A method for quantitatively characterizing the sedimentary subfacies of a nearshore underwater fan

Abstract

The present invention belongs to the technical field of oil and gas exploration and development, and particularly relates to a method for quantitatively characterizing the sedimentary subfacies of a nearshore underwater fan. The method comprises the following steps: establishing a geological identification chart for the sedimentary subfacies of the nearshore underwater fan and conducting single-well facies identification for actual drilled wells; conducting staging of the nearshore underwater fan to achieve qualitative seismic identification of different subfacies of a single-stage nearshore underwater fan; establishing an actual geological model and conducting forward modeling; determining the critical values of sand-to-shale ratio parameters for different subfacies, and fitting the relationship between the formation thickness and reservoir thickness of the fan body varying with the transportation distance; constructing a function for the distribution range of different subfacies zones of a single-stage nearshore underwater fan and determining the distribution ranges of different facies zones. The method of the present invention realizes the quantitative characterization of the boundaries of the sedimentary subfacies of the nearshore underwater fan, and overcomes the drawbacks of the traditional method, such as large human influence and strong multi-solution in the identification of facies zone boundaries.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas exploration and development, and particularly relates to a method for quantitatively characterizing the sedimentary subfacies of a nearshore underwater fan. Background Art

[0002] Continental fault basins are widely developed in the east and west of China. Controlled by tectonic movements, a series of nearshore underwater fan deposits with near-source rapid accumulation develop in front of the valleys in the steep slope zones, which can be subdivided into three subfacies: fan root, fan middle, and fan end. The lithology of such deposits is complex, with a mixed accumulation of sandy conglomerates and mudstones, and the reservoir has strong heterogeneity. The sedimentary facies belt controls the physical properties of the reservoir, which in turn determines the hydrocarbon-bearing property of the reservoir. Among them, the fan middle subfacies is the most favorable hydrocarbon-bearing facies belt. Therefore, it is crucial to accurately characterize the distribution of the sedimentary facies belt of such deposits.

[0003] Currently, the mainstream method for characterizing the distribution of the nearshore underwater fan sedimentary facies belt is [1 -4]: First, based on provenance analysis and guided by Walther's law, single-well sedimentary facies identification of typical wells, sedimentary facies analysis of the cross-well profile, and preliminary judgment of the planar facies belt distribution are carried out; second, based on the calibration of synthetic seismograms, planar seismic facies or sensitive seismic attributes with high correlation with the sedimentary facies of the actual drilled wells are selected; finally, with the single-well facies at the actual drilled well points as the control points and the selected sensitive seismic facies or attributes as the planar guidance, the facies belt boundaries are determined based on personal experience to complete the compilation of the planar sedimentary facies map. Although this method has initially realized the combination of well and seismic data for facies belt prediction, the wellbore data has not been fully utilized, the well and seismic data are isolated from each other, the facies belt boundary identification is greatly affected by humans and has strong multi-solutionity, and it cannot meet the requirements of the increasingly improved exploration and development degree.

[0004] In addition, through a large number of literature investigations, it is found that some literature has carried out quantitative identification of sedimentary facies [5 -6], but it only realizes the quantitative identification of the single-well facies on the actual drilled wells through the intersection of logging curves or lithology and sedimentary facies, and does not study the distribution of the planar facies belt.

[0005] Chinese Patent Application CN109324171A discloses a method for quantitatively identifying sedimentary facies based on lithology statistics. The method includes: data collection: collecting well logging data of lithology and known sedimentary facies data in the work area; data processing: taking the continuous and uninterrupted stratigraphic section corresponding to a sedimentary facies type divided on a single well as a calculation unit, removing the calculation units with a thickness less than 20m, removing the invalid rock layers in each calculation unit, and dividing the remaining rock layers into two categories: sandstone and mudstone; parameter calculation: calculating two parameters, namely the sandstone frequency and the sand-to-shale ratio of each calculation unit respectively; establishing a quantitative identification chart of sedimentary facies: according to the parameter calculation results of each calculation unit, plotting points on the intersection diagram of sandstone frequency - sand-to-shale ratio, and then drawing the identification areas of different sedimentary facies types according to the sedimentary facies type and the plotting position of the calculation unit.

[0006] Chinese Patent Application CN109709155A discloses a method for quantitatively discriminating sedimentary facies by applying apparent resistivity logging curves. The method includes the following steps: Step 1, collecting and comprehensively studying regional geological data; Step 2, establishing a sedimentary system map of typical boreholes in the region; Step 3, establishing a quantitative discrimination function of regional sedimentary facies; Step 4, quantitatively discriminating sedimentary facies of a single well in the region; Step 5, quantitatively discriminating regional sedimentary facies.

[0007] The above methods mostly have the drawbacks of great human influence on the discrimination of facies belt boundaries and strong multi-solutionity. The method for quantitatively depicting sedimentary facies needs to be further improved.

[0008] [1] Shao Xupeng. Study on seismic sedimentology of the upper submember of the Sha-4 Formation in the northern Minfeng area of the Dongying Sag [D]. China University of Petroleum (East China), 2014.

[0009] [2] Pang Jungang, Yang Youyun, Pu Xiugang. Identification characteristics of fan deltas, nearshore subaqueous fans and sublacustrine fans in faulted lake basins [J]. Journal of Lanzhou University (Natural Sciences), 2011, 47(04): 18 - 23 + 32.

[0010] [3] Zhang Xiaojie. Study on sequence stratigraphy and sedimentary facies of the Sha-4 Formation in the middle section of the northern belt of the Dongying Sag [D]. China University of Petroleum, 2011.

[0011] [4] Xu Tianwu, Song Haiqiang, Fan Weiping, Shao Manjun, Fan Shangwu. Quantitative analysis of provenance and sedimentary facies during the sedimentation period of the first member of the Tai Formation in the Late Cretaceous in the Gaoyou Sag [J]. Petroleum Geology and Recovery Efficiency, 2008(06): 29 - 31 + 38 + 112 - 113.

[0012] [5] Wang Haofeng, Liu Bo, Chen Shuang, Wang Wenxu, Peng Ruiqiang. Quantitative discrimination of sedimentary facies by using the morphological characteristics of apparent resistivity logging curves [J]. Journal of East China University of Technology (Natural Science), 2019, 42(04): 392 - 400.

[0013] [6] Zhou Xinping, Zhang Wenxuan, Liu Guanglin, Song Peng, Chu Meijuan, Chen Lu. Quantitative analysis of sedimentary facies by lithology statistics: Taking the Chang 8 section of the Yanchang Formation in northern Shaanxi as an example [J]. Journal of Xi'an Shiyou University (Natural Science Edition), 2016, 31(06): 9-14+22. Summary of the Invention

[0014] The main object of the present invention is to provide a method for quantitatively characterizing the sedimentary subfacies of a nearshore underwater fan. By using mathematical methods to highly integrate well and seismic data, and through forward modeling and well-seismic combination, the present invention realizes the quantitative characterization of the boundaries of the sedimentary subfacies of the nearshore underwater fan, overcoming the disadvantages of large human influence and strong multi-solution in the identification of facies belt boundaries in traditional methods.

[0015] To achieve the above object, the present invention adopts the following technical solutions:

[0016] The present invention provides a method for quantitatively characterizing the sedimentary subfacies of a nearshore underwater fan, which includes the following steps:

[0017] Step 1. Establish a geological identification plate for the sedimentary subfacies of the nearshore underwater fan and conduct single-well facies identification of the actual drilled wells;

[0018] Step 2. Conduct stage division of the nearshore underwater fan to achieve qualitative seismic identification of different subfacies of a single-stage nearshore underwater fan;

[0019] Step 3. Establish an actual geological model and conduct forward modeling;

[0020] Step 4. Determine the critical values of the sand-to-shale ratio parameters of different subfacies, and fit the relationship between the thickness of the fan body stratum and the reservoir thickness with the change of the transportation distance;

[0021] Step 5. Construct the distribution range function of different subfacies belts of a single-stage nearshore underwater fan and determine the distribution ranges of different facies belts.

[0022] Further, in Step 1, based on the core, logging, and well logging data of the actual drilled wells, and guided by sedimentology, a geological identification plate for the sedimentary subfacies of the nearshore underwater fan is established, and the template includes the following geological parameters: logging curves, lithology-related parameters, and core-related parameters.

[0023] Furthermore, the geological identification plate for the sedimentary subfacies of the nearshore underwater fan is established as follows:

[0024] Fan root subfacies: thick massive conglomerates and coarse conglomerates, with little mudstone development, and the SP curve is box-shaped or Christmas tree-shaped; 50≤AC≤59, 2≤CNL≤5, 41≤LLD≤100;

[0025] Fan middle subfacies: thick massive pebbly sandstones, pebble-bearing rocks or fine conglomerates intercalated with thin mudstone layers, and the SP curve is continuously box-shaped or thick finger-shaped, 60≤AC≤72, 6≤CNL≤17, 16≤LLD≤40;

[0026] Fan-end subfacies: thick mudstone intercalated with thin sandstone, SP curve is straight, AC ≤ 73, 18 ≤ CNL ≤ 35, 3 ≤ LLD ≤ 15.

[0027] Further, in step 2, the well-seismic correspondence is established by using fine synthetic seismic record calibration, the stages of the nearshore submarine fan are divided, the seismic reflection bands corresponding to different sedimentary subfacies boundaries are determined, and the seismic reflection characteristics of each sedimentary subfacies are analyzed to realize the qualitative seismic identification of different subfacies of a single-stage nearshore submarine fan.

[0028] Even further, the seismic reflection bands corresponding to different sedimentary subfacies boundaries are as follows:

[0029] The top surface of a single-stage fan body, i.e., the stage interface of the nearshore submarine fan: is the mud-sand interface, showing a stable, continuous, medium-strong reflection;

[0030] Fan-end subfacies: multiple nearly parallel short-axis-shaped strong reflections, obliquely intersecting at an angle with the strong reflection of the front lacustrine deposit, with a relatively high seismic dominant frequency and a relatively narrow reflection waveform;

[0031] Fan-middle subfacies: the amplitude gradually weakens from the strong reflection at the fan end towards the interior of the fan body, with a relatively low seismic dominant frequency and a relatively wide reflection waveform;

[0032] Fan-root subfacies: nearly blank reflection, with a high-angle medium-strong reflection at the boundary with the fan middle, and a wide and gentle reflection waveform.

[0033] Further, in step 3, if the forward modeling result is compared with the actual seismic data and the reflection characteristics representing the same sedimentary subfacies are consistent for both, it is the true reflection characteristic;

[0034] If the reflection characteristics representing the same sedimentary subfacies are inconsistent for both, return to check and correct steps (2) and (3) until the reflection characteristics are consistent for both.

[0035] Further, in step 4, based on the single-well facies identification in step 1, by statistically analyzing the formation parameters of different sedimentary subfacies, pairwise correlation intersection analysis is carried out for each parameter, geological parameters that can effectively distinguish different sedimentary subfacies are optimized, and finally the critical value of the sand-to-ground ratio parameter that can effectively distinguish different subfacies is determined.

[0036] Even further, the statistical formation parameters of different sedimentary subfacies include: formation thickness, sandstone thickness, mudstone thickness, and single-layer sandstone thickness.

[0037] Further, the method further includes the following steps:

[0038] After determining the parameter critical values, a seismic profile is cut along the provenance direction. The bottom interface of the nearshore subaqueous fan in the target period is flattened, and the flattened seismic profile is converted into a mathematical model to construct a function for the distribution range of different subfacies of a single-period nearshore subaqueous fan.

[0039] Compared with the prior art, the present invention has the following advantages:

[0040] The method of the present invention realizes the quantitative characterization of the sedimentary subfacies boundary of the nearshore subaqueous fan, avoiding the disadvantages of large artificial influence and strong multi-solution of the conventional method of extrapolating and characterizing the facies belt boundary based on attributes, and greatly improving the characterization accuracy of the facies belt boundary. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The accompanying drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not constitute an improper limitation to the present invention.

[0042] Figure 1 It is a flow chart of the method for quantitatively characterizing the sedimentary subfacies of the nearshore subaqueous fan described in Embodiment 1 of the present invention;

[0043] Figure 2 It is a crossplot of the maximum single-layer thickness and sand-to-shale ratio of the reservoir in the middle subfacies of the nearshore subaqueous fan in Embodiment 1 of the present invention;

[0044] Figure 3 It is the fan body morphology after flattening the period bottom in Embodiment 1 of the present invention;

[0045] Figure 4 It is a schematic diagram of constructing the fan body width function in Embodiment 1 of the present invention;

[0046] Figure 5 It is the sedimentary facies map before applying the quantitative characterization described in Embodiment 1 of the present invention;

[0047] Figure 6 It is the sedimentary facies map quantitatively characterized by applying the method described in Embodiment 1 of the present invention;

[0048] Figure 7 It is a conventional seismic attribute prediction map. DETAILED DESCRIPTION OF THE INVENTION

[0049] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0050] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they specify the presence of features, steps, operations, and / or combinations thereof.

[0051] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in conjunction with specific embodiments.

[0052] Embodiment 1

[0053] As Figure 1 shown, the method for quantitatively characterizing the subfacies of the nearshore underwater fan deposition includes the following steps:

[0054] Step 1. Based on the actual drilled well cores, logging, and well logging data, under the guidance of sedimentology, establish a geological identification chart for the subfacies of the nearshore underwater fan deposition (the content includes but is not limited to geological parameters such as well logging curves, lithology, and cores), and identify the single well facies of the actual drilled wells according to the identification chart:

[0055] Establish a geological identification chart for the subfacies of the nearshore underwater fan deposition based on the actual drilled well cores, logging, and well logging data:

[0056] Fan root subfacies: A large set of thick-layer conglomerates and coarse conglomerates, with little development of mudstone, and the SP curve is box-shaped or Christmas tree-shaped; 50 ≤ AC ≤ 59, 2 ≤ CNL ≤ 5, 41 ≤ LLD ≤ 100;

[0057] Fan middle subfacies: Thick-layer pebbly sandstone, pebbly rock or fine conglomerate interbedded with thin mudstone layers, the SP curve is continuously box-shaped or thick finger-shaped, 60 ≤ AC ≤ 72, 6 ≤ CNL ≤ 17, 16 ≤ LLD ≤ 40;

[0058] Fan end subfacies: Thick-layer mudstone interbedded with thin sandstone layers, the SP curve is straight, AC ≤ 73, 18 ≤ CNL ≤ 35, 3 ≤ LLD ≤ 15.

[0059] Step 2. Use the fine synthetic seismic record calibration to establish the well-seismic correspondence, conduct the stage division of the nearshore underwater fan, determine the seismic reflection bands corresponding to the interfaces of different sedimentary subfacies, analyze the seismic reflection characteristics of each sedimentary subfacies, and realize the qualitative seismic identification of different subfacies of the single-stage nearshore underwater fan.

[0060] Among them, the seismic reflection bands corresponding to the interfaces of different sedimentary subfacies are:

[0061] The top surface of the single-stage fan body, that is, the stage interface of the nearshore underwater fan: It is the mud-sand interface, showing a stable, continuous, medium-strong reflection;

[0062] Fan tip subfacies: Multiple nearly parallel short-axis-shaped strong reflections, obliquely intersecting at an angle with the strong reflections of the front-lake facies deposits, with a relatively high seismic dominant frequency and a relatively narrow reflection waveform;

[0063] Fan middle subfacies: The amplitude gradually weakens from the strong reflections at the fan tip towards the interior of the fan body, with a relatively low seismic dominant frequency and a relatively wide reflection waveform;

[0064] Fan root subfacies: Nearly blank reflections, with high-angle medium-strong reflections appearing at the boundary with the fan middle, and a wide and gentle reflection waveform.

[0065] Step 3. Combine the actual drilling and seismic data to establish an actual geological model for forward modeling. Compare the forward modeling results with the actual seismic data. If the reflection characteristics representing the same sedimentary subfacies are consistent for both, it is the true reflection characteristic. If the reflection characteristics representing the same sedimentary subfacies are inconsistent for both, return to check the accuracy of the synthetic seismogram calibration in step (2) and the accuracy of the geological model structure and the velocity and density assignments in step (3) until the reflection characteristics of both are consistent, and establish a seismic facies identification chart for different sedimentary subfacies.

[0066] Fan root subfacies: Velocity 4800 m / s, density 2.64 g / m 3 , formation thickness 80 - 100 m

[0067] Fan middle subfacies: Velocity 4600 m / s, density 2.55 g / m 3 , formation thickness 40 - 80 m

[0068] Fan tip subfacies: Velocity 4300 m / s, density 2.53 g / m 3 , formation thickness 20 - 40 m

[0069] Lake facies: Velocity 3500 m / s, density 2.5 g / m 3 , formation thickness 100 m

[0070] The results of forward modeling show that:

[0071] Mudstone interlayers develop between fan bodies of different periods, so the period interfaces are manifested as medium-strong reflections;

[0072] The mudstone is not well developed inside the fan root subfacies, so it is manifested as blank reflections. The high-angle medium-strong reflections at its boundary with the fan middle are caused by the velocity changes due to the differences in reservoir physical properties of different subfacies;

[0073] Medium-strong reflections are formed by the development of sandy deposits inside the fan tip subfacies, and the more towards the fan middle, the more developed the sandy conglomerate reservoir is and the greater the reflection intensity is; at the same time, since the fan body enters the lake in a wedge shape from the slope, it obliquely intersects with the nearly parallel lake facies strata.

[0074] Step 4. On the basis of single-well facies identification in Step 1, statistically analyze the formation thickness, sandstone thickness, mudstone thickness, and single-layer thickness of sandstone in different sedimentary subfacies. Conduct correlation cross-analysis pairwise for each parameter, optimize the geological parameters that can effectively distinguish different sedimentary subfacies, and determine the critical value of the sand-to-shale ratio parameter that can effectively distinguish different subfacies.

[0075] Statistical analysis of sand-to-shale ratio within a single stage: 95% ≤ proximal fan subfacies ≤ 100%, 95% < middle fan subfacies ≤ 25%, distal fan subfacies < 25%;

[0076] Statistical analysis of single-layer thickness of reservoir within a single stage: 50 ≤ proximal fan subfacies ≤ 130, 50 < middle fan subfacies ≤ 10, distal fan subfacies < 10.

[0077] Conduct cross-analysis of the sand-to-shale ratio and the maximum single-layer thickness of the reservoir to determine that the sand-to-shale ratio can effectively distinguish different subfacies. The critical value of this parameter:

[0078] Among them, 95% ≤ proximal fan subfacies, 95% < middle fan subfacies ≤ 25%, distal fan subfacies < 25%.

[0079] Furthermore, it is found by fitting that the formation thickness of the single-stage fan body and the reservoir thickness both decrease linearly with the transportation distance.

[0080] Step 5. Cut a seismic profile along the provenance direction and flatten the bottom interface of the target-stage nearshore submarine fan stage.

[0081] It is found that the top interface of the fan stage intersects the bottom interface in an arc shape, and the two form a right-angled arc closed image with the perpendicular line of the proximal fan boundary, as Figure 3 shown.

[0082] Step 6. Convert the flattened seismic profile into a mathematical model and construct a function for the distribution range of different subfacies of the single-stage nearshore submarine fan.

[0083] According to the linear decrease trend of the formation thickness of the single-stage fan body and the reservoir thickness with the transportation distance obtained in Step 4, a triangle of the two and the formation thickness H can be constructed, and a function for the distribution range of different sedimentary subfacies can be established through mathematical derivation:

[0084] L = (1 - M) * H / (tgβ - M * tgɑ), where L is the distribution width of the subfacies facies belt, ɑ is the formation dip angle, β is the reservoir decreasing angle, H is the formation thickness, and M is the critical value of the sand-to-shale ratio parameter of different subfacies obtained by statistical analysis in Step 4.

[0085] Step 7. Obtain the parameters H, β, and ɑ required in the fitting formula in Step 6 from the actual drilled wells and seismic profiles, determine the distribution range of different subfacies, and realize the planar characterization of sedimentary subfacies.

[0086] Select the Yanjia area with relatively rich actual drilled well data and conduct tests using the method described in Example 1 of the present invention. The area of the test work area is 4.23 km2 , 14 test well locations were involved, and 15 verification well locations. By the prediction method of extracting the root mean square amplitude attribute by opening time windows above and below the conventional top surface of each stage, the sedimentation law of the nearshore underwater fan can be clearly reflected, but the delineation of the facies belt boundary is relatively fuzzy. For example, Figure 5 , Figure 7 as described, and there are relatively large errors between the facies belt divisions of some wells and the actual drilled wells. The sedimentary facies boundary obtained by applying the quantitative prediction method for the sedimentary subfacies of the nearshore underwater fan of the present invention is clear, has a good coincidence with the wells, and the boundary between the two fans is also clearer and more definite, as Figure 6 shown.

[0087] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A method for quantitatively characterizing the sedimentary subfacies of a nearshore underwater fan, characterized in that, It includes the following steps: Step 1: Establish a geological identification plate for the subfacies of the nearshore underwater fan and conduct single-well facies identification for the actual drilled wells; Step 2: Conduct the stage division of the nearshore underwater fan to achieve the qualitative seismic identification of different subfacies of a single-stage nearshore underwater fan; Step 3: Establish an actual geological model and conduct forward modeling; Step 4: Determine the critical values of the sand-to-shale ratio parameters for different subfacies, and fit the relationship between the formation thickness and reservoir thickness of the fan body with the transportation distance; Step 5: Construct the distribution range function of different subfacies zones of a single-stage nearshore underwater fan to determine the distribution ranges of different facies zones; Step 6: Cut the seismic profile along the provenance direction and flatten the bottom interface of the nearshore underwater fan stage of the target stage; transform the flattened seismic profile into a mathematical model and construct the distribution range function of different subfacies of a single-stage nearshore underwater fan; the formation thickness and reservoir thickness of the single-stage fan body both show a linear decreasing trend with the transportation distance, and a triangle of the two and the formation thickness H can be constructed. Through mathematical derivation, establish the distribution range function of different sedimentary subfacies: L = (1 - M) * H / (tgβ - M * tgɑ), where L is the distribution width of the subfacies facies zone, ɑ is the formation dip angle, β is the reservoir decreasing angle, H is the formation thickness, and M is the critical value of the sand-to-shale ratio parameter for different subfacies obtained by statistics in Step 4; Step 7: Obtain the parameters H, β, and ɑ required in the fitting formula in Step 6 from the actual drilled wells and seismic profiles, determine the distribution ranges of different subfacies, and achieve the characterization of the planar sedimentary subfacies; In Step 3, compare the forward modeling results with the actual seismic data. If the reflection characteristics representing the same sedimentary subfacies are consistent for both, it is the true reflection characteristic; If the reflection characteristics representing the same sedimentary subfacies are inconsistent for both, return to check and correct Steps 2 and 3 until the reflection characteristics of both are consistent.

2. The method according to claim 1, characterized in that, In Step 1, based on the core, logging, and well logging data of the actual drilled wells, under the guidance of sedimentology, establish a geological identification plate for the subfacies of the nearshore underwater fan. The plate includes the following geological parameters: well logging curves, lithology-related parameters, and core-related parameters.

3. The method according to claim 1 or 2, characterized in that The establishment of the geological identification plate for the subfacies of the nearshore underwater fan is as follows: Fan root subfacies: large thick layers of conglomerate and coarse conglomerate, with little development of mudstone, and the SP curve is box-shaped or Christmas tree-shaped; 50 ≤ AC ≤ 59, 2 ≤ CNL ≤ 5, 41 ≤ LLD ≤ 100; Fan middle subfacies: thick-layered pebbly sandstone, pebble-bearing rock or fine conglomerate intercalated with thin mudstone layers, the SP curve is continuously box-shaped or thick finger-shaped, 60 ≤ AC ≤ 72, 6 ≤ CNL ≤ 17, 16 ≤ LLD ≤ 40; Fan end subfacies: thick-layered mudstone intercalated with thin sandstone layers, the SP curve is straight, AC ≤ 73, 18 ≤ CNL ≤ 35, 3 ≤ LLD ≤ 15.

4. The method according to claim 1, wherein In Step 2, use the fine synthetic seismic record calibration to establish the well-seismic correspondence, conduct the stage division of the nearshore underwater fan, determine the seismic reflection bands corresponding to different sedimentary subfacies interfaces, analyze the seismic reflection characteristics of each sedimentary subfacies, and achieve the qualitative seismic identification of different subfacies of a single-stage nearshore underwater fan.

5. The method according to claim 4, wherein The seismic reflection bands corresponding to different sedimentary subfacies interfaces are: The top surface of a single-stage fan body, i.e., the nearshore underwater fan stage interface: is the mud-sand interface, showing a stable continuous medium-strong reflection; Fan - end subfacies: Multiple nearly parallel short - axis - shaped strong reflections, obliquely intersecting at an angle with the strong reflections of the frontal lacustrine deposits, with a relatively high seismic dominant frequency and a relatively narrow reflection waveform; Fan - middle subfacies: The amplitude gradually weakens from the strong reflections at the fan - end towards the interior of the fan body, with a relatively low seismic dominant frequency and a relatively wide reflection waveform; Fan - root subfacies: Nearly blank reflections, with high - angle medium - strong reflections appearing at the boundary with the fan - middle, and a wide and gentle reflection waveform.

6. The method according to claim 1, wherein In step 4, based on the single - well facies identification in step 1, by statistically analyzing the formation parameters of different sedimentary subfacies, pairwise correlation cross - analysis of each parameter is carried out to optimize the geological parameters that can effectively distinguish different sedimentary subfacies, and finally the critical value of the sand - to - shale ratio parameter that can effectively distinguish different subfacies is determined.

7. The method according to claim 6, wherein Statistical formation parameters of different sedimentary subfacies include: formation thickness, sandstone thickness, mudstone thickness, and single - layer sandstone thickness.

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