An all-time anchor point identification and anchoring method based on astronomical cycle phase change zone

Abstract

The application discloses a kind of based on astronomical cycle phase change area's full isochronous anchor point identification and anchoring method, and preliminary positioning is carried out according to its fixed space superimposed relationship with target isochronous anchor point by identifying the reference marker layer widely developed in area;Extract 405kyr long eccentricity period signal in preliminary positioning layer section natural gamma ray logging curve, identify and target isochronous anchor point fixed corresponding feature cycle phase section;Combining the feature lithology after phase change with logging curve response characteristics is comprehensively judged, and the accurate positioning of target isochronous anchor point phase change body is realized;Finally, the absolute astronomical age scale is established with the isochronous anchor point of accurate positioning as reference.The application breaks through the dependence of traditional method on the lithological characteristics of marker bed, realizes the accurate identification and absolute dating of phase change area isochronous anchor point, and provides reliable technical support for the construction of regional high-resolution isochronous stratigraphic framework.

Description

Technical Field

[0001] This invention relates to the field of geological exploration technology, specifically to a method for isochronous anchor point identification and anchoring based on astronomical cyclic phase transition zones. Background Technology

[0002] Establishing a high-resolution isochronous framework for energy-bearing basin strata is a core technical means for sedimentary geology and stratigraphy to transition from a paradigm of "lithostratigraphic correlation" to "chronostratigraphic correlation." It has significant scientific and engineering value for clarifying the global tectonic-climate-sea-level co-evolution patterns and revealing the basin-mountain coupling relationship and energy enrichment mechanism.

[0003] The Carboniferous-Permian period was the core stage of the Late Paleozoic tectonic evolution and energy enrichment of the North China Craton. During this period, a set of widely distributed and sequence-complete coal-bearing strata developed, containing abundant coal, coalbed methane, sandstone gas reservoirs, shale gas, and other diverse energy resources. Key marker beds such as the Miaogou Limestone, Beichagou Sandstone, and Camel Neck Sandstone are found within this stratum. These marker beds exhibit significant lithological characteristics and well logging responses, serving as macroscopic indicators for regional stratigraphic division and correlation. The Miaogou Limestone, in particular, represents an isochronous interface throughout the entire North China Plate, exhibiting no transchronous phenomena, and is a core anchor point for establishing isochronous frameworks.

[0004] However, the coal-bearing strata of the Carboniferous-Permian system in North China are affected by a combination of regional sedimentary factors such as the lateral migration of sedimentary facies zones, differences in the intensity of sediment supply, and paleotopographic undulations. As a result, key isochronous anchor points in some zones will undergo sedimentary facies changes to varying degrees, which are manifested in significant changes in lithology, thickness, and logging curve response.

[0005] In extreme cases, the three key isochronous anchor points—Miaogou Limestone, Beichagou Sandstone, and Camel Neck Sandstone—have all undergone sedimentary facies transitions, and their original lithology, development thickness, and typical well logging response characteristics have all disappeared. In such isochronous anchor point facies transition zones, traditional petrographic methods rely heavily on the macroscopic characteristics of marker layers, such as lithology and thickness. When marker layers undergo sedimentary facies transitions, their identification and comparison lose their basis, making it difficult to achieve anchor point identification and stratigraphic calibration. This has become a key technical bottleneck restricting the construction of high-resolution isochronous frameworks in the region.

[0006] Existing technologies for anchor point identification in stratigraphic phase transition zones have the following shortcomings: traditional methods rely solely on comparisons between lithological combinations and well logging curve morphology, lacking isochronous constraints and prone to time-crossing phenomena; while isotopic dating methods can provide absolute age constraints, they are costly, have low spatial resolution, and are difficult to continuously calibrate in phase transition zones; existing astronomical cycle methods are mainly applied to lithologically stable marine or lacustrine strata, and there is no systematic identification and anchoring method for the extreme case where key anchor points in coal-bearing strata undergo phase transitions.

[0007] Therefore, there is an urgent need to develop a method for identifying and anchoring stratigraphic anchor points in isochronous anchor point phase transition zones, breaking through the dependence of traditional methods on the lithological characteristics of marker layers, and achieving accurate identification and dating of isochronous anchor points in phase transition zones. Summary of the Invention

[0008] To address this, the present invention provides a method for identifying and anchoring isochronous anchor points based on astronomical cycle phase transition zones. This method solves the technical problem that traditional methods cannot effectively identify and anchor isochronous interfaces when all key isochronous anchor points in coal-bearing strata undergo sedimentary phase transitions. Relying on the global isochronous characteristics of astronomical cycles and combined with the spatial constraints of regional reference marker layers, this method achieves accurate identification and absolute age calibration of isochronous anchor points in full phase transition zones, providing core technical support for the construction of regional high-resolution isochronous stratigraphic frameworks.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A method for isochronous anchor point identification and anchoring based on astronomical cyclic phase transition regions includes the following steps:

[0011] S1. Obtain basic geological data for the target well;

[0012] S2. Identify regional reference marker layers; through drilling logging lithological description or logging curve response characteristics, identify widely distributed and stably developed reference marker layers in the target study well, and determine the development depth and location of the reference marker layers.

[0013] S3. Perform preliminary positioning of the target isochronous anchor point; based on the known fixed spatial superposition relationship between the reference marker layer and the target isochronous anchor point identified in step S2, and using the reference marker layer as a reference, preliminarily determine the approximate stratigraphic range of the phase variant of the target isochronous anchor point.

[0014] S4. Extract the astronomical cyclic signal of the target formation range; preprocess the natural gamma logging curve corresponding to the formation range initially located in step S3 to eliminate non-cyclic trend error; use bandpass filtering to perform spectral analysis on the detrended natural gamma logging curve to extract the astronomical orbital period signal.

[0015] S5. Identify the target astronomical cycle characteristic phase; In the astronomical cycle signal extracted in step S4, identify the characteristic cycle phase segment that has a fixed correspondence with the target isochronous anchor point. This characteristic cycle phase segment is not affected by local depositional phase transitions.

[0016] S6. Accurately locate the target isochronous anchor point; within the formation depth range corresponding to the characteristic cycle phase segment identified in step S5, make a comprehensive judgment by combining lithological evolution characteristics and well logging curve response characteristics, taking the characteristic cycle phase segment as the core judgment basis, and using the characteristic lithology and well logging curve response characteristics after phase transformation as evidence, to determine the precise depth position of the phase transformation of the target isochronous anchor point.

[0017] S7. Establish an absolute astronomical age scale; using the target isochronous anchor point precisely located in step S6 as the time reference point, and combining the known absolute age data of the target isochronous anchor point, construct an absolute astronomical age scale for the coal-bearing strata of the target study well.

[0018] Furthermore: the basic geological data includes drilling logging lithological descriptions, natural gamma logging curves, and stratigraphic information on the development of regional landmark coal seams; the astronomical orbital periodic signals include a 405 kyr long eccentricity period, a 109 kyr short eccentricity period, and a 21.7 kyr precession period.

[0019] Furthermore: the reference marker layer in S2 is a major coal seam that is widely developed in the region; the fixed spatial superposition relationship in step S3 is: the target isochronous anchor points are generally developed in the upper part of the major coal seam, or directly constitute the direct roof of the major coal seam.

[0020] Furthermore: the non-cyclic trend error mentioned in step S4 includes long-period trend signals caused by factors such as deposition substrate rise and fall and uneven deposition rate; the preprocessing and bandpass filtering are performed using Aycle 2.3 software.

[0021] Furthermore: the characteristic cycle phase segment mentioned in step S5 is a specific phase segment in the 405kyr long eccentricity period that has a fixed correspondence with the target isochronous anchor point; when the target isochronous anchor point is the bottom boundary of Miaogou limestone, the characteristic cycle phase segment is a low value area of ​​the 405kyr long eccentricity period, and the stratigraphic deposition period corresponding to this low value area is a stable period of climate cycle driven by low orbital eccentricity.

[0022] Furthermore: the lithological evolution characteristic determination in step S6 includes: identifying the characteristic lithology after phase transformation of the target isochronous anchor point, the characteristic lithology including marl and calcareous mudstone, the characteristic lithology retains the carbonate mineral composition characteristics and has lithological differences from the underlying clastic rocks.

[0023] Furthermore: the well logging curve response characteristic determination in step S6 includes: identifying the well logging curve response characteristics corresponding to the phase change lithology; the well logging curve response characteristics are a combination of relatively high density, relatively low natural gamma, and relatively high resistivity.

[0024] Furthermore, the construction of the absolute astronomical age scale in step S7 specifically includes: using the precisely located target isochronous anchor point as the fixed time anchor point, using the 405kyr long eccentricity period as the core period benchmark, and performing depth-time quantitative conversion on the natural gamma logging curve of the target layer through the cyclotron counting method.

[0025] Furthermore: when at least one well-developed and identifiable key isochronous anchor point is preserved in the target well, step S7 is executed directly using the preserved key isochronous anchor point as the benchmark to establish an absolute astronomical time scale; the key isochronous anchor point is any one of Miaogou limestone, Beichagou sandstone or Camel Neck sandstone.

[0026] This invention has the following advantages: It breaks through the traditional lithology dependence and takes the global isochronous astronomical cycle characteristics as the core basis, effectively solving the problem of identifying the marker layer of the phase transition zone; it establishes an absolute astronomical age scale based on the precisely located phase transition anchor point, and the division accuracy can reach the orbital scale, so that the stratigraphic division is upgraded from qualitative description to quantitative calibration.

[0027] Other features and advantages of the present invention will be set forth in the following description. Attached Figure Description

[0028] To more intuitively illustrate the prior art and this application, exemplary drawings are provided below. It should be understood that the specific shapes and structures shown in the drawings should not generally be regarded as limiting conditions for implementing this application; for example, based on the technical concept disclosed in this application and the exemplary drawings, those skilled in the art are able to easily make conventional adjustments or further optimizations to the addition / reduction / classification, specific shapes, positional relationships, connection methods, size ratios, etc. of certain units (components).

[0029] Figure 1 The flowchart illustrates a method for isochronous anchor point identification and anchoring based on astronomical cyclic phase transition regions, provided in this embodiment of the invention. Detailed Implementation

[0030] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. It should be understood that these embodiments are merely for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above-described content.

[0031] Example 1

[0032] Age verification and establishment of astronomical time scale for key isochronous anchor points in anchor wells.

[0033] This embodiment demonstrates the application of the method of the present invention in anchored wells where key isochronous anchor points are well-developed and there are no obvious sedimentary facies transitions. It verifies the reliability and accuracy of the technical approach of establishing an absolute astronomical timescale using the Miaogou limestone as a fixed anchor point and combining it with astronomical cycles. Specifically, it can be implemented according to the following steps:

[0034] Step 1: Select anchor wells and obtain basic data;

[0035] Three key isochronous anchor points were selected from the coal-bearing strata of the Carboniferous-Permian system in the North China Craton: Miaogou Limestone, Beichagou Sandstone, and Camel Neck Sandstone. These wells had complete development of anchor points, typical lithological characteristics, clear logging responses, and no obvious sedimentary facies transitions. Drilling logging lithological description data, core data, thin section identification data, and continuous natural gamma logging curve data were obtained from these anchor wells.

[0036] Step 2: Calibrating the lithology and logging response characteristics of key isochronous anchor points;

[0037] A systematic lithological identification and logging response characteristic calibration were performed on the three key isochronous anchor points in the anchored well, and identification markers for each anchor point were established; specifically:

[0038] Camel Neck Sandstone: 3m thick, medium to coarse-grained lithic sandstone, light gray to grayish-white, medium to coarse-grained texture, massive structure, moderately sorted; thin sections show a medium to coarse sandy texture, containing approximately 5% fine sand, 38% medium sand, and 57% coarse sand. The clastic grains are well-rounded, mainly sub-rounded to sub-angular, with uneven to long-line contact, and grain support; the main components of the clastic grains are quartz and potassium feldspar.

[0039] The well logging curves exhibited a "two lows, one high, two negative anomalies" characteristic: the natural gamma curve showed low values ​​(40 to 50 API), the sonic transit time curve showed low values ​​(210 to 230 μs / m), and the density curve showed medium to high values ​​(2.45 to 2.55 g / cm³). 3 The spontaneous potential curve shows a negative anomaly, but the anomaly amplitude is slightly smaller (-10 to -20 mV), and the resistivity curve is of medium to high value (300 to 500 Ω·m).

[0040] Beichagou Sandstone: 5m thick, medium to fine-grained lithic quartz sandstone, light yellowish-white to light gray in color, medium-grained texture, massive structure, medium to good sorting, grain roundness is subangular to subrounded, and there is an abrupt contact with the underlying mudstone; thin sections of the rock show that it is fine to medium-grained lithic quartz sandstone, grain-supported structure, mainly with concave-convex contact, quartz grain content is more than 80%, and a small amount of potassium feldspar and flint are found;

[0041] The logging curves also exhibited a "two lows, one high, two negative anomalies" characteristic, but overall showed a pattern of "low values ​​being even lower and anomalies being even higher"; that is, the natural gamma curve was low (30 to 40 API), the sonic transit time curve was low (200 to 220 μs / m), and the density curve was medium to high (2.5 to 2.6 g / cm³). 3 The spontaneous potential curve shows a significant negative anomaly with a large amplitude (less than -20mV), and the resistivity curve is of medium to high value (500 to 1000Ω·m).

[0042] Miaogou Limestone: The lithology is microcrystalline to fine-crystalline limestone, with typical lithological characteristics within the study area and highly identifiable in the field and drilling logging. The logging curves exhibit a significant "three lows, one high, one flat" characteristic: the natural gamma ray curve shows a significantly low value (less than 30 API), much lower than the underlying mudstone; the sonic transit time curve is low (less than 200 μs / m), reflecting the dense rock and fast sonic propagation speed; the spontaneous potential curve is close to the baseline, without obvious anomalies, and is flat; the density curve is high (greater than 2.6 g / cm³). 3 This corresponds to the high density characteristics of carbonate rocks; the resistivity curve is high (greater than 1000 Ω·m), due to the dense rock, low porosity, and poor electrical conductivity.

[0043] Step 3: Extract the astronomical cyclic signal from the anchored well;

[0044] Natural gamma logging curves are sensitive to changes in the content of radioactive elements in sediments, and the changes in the material composition and sedimentation rate of sediments are controlled by climate cycles caused by astronomical orbit forcing. Therefore, natural gamma logging curves can clearly record astronomical cycle signals in the formation sedimentation process.

[0045] This embodiment uses continuous natural gamma logging curves from anchored wells as data and employs Aycle 2.3 software for system processing, specifically as follows:

[0046] The original natural gamma-ray logging curves were preprocessed to eliminate non-cyclic trend errors caused by factors such as sedimentary basement uplift and uneven sedimentation rates, restoring the original cyclic characteristics of the curves. Subsequently, bandpass filtering was used to perform spectral analysis and astronomical verification on the detrended natural gamma-ray logging curves, accurately extracting characteristic cyclic signals such as the 405 kyr long eccentricity, 109 kyr short eccentricity, and 21.7 kyr precession that conform to the astronomical orbital period. Among them, the 405 kyr long eccentricity period is used as the core period benchmark for the age scale established in this embodiment due to its strong stability and global comparability.

[0047] Step 4: Establish an absolute astronomical timescale and determine the anchor point age;

[0048] Using the known age of 298.9 Ma at the bottom boundary of the Miaogou limestone as a fixed astronomical anchor point, and combined with the astronomical cycle period identified in step 3, the depth-time quantitative conversion of the natural gamma logging curve is performed. The continuous chronology of the entire study section (Benxi Formation-Shanxi Formation) in this embodiment is achieved by using cycle counting, thus constructing a high-precision and continuous absolute astronomical chronology scale for the Carboniferous-Permian coal-bearing strata of the anchor well.

[0049] Based on the aforementioned absolute astronomical age scale, the ages of the bottom boundaries of the Beichagou Sandstone and the Camel Neck Sandstone in the anchoring well were accurately determined, and the astronomical cycle dating results for the two were as follows: the astronomical age of the bottom boundary of the Beichagou Sandstone was 298.15 Ma, and the astronomical age of the bottom boundary of the Camel Neck Sandstone was 297 Ma.

[0050] Step 5: Verify the age of the isochronous anchor point;

[0051] To verify the reliability and accuracy of the astronomical cycle dating results, the astronomical dating results obtained in step 4 were compared with previously published zircon U-Pb isotope dating data. Previous isotope dating data showed that the Beichagou sandstone in the Baode section of the northern North China Plate was determined to have a formation age of 298.18 Ma by volcanic ash zircon U-Pb dating, and the Camel Neck sandstone was determined to have a formation age of 295.346 Ma.

[0052] The comparison results show that the age data obtained by this astronomical cycle dating has a very small deviation from the previous isotope dating data; among them, the age deviation of the Beichagou sandstone is only 0.03 Ma and the age deviation of the Luotuobozi sandstone is only 0.014 Ma, and the deviation range is within the reasonable error range of the dating method.

[0053] The slight discrepancies between the two methods mainly stem from the inherent differences in sampling location and testing methods. Specifically, isotopic dating samples are collected from volcanic ash interlayers near stratigraphic interfaces, and their age represents the instantaneous time of volcanic ash eruption. In contrast, astronomical cycle dating calibrates the depositional age of stratigraphic rock interfaces, which involves a brief time difference in deposition. Additionally, zircon U-Pb dating and astronomical cycle dating have different testing principles and data processing methods, resulting in slight systematic errors.

[0054] The above-mentioned astronomical cycle dating results are in high agreement with the previous isotope dating results, which fully verifies the reliability and accuracy of the method of establishing an absolute astronomical age scale for coal-bearing strata by taking Miaogou limestone as the core fixed anchor point and combining it with astronomical cycles. It also confirms the accuracy of the astronomical dating age of the Beichagou sandstone and Camel Neck sandstone isochronous anchor points.

[0055] Step 6: Astronomical Cyclic Response Characteristic Analysis of Key Isochronous Anchor Points;

[0056] Based on the natural gamma logging curve cyclic signal after detrending and bandpass filtering, combined with the period division and phase calibration of 405 kyr long eccentricity, 109 kyr short eccentricity and 21.7 kyr precession, the precise positions of the three isochronous anchor points in the anchoring well in the astronomical cyclic sequence are determined.

[0057] Each anchor point exhibits unique and stable astronomical cycle response characteristics, and the three together form a continuous and identifiable combination of astronomical cycle markers:

[0058] Miaogou Limestone: Its base precisely corresponds to the low-value area of ​​the 405kyr long eccentricity cycle, and it is in the initial stage of the transition from a low to a high value of the 109kyr short eccentricity cycle. The 21.7kyr precession shows a strong signal peak phase. The development section of Miaogou Limestone is in a stable period of climate cycle driven by the low value of the long eccentricity cycle. The North China Plate is controlled by the low orbital eccentricity, the paleoclimate is warm and humid, the marine transgression of the continental sea reaches its peak, and the deposition of carbonate rocks is intense, ultimately forming the Miaogou Limestone, which is stably developed throughout the region. The fixed correspondence between Miaogou Limestone and the low value area of ​​the 405kyr long eccentricity cycle becomes an important astronomical cycle basis for its use as a core isochronous anchor point.

[0059] Beichagou Sandstone: Its base corresponds to the transitional stage from the low value area to the high value area of ​​the 405kyr long eccentricity, and is in the high value phase of the 109kyr short eccentricity. The 21.7kyr precession signal shows a weak signal trough phase. During this stage, it is controlled by the gradual increase of orbital eccentricity. The paleoclimate of the North China Plate is changing from warm and humid to semi-humid and semi-arid. The continental sea begins to retreat. The sedimentary environment gradually evolves from marine facies, marine-continental transitional facies to terrestrial facies. The hydrodynamic conditions are enhanced and the supply of clastic material increases.

[0060] Camel Neck Sandstone: Its base corresponds to the middle stage of the high value zone of 405 kyr long eccentricity, and is in the transition phase of the transition from high value to low value of 109 kyr short eccentricity. The 21.7 kyr precession recovers to the peak phase of strong signal. During this stage, the 405 kyr long eccentricity is at a high value. The North China Plate is controlled by orbital forcing. The seasonal differences in paleoclimate are intensified. The epicephalomaran sea completely withdraws from the study area. The sedimentary environment is completely transformed into terrestrial fluvial-deltaic sedimentation. The hydrodynamic conditions are strong and the supply of lithic material increases.

[0061] The three key isochronous anchor points exhibit a continuous phase change characteristic in the astronomical cycle sequence, from a low value area with a long eccentricity of 405 kyr to a transition area from low to high value and then to a high value area. This is highly coupled with the sedimentary evolution process of the Carboniferous-Permian system in the North China Plate, which went from marine transgression to marine regression and then to the completion of the marine-continental transition. This reveals the dominant role of astronomical orbital forcing in the evolution of the sedimentary environment and the formation of landmark rock strata in the study area.

[0062] Example 2

[0063] This embodiment focuses on the Carboniferous-Permian coal-bearing strata on the eastern margin of the Ordos Basin in the North China Craton, using the Miaogou Limestone as the target isochronous anchor point and the No. 8 coal seam as the reference marker. The No. 8 coal seam in the North China Plate is characterized by its wide distribution, stable thickness, and strong identifiability, with no missing sections. It is the core stratum for coalbed methane exploration and development in this region, and it has a fixed spatial superposition relationship with the Miaogou Limestone: the Miaogou Limestone is generally developed above the No. 8 coal seam, and in some areas directly constitutes the immediate roof of the coal seam.

[0064] See Figure 1 A method for isochronous anchor point identification and anchoring based on astronomical cyclic phase transition regions; comprising the following steps:

[0065] S1: Obtain basic geological data for the target well;

[0066] The isochronous anchor point phase transition well in the study area was selected as the target study well. The three key isochronous anchor points in the well, namely Miaogou limestone, Beichagou sandstone and Luotuobozi sandstone, have all undergone sedimentary phase transition. Their original lithology, development thickness and typical logging response characteristics have all disappeared. Drilling logging lithological description data and continuous natural gamma logging curve data of the well were obtained.

[0067] S2: Identification area reference marker layer;

[0068] The No. 8 coal seam in the North China Plate is characterized by its wide distribution, stable thickness, and strong identifiability, with no missing seams. It is the core main seam for coalbed methane exploration and development in this region. Through well logging lithology description and well logging response characteristics, the development depth of the No. 8 coal seam in the target study well was accurately identified. The identified development depth of the No. 8 coal seam in this well is 1900m.

[0069] S3: Preliminary positioning of isochronous anchor points;

[0070] Based on the spatial superposition relationship that the Miaogou limestone is generally developed in the upper part of the No. 8 coal seam and directly constitutes its immediate roof in some areas, the No. 8 coal seam identified in step S2 is used as a reference benchmark. The approximate development location of the Miaogou limestone facies variant is initially located in the stratigraphic section adjacent to the upper part of the No. 8 coal seam, and the preliminary stratigraphic range is delineated.

[0071] S4: Extract astronomical cyclic signals within the target stratum;

[0072] For the natural gamma logging curves corresponding to the formation range initially located in step S3, the same processing method as step 3 of Example 1 is used to eliminate non-cyclic trend errors. Bandpass filtering is used for spectrum analysis to extract the 405 kyr long eccentricity, 109 kyr short eccentricity and 21.7 kyr precession characteristic cyclic signals.

[0073] S5: Identify the astronomical cycle characteristic phase of the target;

[0074] The depositional formation of the Miaogou limestone has a fixed coupling relationship with astronomical orbital cycles. Its bottom boundary precisely corresponds to the low value region of the 405kyr long eccentricity period. The phase of this characteristic cycle is controlled by global orbital forcing and is not affected by local sedimentary facies changes. In the astronomical cycle signal extracted in step S4, the first 405kyr long eccentricity low value segment appearing in the upper part of coal seam No. 8 is identified. This low value segment is the core basis for the precise positioning of the facies change anchor point of the Miaogou limestone.

[0075] S6: Precise positioning of isochronous anchor points;

[0076] Within the formation depth range corresponding to the low eccentricity segment of 405kyr identified in step S5, a comprehensive judgment is made based on lithological evolution characteristics and well logging curve response characteristics:

[0077] Lithological evolution characteristics determination: The Carboniferous-Permian period in the study area was under a stable sedimentary background of an epicontinental sea. The sedimentary facies transformation of the Miaogou limestone was not an irregular lithological abrupt change, but a gradual lithological evolution influenced by the reduction in the scale of marine transgression and changes in sedimentation rate. The primary limestone lithology mostly transformed into marl and calcareous mudstone. Lithological identification of the stratigraphic section corresponding to the low value section was carried out, confirming that its lithology is marl. This lithology still retains certain carbonate mineral composition characteristics and has significant lithological differences from the underlying mudstone, siltstone and other clastic rocks.

[0078] Well logging response characteristics: The typical well logging response of the primary lithology of Miaogou limestone is characterized by "three lows, one high, and one flat" (natural gamma less than 30 API, sonic transit time less than 200 μs / m, flat spontaneous potential, density greater than 2.6 g / cm³, and resistivity greater than 1000 Ω·m). The marl after phase transformation still contains carbonate minerals and maintains relatively high rock density, and the content of radioactive minerals is still at a low level. Its well logging curve shows the characteristics of relatively high density, relatively low natural gamma, and relatively high resistivity, which are significantly different from the well logging response of the surrounding rock.

[0079] Using the astronomical cycle characteristic phase (405kyr long eccentricity low value segment) as the core judgment basis, and the lithological characteristics of marl and the logging response characteristics of relatively high density-low natural gamma-high resistivity as evidence, the three mutually corroborate each other, and determine that the marl layer corresponding to the low value segment is the phase transition anchor point of Miaogou Limestone, thus completing the precise positioning of the anchor point.

[0080] S7: Establish an absolute astronomical time scale;

[0081] Using the Miaogou limestone facies transition anchor point precisely located in step S6 as the core benchmark, and the known absolute age of 298.9 Ma of this anchor point as the fixed time anchor point, combined with the astronomical cycle period identified in step S4 (with the 405 kyr long eccentricity cycle as the core cycle benchmark), the absolute astronomical age scale of the Carboniferous-Permian coal-bearing strata in this well is constructed by performing cycle counting and depth-time quantitative conversion on the natural gamma logging curves of the target section, thereby achieving continuous age calibration of the entire study section.

[0082] This invention uses natural gamma logging curves as the extraction carrier of astronomical cycle signals, which are the most common logging data in geological exploration. Bandpass filtering, cycle counting, and other astronomical cycle signal processing methods are common techniques in stratigraphy research and can be adaptively modified and extended to the identification of anchor points in coal-bearing strata phase transition zones in other strata and sedimentary backgrounds. In addition, the method of this invention establishes an absolute astronomical age scale based on precisely located phase transition anchor points, with a division accuracy reaching the orbital scale, upgrading the division of coal-bearing strata in North China from qualitative description to quantitative calibration.

[0083] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for isochronous anchor point identification and anchoring based on astronomical cyclic phase transition regions, characterized in that, Includes the following steps: S1. Obtain basic geological data for the target well; S2. Identify widely distributed and stably developed reference marker layers in the target study well by using drilling logging lithological descriptions or logging curve response characteristics, and determine the development depth and location of the reference marker layers. S3. Based on the known fixed spatial superposition relationship between the reference marker layer and the target isochronous anchor point identified in step S2, and taking the reference marker layer as a reference, the approximate stratigraphic range of the phase variant of the target isochronous anchor point is preliminarily determined. S4. Preprocess the natural gamma logging curves corresponding to the formation range initially located in step S3 to eliminate non-cyclic trend errors; use bandpass filtering to perform spectral analysis on the detrended natural gamma logging curves to extract astronomical orbital period signals. S5. In the astronomical cycle signal extracted in step S4, a characteristic cycle phase segment with a fixed correspondence to the target isochronous anchor point is identified. This characteristic cycle phase segment is not affected by local depositional phase transitions. S6. Within the formation depth range corresponding to the characteristic cycle phase segment identified in step S5, a comprehensive judgment is made by combining lithological evolution characteristics and well logging curve response characteristics. The characteristic cycle phase segment is used as the core judgment basis, and the characteristic lithology and well logging curve response characteristics after phase transformation are used as evidence to determine the precise depth position of the target isochronous anchor point phase transformation. S7. Using the target isochronous anchor point precisely located in step S6 as the time reference point, and combining the known absolute age data of the target isochronous anchor point, construct an absolute astronomical age scale for the coal-bearing strata of the target study well.

2. The method for isochronous anchor point identification and anchoring based on astronomical cycle phase transition regions according to claim 1, characterized in that, The basic geological data includes drilling logging lithological descriptions, natural gamma logging curves, and stratigraphic information on the development of regional landmark coal seams; the astronomical orbital periodic signals include a 405 kyr long eccentricity period, a 109 kyr short eccentricity period, and a 21.7 kyr precession period.

3. The method for isochronous anchor point identification and anchoring based on astronomical cycle phase transition regions according to claim 1, characterized in that, The reference marker layer in S2 is the main coal seam that is widely developed in the region; the fixed spatial superposition relationship in step S3 is that the target isochronous anchor points are generally developed in the upper part of the main coal seam, or directly constitute the direct roof of the main coal seam.

4. The method for isochronous anchor point identification and anchoring based on astronomical cycle phase transition regions according to claim 1, characterized in that, The non-cyclic trend error mentioned in step S4 includes long-period trend signals caused by factors such as deposition substrate rise and fall and uneven deposition rate; the preprocessing and bandpass filtering are performed using Aycle 2.3 software.

5. The method for isochronous anchor point identification and anchoring based on astronomical cycle phase transition regions according to claim 1, characterized in that, The characteristic cycle phase segment mentioned in step S5 is a specific phase segment in the 405kyr long eccentricity period that has a fixed correspondence with the target isochronous anchor point; when the target isochronous anchor point is the bottom boundary of Miaogou limestone, the characteristic cycle phase segment is a low value area of ​​the 405kyr long eccentricity period, and the stratigraphic deposition period corresponding to this low value area is a stable period of climate cycle driven by low orbital eccentricity.

6. The method for isochronous anchor point identification and anchoring based on astronomical cycle phase transition regions according to claim 1, characterized in that, The determination of lithological evolution characteristics in step S6 includes: identifying the characteristic lithology after phase transformation at the target isochronous anchor point. The characteristic lithology includes marl and calcareous mudstone. This characteristic lithology retains the carbonate mineral composition characteristics and has lithological differences from the underlying clastic rocks.

7. The method for isochronous anchor point identification and anchoring based on astronomical cycle phase transition regions according to claim 1, characterized in that, The well logging curve response characteristic determination in step S6 includes: identifying the well logging curve response characteristics corresponding to the phase change lithology; the well logging curve response characteristics are a combination of relatively high density, relatively low natural gamma, and relatively high resistivity.

8. The method for isochronous anchor point identification and anchoring based on astronomical cycle phase transition regions according to claim 1, characterized in that, The construction of the absolute astronomical age scale in step S7 specifically includes: using the precisely located target isochronous anchor point as the fixed time anchor point, using a long eccentricity period of 405 kyr as the core period benchmark, and performing depth-time quantitative conversion on the natural gamma logging curve of the target layer through cyclic counting.

9. The method for isochronous anchor point identification and anchoring based on astronomical cycle phase transition regions according to claim 1, characterized in that, When at least one well-developed and identifiable key isochronous anchor point is preserved in the target well, step S7 is executed directly using the preserved key isochronous anchor point as the benchmark to establish an absolute astronomical time scale; the key isochronous anchor point is any one of Miaogou limestone, Beichagou sandstone or Camel Neck sandstone.

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