Method for dividing stratum sequence of extremely thick shallow water carbonate rock gentle slope

Abstract

The invention discloses a method for dividing a stratum sequence of a giant thick shallow water carbonate rock gentle slope, and the method comprises the following steps: determining a secondary stratum sequence interface based on regional geological survey; establishing a third-level sequence cycle under the control of the second-level sequence; under the control of the third-level sequence, determining a fourth-level sequence interface; and establishing a four-level sequence framework according to the four-level sequence interface. According to the method for dividing the stratigraphic sequence of the giant thick shallow water carbonate rock gentle slope, the giant thick deposition characteristic and the sequence cycle characteristic of the carbonate rock gentle slope are fully considered, the established three-level sequence stratigraphic framework and the development strata series have a good corresponding relation, and three-dimensional geological modeling and oil reservoir development deployment can be effectively guided.

Description

Technical Field

[0001] This invention relates to the field of carbonate stratigraphy, specifically to a method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes. Background Technology

[0002] Gentle slopes are an important sedimentary environment for carbonate rocks, located in wide, low-angle areas where tectonic uplifts transition evenly into basins. In warm, shallow water environments, gentle carbonate slopes exhibit extremely high sedimentation rates, leading to the formation of thick carbonate strata and massive oil reservoirs. These massive carbonate oil reservoirs are commonly found in the Cretaceous strata of the Middle East, and the main oil reservoirs in several giant carbonate oil fields in southeastern Iraq were formed in Cretaceous gentle carbonate slope environments. The thick carbonate rock strata contain multiple sequence cycles of different levels. The following challenges exist in identifying sequence boundaries and determining sequence levels: (1) Insufficient space to accommodate carbonate rock deposits can lead to lateral progradation, but the progradation structure of gentle slope landforms is not obvious; (2) During sea-level rise and fall cycles, strata are easily exposed. In humid climates, leaching and dissolution are common, making it difficult to distinguish levels; (3) Due to the shallow water depositional environment, the maximum flooding surface is blurred and difficult to identify, further increasing the difficulty of identifying sequence boundaries in shallow water carbonate rock gentle slope deposits.

[0003] Oil reservoirs exhibit strong vertical heterogeneity, making stratified development an effective strategy for improving recovery and balancing reserve utilization. The delineation of development strata is closely related to sequence stratigraphy. Typically, third-order sequences can form regional exposure surfaces, and long-term exposure can create relatively good interlayers. Significant differences in sedimentary environments above and below the sequence result in independent reservoir units for different sequences. Within a sequence, reservoir connectivity is good, and reservoir properties typically exhibit gradual changes. Therefore, a reasonable delineation of third-order sequences is crucial for reservoir stratified development, and accurate delineation of fourth-order sequences can effectively guide sub-sequence delineation and correlation. However, the unique sedimentary characteristics and complex petrological features of shallow-water carbonate gentle slopes lead to lower sequence delineation accuracy, hindering efficient reservoir development. New methods and approaches are needed to delineate sequence stratigraphy in thick, shallow-water carbonate gentle slope formations.

[0004] In existing technologies, the commonly used methods for sequence stratigraphy include geological analysis, well logging, seismic analysis, paleontological analysis, geochemical analysis, paleomagnetic analysis, and mathematical analysis. However, these methods have limited applicability to thick bioclastic limestone. Summary of the Invention

[0005] The purpose of this invention is to provide a method for dividing the sequence stratigraphy of thick, shallow-water carbonate rocks on gentle slopes, in order to solve the technical problem of low accuracy in the sequence stratigraphy of thick, shallow-water carbonate rocks on gentle slopes in the prior art.

[0006] To achieve the above objectives, one embodiment of the present invention provides a method for dividing a sequence of thick, shallow-water carbonate rock strata on a gentle slope, comprising the following steps:

[0007] Based on regional geological surveys, the second-order sequence boundaries were identified;

[0008] Under the control of the second-level sequence, a third-level sequence cycle is established;

[0009] Under the control of the three-level hierarchy, the interface of the four-level hierarchy is determined;

[0010] Based on the four-level hierarchical interface, establish a four-level hierarchical framework;

[0011] The method for establishing a three-level sequence cycle includes the following steps:

[0012] Based on the regional tectonics and sedimentary background, a framework profile was established;

[0013] Based on the established framework profile, stratigraphic contact relationships are observed through seismic data and outcrop analogy;

[0014] After the formation contact relationship observation is completed, key sequence interfaces are identified based on core, cast thin section data and well logging curves;

[0015] After the key sequence boundary was identified, phase sequence analysis was carried out under the control of the deposition mode.

[0016] After the phase sequence analysis is completed, the stratigraphic structure analysis is performed to identify the maximum flooding surface.

[0017] In one preferred embodiment of the present invention, when observing the contact relationship of strata, it is necessary to determine whether the strata have at least one of the contact relationships of cut-off, top overshoot, top overshoot, and bottom overshoot.

[0018] In one preferred embodiment of the present invention, when the strata have at least one of the contact relationships of cut-off, top overflow, overflow, and underflow, a third-order sequence cycle is established under the control of the second-order sequence through seismic data and outcrop analogy; when the strata do not have at least one of the contact relationships of cut-off, top overflow, overflow, and underflow, key sequence interfaces are identified based on core, cast thin section data, and well logging curves, and a third-order sequence cycle is established under the control of the second-order sequence.

[0019] One preferred embodiment of the present invention is that the third-order sequence cycle established under the control of the second-order sequence through seismic data and outcrop analogy needs to further identify key sequence interfaces through core, cast thin section data and well logging curves, and further clarify the third-order sequence cycle on the established third-order sequence cycle.

[0020] In one preferred embodiment of the present invention, before identifying a key sequence interface, it is necessary to determine whether a key sequence interface exists. When a key sequence interface exists, it is identified based on core, cast thin section data, and well logging curves. Under the control of the second-order sequence, a third-order sequence cycle is established, or a third-order sequence cycle is further clarified on the existing third-order sequence cycle. When a key sequence interface does not exist, a third-order sequence cycle is established through phase sequence analysis, or a third-order sequence cycle is further clarified on the existing third-order sequence cycle.

[0021] In one preferred embodiment of the present invention, when a critical sequence interface is present, the leaching and dissolving surface is classified after the critical sequence interface is identified.

[0022] One preferred embodiment of the present invention is a method for classifying leaching and dissolution surfaces, which specifically involves classifying leaching and dissolution surfaces based on rock structure, particle composition, dissolution intensity, and stratum preservation, thereby identifying third-order and fourth-order sequence leaching and dissolution surfaces.

[0023] In one preferred embodiment of the present invention, the key sequence interfaces include a strongly leached and dissolved surface, a weathered soil surface, an uptidal flat indicator, a dolomitized surface, and a terrestrial indicator.

[0024] In one preferred embodiment of the present invention, phase sequence analysis requires determining whether a phase sequence abrupt change surface exists.

[0025] In one preferred embodiment of the present invention, when a phase sequence abrupt change surface exists in the phase sequence analysis, the original sequence is divided into two tertiary sequences; when no phase sequence abrupt change surface exists in the phase sequence analysis, the original sequence is maintained.

[0026] One preferred embodiment of the present invention is a method for identifying the maximum flooding surface: within the identified third-order sequence, a microfacies sequence is established, and based on the rock structure and biological type in the microfacies, the turning point from deep to shallow water is found, and the water body corresponding to the point is the deepest, which is determined as the maximum flooding surface.

[0027] One preferred embodiment of the present invention is as follows: under the control of the third-order sequence, based on the guidance of the sedimentary model, diagenetic analysis is carried out, and the fourth-order sequence interface is determined by coupling the microfacies sequence and the intensity of dissolution.

[0028] One preferred embodiment of the present invention is a method for establishing a fourth-order sequence framework: based on the logging response of the fourth-order sequence interface and under the guidance of the sedimentation mode, a fourth-order sequence comparison is carried out in non-cored wells to establish a fourth-order sequence framework.

[0029] In summary, the beneficial effects of the present invention are as follows:

[0030] 1. The method for dividing the sequence stratigraphy of thick, shallow-water carbonate rock strata in this invention is based on the principle that in sequence stratigraphy, the higher the order of the sequence, the more obvious its interface features and the higher the accuracy. Therefore, higher-order sequences are used to constrain lower-order sequences to ensure the accuracy of the results. Simultaneously, because there are many dissolution interfaces in shallow-water carbonate rock strata deposits, and the maximum flooding surface features are not obvious, it is necessary to classify the leaching dissolution surfaces based on rock structure, grain composition, dissolution intensity, and stratigraphic preservation. Furthermore, under normal circumstances, the third-order sequence interface reflects a greater drop in water volume, indicating a more significant impact. Longer exposure time leads to stronger sedimentary hydrodynamics, higher content of high-energy bioclastics, larger particle size, and more intense dissolution. However, it also results in more severe damage to the stratigraphic structure, lower preservation, and complete erosion of high-quality reservoirs, often accompanied by cemented zones. Determining the maximum floodplain requires identifying the turning point from deep to shallow water based on the rock structure and biotype within the microfacies. The point corresponding to the deepest water body is determined as the maximum floodplain. Furthermore, the fourth-order sequence boundary should be subject to short-term exposure and dissolution. The sub-layers are delineated by coupling the microfacies sequence and the intensity of dissolution. This invention enables the delineation of thick bioclastic limestone sequence units on gentle slopes of shallow-water carbonate rocks, and the research results have significant guiding significance for development stratigraphic division and reservoir prediction.

[0031] 2. The method for dividing the sequence stratigraphy of thick, shallow-water carbonate rocks on gentle slopes in this invention fully considers the sedimentary characteristics and sequence cyclic characteristics of the thick carbonate rocks on gentle slopes. The established three-level sequence stratigraphic framework has a good correspondence with the development strata and can effectively guide three-dimensional geological modeling and reservoir development deployment.

[0032] 3. The research scale of this invention decreases step by step, and lower-level sequences are controlled by higher-level sequences, which ensures the accuracy of the sequence division results. At the same time, considering the fact that there are many types of key sequence interfaces, nested leaching and dissolution surfaces of different levels, and difficulty in identifying the maximum flooding surface during the research process, and combined with the characteristics of shallow-water carbonate rock gentle slope deposition, multiple phases and multiple methods are used to ensure the reliability and correctness of the research results.

[0033] 4. This invention clarifies the correspondence between the fourth-order sequence stratigraphy and the sub-layers. Based on the sedimentary model and diagenesis, it guides the division and correlation of sub-layers. The divided sub-layers have certain isochronous significance, and the reservoirs in the same sub-layer have similar sedimentary and diagenetic coupling results. The reservoirs have certain genetic correlations, and the sub-layer correlation results are reliable.

[0034] 5. This invention proposes the relationship between shallow-water carbonate rock gentle slope sequence stratigraphy and the development stratigraphy of thick carbonate rock reservoirs, identifies several key interfaces of the third-order stratigraphy, and proposes a method for identifying the maximum flooding surface based on microfacies sequence, sedimentary structure, and biotype. The established third-order sequence stratigraphy framework can effectively guide the division of reservoir development stratigraphy.

[0035] 6. This invention proposes a method for distinguishing between third-order and fourth-order sequence leaching and dissolution surfaces. The leaching and dissolution surfaces are classified based on rock structure, grain composition, dissolution intensity, and stratigraphic preservation. Furthermore, this method controls lower-order sequences based on higher-order sequences to ensure the accuracy of sequence delineation results. Simultaneously, considering the lack of clear sequence boundaries during the research process, multiple phases and methods are employed to guarantee sequence delineation. Attached Figure Description

[0036] Figure 1 This is a flowchart illustrating a method for dividing a thick, shallow-water carbonate rock strata sequence according to an embodiment of the present invention;

[0037] Figure 2 This is a north-south seismic profile of the study area M group in one embodiment of the present invention;

[0038] Figure 3 This is a schematic diagram of a strongly leaching and dissolution surface identified in group M based on core, cast thin section data and well logging curves in one embodiment of the present invention.

[0039] Figure 4 This is a schematic diagram of weathered soil surfaces identified in group M based on core, cast thin section data and well logging curves in one embodiment of the present invention.

[0040] Figure 5 This is a schematic diagram of the tidal flat identified in group M based on core, cast thin section data and well logging curves in one embodiment of the present invention.

[0041] Figure 6 This is a schematic diagram of terrestrial indicator markers identified in group M based on core, cast thin section data and well logging curves in one embodiment of the present invention;

[0042] Figure 7 This is a schematic diagram of a three-level hierarchical lattice structure (M groups) in one embodiment of the present invention;

[0043] Figure 8 This is a core diagram of a third-order sequence leaching and dissolution surface in one embodiment of the present invention;

[0044] Figure 9 This is a microstructure diagram of a three-level sequence leaching and etching surface in one embodiment of the present invention;

[0045] Figure 10 This is a core diagram of a fourth-order sequence leaching and dissolution surface in one embodiment of the present invention;

[0046] Figure 11 This is a microstructure diagram of a four-level sequence leaching and etching surface in one embodiment of the present invention;

[0047] Figure 12This is a schematic diagram showing that the biodebris in the largest flood surface feature of the present invention is planktonic foraminifera;

[0048] Figure 13 This is a schematic diagram showing that the biodebris in the largest flood surface feature of the present invention is sponge spicules;

[0049] Figure 14 This is a sequence diagram of sequence cycles and sedimentary evolution in one embodiment of the present invention;

[0050] Figure 15 This is a contact relationship diagram of normal sedimentary facies from land to sea in a shallow-water carbonate rock gentle slope depositional process according to one embodiment of the present invention. Detailed Implementation

[0051] This invention provides a method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes, such as... Figure 1 As shown, it includes the following steps:

[0052] Based on regional geological surveys, the second-order sequence boundary was identified as a control condition for identifying the third-order sequence.

[0053] Under the control of the second-level sequence, a third-level sequence cycle is established;

[0054] Under the control of the third-order sequence, and guided by the sedimentary model, diagenetic analysis was carried out. The fourth-order sequence boundary was determined by the coupling of microfacies sequence and dissolution intensity. Specifically, under the control of the third-order sequence, and guided by the sedimentary model, diagenetic analysis was carried out using thin section data. The microfacies corresponding to the change from strong to weak hydrodynamics and the strongest coupling with dissolution can be identified as the fourth-order sequence boundary.

[0055] Based on the logging response of the fourth-order sequence boundary and under the guidance of the sedimentary model, fourth-order sequence correlation of non-cored wells was carried out to establish a fourth-order sequence framework. Specifically, based on the logging response of the fourth-order sequence boundary and under the guidance of the sedimentary model, the fourth-order sequence division of non-cored wells was achieved by comprehensively considering factors such as gentle slope topography and planar facies change.

[0056] The method for establishing a three-level sequence cycle includes the following steps:

[0057] Using the second-order sequence boundary as a constraint, a framework profile was established based on the regional tectonics and sedimentary background.

[0058] Based on the established framework profile, stratigraphic contact relationships are observed through seismic data and outcrop analogy. When observing stratigraphic contact relationships, it is necessary to determine whether the strata exhibit at least one of the following contact relationships: cut-off, top-overlap, overlap, and underlap. The specific criteria and reasons for this judgment are as follows: Normally deposited strata exhibit conformable contact, with different interfaces parallel to each other. In seismic profiles, the seismic axes formed by reflections from stratigraphic interfaces are continuous and stable. In field outcrops, different strata are continuous and parallel. If phenomena such as cut-off, top-overlap, overlap, and underlap are observed, it represents changes in sedimentation caused by sea-level changes. Cut-off reflects a rapid and significant drop in sea level, and the cut section represents the top interface of the sequence stratigraphy. Top-over reflects a continuous rise in sea level to its maximum, with no sedimentation forming a sedimentary hiatus; the top-over surface represents the maximum flooding surface. Underlap reflects a continuous drop in sea level and continuous progradation of strata; the underlap surface represents the boundary between the lowstand systems tract and the transgressive systems tract. Overlap reflects a continuous rise in sea level and continuous regression of strata; the overlap surface represents the boundary between the transgressive systems tract and the highstand systems tract.

[0059] The judgment results are divided into the following two situations: (1) When the strata have at least one of the contact relationships of cut-off, top overlap, overlap and underlap, a third-order sequence cycle is established under the control of the second-order sequence through seismic data and outcrop analogy; (2) When the strata do not have at least one of the contact relationships of cut-off, top overlap, overlap and underlap, a third-order sequence cycle cannot be established through seismic data and outcrop analogy.

[0060] After observing stratigraphic contact relationships, key sequence boundaries are identified based on core, cast thin section, and well logging data. These key sequence boundaries include strongly leached and dissolved surfaces, weathered soil surfaces, supratidal indicators, dolomitization surfaces, and terrestrial indicators. These boundaries are common in shallow-water carbonate rock slopes. Before identifying key sequence boundaries, it is necessary to determine whether a key sequence boundary exists. The specific criteria and reasons for this determination are as follows:

[0061] Leaching and dissolution surfaces: As sea levels drop and strata are exposed, under humid climate conditions, strata are subjected to leaching and dissolution by atmospheric freshwater. The strata undergo varying degrees of change depending on factors such as exposure time and dissolution intensity. If the exposure time is long and the dissolution is intense, the original structure of the strata will be destroyed, and cement will form near the dissolution surface. If the exposure time is short and the dissolution is weak or moderate, the dissolution process will create numerous secondary pores. In this case, without destroying the original structure of the strata, the porosity and permeability of the rock will significantly increase.

[0062] Weathered soil surface: With a significant drop in sea level and long-term exposure of the strata, under arid climate conditions, the strata are weathered, and the original rock is gradually transformed into soil, losing its original rock structure;

[0063] Upland flat indicator: Upland flat indicator includes bird's eye structure and top and bottom structure, reflecting the continuous drop in sea level and the continuous migration of sedimentary facies towards the basin. Upland flat indicator represents the facies zone corresponding to the lowest point of sea level.

[0064] Dolomitization: When sea level drops to its lowest point, quasi-syngenetic dolomitization occurs if climatic and aquatic conditions are suitable. The dolomite crystals are clean, with low euhedrality and a grain size less than 100 μm. Normally, limestone is deposited on gentle carbonate slopes; the presence of dolomite indicates that sea level has dropped to its lowest point. The limestone-dolomite-limestone pattern reflects a sea level that initially drops and then rises.

[0065] Terrestrial indicators: Terrestrial indicators refer to coal seams or carbonaceous mudstone. Both coal seams and carbonaceous mudstone are products of terrestrial origin. The presence of coal seams or carbonaceous mudstone within carbonate strata indicates a significant drop in sea level and prolonged exposure of the strata. During the exposure process, the strata are covered with lush vegetation and submerged in silt. After the vegetation dies, it decomposes rapidly, preserving organic matter in a reducing environment, gradually forming a swampy environment. When the sediments are buried, they undergo temperature and pressure conditions similar to those of coal-forming environments, gradually forming coal seams or carbonaceous mudstone.

[0066] The judgment results are specifically divided into the following situations: (1) When there is a key sequence interface, and a third-order sequence cycle has been established through seismic data and outcrop analogy, the key sequence interface is identified based on core, cast thin section data and well logging curves, and the third-order sequence cycle is further clarified on the established third-order sequence cycle; (2) When there is a key sequence interface, and a third-order sequence cycle has not yet been established through seismic data and outcrop analogy, the key sequence interface is identified based on core, cast thin section data and well logging curves, and a third-order sequence cycle is established under the control of the second-order sequence; (3) When there is no key sequence interface, it is impossible to identify the key sequence interface based on core, cast thin section data and well logging curves, establish a third-order sequence cycle or further clarify the third-order sequence cycle on the established third-order sequence cycle;

[0067] When a key sequence boundary is present, after the key sequence boundary is identified, the leaching and dissolution surfaces need to be classified. Specifically, the leaching and dissolution surfaces are classified according to rock structure, grain composition, dissolution intensity, and stratigraphic preservation, identifying third-order and fourth-order sequence leaching and dissolution surfaces to further clarify the established third-order sequence. The criteria for determining third-order and fourth-order sequence leaching and dissolution surfaces are as follows: Third-order sequence leaching and dissolution surfaces: The rock structure is a sparry-supported structure with a sparry cement content exceeding 50%, and the grain composition contains relatively large-sized thick-shelled clam shells. Large, with a content exceeding 50%, exhibiting high dissolution intensity, destruction of the stratigraphic structure, visible honeycomb-like pores, and significant differences from the underlying strata. A dense layer is formed near the interface due to cementation. Fourth-order sequence leaching dissolution surface: The rock structure is a grain-supported structure, with a spar crystal cement content of less than 50%. The granular component contains 20%-50% thick-shelled clam debris, and about 20%-50% bivalves, echinoderms, or spheroids. The dissolution intensity is relatively weak or moderate, the stratigraphic structure is not destroyed, and there is little difference from the underlying strata. The porosity and permeability near the interface are usually high.

[0068] After key sequence boundary identification, facies sequence analysis is conducted under the control of the sedimentary model. This analysis requires determining whether a facies sequence abrupt change occurs. Specifically, under the control of the sedimentary model, facies sequence analysis is performed within the third-order sequence. If a facies sequence abrupt change is found, the original sequence is divided into two third-order sequences; if no facies sequence abrupt change is found, the original sequence is maintained. The basis and rationale for determining the existence of facies sequence abrupt changes are as follows: In shallow-water carbonate rock slope deposition, from land to sea, the normal sedimentary facies contact relationship is supratidal flat - lagoon (tidal channel) - hill shoal - grain shoal - foreshore - slope foot - open shelf. Figure 15 As shown, within a sequence, the facies sequence of sedimentary facies should conform to the above relationship; if facies that should not be in contact in the well are in contact with each other, such as deep-water shelf-slope foot-foreshore-bioclastic shoal-deep-water shelf, the bioclastic shoal and deep-water shelf are abrupt changes in facies sequence, reflecting a sudden rise in sea level, and there is a third-order sequence boundary between the bioclastic shoal and deep-water shelf.

[0069] The specific judgment results are divided into the following situations: (1) When there is a phase sequence abrupt change surface, a third-order sequence cycle has been established through seismic data and outcrop analogy. Based on core, cast thin section data and well logging curves, key sequence interfaces are identified, and the third-order sequence cycle is further clarified on the established third-order sequence cycle. Based on the third-order sequence cycle further clarified through phase sequence analysis, the third-order sequence cycle is further clarified; (2) When there is a phase sequence abrupt change surface, a third-order sequence cycle has been established through seismic data and outcrop analogy. The third-order sequence cycle is further clarified on the established third-order sequence cycle through phase sequence analysis; (3) When there is a phase sequence abrupt change surface, a third-order sequence cycle has been established based on core, cast thin section data and well logging curves. The third-order sequence cycle is further clarified on the established third-order sequence cycle through phase sequence analysis; (4) When there is no phase sequence abrupt change surface. (5) If a facies change surface is not present, and ...

[0070] After the facies sequence analysis is completed, sequence structure analysis is performed to identify the maximum flooding surface. Specifically, within the already identified third-order sequence, a microfacies sequence is established based on core and thin section data. Based on the rock structure and biotype in the microfacies, the turning point from deep to shallow water is found. The point corresponding to the deepest water body is determined as the maximum flooding surface. The basis and reasoning for identifying the maximum flooding surface are as follows: Each microfacies contains five types of information: indicator organisms + structure + grain size + sorting + tectonics. In shallow-water carbonate rock slopes, organisms indicating deep-water environments include sponge spicules and planktonic foraminifera; structures indicating deep-water environments include granular-muddy structures and micritic structures; grain size indicating deep-water environments is less than 10 μm; sorting indicating deep-water environments is relatively high; and tectonic features indicating deep-water environments are slump structures or horizontal bedding. Specifically, the presence of any one of the following—sponge spicules or planktonic foraminifera, grain content less than 30%, grain size less than 10 μm, relatively good grain sorting, or slump structures / horizontal bedding—can determine the maximum floodplain. In shallow-water carbonate rock slopes, not all five factors may be present. When all five are present, the influence weights from highest to lowest are: indicator organisms, structure, grain size, sorting, and tectonic features.

[0071] Example

[0072] Using the method of this invention, a sequence lattice framework for the massive, thick bioclastic limestone formation of the Cretaceous M Group in southeastern Iraq was established.

[0073] Step 1: Based on literature review, it was determined that the M group strata are in a second-order sequence subsidence half-cycle, with the bottom of the M group being the maximum flooding surface of the second-order sequence and the top being the top boundary of the second-order sequence. According to the regional tectonic-sedimentary background, the carbonate rocks in the study area trend gently from north to south. Multiple north-south oriented framework sections were established. Based on seismic data, stratigraphic contact relationships were observed. Although multiple progradational bodies developed from north to south, no obvious sequence boundaries such as truncation, top-overlap, or overlap were found. Figure 2 As shown;

[0074] Step Two: Based on core, cast thin section data, and well logging curves, key sequence boundaries such as strongly leached and dissolved surfaces, weathered soil surfaces, supratidal flats, and terrestrial indicators are identified in Group M. Figures 3-6 As shown, where Figure 3 For a strongly leached and dissolved surface, Figure 4 For weathered soil surface, Figure 5 This is a signpost for Chaoshangping. Figure 6 It serves as a terrestrial indicator; four third-order sequence cycles were identified, such as Figure 7 As shown;

[0075] Step 3: Group M contains numerous leaching and dissolution surfaces. These surfaces are classified based on rock structure, grain composition, dissolution intensity, and stratigraphic preservation, distinguishing between fourth-order and third-order sequence leaching and dissolution surfaces. Figures 8-11 As shown, where Figure 8 A schematic diagram of the core of a third-order sequence leaching and dissolution surface. Figure 9 This is a microstructure diagram of a three-level sequence leaching and dissolution surface. Figure 10 This is a schematic diagram of a core sample showing a fourth-order sequence leaching and dissolution surface. Figure 11 Microstructure diagram of the fourth-order sequence leaching and dissolution surface;

[0076] Step 4: The key sequence boundary features of Group M are obvious. Group M is a typical carbonate rock gentle slope depositional model. Under the control of the depositional model, facies sequence analysis was carried out within the sequence, and there were no abrupt facies sequence transitions within the sequence.

[0077] Step 5: Identify the maximum flooding surface. Within the already identified third-order sequence stratigraphy, establish a microfacies sequence based on core and thin-section data. Based on the rock structure and biotypes within the microfacies, locate the turning point where the water becomes shallower; the point corresponding to the deepest water body is identified as the maximum flooding surface. The largest flooding surface in Group M is dominated by granulite marl, with bioclastic materials primarily consisting of sponge spicules, ostracods, and planktonic foraminifera. Figure 12 and Figure 13 As shown, where Figure 12 The detritus consists of planktonic foraminifera. Figure 13 The biological debris consists of sponge-like spicules.

[0078] Step 6: Under the control of the third-order sequence stratigraphy, based on the sedimentary model, and using thin section data, diagenetic analysis is carried out. The microfacies sequence and dissolution intensity are coupled to divide the layers. The microfacies that reflect the strongest hydrodynamics and the microfacies with the strongest dissolution intensity in terms of sedimentary structure and grain composition are identified as the fourth-order sequence boundary.

[0079] Step 7: Based on the logging response of the fourth-order sequence boundary, and guided by the sedimentary model, taking into account factors such as gentle slope topography and planar facies transition, conduct fourth-order sequence correlation using non-cored wells to establish a fourth-order sequence framework, such as... Figure 14 As shown.

[0080] In summary, the method for dividing the sequence stratigraphy of thick, shallow-water carbonate rocks on gentle slopes according to the present invention provides important guidance for the stratigraphic division and reservoir prediction of thick bioclastic limestone oil reservoirs.

[0081] Although specific embodiments of the present invention have been described in detail with reference to the accompanying drawings, this should not be construed as limiting the scope of protection of this patent. Various modifications and variations that can be made by those skilled in the art without inventive effort within the scope described in the claims still fall within the scope of protection of this patent.

Claims

1. A method for sequence stratigraphy of thick, shallow-water carbonate rocks on gentle slopes, characterized in that, Includes the following steps: Based on regional geological surveys, the second-order sequence boundaries were identified; Under the control of the second-level sequence, a third-level sequence cycle is established; Under the control of the three-level hierarchy, the interface of the four-level hierarchy is determined; Based on the four-level hierarchical interface, establish a four-level hierarchical framework; The method for establishing a three-level sequence cycle includes the following steps: Based on the regional tectonics and sedimentary background, a framework profile was established; Based on the established framework profile, stratigraphic contact relationships are observed through seismic data and outcrop analogy; After the formation contact relationship observation is completed, key sequence interfaces are identified based on core, cast thin section data and well logging curves; After the key sequence boundary was identified, phase sequence analysis was carried out under the control of the deposition mode. After the phase sequence analysis is completed, the stratigraphic structure analysis is performed to identify the maximum flooding surface.

2. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 1, characterized in that: When observing the contact relationship of strata, it is necessary to determine whether the strata have at least one of the following contact relationships: cut-off, top overlap, top overlap, and bottom overlap.

3. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 2, characterized in that: When the strata have at least one of the contact relationships of cut-off, top overflow, overflow, and underflow, a third-order sequence cycle is established under the control of the second-order sequence through seismic data and outcrop analogy; when the strata do not have at least one of the contact relationships of cut-off, top overflow, overflow, and underflow, key sequence interfaces are identified based on core, cast thin section data, and well logging curves, and a third-order sequence cycle is established under the control of the second-order sequence.

4. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 3, characterized in that: The third-order sequence cycles established under the control of the second-order sequence through seismic data and outcrop analogy need to be further clarified by identifying key sequence interfaces through core, cast thin section data and well logging curves, based on the established third-order sequence cycles.

5. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 4, characterized in that: Before identifying key sequence interfaces, it is necessary to determine whether a key sequence interface exists. When a key sequence interface exists, it is identified based on core, cast thin section data, and well logging curves. Under the control of the second-order sequence, a third-order sequence cycle is established, or a third-order sequence cycle is further clarified on the existing third-order sequence cycle. When a key sequence interface does not exist, a third-order sequence cycle is established through facies analysis, or a third-order sequence cycle is further clarified on the existing third-order sequence cycle.

6. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 5, characterized in that: When a key sequence interface is present, the leaching and dissolving surface is classified after the key sequence interface is identified.

7. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 6, characterized in that: The grading method for the leaching and dissolution surfaces is as follows: the leaching and dissolution surfaces are graded according to the rock structure, particle composition, dissolution intensity and the degree of formation preservation, and the third-order and fourth-order sequence leaching and dissolution surfaces are determined.

8. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 1, characterized in that: The key sequence interfaces include the intensely leached and dissolved surface, the weathered soil surface, the tidal flat indicator, the dolomitization surface, and the terrestrial indicator.

9. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 1, characterized in that: The phase sequence analysis needs to determine whether there is a phase sequence abrupt change surface.

10. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 9, characterized in that: When a phase sequence abrupt change surface exists in the phase sequence analysis, the original sequence is divided into two third-order sequences; when no phase sequence abrupt change surface exists in the phase sequence analysis, the original sequence is preserved.

11. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 1, characterized in that, The method for identifying the maximum flooding surface is as follows: within the identified third-order sequence, a microfacies sequence is established. Based on the rock structure and biological type in the microfacies, the turning point from deep to shallow water is found. The water body corresponding to this point is the deepest and is determined as the maximum flooding surface.

12. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 1, characterized in that, The method for determining the fourth-order sequence boundary is as follows: under the control of the third-order sequence, based on the guidance of the sedimentary model, diagenetic analysis is carried out, and the fourth-order sequence boundary is determined by coupling the microfacies sequence and the intensity of dissolution.

13. The method for dividing the stratigraphic sequence of thick, shallow-water carbonate rocks on gentle slopes as described in claim 1, characterized in that, The method for establishing the fourth-order sequence framework is as follows: based on the logging response of the fourth-order sequence interface, and under the guidance of the sedimentation mode, conduct fourth-order sequence comparison of non-cored wells to establish the fourth-order sequence framework.

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