Ancient karst landform restoration method

Abstract

The invention discloses an ancient karst landform restoration method, which belongs to the field of oil-gas exploration, and comprises the following steps: S1, obtaining petrology characteristics and geochemical sign characteristics of a research area; acquiring earthquake identification information and logging response characteristics of the research area; s2, determining macroscopic and microcosmic petrology markers of the karst landform of the research area; establishing a landform unit identification mode and method; s3, carrying out landform unit division, landform shape refinement and trench development rule refinement; s4, representing the ancient geomorphic pattern of the karst and the development system of the ancient karst; and S5, making an ancient terrain map, and judging the positive terrain, the negative terrain, the ancient structure highland and the ancient trench development position of the karst ancient landform. According to the method, on the basis of rich drilling layering data, the latest earthquake and logging information is combined, the combination of the'residual thickness method 'and the'impression method' is selected, the advantages of the'residual thickness method 'and the'impression method' are adopted, and the ancient karst landform unit in the research area is comprehensively recovered.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas exploration, and in particular to a method for restoring ancient karst landforms. Background Art

[0002] Karst paleo-geomorphology is the result of the combination of karstification and various geological processes, and its morphology itself is one of the important controlling factors for the development of karst. Therefore, restoring the karst paleo-geomorphology of the pre-Carboniferous, i.e., the top of the Ordovician, has important practical and theoretical significance for natural gas exploration in the eastern region. The study of weathering crust karst paleo-geomorphology and the laws and controlling factors of natural gas enrichment has become the focus of continuous research in the Ordos region for many years.

[0003] like Figure 2 As shown, previous studies have shown that it is effective to apply the "imprint method" or "residual thickness method" to restore the karst paleogeomorphology in stable platform areas. The degree of erosion of the Caledonian weathering crust is positively correlated with the paleogeomorphological state of the weathering crust. That is, where the paleogeomorphology is relatively high, the more the strata are eroded, the thinner the overlying strata are; where the paleogeomorphology is relatively low, the strata are eroded less. Correspondingly, the filling degree of the weathering crust paleogeomorphology is exactly the opposite to the paleogeomorphological state, that is, where the paleogeomorphology is relatively high, the thinner the overlying strata are; where the paleogeomorphology is relatively low, the thicker the overlying strata are. This complementary relationship between the residual thickness of the weathering crust and the thickness of the Carboniferous sediments provides important technical ideas and methodological basis for the study of paleogeomorphology. But in terms of regional or local areas, what we usually observe in field outcrops is often that the lower the ancient landform is, that is, the deeper the valley is, the more the strata are eroded, and the older the exposed strata are; while the higher the mountain is, the newer the exposed strata are, and the more completely the strata are preserved.

[0004] Therefore, for the Caledonian paleo-geomorphology, it is necessary not only to analyze the thickness of the overlying Carboniferous sediments, but also to carefully study the residual thickness of the weathering crust itself. Only by closely combining the two can we more truly reflect the Caledonian paleo-geomorphology. Summary of the invention

[0005] In order to solve the technical problems existing in the above-mentioned prior art that a single ancient karst landform restoration method cannot adapt to some landform conditions, and the results obtained by using a single ancient karst landform restoration method do not conform to the actual situation, the present invention provides a ancient karst landform restoration method.

[0006] To achieve the above technical solution, the present invention provides a method for restoring ancient karst landforms, comprising the steps of: S1: Obtain petrological characteristics and geochemical characteristics of the study area through field outcrop, core observation and sampling analysis; obtain seismic identification information and logging response characteristics of the study area by analyzing seismic data and well logging data in the study area; S2: According to the petrological characteristics and geochemical characteristics obtained in step S1, the macroscopic and microscopic petrological characteristics of the karst landforms in the study area are determined; the geomorphological unit identification model and method are established according to the seismic identification information and the logging response characteristics; S3: Processing the geomorphological data of the study area obtained by integrating the macroscopic and microscopic petrological signs, geomorphological unit identification patterns and methods of the karst landforms obtained in step S2, and performing geomorphological unit division, geomorphological morphology refinement and groove development law refinement; S4: Based on the results obtained in step S3, the karst paleo-geomorphological pattern and the development system of paleo-karst are reproduced; S5: The paleotopography map is prepared through statistical analysis of stratum thickness, and the karst landform characteristics and paleo-groove distribution trends are analyzed from point to surface through single well dissection and profile comparison, so as to determine the positive topography, negative topography, paleo-tectonic highlands and paleo-groove development locations of the karst paleo-topography.

[0007] Furthermore, in step S1, the petrological characteristics include structural characteristics and clastic component characteristics of the study area; and the geochemical marker characteristics include major elements and rare earth elements.

[0008] Furthermore, the debris component characteristic analysis step comprises: In the study area, 30 common thin sections and 30 cast thin section samples were selected for sandstone component identification, and a total of 60 sets of thin section sandstone component identification data were obtained for systematic analysis; The single mineral analysis and multi-mineral combination of the selected thin sections were analyzed. The single mineral analysis included three types of skeleton minerals: quartz, feldspar, and rock fragments. The multi-mineral combination analysis used Dickinson's triangular diagram to analyze the mineral combination characteristics of sandstone fragments in the study area.

[0009] Furthermore, the major element measurements include SiO2, MgO, MnO, A12O3, TiO2, Fe2O3, P2O5, K2O, NaO2 and CaO; the rare earth elements are identified using the sedimentary rock matrix composition combined with a trace element value table.

[0010] Furthermore, in step S2, the macroscopic and microscopic petrological markers of the karst landform include: residual thickness, stratigraphic composition, topographic and geomorphic development, and isotopic composition of detrital zircon.

[0011] Furthermore, in step S2, the unit identification model is formed by combining the residual thickness method, the impression method, the layer flattening method and the paleogeological map method, and performing comprehensive analysis and comparison so that they can be mutually verified.

[0012] Furthermore, in step S3, the step of dividing the landform units includes: S11: Based on the existing drilling data, the paleo-geomorphology of the Ordovician weathering crust karst was restored by the “impression method” and “residual thickness method” respectively; S12: Based on the total thickness of the Carboniferous and its changes, combined with paleotopography and paleohydrodynamic analysis, the study area is divided into three secondary paleo-geomorphological units or geomorphological intervals: karst highland, karst slope and karst basin; S13, based on the relative height difference of the residual thickness of the Ordovician top weathering crust layer, karst properties and topographic characteristics, seven second- and third-level paleo-geomorphic units or morphological intervals are further divided into karst highland, karst slope, karst basin, karst residual hill, karst platform, karst valley and karst residual platform; S14. Combining the results of the impression method and the residual thickness method, on the basis of the above-mentioned second- and third-level paleogeomorphological unit divisions, combined with the regional geological structural evolution and paleotopographic background, the paleogeomorphological units are systematically analyzed, sorted, arranged and classified again to obtain the final result of the restoration of the Ordovician weathering crust karst paleogeomorphology in this area.

[0013] Furthermore, in step S3, the ancient landforms are further refined according to the restoration of the ancient karst landforms of the Ordovician weathering crust in the Yanchang exploration area, and the law of trough development is judged.

[0014] Furthermore, in step S5, the basis for the classification of paleokarst landform types includes: ① selecting the Carboniferous No. 8 coal seam, which is the overlying stratum of the Ordovician weathering crust with stable distribution in the region, as the base plane, and counting the thickness from this base plane to the top of the Majiagou Formation to study its distribution trend; ② studying the distribution trend of the pre-Carboniferous strata; ③ analyzing the characteristics of ancient water flow; ④ the erosion and dissolution characteristics of the weathering crust surface and the properties of the sediments.

[0015] In summary, the present invention has the following beneficial effects compared with the prior art: According to the actual situation, based on the relatively rich drilling stratification data and combined with the latest seismic and logging data, the present invention selects the combination of "residual thickness method" + "impression method", adopts their respective advantages, and comprehensively restores the paleokarst landform units in the study area. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings: Figure 1 A flow chart of a method for restoring ancient karst landforms provided by the present invention; Figure 2 To provide a basic roadmap for the restoration of paleokarst landforms; Figure 3 Schematic diagram of ancient landform restoration using the impression method and the residual thickness method (the upper part is the impression method, the lower part is the residual thickness method); Figure 4 This is the background map of the paleo-geomorphological development in the eastern part of the Ordos Basin during the pre-Carboniferous period; Figure 5 This is the triangular distribution diagram of sandstone types of Benxi Formation samples in the study area; Figure 6 It is a diagram illustrating the distribution pattern of sandstone clastic components in different source areas of the Benxi Formation in the study area; Figure 7 To extend the paleotopography of the top of the Majiagou Formation in the exploration area; Figure 8 The paleotopographic map of Mawuding in the Majiagou Formation in the extended exploration area; Fig. 9 This is a schematic cross-section diagram of the karst paleo-geomorphology of the Majiagou Formation; Fig.10 This is the seismic profile of the Majiagou Formation; Fig.11 To extend the ancient karst landform map of the exploration area. DETAILED DESCRIPTION

[0017] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0018] See also Figure 1 As shown, the present invention provides a method for restoring ancient karst landforms, comprising the steps of: S1: Obtain petrological characteristics and geochemical characteristics of the study area through field outcrop, core observation and sampling analysis; obtain seismic identification information and logging response characteristics of the study area by analyzing seismic data and well logging data in the study area; S2: According to the petrological characteristics and geochemical characteristics obtained in step S1, the macroscopic and microscopic petrological characteristics of the karst landforms in the study area are determined; the geomorphological unit identification model and method are established according to the seismic identification information and the logging response characteristics; S3: Processing the geomorphological data of the study area obtained by integrating the macroscopic and microscopic petrological signs, geomorphological unit identification patterns and methods of the karst landforms obtained in step S2, and performing geomorphological unit division, geomorphological morphology refinement and groove development law refinement; S4: Based on the results obtained in step S3, the karst paleo-geomorphological pattern and the development system of paleo-karst are reproduced; S5: The paleotopography map is prepared through statistical analysis of stratum thickness, and the karst landform characteristics and paleo-groove distribution trends are analyzed from point to surface through single well dissection and profile comparison, so as to determine the positive topography, negative topography, paleo-tectonic highlands and paleo-groove development locations of the karst paleo-topography.

[0019] Paleomorphological restoration is the key to studying paleokarst landforms. Based on the analysis of basic geological data, relevant basic data are statistically analyzed, and the paleomorphology is restored using the "residual thickness method + impression method". This method is combined with relevant logging and seismic data to finally complete the division of paleomorphological patterns. The specific research route adopted is discussed in detail. Figure 2 shown.

[0020] Paleogeomorphological restoration is one of the contents of basin analysis and research. Paleogeomorphological map refers to the concave-convex state map of the interface surface represented by the equal depth lines or stratigraphic boundaries on a certain interface during a certain period of basin development. Currently commonly used methods include: residual thickness method, impression method, geophysical method, sequence stratigraphic method, sedimentological method, etc., and these methods have their own advantages and disadvantages. According to the actual situation, the present invention selects the combination of "residual thickness method" + "impression method" on the basis of relatively rich drilling stratification data, combined with the latest seismic and logging data, and uses their respective advantages to comprehensively restore the paleokarst geomorphological units in the study area. Previous studies have shown that it is effective to apply the "impression method" and "residual thickness method" to restore the paleokarst geomorphological morphology in stable platform areas.

[0021] like Figures 2 to 4 As shown, taking the Yanchang exploration area in the south-central part of the Ordos Basin as an example, the Yanchang study area is adjacent to the Wuqi karst platform and Yulin karst terrace in the west and the Mizhi-Qingjian karst basin in the east, spanning three large secondary paleogeomorphic units including karst highland, karst slope and karst basin. The paleotopography is generally high in the west and low in the east, and the erosion is generally strong in the west and weak in the east.

[0022] The first set of sediments overlying the weathering crust is the coal-bearing clastic rock of the Middle and Late Carboniferous marine-continental alternating phase (Benxi Formation-Taiyuan Formation), with a total thickness of 20m-103m. The sediment thickness is complementary to the Caledonian erosion surface or weathering crust karst paleo-geomorphology. The sediments are widely distributed, reflecting that at the end of the Carboniferous deposition, the Caledonian weathering crust had been completely overlain by the Carboniferous deposition and the weathering crust paleo-geomorphology was filled and leveled. The sediment thickness is mainly controlled by the paleo-geomorphology. The sediments are generally thicker in the depressions and thinner or absent in the highlands.

[0023] The present invention firstly uses the residual thickness method and the impression method to analyze the morphology of the paleo-geomorphology of the Yanchang exploration area respectively.

[0024] (1) About the residual thickness method First, a stratum with the property of filling and complementing the erosional landform is selected in the overlying strata as the datum plane, and then a special layer section in the sedimentary stratum is selected as the datum plane and flattened. The size of the residual thickness above this surface represents the shape of the ancient landform.

[0025] (2) About the impression method The basic morphology of the Caledonian paleo-geomorphology can be indirectly restored based on the sedimentary thickness of the Carboniferous. This method mainly uses the thickness of the overlying specific layer (Benxi-Taiyuan) plus the control of the regional marker layer (Taiyuan Group limestone) for research.

[0026] (3) Compare and analyze the results of the residual thickness method and the impression method, and combine them It is found through actual results that the combination of the "impression method" and the "residual thickness method" can verify each other through comprehensive analysis and comparison, so as to obtain a more practical effect.

[0027] Preferably, in step S1, the petrological characteristics include structural characteristics and clastic component characteristics of the study area; and the geochemical marker characteristics include major elements and rare earth elements.

[0028] As a preferred embodiment, Figure 5 and Figure 6 As shown in the figure, according to the sedimentary background of the study area, the study area is located in the center of the lake basin during the sedimentary period of the Benxi Formation, and belongs to the intersection of the north and south provenances. In order to better analyze and compare the characteristics of the sandstone components of the Benxi Formation in the study area, so as to have a clearer understanding of the provenance, 30 ordinary thin sections and 30 cast thin section samples of the Benxi Formation in the study area were used to identify the sandstone components, and a total of 60 groups of thin section sandstone component identification data were obtained for systematic analysis. The analysis content mainly includes single mineral analysis and multi-mineral combination analysis. The single mineral analysis mainly analyzes the three types of skeleton minerals, quartz, feldspar, and rock fragments. The multi-mineral combination analysis mainly uses Dickinson's triangular diagram to analyze the characteristics of the sandstone clastic mineral combination in the study area. On the whole, the lithology type of the sandstone samples of the Benxi Formation in the study area is mainly quartz sandstone.

[0029] As a preferred option, major elements generally refer to elements with high abundance in the crust and mantle. Representative samples are selected in the study area, and the components such as SiO2, MgO, MnO, A12O3, TiO2, Fe2O3, P2O5, K2O, NaO2, CaO, etc. are measured.

[0030] Table 1 Tectonic environment of sandstone corresponding to different trace element characteristic values During the sedimentation process, rare earth elements are less affected by factors such as weathering and diagenesis. They enter fine-grained sediments in a short period of time during the sedimentation process, exist in the fluid for a short time, and the dissolution effect is not obvious. In addition, they are less affected by dissolution during the diagenesis process, so they are very suitable for the identification of the composition of sedimentary parent rocks. In addition, according to the corresponding trace element value tables (Table 1, Table 2) and REE characteristic values ​​and model curves under different tectonic environments, representative samples of the Benxi Formation in the study area were selected for rare earth element analysis (Table 1).

[0031] Table 2 Rare earth element characteristics of greywacke in sedimentary basins with different tectonic settings Preferably, in step S2, the macroscopic and microscopic petrological markers of the karst landform include: residual thickness, stratigraphic composition, topographic and geomorphic development, and isotopic composition of detrital zircon.

[0032] Isotope research is widely used in provenance analysis and basin-mountain coupling research. Zircon is usually used to trace the provenance of current sedimentary strata and compare it with the ancient continental basement due to its extremely high stability and extremely wide distribution. To determine the provenance of a sedimentary basin by detrital zircon age, it is necessary to combine the characteristics of the provenance area with the zircon age spectrum, conduct an overall analysis based on previous geological understanding of the region and the surrounding sedimentary evolution events, and compare the chronological characteristics of detrital zircons in the surrounding sediments. In order to further clarify the provenance characteristics of the study area, systematic sampling and isotope analysis were carried out on the Benxi Formation in the southwest of the Yan'an gas field. It is intended to determine the age and composition characteristics of the provenance area through isotope age research, thereby dissecting the geological history information contained in the zircon isotope characteristics and further clarifying the provenance characteristics of the study area.

[0033] As a preference, in step S2, the unit identification model is formed by combining the residual thickness method, the impression method, the layer flattening method and the paleogeological map method, and performing comprehensive analysis and comparison so that they can be mutually verified.

[0034] As a preference, in step S3, the step of dividing the landform units comprises: S11: Based on the existing drilling data, the paleo-geomorphology of the Ordovician weathering crust karst was restored by the “impression method” and “residual thickness method” respectively; S12: Based on the total thickness of the Carboniferous and its changes, combined with paleotopography and paleohydrodynamic analysis, the study area is divided into three secondary paleo-geomorphological units or geomorphological intervals: karst highland, karst slope and karst basin; S13, based on the relative height difference of the residual thickness of the Ordovician top weathering crust layer, karst properties and topographic characteristics, seven second- and third-level paleo-geomorphic units or morphological intervals are further divided into karst highland, karst slope, karst basin, karst residual hill, karst platform, karst valley and karst residual platform; S14. Combining the results of the impression method and the residual thickness method, on the basis of the above-mentioned second- and third-level paleogeomorphological unit divisions, combined with the regional geological structural evolution and paleotopographic background, the paleogeomorphological units are systematically analyzed, sorted, arranged and classified again to obtain the final result of the restoration of the Ordovician weathering crust karst paleogeomorphology in this area.

[0035] In the analysis of weathering crust paleogeomorphology, we first used the existing drilling data as the main basis, and restored the paleogeomorphology of the Ordovician weathering crust karst by the "impression method" and "residual thickness method", and obtained two research results with different focuses. According to the pre-Carboniferous paleogeomorphological development background of the basin, this study area is located in the central and southern part of the basin, backed by the central paleo-uplift, and deep into the northern Shaanxi depression in the east, spanning three secondary paleogeomorphological units of karst highland, karst slope and karst basin, with karst slope and karst basin areas as the main body. In order to further realize the characterization of the third-level paleogeomorphological units on this basis, firstly, according to the total thickness of the Carboniferous and its changes, combined with paleotopography and paleohydrodynamic analysis, the study area is divided into three secondary paleogeomorphological units or geomorphological intervals, namely karst highland, karst slope and karst basin.

[0036] Secondly, based on the relative height difference of the residual thickness of the top weathering crust section of the Ordovician System (Ma55-Ma6), the karst properties, and topographic characteristics, seven second- and third-level paleo-geomorphological units or morphological intervals were further divided, including karst highlands, karst slopes, karst basins, karst residual hills, karst platform, karst valleys, and karst residual platforms.

[0037] Finally, the results of the impression method and the residual thickness method were combined. On the basis of the above-mentioned division of the second and third level paleogeomorphic units, the paleogeomorphic units were systematically analyzed, sorted, arranged and classified again in combination with the regional geological structure evolution and paleotopographic background, and the final result of the restoration of the Ordovician weathering crust karst paleogeomorphology in this area was obtained. A total of three second-level paleogeomorphologies, namely karst highland, karst slope and karst basin, and seven third-level paleogeomorphic units, namely residual hill, platform, platform edge, terrace, depression, valley and residual platform, were identified, as shown in Table 3.

[0038] Table 3 Classification of Ordovician weathering crust karst paleo-geomorphology types in the eastern part of the basin The conceptual characteristics of various paleomorphic units are as follows.

[0039] Ⅰ. Karst highland: generally distributed in the periphery of the ancient land or central uplift, and transitioning to the karst slope belt to the east. The overall ancient terrain is relatively high, mainly subject to erosion and dissolution; the exposed strata on the surface of the weathering crust are mainly the lower part of Mawu 1 and the top of Mawu 2. The karst action is mainly vertical infiltration, forming karst forms such as vertical dissolution zones and sinkholes, with obvious heterogeneity. The transition area between its east side and the karst slope belt is a hydrodynamic energy concentration zone and conversion zone, where groundwater changes from a vertical recharge state to a horizontal runoff state, and karst is strongly developed, forming a relatively good dissolution pore reservoir.

[0040] Ⅱ. Karst slope: between the karst highland and the karst basin, it is located in the ancient terrain slope position, and there are many alternating positive and negative terrains on the slope. It is mainly a dolomite and mud dolomite microfacies distribution area, which is conducive to karstification and the formation of dissolution holes. The weathering crust surface mainly exposes the middle and upper layers of Mawu 1, and the distribution is relatively stable and continuous. The Mawu 1 sub-segment is partially missing in the northwest of this area; the Maliu segment is retained in the middle and eastern part. Because it is located in the middle zone between the karst highland and the karst basin, the groundwater is mainly in the horizontal runoff state, the terrain is gentle, and the hydraulic gradient is small. The karstification is relatively strong, and the horizontal layer dissolution of slow diffusion flow is the main form, forming good dissolution holes.

[0041] Ⅲ. Karst basin: Karst basin is a negative topography formed by the continuous expansion of karst quasi-plain. It appears in the eastern part of this area and occupies about one-third of the area. Its west side is in transition with the wider karst slope development area. The ancient terrain is flat and open, with residual hills and isolated peaks in some areas. The surrounding areas are slightly higher than the central area, so it is called a karst basin type area. The sedimentary microfacies are still dominated by dolomite and mud dolomite phases; the surface layer of the weathering crust is mainly exposed in the Ma Liu section and the middle and upper sections of Ma Wu 1. The karstification mode is significantly different from that of the karst platform area, which is manifested in weak layered dissolution, mainly dissolution along the surface erosion zone and rock dissolution in the shallow underground runoff zone, and the heterogeneity is more obvious. At the same time, because this area is in the catchment and discharge area of ​​water flow, the filling and sedimentation are strong, and the filling degree of karst space is high. Due to the filling and cementation of calcium and mud, a dense lithology zone is formed, which affects the reservoir performance of the reservoir. However, the karstification intensified during the burial period, and the micro-uplift zones in the basin were more favorable for the transformation of karst reservoirs due to their good pore connectivity in the early stage.

[0042] The basic definitions and main characteristics of the three-level paleo-geomorphic units developed in this area are as follows: Platform: It is a relatively high and flat area on the secondary paleo-geomorphic background. It can be developed in karst highlands, karst slopes and karst basins respectively; it is the main component of karst highlands; it is a large-scale obvious uplift zone in the karst slope background; it is a local residual micro-uplift platform (residual platform) in the karst basin.

[0043] Residual hill: It is an ancient highland that is relatively isolated on the secondary paleo-geomorphic background, with a large elevation difference and a small distribution area, and is approximately hilly. It can be formed in karst highlands, karst slopes and karst depressions. It is higher than the terrace in terms of stratigraphic position.

[0044] Platform margin: It is the transition zone between the karst slope platform and the karst slope terrace; it is generally narrow and steep, with dense contour lines, distributed around the platform.

[0045] Terraces: These are relatively low-lying, flat and open areas between the erosion valley and the slope platform edge in the karst slope. They are distributed over a large area and constitute the main part of the karst slope in this area. The karstification is also relatively strong, but the reservoir-forming conditions are slightly worse than those of the terrace and residual hill type areas.

[0046] Valley: It is a dendrite-shaped, long strip of low-lying area formed by strong erosion and dissolution of the ancient surface. It is the lowest level in the three-level geomorphic unit. It can be further divided into main and branch valleys; it extends and merges from the karst highland karst basin, with an overall orientation of nearly east-west, and its width gradually changes from narrow to wide, and its cutting depth changes from small to large and then from large to small.

[0047] Depression: It is the main part of the karst basin. The terrain is relatively flat and open. The surface microkarst forms include karst depressions and karst buckets. Accumulation and filling play a dominant role. The underground karst is well developed with obvious heterogeneity and relatively concentrated subsurface trenches. The overall reservoir of this type of area is poor, which is an unfavorable area for oil and gas exploration.

[0048] As a preferred embodiment, in step S3, the ancient landform is further refined according to the restoration of the ancient karst landform of the Ordovician weathering crust in the Yanchang exploration area, and the law of trough development is judged.

[0049] The ancient karst platform is mainly located in the central and southwestern part of the exploration area, divided by grooves in the middle, and the top lithology is mainly dolomite and limy dolomite of the Mawu section; the ancient karst slope is generally distributed in the northwest-southeast direction, and the distribution range in the exploration area is relatively large; the ancient residual hills are scattered in the southern and southeastern parts of the exploration area, and the top is mainly limestone and dolomitic limestone of the Maliu section; the ancient karst basin is located in the eastern part of the exploration area, distributed in the north-south direction and extending eastward. It is a low-lying area and an area where ancient grooves converge. There are also local micro-uplift areas in the karst basin. Due to the cutting of the ancient grooves, the ancient landforms in the area present a situation of mainly terraces and coexistence of grooves and platforms. It also makes the Majiagou Formation strata in the study area have relatively intact Maliu residual areas and eroded Mawu areas. The eroded groove is the maximum erosion depth. In the grooves, most of the Mawu 1 strata are missing.

[0050] Based on the previous work, the Ordovician weathering crust paleokarst landform restoration in the Yanchang exploration area was carried out. Fig. 9), divided the paleo-landforms into three secondary landform types, namely karst platform, karst slope and karst basin, and nine tertiary landform types, namely highland, platform, residual hill, slope, karst hill, depression, basin, residual hill and trough. It established that the general trend of the karst paleo-trough is nearly east-west and the distribution pattern is dendritic.

[0051] As a preferred embodiment, in step S5, the basis for the classification of ancient karst landform types includes: ① selecting the Carboniferous No. 8 coal seam, which is a stable stratum overlying the Ordovician weathering crust in the region, as the base plane, and counting the thickness from this base plane to the top of the Majiagou Formation to study its distribution trend; ② studying the distribution trend of the pre-Carboniferous strata; ③ analyzing the characteristics of ancient water flow; ④ the erosion and dissolution characteristics of the weathering crust surface and the properties of the sediments.

[0052] By counting the thickness of the coal seam from the top of the Majiagou Formation to the No. 8 coal seam of the Benxi Formation, a paleotopographic map was produced ( Figure 7 , Figure 8 ), and through single well dissection and profile comparison, the karst landform characteristics and paleo-groove distribution trends are analyzed from point to surface, so as to determine the positive topography, negative topography, paleo-tectonic highlands, and paleo-groove development locations of the karst paleo-landform.

[0053] At the same time, the seismic interpretation model was used to identify and define the grooves in combination with drilling data ( Fig.10 and Fig.11 ). From the seismic profile, it can be seen that the wave impedance of the top of the Majiagou Formation in Well Yan 221 in the southern part of the exploration area has changed. It is located in the paleokarst slope zone and is strongly affected by paleokarst. From the ups and downs of the seismic profile, it can be seen that the changing characteristics of the formation are reflected. The terrain becomes higher from Well Yan 221 to Well Test 25. Combined with the lithological characteristics, there may be good reservoirs in the Well Yan 221 area.

[0054] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for restoring ancient karst landforms, characterized in that: Includes steps: S1: Obtain petrological characteristics and geochemical characteristics of the study area through field outcrop, core observation and sampling analysis; obtain seismic identification information and logging response characteristics of the study area by analyzing seismic data and well logging data in the study area; S2: According to the petrological characteristics and geochemical characteristics obtained in step S1, the macroscopic and microscopic petrological characteristics of the karst landforms in the study area are determined; the geomorphological unit identification model and method are established according to the seismic identification information and the logging response characteristics; S3: Processing the geomorphological data of the study area obtained by integrating the macroscopic and microscopic petrological signs, geomorphological unit identification patterns and methods of the karst landforms obtained in step S2, and performing geomorphological unit division, geomorphological morphology refinement and groove development law refinement; S4: Based on the results obtained in step S3, the karst paleo-geomorphological pattern and the development system of paleo-karst are reproduced; S5: The paleotopography map is prepared through statistical analysis of stratum thickness, and the karst landform characteristics and paleo-groove distribution trends are analyzed from point to surface through single well dissection and profile comparison, so as to determine the positive topography, negative topography, paleo-tectonic highlands and paleo-groove development locations of the karst paleo-topography.

2. The method for restoring ancient karst landforms according to claim 1, characterized in that: In step S1, the petrological characteristics include structural characteristics and clastic component characteristics of the study area; the geochemical marker characteristics include major elements and rare earth elements.

3. The method for restoring ancient karst landforms according to claim 2, characterized in that: The debris component characteristic analysis step comprises: In the study area, 30 common thin sections and 30 cast thin section samples were selected for sandstone component identification, and a total of 60 sets of thin section sandstone component identification data were obtained for systematic analysis; The single mineral analysis and multi-mineral combination of the selected thin sections were analyzed. The single mineral analysis included three types of skeleton minerals: quartz, feldspar, and rock fragments. The multi-mineral combination analysis used Dickinson's triangular diagram to analyze the mineral combination characteristics of sandstone fragments in the study area.

4. The method for restoring ancient karst landforms according to claim 2, characterized in that: The major element measurements include SiO2, MgO, MnO, A12O3, TiO2, Fe2O3, P2O5, K2O, NaO2 and CaO; the rare earth elements are determined using the sedimentary rock matrix composition combined with a trace element value table.

5. The method for restoring ancient karst landforms according to claim 1, characterized in that: In step S2, the macroscopic and microscopic petrological markers of the karst landform include: residual thickness, stratigraphic composition, topographic and geomorphic development, and isotopic composition of detrital zircon.

6. The method for restoring ancient karst landforms according to claim 1, characterized in that: In step S2, the unit identification model is formed by combining the residual thickness method, the impression method, the layer flattening method and the paleogeological map method, and performing comprehensive analysis and comparison so that they can be verified with each other.

7. The method for restoring ancient karst landforms according to claim 1, characterized in that: In step S3, the step of dividing the landform units includes: S11: Based on the existing drilling data, the paleo-geomorphology of the Ordovician weathering crust karst was restored by the "impression method" and "residual thickness method" respectively; S12: Based on the total thickness of the Carboniferous and its changes, combined with paleotopography and paleohydrodynamic analysis, the study area is divided into three secondary paleo-geomorphological units or geomorphological intervals: karst highland, karst slope and karst basin; S13, based on the relative height difference of the residual thickness of the Ordovician top weathering crust layer, karst properties and topographic characteristics, seven second- and third-level paleo-geomorphological units or morphological intervals are further divided into karst highland, karst slope, karst basin, karst residual hill, karst platform, karst valley and karst residual platform; S14. Combining the results of the impression method and the residual thickness method, on the basis of the above-mentioned second- and third-level paleogeomorphological unit divisions, combined with the regional geological structural evolution and paleotopographic background, the paleogeomorphological units are systematically analyzed, sorted, arranged and classified again to obtain the final result of the restoration of the Ordovician weathering crust karst paleogeomorphology in this area.

8. The method for restoring ancient karst landforms according to claim 1, characterized in that: In step S3, the ancient landforms are further divided into detailed geomorphological forms according to the restoration of the ancient karst landforms of the Ordovician weathering crust in the Yanchang exploration area, and the law of trough development is judged.

9. The method for restoring ancient karst landforms according to claim 1, characterized in that: In step S5, the basis for the classification of paleokarst landform types includes: ① selecting the Carboniferous No. 8 coal seam, which is the overlying stratum of the Ordovician weathering crust with stable distribution in the region, as the base plane, and counting the thickness from this base plane to the top of the Majiagou Formation to study its distribution trend; ② studying the distribution trend of the pre-Carboniferous strata; ③ analyzing the characteristics of ancient water flow; ④ the erosion and dissolution characteristics of the weathering crust surface and the properties of the sediments.

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