Method for predicting scale of sedimentary system

Abstract

The invention belongs to the field of petroleum and natural gas geological exploration, and particularly relates to a method for predicting the scale of a sedimentary system. The method comprises the steps that a mathematical model with the gravel diameter changing along with the carrying distance is obtained from a modern sedimentary system, and the method for predicting distribution of the coarse-grain river fan and alluvial fan sedimentary system is formed by comparing and verifying the mathematical model with underground drilling core data. The method provided by the invention can quantitatively describe the change rule of the diameter of the sediment in the source-to-sink process of the coarse grain deposition system, so that the research on the distribution of the deposition system is changed from qualitative to quantitative; the mathematical model provided by the invention has good reliability and applicability.

Description

Technical Field

[0001] The invention belongs to the field of petroleum and natural gas geological exploration, and in particular relates to a method for predicting the scale of a sedimentary system. Background Art

[0002] The recognition and research of gravel diameter decrease phenomenon originated from modern river sedimentation. As early as the 1880s, some foreign sedimentologists began to do a lot of work on the phenomenon of gravel diameter decrease. By the 1930s, a set of basic theories for quantitatively describing the gravel diameter decrease process was formed, laying a solid foundation for the development of gravel diameter decrease research. At the beginning of the 21st century, with the deepening of oil and gas exploration, conglomerate reservoirs have become the key exploration targets, and it is necessary to make a quantitative description of the distribution of coarse-grained sedimentary systems.

[0003] The lower assemblage on the southern edge of the Junggar Basin has great potential for oil and gas exploration. The wells Gaotan 1, Hutan 1, Letan 1 and Tianwan 1 have successively obtained high-yield industrial oil and gas flows. The main producing layer encountered is a set of coarse-grained clastic reservoirs. However, due to the lack of clear understanding of the distribution range of this set of coarse-grained clastic reservoirs, the distribution of favorable reservoirs and exploration prospects are difficult to implement.

[0004] Gao Zhiyong et al. reported in the Journal of Petroleum Exploration and Development that: In view of the difficulty in studying the lithofacies paleogeography of the prototype basin and the low credibility of the distribution range of sedimentary sand bodies due to the small number of deep drillings, complex seismic imaging and low exploration level in the southern margin of the Junggar Basin, a new method was used to carry out lithofacies paleogeography restoration and sand-conglomerate distribution prediction research based on the source-sink concept, and the macro-source area and uplift time were determined by apatite fission track age; the source area and lakeshore migration process were restored based on the quantitative relationship between gravel diameter and transportation distance, structural shortening and other geological parameters; drilling cores and field outcrop sedimentary structures were analyzed, and a series of lithofacies paleogeographic evolution and sand-conglomerate distribution range maps were compiled. This understanding can provide a geological parameter basis for deep-seated combined oil and gas exploration in the southern margin of the Junggar Basin.

[0005] In addition, Gao Zhiyong et al. reported in the Journal of Paleogeography 2021, 23(03) that "the role of gravel in analyzing the migration of basin provenance and the evolution of lakeshores" specifically includes: the basin-mountain junction of basins in central and western China has a low degree of exploration, few wells, and complex seismic imaging, which makes it difficult to study the lithofacies paleogeographic characteristics of prototype basins using traditional seismic phases, logging phases, and core data. Based on the understanding that the thick conglomerate development intervals in the outcrop correspond to the widest planar distribution of reservoirs in the basin, the "using the present to discuss the past" method was used to establish the relationship between gravel and transportation distance in the pre-modern alluvial fan, fan delta plain, river and other sedimentary systems in the South Tianshan Mountains, as well as the relationship between the occurrence of gravel and the distance of the lakeshore in different sedimentary facies types. The distance between the current position of the Jurassic-Cretaceous in multiple key sections of the Junggar Basin and the sediment source area and lakeshore evolution was quantitatively calculated, and the range of the Jurassic-Cretaceous provenance and the lakeshore evolution process were restored.

[0006] The existing research on the distribution of sedimentary systems mainly focuses on the study of sedimentary phases, and qualitatively describes the sedimentary system based on the phase model. By identifying the sedimentary phases of cores and outcrops, the sedimentary system type of the study area is determined, and the sedimentary phase model of the study area is established. Although these methods can provide references for exploration, they cannot meet the quantitative research on the distribution of sedimentary systems and cannot effectively obtain the scale of the sedimentary system in the study area, thus restricting the progress of oil and gas exploration. Some scholars use the method of "using the present to discuss the past" to study the distribution of sedimentary systems, and apply the relationship between gravel and transportation distance of modern sedimentary systems to the underground, but it is difficult to effectively characterize the trend of underground sediment particle size changes in different study areas. Therefore, it is necessary to combine the underground conditions and establish a relationship between sediment particle size and transportation distance that conforms to the underground, so that it can be applied to more accurate oil and gas exploration.

[0007] There are many studies on the phenomenon of gravel diameter decreasing downstream, but few studies have applied this phenomenon underground. Traditional studies on the relationship of gravel diameter decreasing downstream are mainly concentrated in modern sedimentary systems. Due to the lack of research on the trend of underground gravel diameter decrease, this technology has not been applied to the study of underground sedimentary system distribution. Summary of the invention

[0008] In view of the shortcomings of the existing quantitative analysis of sedimentary system distribution, which is difficult to apply to more accurate oil and gas exploration, the present invention provides a method for predicting the scale of a sedimentary system. Specifically, it is a method for predicting the distribution of a sedimentary system using the law of decreasing gravel diameter. The method can quantitatively describe the law of change of sediment diameter in the process from source to sink of a coarse-grained sedimentary system, so that the study of sedimentary system distribution can move from qualitative to quantitative.

[0009] In order to achieve the above-mentioned purpose of the present invention, the specific technical solution adopted by the present invention is:

[0010] A method for predicting the scale of a sedimentary system, the method comprising: obtaining a mathematical model of the variation of gravel diameter with transportation distance from a modern sedimentary system, and verifying it by comparing it with underground drilling core data to form a method for predicting the distribution of coarse-grained river fan and alluvial fan sedimentary systems.

[0011] Preferably, the mathematical model is based on a negative exponential function model of the relationship between gravel diameter and transport distance in the sedimentation of a modern large-scale branching river system, and is analyzed using outcrop and downhole gravel diameter data; the mathematical model is used to analyze the distribution range of the Kalaza Formation and Qingshuihe Formation sedimentary systems in the southern margin of the Junggar Basin

[0012] Preferably, the mathematical model is L=-35.71lnd+136.89, wherein L is the transportation distance and d is the gravel particle size.

[0013] Preferably, the method for establishing the mathematical model comprises:

[0014] (1) By comparing modern and ancient sedimentary systems, find the modern sedimentary system that matches the underground conditions;

[0015] (2) Select a modern large-scale branching river system and select test points by analyzing the general law of decreasing sediment gravel diameter along the river;

[0016] (3) Sampling and measuring gravel diameter at the test point, and measuring the distance from a single test point to the fan top, to conduct particle size analysis of gravel diameter;

[0017] (4) Based on the gravel diameter data of each test point and the distance to the fan top, a mathematical model for the decreasing gravel diameter along the way is established.

[0018] Further preferably, the comparison of modern and ancient sedimentary systems in step (1) includes: comparing the development of the underground sedimentary system with the modern sedimentary system, so as to find a modern sedimentary system that is more consistent with the underground conditions.

[0019] Further preferably, the modern sedimentary system described in step (1) is observed and screened through Google Earth, and it is found that the climate in the Hexi Corridor is dry, and the sedimentary environment is similar to that in the deposition period of the Kalaza Formation-Qingshuihe Formation. A coarse-grained sedimentary system is also developed in front of the mountain, which is large in scale and extends far.

[0020] Further preferably, the modern large-scale branch river system in step (2) is the branch river system of Shule River.

[0021] Further preferably, the number of test points in step (3) is 4-8; the test points are well exposed and the gravel particles are obvious; the sampling area of ​​a single test point is 0.8-1.2m×0.8-1.2m, and the number of samples of a single test point is 280-320.

[0022] Further preferably, the results of the particle size analysis of the gravel diameter in step (3) include: the overall particle size is coarse at the proximal end, the particle size is more dispersed, and the span is wide, and the particle size gradually becomes finer and smaller downstream, and the particle size becomes more concentrated, which conforms to the general law of decreasing sediment particle size.

[0023] Preferably, the step of analyzing the distribution range of the deposition system comprises:

[0024] (1) Conduct outcrop sediment grain size analysis and calculate gravel diameter data;

[0025] (2) Substitute the gravel diameter data into the mathematical model and calculate to obtain the gravel transport distance;

[0026] (3) Based on the mathematical model, the distance required for gravel deposits to be transported is predicted when they are transformed into sandy deposits, and the distribution range of the sedimentary system is analyzed.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) The present invention provides a new method for predicting the distribution of coarse-grained river fan and alluvial fan sedimentary systems, which can quantitatively describe the change law of sediment diameter in the process of coarse-grained sedimentary systems from source to sink, thus making the study of sedimentary system distribution move from qualitative to quantitative;

[0029] (2) The mathematical model in the present invention has good reliability and applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the location map of Shule River and test points;

[0031] Figure 2 Gravel diameter sampling map for the test points;

[0032] Figure 3 To fit the mathematical model diagram of the decreasing diameter of gravel in Shule River;

[0033] Figure 4 This is the location map of the core sampling points in Tianan 1 Well;

[0034] Figure 5 This is a model calculation and prediction diagram, specifically the Shule River model in Example 3 calculates the gravel transportation distance at the outcrop and well location and predicts the transportation distance required for all gravel to be converted into sand. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present invention are further clearly described. The described embodiments are only a part of the present invention and are used to explain the present invention but not to limit the present invention. Therefore, other embodiments obtained by other technicians in the field without creative work all belong to the protection scope of the present invention.

[0036] Example 1

[0037] 1. Comparison between modern and ancient sedimentary systems: It is necessary to compare the development of underground sedimentary systems with modern sedimentary systems, so as to find modern sedimentary systems that are more in line with underground conditions for subsequent steps. A large set of coarse-grained sedimentary systems developed in the Kalaza Formation-Qingshuihe Formation on the southern edge. The onshore part of these sedimentary systems can be called branch river systems. The development environment is relatively arid, mainly developed in front of the mountains, and extended far. Through the observation and screening of modern sedimentary systems by Google Earth, it was found that the climate in the Hexi Corridor is dry, and the sedimentary environment is similar to that of the Kalaza Formation-Qingshuihe Formation during the deposition period. A coarse-grained sedimentary system also developed in front of the mountains, which is large in scale and extends far.

[0038] 2. Selection of modern large-scale branch river system: Based on the analysis of remote sensing image data, the branch river system of Shule River was selected as the research object. On the plane, the branch river system of Shule River is symmetrical fan-shaped, with a fan radius of 59.6km and an arc length of 128km. The total length of Shule River is 620km. A large amount of gravel deposition can be seen at the proximal and middle parts, and the grain size is obviously finer at the distal end, but a small amount of gravel can still be seen ( Figure 1 ).

[0039] 3. Test point selection: With the assistance of drones, the gravel exposure of the Shule River was observed. Combined with the changes in river morphology, a total of 5 test points with good exposure and obvious gravel particles were selected, and the distance from a single test point to the fan top was measured ( Figure 1 ).

[0040] 4. Gravel diameter sampling and measurement: The sampling area of ​​a single test point is 1x1m, and the number of samples at each sampling point is about 300. The long axis (a axis) of the gravel is extracted from the image using image software ( Figure 2 ).

[0041] 5. Grain size analysis of gravel diameter: By analyzing the gravel diameter at the sampling point, it is found that the overall grain size is coarse at the proximal end, the grain size is more dispersed, and the span is wide. The grain size gradually becomes finer and smaller toward the downstream, and the grain size becomes more concentrated, which conforms to the general law of decreasing sediment grain size ( Figure 3 ).

[0042] 6. Establish a mathematical model for the decrease of gravel diameter along the way: According to the gravel diameter data of each test point and the distance to the fan top, the mathematical model of the decrease of gravel diameter along the way of Shule River can be obtained by fitting in the data analysis software ( Figure 3 ),Right now:

[0043] L=-35.71lnd+136.89 (Formula 1)

[0044] Where L is the transportation distance (km) and d is the gravel particle size (mm).

[0045] Example 2

[0046] The constructed model is used to predict the sedimentary system in the southern margin of the Junggar Basin to verify the reliability and applicability of the model.

[0047] 1. Outcrop sediment grain size analysis: The outcrop selected is the Jihai River section in Nan'an, and the stratum is the bottom conglomerate of the Qingshuihe Formation. The average diameter of the gravel is mainly extracted. And other related auxiliary parameters (Table 1).

[0048] 2. Selection of verification wells: Tianan 1 Well was selected as the verification well. The distribution direction of the sedimentary system from the Nan'an Jihai profile to Tianan 1 Well is consistent with that from south to north, and the well has sufficient data in the target stratum.

[0049] 3. Acquisition of sediment particle size information of the verification well: The gravel diameter in the core was extracted by image measurement (Table 1), and a total of 4 sampling points were set up ( Figure 4 ), the area of ​​a single sampling point is about 0.5x0.1m 2 , about 100 samples were collected, and the sampling points were spaced about 1.5m apart.

[0050] Table 1 Anjihai profile and gravel diameter data of Tianan 1 well

[0051]

[0052] 4. Calculate the theoretical value of the change in sediment grain size from outcrop to well point: Based on the mathematical model of the decreasing gravel diameter of Shule River (Formula 1), calculate the theoretical average gravel diameter from the Nan'an Jihai profile to the Tian'an 1 well; the current straight-line distance from the Nan'an Jihai profile to the Tian'an 1 well is 31km, plus the structural shortening of the area, a total of 12.95km, the distance between the two points at the time of deposition can be restored to 43.95km.

[0053] From this we can list the relationship: L1 = -35.71lnd1 + 136.89

[0054] L2=-35.71lnd2+136.89

[0055] The solution is: D = 6.75 mm

[0056] 5. Compare the calculated results with the actual gravel diameter at the well point: The comparison shows that the theoretical value is smaller than the actual value. The theoretical particle size of Tianan 1 Well is 6.75mm, and the actual measurement of Tianan 1 Well is 7.83mm, a difference of 1.08mm.

[0057] Example 3

[0058] The steps for predicting the transport distance of the conglomerate at the bottom of the Qingshuihe Formation and the distribution of the sedimentary system are as follows:

[0059] Step 1: Bring the gravel diameter data of the Anjihai River profile and Tianan 1 well into the mathematical model.

[0060] Step 2: Calculate the gravel transport distance of the Anjihai section to be 24.70 km and the gravel transport distance of the Tianan 1 well to be 63.4 km.

[0061] Step 3: Figure 5 Based on the mathematical model in, it is predicted that the required transportation distance for gravel deposits to transform into sandy deposits is 112.14 km. Therefore, it can be considered that if there are no other factors affecting the coarse-grained deposits in this sedimentary system, they can extend for 112.14 km.

[0062] Comparative Examples 1-10

[0063] Comparative Examples 1-10 used different mathematical models to predict the gravel diameter of Tianan 1 Well and compared them with Example 2. The results are shown in Table 2: The mathematical models of gravel diameter decrease of 8 modern branch river systems established by predecessors in the world in the past 10 years were selected and compared with the mathematical model of Shule River. It was found that the calculation results based on the mathematical model of Shule River were closest to the actual data.

[0064] Table 2 Prediction results of gravel diameter in Tianan 1 Well by different mathematical models

[0065]

[0066] The above detailed description is a specific description of one feasible embodiment of the present invention. The embodiment is not intended to limit the patent scope of the present invention. Any equivalent implementation or modification that does not deviate from the present invention should be included in the scope of the technical solution of the present invention.

Claims

1. A method for predicting the scale of a deposition system, characterized in that: The method comprises: obtaining a mathematical model of gravel diameter variation with transport distance from a modern sedimentary system, and predicting the distribution of coarse-grained river fan and alluvial fan sedimentary systems by comparing and verifying with underground drilling core samples.

2. The method according to claim 1, characterized in that The mathematical model is based on a negative exponential function model of the relationship between gravel diameter and transportation distance in the sedimentation of a modern large-scale branching river system, and is analyzed using outcrop and downhole gravel diameter data; the mathematical model is used to analyze the distribution range of the Kalaza Formation and Qingshuihe Formation sedimentary systems in the southern margin of the Junggar Basin.

3. The method according to claim 2, characterized in that The mathematical model is L=-35.71lnd+136.89, wherein L is the transportation distance and d is the gravel particle size.

4. The method according to claim 1, characterized in that The method for establishing the mathematical model comprises: (1) By comparing modern and ancient sedimentary systems, find the modern sedimentary system that matches the underground conditions; (2) Select a modern large-scale branch river system and select test points by analyzing the law of decreasing sediment gravel diameter along the river; (3) Sampling and measuring gravel diameter at the test point and conducting particle size analysis of gravel diameter; (4) Based on the gravel diameter data of each test point and the distance to the fan top, a mathematical model for the decreasing gravel diameter along the way is established.

5. The method according to claim 4, characterized in that The comparison of modern and ancient sedimentary systems in step (1) includes: comparing the development of the underground sedimentary system with the modern sedimentary system, so as to find a modern sedimentary system that is more consistent with the underground conditions.

6. The method according to claim 4, characterized in that The modern sedimentary system described in step (1) is obtained by observing and screening the modern sedimentary system through Google Earth.

7. The method according to claim 4, characterized in that The modern large-scale branch river system described in step (2) is the branch river system of Shule River.

8. The method according to claim 4, characterized in that The number of test points in step (3) is 4-8; the test points are well exposed and have obvious gravel particles; the sampling area of ​​a single test point is 0.8-1.2m×0.8-1.2m, and the number of samples of a single test point is 280-320.

9. The method according to claim 3, characterized in that: The measurement in step (3) includes: measuring the distance from a single test point to the fan top; the particle size analysis includes proximal particle size analysis and downstream particle size analysis.

10. The method according to claim 2, characterized in that The steps of analyzing the distribution range of the deposition system include: (1) Conduct outcrop sediment grain size analysis and calculate gravel diameter data; (2) Substitute the gravel diameter data into the mathematical model and calculate to obtain the gravel transport distance; (3) Based on the mathematical model, the distance required for gravel deposits to be transported is predicted when they are transformed into sandy deposits, and the distribution range of the sedimentary system is analyzed.

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