A method for calculating weathering alteration index of orthometamorphic weathering crust
By measuring the mass percentage of elements in logging cuttings and calculating the oxide molar ratio, a new weathering and alteration index was established, which solved the reliability problem of weathering crust identification in orthomorphic rocks and improved the efficiency and benefits of oil and gas exploration.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CHINA PETROLEUM & CHEMICAL CORP
- Filing Date
- 2024-12-23
- Publication Date
- 2026-06-23
AI Technical Summary
Existing technologies have failed to effectively quantify the degree of chemical weathering and alteration of the weathering crust of orthomorphic bedrock, resulting in significant discrepancies between calculated and theoretical results in oil and gas exploration, which affects the efficiency of oil and gas reservoir identification and exploration.
The mass percentage of elements in logging cuttings was determined by X-ray fluorescence spectrometry, a chemical analysis method for silicate rocks. Through normalization and oxide molar ratio calculation, a new weathering and alteration index formula was established to identify the structure and reservoir characteristics of the weathering crust of orthometamorphic rocks.
It enables simple and reliable identification of the weathering crust of deeply buried orthomorphic rocks, shortens the exploration time window, reduces exploration costs, and improves the efficiency of oil and gas exploration.
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Figure CN122259631A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of oil and gas geological exploration technology, specifically a method for calculating the weathering and alteration index of the weathering crust of orthomorphic rocks. Background Technology
[0002] Weathering crusts of orthomorphic rocks can serve as important oil and gas reservoirs, and numerous oil and gas discoveries have been made there. In recent years, domestic researchers have generally focused on the study of bedrock reservoirs, including igneous and metamorphic rocks. The development of igneous and metamorphic rock reservoirs is mainly controlled by tectonic movements and weathering, forming fractured reservoirs and weathering-dissolution reservoirs. Controlled by the intensity of weathering, the weathering crusts of orthomorphic rocks have different structures and form an orderly distribution in the vertical direction. Different weathering crust structures exhibit different degrees of alteration and different levels of atmospheric freshwater leaching, resulting in different porosity and permeability characteristics, which determine their different oil and gas storage capacities. Therefore, strengthening the identification of weathering crusts of orthomorphic rocks has important practical significance for oilfield exploration and development.
[0003] Weathering is a continuous evolutionary process. Rocks and their fragments break down under long-term physical, chemical, and biological conditions, and unstable minerals decompose. Therefore, different degrees of weathering result in variations in the mineral and elemental content of rocks. Studies on elemental changes during weathering primarily focus on silicate rocks, especially granites. The most significant aspect of chemical weathering in silicate rocks is the leaching of primary aluminosilicate minerals, particularly feldspar, i.e., the loss of alkali metals (Na₂O₃) from the primary minerals. + K + ) and alkaline earth metals (Mg 2+ Plagioclase undergoes hydrolysis to form water-soluble salts, which are then dissolved and leached away by water bodies. Because plagioclase weathers faster than potassium feldspar, and due to differences in ionic radii, Na+ undergoes a greater precipitation process during weathering. + The rate of hydrolysis leaching is greater than K + Faster, and the losses were also greater than K's. + Large. Mn, Fe, and Ti have relatively weak activity during weathering, but their content in the weathered layer increases significantly due to the relative loss of other components.
[0004] Commonly used chemical weathering indices include the Parker Indices (WIP), Wiggett Residual Indices (V), Chemical Alteration Indices (CIA), Chemical Weathering Indices (CIW), Plagioclase Alteration Indices (PIA), TiO2 Index, and Si / Al R (Table 1). These indices are widely used in the study of weathering crusts of surface rocks such as carbonate rocks and clastic rocks, and also show good results in soils such as loess and red clay.
[0005] In oil and gas exploration, the bedrock weathering crust is typically buried at a depth exceeding 2000m, which differs significantly from the temperature and pressure fields of the surface weathering crust. This leads to substantial discrepancies between the calculated chemical weathering and alteration indices and the theoretical results. Furthermore, the degree of chemical weathering and alteration of the bedrock weathering crust in orthomorphic rocks has not been thoroughly studied, and no effective index has been developed for quantitative characterization. Summary of the Invention
[0006] In view of the above problems, the present invention is proposed to provide a method for calculating the weathering and alteration index of the weathering crust of orthometamorphic rocks that overcomes or at least partially solves the above problems.
[0007] To achieve the above objectives, the present invention adopts the following technical solution:
[0008] A method for calculating the weathering alteration index of the weathering crust of orthometamorphic rocks, the method comprising:
[0009] S1. Determine the mass percentage of elements in logging cuttings;
[0010] S2. Normalize the mass percentage of the elements;
[0011] S3. Convert the mass percentage of the normalized elements into the molar ratio of oxides;
[0012] S4. Based on the changing trend of the element oxide molar ratio, determine the relative loss and relative enrichment trends of various elements in the weathering crust of orthomorphic rocks.
[0013] S5. Based on the trend of relative loss or enrichment of elements, determine the easily soluble elements and the easily residual elements, and fit a new calculation formula for the weathering and alteration index of the weathering crust of orthomorphic rocks.
[0014] S6. Substitute the molar ratio of element oxides into the newly established calculation formula, identify the weathering crust of orthomorphic rocks based on the calculated weathering and alteration index, and classify the structure of the weathering crust of orthomorphic rocks.
[0015] Optionally, in step S1, the mass percentage of elements in the logging cuttings is determined using X-ray fluorescence spectrometry, a chemical analysis method for silicate rocks.
[0016] Optionally, in step S2, the mass percentage of the elements is normalized, and the mass percentages of each element are added together to get 100%.
[0017] Optionally, in step S3, the mass percentage of the normalized elements is converted into the molar ratio of oxides, which is described by the formula Xa=Wa / Aa;
[0018] Where: Xa is the molar ratio of element oxides; Wa is the mass percentage of the element after normalization; Aa is the atomic weight of the element oxides.
[0019] Optionally, in step S4, the trend of relative loss and relative enrichment of various elements in the weathering crust of orthomorphic rocks is determined based on the changing trend of the element oxide molar ratio, including a leftward shift of the element oxide molar ratio indicating relative element loss, and a rightward shift of the element oxide molar ratio indicating relative element enrichment.
[0020] Optionally, in step S5, the calculation formula for the new weathering and alteration index of the weathering crust of the orthometamorphic rock is: Weathering index of the weathering crust of the orthometamorphic rock = Sum of the molar ratios of easily soluble element oxides / Sum of the molar ratios of insoluble element oxides.
[0021] Optionally, in step S6, the weathering crust structure of the orthomorphic rock is divided into weathered rocks with a value ≤ 4 from the top of the orthomorphic rock downwards, and unweathered rocks with a value > 4, in order to identify the weathering crust of the orthomorphic rock.
[0022] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:
[0023] 1. Based on a large amount of data obtained from analyzing the elemental content of logging cuttings, this invention establishes a new formula for calculating the chemical weathering and alteration index and proposes a new method for identifying the weathering crust of orthometamorphic rocks.
[0024] 2. Unlike parametamorphic rocks, which are formed from sedimentary rocks, orthometamorphic rocks are formed from igneous rocks. Their lithological characteristics are controlled by the parent rock and have a certain degree of inheritance, but due to different metamorphic processes, they also exhibit new characteristics in mineral composition and structure (such as the presence of metamorphic minerals and directional structures). Therefore, when forming weathering crusts, the weathering crusts of orthometamorphic rocks in different regions and at different depths exhibit different atmospheric freshwater leaching, resulting in different structures, porosity, and permeability characteristics, thus determining their different hydrocarbon reservoir capacities. This invention achieves a simpler and more reliable identification of orthometamorphic rock weathering crusts based on well logging cuttings elemental analysis data. It provides a reliable basis for identifying weathering crusts of relatively deep-buried (usually exceeding 2000m) orthometamorphic rocks, providing important technical support for further clarifying the reservoir characteristics of orthometamorphic rocks in this stratum. This can effectively shorten the exploration time window, reduce exploration costs, and achieve higher oil and gas exploration efficiency. Attached Figure Description
[0025] Figure 1 This application provides a method for calculating the weathering and alteration index of the weathering crust of orthometamorphic rocks.
[0026] Figure 2This is a scatter plot showing the molar ratio of each element's oxide as a function of depth in the embodiments of this application.
[0027] Figure 3 This is a composite bar chart showing the calculation results of the weathering index WIO of the weathering crust of the orthomorphic rock in the embodiments of this application. Detailed Implementation
[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.
[0029] Please see Figure 1 This embodiment provides a method for calculating the weathering and alteration index of the weathering crust of orthometamorphic rocks, the method comprising:
[0030] S1. Determine the mass percentage of elements in logging cuttings.
[0031] In step S1, the mass percentage of elements in the logging cuttings is determined using X-ray fluorescence spectrometry, a chemical analysis method for silicate rocks.
[0032] The mass percentages of major elements were determined. The mass percentages of elements in logging cuttings were determined using X-ray fluorescence spectrometry, as per GB / T14506.28-93, Chemical Analysis Methods for Silicate Rocks. Table 1 shows the mass percentages of major elements (Mn, Fe, Na, Mg, Al, Si, K, Ca, Ti) in specific embodiments.
[0033] Table 1. List of mass percentages of constant elements in specific embodiments.
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[0040] S2. Normalize the mass percentage of the elements.
[0041] In step S2, the mass percentage of the elements is normalized, and the mass percentages of each element are added together to get 100%.
[0042] The mass percentage of each element is normalized. This is done by dividing the mass percentage of each element by the sum of the mass percentages of all elements, and multiplying by 100 to obtain the normalized mass percentage of the element. Table 2 shows the normalized mass percentage of constant elements in a specific embodiment.
[0043] Table 2. List of mass percentages of constant elements after normalization in specific embodiments.
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[0051] S3. Convert the mass percentage of the normalized elements into the molar ratio of oxides.
[0052] In step S3, the mass percentage of the normalized elements is converted into the molar ratio of oxides, which is described by the formula Xa=Wa / Aa.
[0053] Where: Xa is the molar ratio of element oxides, in mol; Wa is the mass percentage after element normalization, in %; Aa is the atomic weight of element oxides, in 1.
[0054] The normalized mass percentages of elements were converted into molar ratios of oxides. The mass percentages of constant elements were converted into molar ratios of oxides by dividing by the relative atomic mass of the oxide. Table 3 shows a list of oxide molar ratios for specific embodiments.
[0055] Table 3. List of oxide molar ratios for specific embodiments
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[0066] S4. Based on the changing trend of the element oxide molar ratio, determine the trend of relative loss and relative enrichment of various elements in the weathering crust of orthomorphic rocks.
[0067] In step S4, the trends of relative element loss and relative enrichment are determined. The relationship between the molar ratio of each element's oxide and depth is established. Based on the changing trend of the element oxide molar ratio, the trends of relative loss and relative enrichment of various elements in the weathering crust of orthomorphic rocks are determined, including a leftward shift of the element oxide molar ratio indicating relative element loss, and a rightward shift indicating relative element enrichment.
[0068] Figure 2 The scatter plot shows the molar ratio of various elemental oxides as a function of depth in a specific embodiment. As can be seen from the plot, starting at 1966.5m where the rock reaches the top surface of the orthomorphic rock, a clear inflection point appears at 2060m. In this depth range, compared to the 2060m to 2140m depth range, MnO, Fe2O3, CaO, and TiO2 show an increasing trend, indicating relative enrichment, while Na2O, MgO, Al2O3, SiO2, and K2O show a decreasing trend, indicating relative loss. Therefore, the 1966.5m to 2060m depth range can be identified as the weathering crust development section of the orthomorphic rock.
[0069] S5. Based on the trend of relative loss or enrichment of elements, determine the easily soluble elements and the easily residual elements, and fit a new calculation formula for the weathering and alteration index of the weathering crust of orthomorphic rocks.
[0070] A formula for the weathering index of orthometamorphic rocks was established. Based on the identification of relative enrichment and relative loss of elements in the weathering crust of orthometamorphic rocks in step S4, easily soluble and easily residual element oxides were determined. A new formula for calculating the weathering index of orthometamorphic rocks was then fitted: Weathering index of orthometamorphic rocks = Sum of the molar ratios of easily soluble element oxides / Sum of the molar ratios of insoluble element oxides.
[0071] The above process can be described by a formula.
[0072] WIO=(Na2O+MgO+Al2O3+SiO2+K2O) / (MnO+Fe2O3+CaO+TiO2)
[0073] S6. Substitute the molar ratio of element oxides into the newly established calculation formula, identify the weathering crust of orthomorphic rocks based on the calculated weathering and alteration index, and classify the structure of the weathering crust of orthomorphic rocks.
[0074] The weathering crust structure of orthomorphic rocks is divided into two parts: those with a value ≤ 4 from the top of the orthomorphic rock downwards are weathered, and those with a value > 4 are unweathered. This division is used to identify the weathering crust of orthomorphic rocks.
[0075] The weathering and alteration index is calculated to identify the weathering crust. The molar ratio of major element oxides is substituted into a newly established calculation formula to obtain the weathering and alteration index of the weathering crust. Table 3 lists the weathering and alteration indices for specific embodiments. Values ≤4 for the entire normal metamorphic rock from the top downwards are considered weathered, while values >4 are considered unweathered, thus identifying the weathering crust of the normal metamorphic rock. To further subdivide the structure of the weathering crust, the top layer with values >4 is classified as the hydrolysis layer, and the interior layer with values ≤4 is classified as the weathering leaching layer. For example... Figure 3 The figure shows a comprehensive bar chart of the calculated weathering alteration index (WIO) of the weathering crust of orthomorphic rocks in a specific embodiment. The area from 1966.5 to 2060 m represents the weathering crust, and from 2060 to 2140 m represents the original rock. The weathering crust can be further subdivided into structural layers based on its numerical value: the area from 1966.5 to 2000 m represents the hydrolysis layer, and the area from 2000 to 2060 m represents the weathering leaching layer.
[0076] Table 3 lists the weathering and alteration indices for specific embodiments.
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[0083] Based on a large amount of data obtained from analyzing the elemental content of logging cuttings, this embodiment establishes a new formula for calculating the chemical weathering and alteration index and proposes a new method for identifying the weathering crust of orthometamorphic rocks.
[0084] This embodiment enables a simpler and more reliable identification of weathering crusts of orthometamorphic rocks based on logging cuttings elemental analysis data. It provides a reliable basis for identifying weathering crusts of orthometamorphic rocks that are buried relatively deep (usually more than 2000m). It can provide important technical support for further clarifying the reservoir characteristics of orthometamorphic rocks in this layer, effectively shorten the time window of exploration work, reduce exploration costs, and achieve higher oil and gas exploration benefits.
[0085] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.
Claims
1. A method of calculating a weathering alteration index of a orthometamorphic weathering crust, characterized by, The method includes: S1. Determine the mass percentage of elements in logging cuttings; S2. Normalize the mass percentage of the elements; S3. Convert the mass percentage of the normalized elements into the molar ratio of oxides; S4. Based on the changing trend of the element oxide molar ratio, determine the relative loss and relative enrichment trends of various elements in the weathering crust of orthomorphic rocks. S5. Based on the trend of relative loss or enrichment of elements, determine the easily soluble elements and the easily residual elements, and fit a new calculation formula for the weathering and alteration index of the weathering crust of orthomorphic rocks. S6. Substitute the molar ratio of element oxides into the newly established calculation formula, identify the weathering crust of orthomorphic rocks based on the calculated weathering and alteration index, and classify the structure of the weathering crust of orthomorphic rocks.
2. The method for calculating the weathering and alteration index of the weathering crust of orthometamorphic rocks as described in claim 1, characterized in that, In step S1, the mass percentage of elements in the logging cuttings is determined using X-ray fluorescence spectrometry, a chemical analysis method for silicate rocks.
3. The method for calculating the weathering and alteration index of the weathering crust of orthometamorphic rocks as described in claim 1, characterized in that, In step S2, the mass percentage of the elements is normalized, and the mass percentages of each element are added together to get 100%.
4. The method for calculating the weathering and alteration index of the weathering crust of orthometamorphic rocks as described in claim 1, characterized in that, In step S3, the mass percentage of the normalized elements is converted into the molar ratio of oxides, which is described by the formula Xa=Wa / Aa; Where: Xa is the molar ratio of element oxides; Wa is the mass percentage of the element after normalization; Aa is the atomic weight of the element oxides.
5. The method for calculating the weathering and alteration index of the weathering crust of orthometamorphic rocks as described in claim 1, characterized in that, In step S4, the trend of relative loss and relative enrichment of various elements in the weathering crust of orthomorphic rocks is determined based on the changing trend of the element oxide molar ratio. This includes a leftward shift of the element oxide molar ratio indicating relative element loss, and a rightward shift indicating relative element enrichment.
6. The method for calculating the weathering and alteration index of the weathering crust of orthometamorphic rocks as described in claim 1, characterized in that, In step S5, the new formula for calculating the weathering and alteration index of the weathering crust of orthometamorphic rocks is: Weathering index of weathering crust of orthometamorphic rocks = Sum of molar ratios of easily soluble element oxides / Sum of molar ratios of insoluble element oxides.
7. The method for calculating the weathering alteration index of the weathering crust of orthometamorphic rocks as described in claim 1, characterized in that, In step S6, the weathering crust structure of the orthomorphic rock is divided into weathered rocks with an overall value ≤ 4 from the top of the orthomorphic rock downwards, and unweathered rocks with a value > 4, in order to identify the weathering crust of the orthomorphic rock.