coefficient obtained by using the exponential relationship between the difference between well logging sonic transit time and the sonic transit time of the mudstone matrix and the burial depth.

[0024] Step 3.2: Determine the relationship between density and burial depth for the normally compacted mudstone layer, as shown in the following formula:

[0025] ρ(z)=ρ ma -c·e -d·z

[0026] Where ρ(z) is the density of the mudstone layer, ρ ma denoted as ρ, where c is the density of the mudstone matrix, c is the fitting coefficient obtained by using the exponential relationship between the difference between the mudstone matrix density and the logging density and the burial depth, and d is the mudstone compaction coefficient obtained by using the exponential relationship between the difference between the mudstone matrix density and the logging density and the burial depth.

[0027] Step 3.3: Establish a model for the relationship between acoustic velocity and density in normally compacted mudstone layers. First, use the reciprocal relationship between acoustic velocity and acoustic time difference to calculate the acoustic velocity in the mudstone layer, as shown in the following formula:

[0028]

[0029] Among them, V s To determine the acoustic velocity of the mudstone layer, the acoustic transit time formula for normally compacted mudstone in step 3.1 is used to calculate the acoustic transit time for mudstone layers at different burial depths under normal compaction. Furthermore, using the reciprocal relationship between acoustic velocity and acoustic transit time established in this step, the acoustic velocity of mudstone layers at different burial depths under normal compaction is calculated. Combined with the density versus burial depth relationship model in step 3.2, a relationship model between acoustic velocity and density in normally compacted mudstone layers is established, as shown in the following formula:

[0030]

[0031] Step 3.4: Establish a model relating density to vertical effective stress in normally compacted mudstone strata. First, establish a model relating formation density to burial depth, as shown in the following formula:

[0032] ρ(z) layer =j·z k

[0033] Where, ρ(z) layerLet j and k be the density of the formation at a certain burial depth, and j and k be the fitting coefficients obtained by fitting the formation density logging data with a power function of the burial depth. Further, a calculation model for the overburden load stress of the formation is established, as shown in the following equation:

[0034]

[0035] Where g is the acceleration due to gravity, a model is established to show the relationship between the effective vertical stress of a normally compacted stratum and its burial depth, as shown in the following equation:

[0036]

[0037] Where δ is the effective vertical stress of the formation, ρ w Given the formation water density, and combined with the mudstone density calculation model from step 3.2, a relationship model between the density and vertical effective stress of a normally compacted mudstone layer is established, as shown in the following formula:

[0038]

[0039] Step 3.5: Using the sonic velocity-density relationship model established in Step 3.3 for normally compacted mudstone sections, generate a density-sonic velocity relationship chart to identify overpressure origins; using the density-vertical effective stress relationship model established in Step 3.4 for normally compacted mudstone sections, generate a density-vertical effective stress relationship chart to identify overpressure origins; calculate the vertical effective stress using measured formation pressure data and overlying load stress, using the following formula:

[0040]

[0041] Among them, P 实测 To measure the formation pressure in the reservoir, the vertical effective stress of the overpressured section, the sonic velocity and density data points of the adjacent mudstone are projected onto the established density-sonic velocity relationship chart and density-vertical effective stress relationship chart. Combined with the undercompacted section identified in step 2, the overpressured section's overpressure origin is comprehensively determined: undercompacted overpressure origin or unloading overpressure origin. Based on the actual geological conditions, the specific unloading overpressure origin is comprehensively determined.

[0042] Furthermore, step 4 specifically includes:

[0043] Substituting the measured density of the mudstone adjacent to the overpressured section into the relationship model between the density and vertical effective stress of the normally compacted mudstone section established in step 3, the vertical effective stress of the measured overpressured reservoir section before the unloading overpressure occurs is calculated. Using the vertical effective stress of the measured overpressured reservoir section calculated in step 3, this is the vertical effective stress of the overpressured reservoir section after the formation of unloading overpressure. The vertical effective stress of the overpressured reservoir section before the formation of unloading overpressure is then compared with the overpressured reservoir... Subtracting the effective vertical stress of the layer after unloading overpressure formation yields the magnitude of the unloading overpressure of the overpressured reservoir section. Subtracting the calculated unloading overpressure from the total overpressure of the overpressured reservoir section yields the magnitude of the undercompacted overpressure of that overpressured reservoir section. Dividing the calculated unloading overpressure and the undercompacted overpressure of that overpressured reservoir section by the total overpressure yields the contribution of the unloading overpressure and the undercompacted overpressure of that overpressured reservoir section to the total overpressure.

[0044] Furthermore, step 4 specifically includes the following steps:

[0045] Step 4.1: Determine the effective vertical stress in the measured overpressured reservoir section before the overpressure is generated during unloading.

[0046] Substitute the measured density of the mudstone adjacent to the overpressured reservoir section into the relationship model between the density and vertical effective stress of the normally compacted mudstone section, that is, to obtain the vertical effective stress of the measured overpressured reservoir section before the unloading overpressure occurs.

[0047] Step 4.2: Calculate the effective vertical stress in the measured overpressured reservoir section after the formation of unloading overpressure.

[0048] The calculated effective vertical stress of the overpressured reservoir section is taken as the effective vertical stress of the overpressured reservoir section after the formation of unloading overpressure.

[0049] Step 4.3: Determine the magnitude of the unloading overpressure in the measured overpressure reservoir section.

[0050] The magnitude of the unloading overpressure in the overpressure reservoir section is obtained by subtracting the effective vertical stress in the overpressure reservoir section before the formation of unloading overpressure from the effective vertical stress in the overpressure reservoir section after the formation of unloading overpressure.

[0051] Step 4.4: Determine the magnitude of the underpressured actual overpressure in the measured overpressured reservoir section.

[0052] The undercompacted overpressure of the overpressure reservoir section is obtained by subtracting the calculated unloading overpressure from the total overpressure of the overpressure section.

[0053] Step 4.5: Calculate the contribution of unloading overpressure and underpressure actual overpressure in the measured overpressured reservoir section to the total overpressure.

[0054] Dividing the calculated unloading overpressure of the overpressured reservoir segment and the undercompaction overpressure of that overpressured reservoir segment by the total overpressure, yields the contribution of the unloading overpressure and undercompaction overpressure of the overpressured reservoir segment to the total overpressure.

[0055] Compared with the prior art, the present invention has the following beneficial technical effects:

[0056] Overpressure is commonly found in deep oil and gas basins, and it often arises from a combination of multiple mechanisms. Undercompaction overpressure and unloading overpressure are the two main types. Previous quantitative evaluations of their contribution have primarily utilized models relating logging parameters such as sonic transit time and resistivity to effective stress, first determining the magnitude of unloading overpressure and then the magnitude of undercompaction overpressure. However, unloading overpressure, often caused by hydrocarbon generation expansion or overpressure transmission in deep formations, can lead to increased sonic transit time and decreased resistivity. In the chart showing the relationship between resistivity and vertical effective stress, hydrocarbon expansion or overpressure transmission in strata composed of multiple lithologies in the same region will produce different overpressure magnitudes. The resulting increase in acoustic transit time and decrease in resistivity will have different paths. The change paths caused by this increase in acoustic transit time and decrease in resistivity are quite complex and difficult to determine. Previous researchers often assumed the evolution paths when acoustic transit time and resistivity remained constant or assumed that their evolution paths were the same when calculating the magnitude of unloading overpressure, resulting in a large error in the evaluation of the overpressure contribution. Since the density remains essentially unchanged after unloading overpressure caused by hydrocarbon generation expansion or overpressure transmission in deep layers, the evolution path of the density and vertical effective stress during the increase of unloading overpressure in the charts follows the direction of constant density and decreasing vertical effective stress. This evolution path is relatively easy to determine. In addition, when previous researchers used charts showing the relationship between acoustic transit time or resistivity and vertical effective stress to establish models of the relationship between the two under normal compaction conditions, they all used mudstone data from the normal compaction section for fitting. This fitting trend cannot well reflect the changing trend of deep mudstone. This invention uses universally accepted basic formulas to derive and establish a relationship model between the two parameters in the newly selected chart, which can better reflect the changing trend of deep mudstone. Therefore, this invention utilizes universally accepted fundamental formulas to derive a graph relating density to vertical effective stress, thereby quantitatively evaluating the contribution of overpressure to deep multi-mechanism composite structures. Compared to previous methods using graphs of acoustic transit time versus vertical effective stress and resistivity versus vertical effective stress, this method yields more reasonable and accurate results, while also being simpler to operate. In summary, the method of this invention is characterized by its simplicity, accuracy, and rationality, providing a new approach for accurately evaluating the contribution of various overpressure factors in deep multi-mechanism composite structures. Attached Figure Description

[0057] Figure 1 This is a schematic diagram of the process of the present invention.

[0058] Figure 2 The graph shows the relationship between the sonic transit time, density, neutron porosity, formation resistivity, pressure coefficient converted from mud density, and measured formation pressure coefficient as a function of depth in well F1.

[0059] Figure 3 This is a graph showing the relationship between the difference between the logging sonic transit time and the matrix sonic transit time in the normal compaction section of well F1 and the burial depth.

[0060] Figure 4 This is a graph showing the relationship between the difference between the matrix density and the logging density in the normally compacted section of well F1 and the burial depth.

[0061] Figure 5 This is a graph showing the relationship between formation density logging data and burial depth in well F1.

[0062] Figure 6 The graph shows the relationship between density and acoustic velocity in well F1.

[0063] Figure 7 This is a graph showing the relationship between density and effective vertical stress in well F1. Detailed Implementation

[0064] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0065] Taking Well F1 in the central depression of the Junggar Basin as an example, a quantitative evaluation method for the contribution of deep overpressure in oil and gas basins is presented, such as... Figure 1 As shown, it includes the following steps:

[0066] Step 1: Collect and organize the pressure data and logging data of well F1 in the study area. The pressure data includes measured formation pressure and drilling fluid density. The logging data includes natural gamma, sonic transit time, density, neutron porosity and resistivity of mudstone at different burial depths.

[0067] Step 2: Using the data collected in Step 1 regarding the changes in sonic transit time, density, neutron porosity, resistivity, drilling fluid density, and measured formation pressure coefficient of mudstone at different burial depths in Well F1, based on the exponential decrease in sonic transit time and neutron porosity with burial depth and the exponential increase in density and resistivity with burial depth in the normally compacted section of the well, and the negative anomalies in density and resistivity and positive anomalies in sonic transit time and neutron porosity in the undercompacted section, and considering the increase in drilling fluid density in the undercompacted section, a comprehensive judgment is made regarding the normally compacted and undercompacted sections. The overall judgment concludes that the normally compacted section of Well F1 is above 4146m, and the undercompacted section is below 4146m. Figure 2 );

[0068] Step 3: Establishment of a deep overpressure identification map and comprehensive identification of its causes, specifically including:

[0069] Step 3.1: Establish a model showing the relationship between sonic transit time and burial depth in the mudstone section of the normally compacted zone of well F1, as shown in the following formula:

[0070] Δt n =Δt ma +a·e -b·z

[0071] Where, Δt n The sonic transit time of the mudstone layer under normal compaction is Δt. ma Let z be the sonic transit time of the mudstone matrix, taken as 176.5, z be the burial depth, a be the fitting coefficient obtained by using the exponential relationship between the difference between the well logging sonic transit time and the sonic transit time of the mudstone matrix and the burial depth (the value of a in the F1 example well is 301.4), and b be the mudstone compaction coefficient obtained by using the exponential relationship between the difference between the well logging sonic transit time and the sonic transit time of the mudstone matrix and the burial depth (the value of b in the F1 example well is 0.00034). Figure 3 );

[0072] Step 3.2: Determine the relationship between the density of the mudstone layer in the normally compacted section of well F1 and the burial depth, as shown in the following formula:

[0073] ρ(z)=ρ ma -c·e -d·z

[0074] Where ρ(z) is the density of the mudstone layer, ρ ma Let be the density of the mudstone matrix, taken as 2.71. Let c be the fitting coefficient obtained using the exponential relationship between the difference between the mudstone matrix density and the logging density and the burial depth; the c value determined in the F1 example well is 0.686. Let d be the mudstone compaction coefficient obtained using the exponential relationship between the difference between the mudstone matrix density and the logging density and the burial depth; the d value determined in the F1 example well is 0.00045. Figure 4 );

[0075] Step 3.3: Establish a model for the relationship between acoustic velocity and density in the normally compacted mudstone section of well F1. First, use the reciprocal relationship between acoustic velocity and acoustic time difference to calculate the acoustic velocity in the mudstone section of well F1, as shown in the following formula:

[0076]

[0077] Among them, V s Let represent the acoustic velocity of the mudstone layer. Using the acoustic transit time calculation formula for the normally compacted mudstone layer in step 3.1, the acoustic transit time of the mudstone layer at different burial depths under normal compaction in well F1 is calculated. Furthermore, using the reciprocal relationship between acoustic velocity and acoustic transit time established in this step, the acoustic velocity of the mudstone layer at different burial depths under normal compaction in well F1 is calculated. Combined with the density versus burial depth relationship model in step 3.2, a relationship model between acoustic velocity and density in the normally compacted mudstone layer of well F1 is established, as shown in the following formula:

[0078]

[0079] Step 3.4: Establish a model relating the density of the normally compacted mudstone section in well F1 to the vertical effective stress. First, establish a model relating the formation density of well F1 to the burial depth, as shown in the following formula:

[0080] ρ(z) layer =j·z k

[0081] Where, ρ(z) layer For density logging of a formation at a certain burial depth, j and k are the fitting coefficients obtained by fitting the formation density logging data to a power function of the burial depth. The j and k values ​​determined in the F1 example well are 1.355 and 0.0754, respectively. Figure 5 Further, a calculation model for the overburden load stress of the F1 well formation was established, as shown in the following formula:

[0082]

[0083] Where g is the acceleration due to gravity, taken as 9.8. Further calculations yield:

[0084] σ v =0.01235·z 1.0754

[0085] Therefore, a model can be established to show the relationship between the effective vertical stress of the normally compacted strata in well F1 and the burial depth, as shown in the following equation:

[0086] δ=σ v -0.001·ρ w·g·z=0.01235·z 1.0754 -0.001·ρ w ·g·z

[0087] Where δ is the effective vertical stress of the formation, ρ w The density of formation water is taken as 1.02. Further calculations yielded:

[0088] δ=0.01235·z 1.0754 -0.009996·z

[0089] Combined with the mudstone density calculation model of well F1 in step 3.2, a relationship model between the density and vertical effective stress of the normally compacted mudstone section of well F1 is established, as shown in the following formula:

[0090]

[0091] Substituting the relevant parameters into the formula and further calculating, the relationship model between the density and vertical effective stress of the normally compacted mudstone section in well F1 is obtained as follows:

[0092]

[0093] Step 3.5: Using the sonic velocity-density relationship model of the normally compacted mudstone section in well F1 established in Step 3.3, generate a density-sonic velocity relationship chart to identify the cause of overpressure in well F1. Figure 6 Using the density-vertical effective stress relationship model of the normally compacted mudstone section of well F1 established in step 3.4, a density-vertical effective stress relationship chart for identifying the cause of overpressure in well F1 is generated. Figure 7 The effective vertical stress of the reservoir is calculated using measured formation pressure data and overlying load stress. Well F1 has only one measured formation pressure data point, which is 76.78 MPa, corresponding to a burial depth of 5081-5107 m. The formula for calculating the effective vertical stress in this overpressure zone is as follows:

[0094]

[0095] Among them, P 实测 Based on the measured formation pressure of the reservoir, z is taken as the top of this depth range, 5081m. The effective vertical stress of this overpressure range in well F1 can be calculated to be 44.72MPa.

[0096] The data points of the vertical effective stress (44.72 MPa) of the overpressure section, the acoustic velocity (3.56) and density (2.59) of the adjacent mudstone were projected onto the established density vs. acoustic velocity graph. Figure 6 ) and a graph showing the relationship between density and effective vertical stress ( Figure 7Based on the undercompacted section identified in step 2, the overpressure formation type of this overpressure section is comprehensively determined. It is mainly caused by undercompacted overpressure and unloading overpressure. Based on the actual geological conditions of well F1, the specific unloading overpressure formation type is determined to be overpressure transmission.

[0097] Step 4: Quantitative evaluation of the contribution of deep reservoir composite-genetic overpressure, specifically including:

[0098] Step 4.1: Determine the effective vertical stress of the measured overpressure reservoir section in Well F1 before the overpressure is generated. Substitute the density (2.5g) of the mudstone adjacent to the measured overpressure reservoir section (5081-5107m) into the relationship model between the density and effective vertical stress of the normally compacted mudstone section determined in Step 3.4, and the effective vertical stress of the measured overpressure reservoir section in Well F1 before the overpressure is generated can be determined to be 50.48 MPa. Figure 7 );

[0099] Step 4.2: Calculate the effective vertical stress of the overpressured reservoir section after unloading and overpressure formation. The effective vertical stress of the overpressured section calculated in Step 3.5 is the effective vertical stress of the overpressured reservoir section after unloading and overpressure formation, and its magnitude is 44.72 MPa;

[0100] Step 4.3: Determine the magnitude of the unloading overpressure in the measured overpressure section. Based on the principle that the formation density remains constant before and after the unloading overpressure occurs, and that the effective vertical stress continuously decreases during the increase of the unloading overpressure, subtract the effective vertical stress of the overpressure section before the formation of the unloading overpressure from the effective vertical stress of the overpressure section after the formation of the unloading overpressure. This yields the magnitude of the unloading overpressure in the 5081-5107m overpressure section of well F1 as 5.76 MPa. Figure 7 );

[0101] Step 4.4: Determine the magnitude of the under-compacted overpressure in the measured overpressured reservoir section. Subtract the calculated unloading overpressure from the total overpressure of the overpressured reservoir section. The total overpressure of this overpressured reservoir section is 25.99 MPa. Therefore, the magnitude of the under-compacted overpressure in the 5081-5107m overpressured reservoir section of well F1 can be determined to be 20.23 MPa.

[0102] Step 4.5: Calculate the contribution of unloading overpressure and undercompactment overpressure in the measured overpressured reservoir section relative to the total overpressure. Divide the calculated unloading overpressure and undercompactment overpressure of the overpressured reservoir section by the total overpressure. This yields the contribution of unloading overpressure and undercompactment overpressure in the 5081-5107m overpressured reservoir section of Well F1 to the total overpressure, which are 22.16% and 77.84%, respectively. In other words, the contribution of overpressure transmission overpressure and undercompactment overpressure in the 5081-5107m overpressured reservoir section of Well F1 to the total overpressure are 22.16% and 77.84%, respectively.

[0103] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for quantitatively evaluating the contribution of deep overpressure in oil and gas basins, characterized in that, Includes the following steps: Step 1: Organize pressure data and logging data. The pressure data includes measured formation pressure and drilling fluid density. The logging data includes natural gamma, sonic transit time, density, neutron porosity, and resistivity of mudstone at different burial depths. Step 2: Based on pressure data and well logging data, determine the normal compaction zone and the undercompacted zone. Specifically, use the sonic transit time, density, neutron porosity, resistivity, drilling fluid density, and measured formation pressure data with burial depth of mudstone at different burial depths from Step 1 to determine the normal compaction zone and the undercompacted zone. Step 3: Based on the normally compacted section of the well determined in Step 2, using the sonic transit time and density of mudstone sections at different burial depths within the normally compacted section, determine the relationship models between sonic transit time and burial depth, and density and burial depth. Combine these two models to calculate the magnitude of sonic transit time and density of mudstone sections at different burial depths under normal compaction. Further, using the reciprocal relationship between sonic transit time and sonic velocity, calculate the sonic transit time of mudstone sections at different burial depths and the corresponding sonic velocities for the sonic transit time of mudstone sections at different burial depths under normal compaction. Combine the calculated relationship models between sonic velocity and burial depth, and density and burial depth, to establish a relationship model between sonic velocity and density in the normally compacted mudstone section, thus forming a density-sonic velocity relationship chart for identifying the cause of overpressure. Using density logging data from the entire well section, determine the geological... A model relating formation density to burial depth is used to further establish a calculation model for overburden stress. This model, in turn, is used to establish a model relating vertical effective stress of normally compacted strata to burial depth. Combined with the calculated density of normally compacted mudstone strata, a model relating density to vertical effective stress of normally compacted mudstone strata is established, thus forming a density-vertical effective stress relationship chart for identifying overpressure origins. The vertical effective stress of overpressured strata is calculated using measured formation pressure data. The vertical effective stress, sonic velocity, and density data points of these overpressured strata are then projected onto the established density-sonic velocity relationship chart and density-vertical effective stress relationship chart. Combined with the undercompacted strata identified in step 2, the overpressure origin type of the overpressured strata is comprehensively determined. The overpressure origin types include undercompacted overpressure and unloading overpressure. Step 4: Based on Step 3, calculate the magnitude of the unloading overpressure of the overpressure reservoir section; subtract the calculated unloading overpressure from the total overpressure of the overpressure reservoir section to obtain the undercompacted overpressure of the overpressure reservoir section; divide the calculated unloading overpressure and the undercompacted overpressure of the overpressure reservoir section by the total overpressure respectively to obtain the contribution of the unloading overpressure and the undercompacted overpressure of the overpressure reservoir section to the total overpressure.

2. The method for quantitatively evaluating the contribution of deep overpressure in oil and gas basins according to claim 1, characterized in that, Step 3 specifically includes the following steps: Step 3.1: Establish a model showing the relationship between sonic transit time and burial depth in the normally compacted mudstone layer, as shown in the following formula: in, The acoustic transit time of the mudstone layer under normal compaction. For the acoustic transit time of the mudstone matrix, z For burial depth, a is the fitting coefficient obtained by using the difference between the sonic transit time of well logging and the sonic transit time of mudstone matrix with the exponential relationship between burial depth, and b is the mudstone compaction coefficient obtained by using the difference between the sonic transit time of well logging and the sonic transit time of mudstone matrix with the exponential relationship between burial depth. Step 3.2: Determine the relationship between density and burial depth for the normally compacted mudstone layer, as shown in the following formula: in, The density of the mudstone layer. denoted as ρ, where c is the density of the mudstone matrix, c is the fitting coefficient obtained by using the exponential relationship between the difference between the mudstone matrix density and the logging density and the burial depth, and d is the mudstone compaction coefficient obtained by using the exponential relationship between the difference between the mudstone matrix density and the logging density and the burial depth. Step 3.3: Establish a model for the relationship between acoustic velocity and density in normally compacted mudstone layers. First, use the reciprocal relationship between acoustic velocity and acoustic time difference to calculate the acoustic velocity in the mudstone layer, as shown in the following formula: in, V s To determine the acoustic velocity of the mudstone layer, the acoustic transit time formula for normally compacted mudstone in step 3.1 is used to calculate the acoustic transit time for mudstone layers at different burial depths under normal compaction. Furthermore, using the reciprocal relationship between acoustic velocity and acoustic transit time established in this step, the acoustic velocity of mudstone layers at different burial depths under normal compaction is calculated. Combined with the density versus burial depth relationship model in step 3.2, a relationship model between acoustic velocity and density in normally compacted mudstone layers is established, as shown in the following formula: Step 3.4: Establish a model relating density to vertical effective stress in normally compacted mudstone strata. First, establish a model relating formation density to burial depth, as shown in the following formula: in, Let j and k be the density of the formation at a certain burial depth, and j and k be the fitting coefficients obtained by fitting the formation density logging data with a power function of the burial depth. Further, a calculation model for the overburden load stress of the formation is established, as shown in the following equation: Where g is the acceleration due to gravity, a model is established to show the relationship between the effective vertical stress of a normally compacted stratum and its burial depth, as shown in the following equation: in, The effective vertical stress of the formation, Given the formation water density, and combined with the mudstone density calculation model from step 3.2, a relationship model between the density and vertical effective stress of a normally compacted mudstone layer is established, as shown in the following formula: Step 3.5: Using the sonic velocity-density relationship model established in Step 3.3 for normally compacted mudstone sections, generate a density-sonic velocity relationship chart to identify overpressure origins; using the density-vertical effective stress relationship model established in Step 3.4 for normally compacted mudstone sections, generate a density-vertical effective stress relationship chart to identify overpressure origins; calculate the vertical effective stress using measured formation pressure data and overlying load stress, using the following formula: in, P 实测 To measure the formation pressure in the reservoir, the vertical effective stress of the overpressured section, the sonic velocity and density data points of the adjacent mudstone are projected onto the established density-sonic velocity relationship chart and density-vertical effective stress relationship chart. Combined with the undercompacted section identified in step 2, the overpressured section's overpressure origin is comprehensively determined: undercompacted overpressure origin or unloading overpressure origin. Based on the actual geological conditions, the specific unloading overpressure origin is comprehensively determined.

3. The method for quantitatively evaluating the contribution of deep overpressure in oil and gas basins according to claim 1, characterized in that, Step 4 specifically involves: Substituting the measured density of the adjacent mudstone in the overpressured section into the relationship model between the density and vertical effective stress of the normally compacted mudstone section established in step 3, the vertical effective stress of the measured overpressured reservoir section before the unloading overpressure occurs is calculated. Using the vertical effective stress of the measured overpressured reservoir section calculated in step 3, this is the vertical effective stress of the overpressured reservoir section after the unloading overpressure occurs. The vertical effective stress of the overpressured reservoir section before the unloading overpressure occurs is then compared with the vertical effective stress of the overpressured reservoir section before the unloading overpressure occurs. The magnitude of the unloading overpressure of the overpressure reservoir segment is obtained by subtracting the effective vertical stress after the formation of unloading overpressure. The magnitude of the undercompacted overpressure of the overpressure reservoir segment is obtained by subtracting the obtained unloading overpressure from the total overpressure of the overpressure reservoir segment. The contribution of the unloading overpressure and the undercompacted overpressure of the overpressure reservoir segment to the total overpressure is obtained by dividing the obtained unloading overpressure and the undercompacted overpressure of the overpressure reservoir segment by the total overpressure.

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