A shale sublayer division method based on comprehensive analysis of three stratum characteristics

Abstract

The present application belongs to the field of oil and gas geological exploration, and provides a method for shale sublayer division by comprehensive analysis of three kinds of stratum characteristics. Firstly, the method divides the three-level sequence by the conversion interface of sedimentation and lithology, and determines the division basis of sequence stratigraphy by using sedimentary structure characteristics such as lamination. Secondly, the lithological characteristics such as biogenic silica band, banded iron formation and barite are analyzed. Thirdly, the four-level sequence division standard is established by analyzing the chemical characteristics such as the change trend of mineral elements in each sequence. Finally, the final fine division result of the four-level sequence is obtained by comprehensively using the marks of sequence stratigraphy, biostratigraphy and chemical stratigraphy. Based on the method, a more fine four-level sequence division scheme can be established, which can be used for fine stratigraphic correlation of the study area, the isochronism is greatly improved, and the operability is strong, so that more accurate results are provided for subsequent preparation of lithofacies paleogeographic evolution map.

Description

Technical Field

[0001] This invention relates to the field of oil and gas geological exploration technology, specifically to a method for dividing shale sublayers based on a comprehensive analysis of three stratigraphic characteristics. Background Technology

[0002] The organic-rich shale of the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation (Kardian-Elonian stages) is currently the main shale gas exploration and development zone in China. The marine shale of the Wufeng-Longmaxi Formation in the southern Sichuan Basin has abundant well logging and core data to support its development, along with detailed paleontological zoning as a timescale. Furthermore, its heterogeneity in both vertical and horizontal directions makes it an excellent sample for fine-grained sequence stratigraphy. As a comprehensive stratigraphic method in oil and gas exploration, sequence stratigraphy based on fine-grained rocks is a key and challenging aspect.

[0003] Shortly after the establishment of the geological stratification scheme in the Sichuan Basin, some scholars had already recognized the serious time-dip phenomenon in the scheme when performing cross-regional well correlation. Furthermore, due to the lack of drilling data in the transitional areas between the Zhaotong and Changning demonstration zones and the Weiyuan demonstration zone in southern Sichuan at that time, the stratification scheme was not adjusted in a timely manner. Therefore, it is particularly important to conduct detailed fourth-order sequence stratigraphy and correlation work on the target strata in the study area. Research on the detailed sub-strata division method of shale in the study area is urgently needed, which can provide a research foundation for lithofacies paleogeographic reconstruction and shale gas exploration and development. Summary of the Invention

[0004] The purpose of this invention is to provide a method for shale sub-layer division based on a comprehensive analysis of three stratigraphic characteristics. It establishes a rapidly applicable production stratification recommendation scheme based on lithology, sedimentary structures, graptolite zones, inorganic geochemical characteristics, and electrical properties from drill cores, combined with Mitchum's sequence hierarchical naming method, thereby better guiding the selection of favorable strata. Using this method to perform sequence stratigraphy on strata corresponding to the Kerdi-Treichian stages can make the planar characterization results related to sedimentation and reservoirs tend to be equal, thus better guiding the selection of favorable strata. The method of this invention includes the following main steps: (1) Based on the technical aspects of maintaining the results of the three-level sequence stratigraphy scheme, the target layer of the Wufeng-Longmaxi Formation where the data of each drilling well in the work area is located is divided into three levels of sequence stratigraphy. (2) Within the third-order sequence, bedding and lamellar analysis are performed on each sequence to divide it into fourth-order sequences. The lamellar types in the fourth-order sequences are determined, and based on this, the lamellar characteristics of each type of sequence division are determined: G = Analyse(H) In the formula, H represents the laminated rock sample, Analyse() indicates the characteristic analysis of the laminae, and G represents the obtained laminae characteristic results; (3) By using argon ion polishing scanning electron microscopy for quantitative mineral analysis and large-area high-resolution imaging, the lithology, biogenic silica bands, bentonite, and barite in the target layer were analyzed, and the sequence division was further improved by utilizing the lithological characteristics within different sequences. (4) Based on the relationship between mineral element content and mineral content variation, as well as the trend of mineral element variation in each fourth-order sequence, a standard for classifying fourth-order mineral element sequences was established: Kiff = Est(XRD, WE) In the formula, XRD represents the content of different mineral elements, WE represents the relationship between the mineral element content and XRD, Est() sets the fourth-order sequence classification standard for mineral elements, and Kiff represents the established fourth-order sequence classification method for mineral elements. (5) Combining the fourth-order sequence stratigraphy results obtained in step (3) based on laminar and lithological characteristics and the fourth-order sequence stratigraphy results obtained in step (4) based on mineral elements, a fourth-order sequence stratigraphy classification standard based on the comprehensive analysis of sequence stratigraphy, biostratigraphy, and chemostratigraphy indicators is established. Stand = Com(Seq, Bio Chem) In the formula, Seq represents sequence stratigraphic markers, Bio represents biostratigraphic markers, Chem represents chemostratigraphic markers, Com() represents comprehensive analysis, and Stand represents the established fourth-order sequence stratigraphic criteria. Repeat steps (2) to (5) above to complete the division of the fourth-level sequence within each third-level sequence. Then repeat steps (1) to (5) above until all drilling data within the entire work area has been processed. Based on structural analysis, the fourth-level sequence within the target layer can be accurately divided, laying a solid foundation for the study of sedimentary facies, reservoir, sedimentary relationships and reservoir coupling relationships, and the selection of favorable areas.

[0005] The shale sublayer division method based on comprehensive analysis of three stratigraphic characteristics of the present invention has the following advantages: (1) Core data were fully utilized. The mineral element content was determined by quantitative analysis of minerals using argon ion polishing scanning electron microscopy and large-area high-resolution imaging. A fourth-order sequence classification standard for mineral elements was established, providing a favorable basis for sub-layer classification. (2) A four-level sequence stratigraphic standard based on comprehensive analysis of lithology, sedimentary structure and chemical properties was further established. The accuracy of sequence stratigraphy can be improved by using the characteristics of sequence stratigraphy, biostratigraphy and chemostratigraphy together as constraints. Attached Figure Description

[0006] Figure 1 The columnar section of well Yang 101H2-7.

[0007] Figure 2 This invention relates to the fourth-order sequence stratigraphy and characteristic map of the Lu 205 well, obtained based on sedimentary structural and lithological characteristics.

[0008] Figure 3 This invention provides a fourth-order sequence chemostratigraphic feature map of the Lu 210 well, obtained based on mineral composition and chemostratigraphic characteristics.

[0009] Figure 4 This is a comparison diagram of four wells connected in four layers, obtained by sequence division of the work area in the embodiment of the present invention.

[0010] Figure 5 This is a comparison diagram of the proposed modification schemes for the work area layering of the present invention to the layering schemes currently used in production. Detailed Implementation

[0011] The specific embodiments of the present invention are as follows: (1) The core data of the study area were sorted and analyzed. By identifying the lithological abrupt changes and sedimentary tectonic transition interfaces caused by marine transgression and regression, the main geological features within the third-order sequence were determined, and the third-order sequence of the Wufeng-Longmaxi Formation was divided. (2) Within the third-order sequence, the bedding and lamination analysis of each sequence is carried out to divide it into fourth-order sequences, determine the lamination types and filling minerals in the fourth-order sequences, and on this basis, determine the lamination characteristics of each type of sequence division. (3) Analyze the lithology, biogenic silica bands, bentonite, and barite in the target layer by argon ion polishing scanning electron microscopy (QEMSCAN) and large-area high-resolution imaging (MAPS). Further refine the sequence division by utilizing the lithological characteristics within different sequences. (4) Based on the elemental characteristics of the strata, combined with the relationship between the mineral element content and the changes in mineral content, and the trend of mineral element changes in each fourth-order sequence, a standard for classifying the fourth-order sequence of mineral elements was formulated. (5) Combining the fourth-order sequence stratigraphy results obtained in step (3) based on laminar and lithological characteristics and the fourth-order sequence stratigraphy results obtained in step (4) based on mineral elements, a fourth-order sequence stratigraphy standard based on comprehensive analysis of lithology, sedimentary structure and chemical properties is established. (6) Repeat steps (2) to (5) to complete the division of the fourth-level sequence within each third-level sequence; (7) Repeat steps (1) to (6) to complete the fourth-order sequence division of all wells in the study area, and use sequence stratigraphy to obtain a fine fourth-order sequence division of the Cadillac-Elonian stage in southern Sichuan.

[0012] Figure 1 The columnar section of well Yang 101H2-7.

[0013] Figure 2This invention utilizes the technology of this invention to stratify the laminar features in sedimentary structures. Combined with the results of quantitative mineral analysis using argon-ion polished scanning electron microscopy (QEMSCAN) and large-area high-resolution imaging (MAPS), a four-level sequence stratigraphy (PSS) was performed. The figures show that the sedimentary structure features are as follows: PSS1 is generally massive with few laminar features; PSS2 is mostly composed of extremely thin to very thin laminar features (0.5–1 mm); PSS3 is mostly composed of massive bedding and discontinuous extremely thin laminar features; PSS4 has extremely well-developed laminar features, mostly dense pyrite or silt laminar features less than 0.5 mm; PSS5 consists of superimposed massive bedding sections and laminar features; PSS6 has extremely well-developed laminar features, dense and thick, mainly consisting of thin to medium laminar features greater than 1 mm; PSS7 has laminar features superimposed with massive bedding; and PSS8 has numerous, thick laminar features with large intervals.

[0014] Figure 3 This invention presents a fourth-order sequence chemostratigraphic feature map of Well Lu 210, obtained based on mineral composition and chemostratigraphic characteristics. The map shows a clear relationship between the elemental composition and mineral content of the indicated minerals: Si is positively correlated with quartz and illite; Th and Al are positively correlated with total clay content; Ca is positively correlated with calcite; Mg is positively correlated with dolomite; Na and K are somewhat correlated with feldspar, chlorite, and kaolinite; Fe is positively correlated with pyrite; and Ba exhibits a Pss8 anomaly in barite nodules. The vertical distribution and variations of the 16 elements (Si, K, Al, Ca, Mg, Ba, Fe, Na, Th, P, V, Ni, Cu, Zn, Co, and U) all corroborate the rationality of the aforementioned fourth-order sequence division to varying degrees, and indirectly demonstrate significant differences in the sedimentary environments of different fourth-order sequences.

[0015] Figure 4 This is a sequence stratigraphic comparison diagram of four wells in the work area of ​​the embodiment obtained by the method of the present invention, which integrates sequence stratigraphy, lithostratigraphy and chemostratigraphy. The diagram shows that the thickness of each layer increases from the location of well Wei 204H10-2 to well Ning 211.

[0016] Figure 5 This invention proposes a modification to the current production stratification scheme based on the stratigraphic division scheme of the work area in the embodiment. The Wufeng Formation can still be divided into the lower shale section and the upper shale section of the Wufeng Formation according to Pss1 and Pss2. The original Longmaxi Formation Long-1 sub-section 1 layer can basically correspond to Pss3, Pss4, and Pss5 (referred to as 1 lower, 1 middle, and 1 upper within Sichuan Qingyuan), that is, the original layer 1 is suggested to be changed to layers 1, 2, and 3. The original Long-1 sub-section 2 layer corresponds to Pss6, and it is suggested to change it to layer 4 (new). The original Long-1 sub-section 3 layer corresponds to Pss7, and it is suggested to change it to layer 5. The original Long-1 sub-section 4 layer corresponds to Pss8, and it is suggested to change it to layers 6 and 7.

[0017] The advantages of this invention are as follows: It utilizes sedimentary structural features such as bedding to establish evaluation criteria for fourth-order sequence stratigraphy, providing an advantage for preliminary and rapid fourth-order sequence division. It further refines the fourth-order sequence division using lithological features such as biogenic silica bands, bentonite, and barite. Furthermore, based on the variation trends of mineral elements within each sequence, it develops more refined chemical characteristic sequence division criteria. Through comprehensive multi-factor analysis, it obtains the final fourth-order sequence division results. By integrating the characteristics of sequence stratigraphy, lithostratigraphy, and chemostratigraphy analysis methods, a more refined fourth-order sequence division scheme is established, which can provide more accurate results for subsequent lithofacies paleogeographic evolution.

[0018] The above embodiments are only used to illustrate the present invention. The implementation steps of the method can be varied. Any equivalent transformations and improvements made on the basis of the technical solution of the present invention should not be excluded from the protection scope of the present invention.

Claims

1. A method for dividing shale sublayers based on a comprehensive analysis of three stratigraphic characteristics, comprising the following steps: (1) The core data of the study area were sorted and analyzed. By identifying the lithological abrupt changes and sedimentary tectonic transition interfaces caused by marine transgression and regression, the main geological features within the third-order sequence were determined, and the third-order sequence of the target layer was divided. (2) Within the third-order sequence, the bedding and lamination analysis of each sequence is carried out to divide it into fourth-order sequences, determine the lamination types and filling minerals in the fourth-order sequences, and on this basis, determine the lamination characteristics of each type of sequence division. G = Analyse(H) In the formula, H represents the laminated rock sample, Analyse() indicates the characteristic analysis of the laminae, and G represents the obtained laminae characteristic results; (3) By using argon ion polishing scanning electron microscopy for quantitative mineral analysis and large-area high-resolution imaging, the lithology, biogenic silica bands, bentonite, and barite in the target layer were analyzed, and the sequence division was further improved by utilizing the lithological characteristics within different sequences. (4) Based on the elemental characteristics of the strata, combined with the relationship between the mineral element content and the changes in mineral content, and the trend of mineral element changes in each fourth-order sequence, a standard for classifying the fourth-order sequence of mineral elements was formulated. Kiff = Est(XRD, WE) In the formula, XRD represents the content of different mineral elements, WE represents the relationship between the mineral element content and XRD, Est() sets the fourth-order sequence classification standard for mineral elements, and Kiff represents the established fourth-order sequence classification method for mineral elements. (5) Combining the fourth-order sequence stratigraphy results obtained in step (3) based on laminar and lithological characteristics with the fourth-order sequence stratigraphy results obtained in step (4) based on mineral elements, a fourth-order sequence stratigraphy standard based on comprehensive analysis of lithology, sedimentary structure, and chemical properties is established. Stand = Com(Seq, Bio Chem) In the formula, Seq represents sequence stratigraphic markers, Bio represents biostratigraphic markers, Chem represents chemostratigraphic markers, Com() represents comprehensive analysis, and Stand represents the established fourth-order sequence stratigraphic criteria. (6) Repeat steps (2) to (5) to complete the division of the fourth-level sequence within each third-level sequence; (7) Repeat steps (1) to (6) to complete the fourth-order sequence division of all wells in the study area, and a detailed division result of the fourth-order sequence of the shale in the target stratum of the study area can be obtained.

2. The shale sublayer division method based on comprehensive analysis of three stratigraphic characteristics according to claim 1, characterized in that: By making full use of core data, the mineral element content was determined through quantitative mineral analysis using argon ion polishing scanning electron microscopy and large-area high-resolution imaging, and a fourth-order mineral element sequence classification standard was established, providing a favorable basis for sub-layer classification.

3. The shale sublayer division method based on comprehensive analysis of three stratigraphic characteristics according to claim 1, characterized in that: A four-level sequence stratigraphic standard based on comprehensive analysis of lithology, sedimentary structure, and chemical properties was further established. The accuracy of sequence stratigraphy can be improved by using the characteristics of sequence stratigraphy, biostratigraphy, and chemostratigraphy as constraints.

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