A method for calculating weathering alteration index of para-metamorphic rock weathering crust
By measuring the mass percentage of elements in logging cuttings and converting it into oxide molar ratios, a new weathering and alteration index formula was fitted, solving the problem of quantitative identification of the weathering crust of secondary metamorphic rocks and improving the efficiency and accuracy of oil and gas exploration.
Patent Information
- Application Number
- CN202411899780.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-12-23
- Publication Date
- 2026-06-23
AI Technical Summary
Existing technologies are insufficient to effectively quantify and characterize the degree of chemical weathering and alteration of the weathering crust of secondary metamorphic rocks buried at depths exceeding 2000m, leading to significant biases in identification and assessment during oil and gas exploration.
The elemental mass percentages of logging cuttings were determined using X-ray fluorescence spectrometry, a chemical analysis method for silicate rocks. After normalization and conversion to oxide molar ratios, a new weathering and alteration index formula was fitted based on the trend of elemental oxide molar ratio changes, and the structure of the weathering crust of the parametamorphic rock was identified.
It enables simple and reliable identification of weathering crust of secondary metamorphic rocks, provides reliable evidence for weathering crust buried deep, shortens exploration time and costs, and improves the efficiency of oil and gas exploration.
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Figure CN122259632A_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 secondary metamorphic rocks. Background Technology
[0002] The weathering crust of parametamorphic rocks can serve as an important oil and gas reservoir, 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 crust of parametamorphic rocks has different structures and forms an orderly distribution in the vertical direction. Different weathering crust structures have different degrees of alteration and different levels of atmospheric freshwater leaching, exhibiting different porosity and permeability characteristics, which determine their different oil and gas storage capacities. Therefore, strengthening the identification of the weathering crust of parametamorphic 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. + ) 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. Al and Si are relatively inactive 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 parametamorphic 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 a secondary metamorphic rock 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 a secondary metamorphic rock, 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. Determine the relative loss and relative enrichment trends of various elements in the weathering crust of parametamorphic rocks based on the changing trend of the element oxide molar ratio.
[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 the secondary metamorphic rock.
[0014] S6. Substitute the molar ratio of elemental oxides into the newly established calculation formula, identify the weathering crust of the parametamorphic rock based on the calculated weathering and alteration index, and classify the structure of the weathering crust of the parametamorphic rock.
[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 the parametamorphic rock is judged 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 indicating relative element enrichment.
[0020] Optionally, in step S5, the calculation formula for the new weathering and alteration index of the metamorphic rock weathering crust is: Metamorphic rock weathering crust weathering index = Sum of molar ratios of oxides of elements that are not easily soluble / Sum of molar ratios of oxides of elements that are easily soluble.
[0021] Optionally, in step S6, the weathering crust structure of the parametamorphic rock is divided into weathered rocks with a value ≥4 from the top of the parametamorphic rock downwards, and unweathered rocks with a value <4, in order to identify the weathering crust of the parametamorphic 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 parametamorphic rocks.
[0024] 2. Unlike orthometamorphic rocks, which are formed from the metamorphism of igneous rocks, parametamorphic rocks are formed from the metamorphism of sedimentary 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 parametamorphic rocks in different regions and at different depths exhibit different structures, porosity, and permeability characteristics due to variations in atmospheric freshwater leaching, thus determining their different hydrocarbon reservoir capacities. This invention achieves a simpler and more reliable identification of parametamorphic 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) parametamorphic rocks, providing important technical support for further clarifying the reservoir characteristics of parametamorphic 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 A schematic flowchart illustrating a method for calculating the weathering and alteration index of a secondary metamorphic rock weathering crust, provided in an embodiment of this application;
[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 (WIP) of the weathering crust of the secondary metamorphic 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 a secondary metamorphic rock, 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.
[0034]
[0035]
[0036]
[0037] S2. Normalize the mass percentage of the elements.
[0038] In step S2, the mass percentages of the elements are normalized, and the sum of the mass percentages of all elements after normalization is 100%. This is calculated by multiplying the mass percentage of each element by the sum of the mass percentages of all elements by 100 to obtain the normalized mass percentage of the element. Table 2 shows a list of the normalized mass percentages of constant elements in a specific embodiment.
[0039] Table 2. List of mass percentages of constant elements after normalization in specific embodiments.
[0040]
[0041]
[0042]
[0043] S3. Convert the mass percentage of the normalized elements into the molar ratio of oxides.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] Table 3. List of oxide molar ratios for specific embodiments
[0048]
[0049]
[0050]
[0051] 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 the metamorphic rock.
[0052] In step S4, the relationship between the molar ratio of each element oxide and the depth is established. Based on the trend of the change of the molar ratio of element oxides, the trend of relative loss and relative enrichment of various elements in the weathering crust of the metamorphic rock is judged, including the leftward shift of the molar ratio of element oxides indicating relative loss of elements and the rightward shift of the molar ratio of element oxides indicating relative enrichment of elements.
[0053] Figure 2The 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 figure, from 2382m, when entering the top surface of the parametamorphic rock, to 2396m, a clear inflection point appears. In this depth range, compared to the 2396m to 2500m depth range, Al₂O₃, SiO₂, and K₂O show an increasing trend, indicating relative enrichment; while Na₂O, MgO, MnO, Fe₂O₃, CaO, and TiO₂ show a decreasing trend, indicating relative loss. It can be determined that the 2382m to 2396m depth range is the section where the weathering crust of the parametamorphic rock is developed.
[0054] S5. Based on the relative loss or enrichment trend 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 the secondary metamorphic rock.
[0055] A formula for the weathering and alteration index is established. Based on the identification of the relative enrichment and relative loss of elements in the weathering crust of metamorphic rocks in step S4, easily soluble and easily residual element oxides are determined. A new formula for calculating the weathering and alteration index of the weathering crust of metamorphic rocks is fitted, namely: Weathering index of weathered crust of metamorphic rocks = Sum of molar ratios of insoluble element oxides / Sum of molar ratios of easily soluble element oxides.
[0056] The above process can be described by the following formula:
[0057] WIP=(Al2O3+S iO2+K2O) / (Na2O+MgO+MnO+Fe2O3+CaO+TiO2).
[0058] S6. Substitute the molar ratio of elemental oxides into the newly established calculation formula, identify the weathering crust of the parametamorphic rock based on the calculated weathering and alteration index, and classify the structure of the weathering crust of the parametamorphic rock.
[0059] The weathering crust structure of parametamorphic rocks is divided into three parts: those with a value ≥4 from the top of the parametamorphic rock downwards are weathered, and those with a value <4 are unweathered. This division is used to identify the weathering crust of parametamorphic rocks.
[0060] The weathering and alteration index is calculated to identify the weathering crust. The molar ratios of major element oxides are 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. The weathering crust structure of the parametamorphic rock is divided into two categories: those with a value ≥ 4 from the top of the parametamorphic rock downwards are considered weathered, and those with a value < 4 are considered unweathered. This division is used to identify the weathering crust of the parametamorphic rock. For example... Figure 3The figure shown is a composite bar chart of the calculation results of the weathering and alteration index (WIP) of the weathering crust of the secondary metamorphic rock in a specific embodiment, where 2382-2396m is the weathering crust and 2396-2500m is the original rock.
[0061] Table 3 lists the weathering and alteration indices for specific embodiments.
[0062]
[0063]
[0064]
[0065] 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 parametamorphic rocks.
[0066] This embodiment of the invention enables a simpler and more reliable identification of the weathering crust of parametamorphic rocks based on the elemental analysis data of logging cuttings. It provides a reliable basis for the identification of weathering crust of relatively deep parametamorphic rocks (usually more than 2000m deep), and can provide important technical support for further clarifying the reservoir characteristics of parametamorphic rocks in this layer. It can effectively shorten the time window of exploration work, reduce exploration costs, and achieve higher oil and gas exploration benefits.
[0067] 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 para-metamorphic 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. Determine the relative loss and relative enrichment trends of various elements in the weathering crust of parametamorphic rocks based on the changing trend of the element oxide molar ratio. 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 the secondary metamorphic rock. S6. Substitute the molar ratio of elemental oxides into the newly established calculation formula, identify the weathering crust of the parametamorphic rock based on the calculated weathering and alteration index, and classify the structure of the weathering crust of the parametamorphic rock.
2. The method for calculating the weathering alteration index of a para-metamorphic rock weathering crust according to 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 a secondary metamorphic rock 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 a secondary metamorphic rock 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 a secondary metamorphic rock 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 the parametamorphic rock is judged based on the changing trend of the element oxide molar ratio, including the leftward shift of the element oxide molar ratio indicating relative loss of elements and the rightward shift of the element oxide molar ratio indicating relative enrichment of elements.
6. The method for calculating the weathering and alteration index of the weathering crust of a secondary metamorphic rock 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 the metamorphic rock is: Weathering index of the weathering crust of the metamorphic rock = Sum of the molar ratios of oxides of elements that are not easily soluble / Sum of the molar ratios of oxides of elements that are easily soluble.
7. The method for calculating the weathering and alteration index of the weathering crust of a secondary metamorphic rock as described in claim 1, characterized in that, In step S6, the weathering crust structure of the parametamorphic rock is divided into weathered rocks with a value ≥4 from the top of the parametamorphic rock downwards, and unweathered rocks with a value <4, in order to identify the weathering crust of the parametamorphic rock.