A method for reconstructing paleogeographic pattern of sedimentary basin based on geochemical index

Abstract

The application discloses a sedimentary basin paleogeographic pattern reconstruction method based on geochemical indexes, and comprises the following specific steps: analyzing and measuring the major element, trace element and rare earth element compositions of a sampling sample of a sedimentary basin, judging the tectonic background and parent rock lithology of the sedimentary basin, and analyzing the secondary components in the rock debris in the sampling sample to constrain the judgment result of the tectonic background of the sedimentary basin; analyzing and calculating the heavy mineral composition proportion of the sampling sample to obtain a heavy mineral combination maturity index, and judging the sediment transport distance of the sedimentary basin; determining the sedimentary basin paleoenvironment reconstruction index by analyzing the trace elements of the sampling sample, combining the sediment characteristics of the sedimentary basin and the sedimentary facies division, and calculating the sediment dispersion law indicated by the internal strata of the sedimentary basin; and combining the geochemical indexes obtained in steps one, two and three in a coupling mode to reconstruct the paleogeographic pattern of the sedimentary basin.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of reconstruction of paleogeographic pattern of sedimentary basin, and particularly relates to a reconstruction method of paleogeographic pattern of sedimentary basin based on geochemical indexes. BACKGROUND

[0002] The reconstruction of the geographic pattern is to reveal the information of past crustal movement, evolution of the earth's surface structure, climate change, etc. by analyzing the geological structure, topographic and geomorphic features and lithology of different regions. In recent years, with the development of geochemical means under the support of multi-disciplinary intersection, the traditional provenance analysis has developed from "qualitative" to "quantitative", and certain progress has been made in the restoration of the paleogeographic pattern of the sedimentary basin and the prediction of the sediment distribution rule.

[0003] The "source-sink" system pair in the concept of paleogeography is the organic combination of basin-mountain dynamics, material transport process and sediment dispersion difference, which divides the denudation source area, transport area and final deposition area from the spatial concept, and regards them as a complete system (also known as the sediment path system), which is no longer limited to the sedimentary area in the traditional sedimentology research.

[0004] However, the existing reconstruction of the paleogeographic pattern of the sedimentary basin has the following technical problems:

[0005] The previous research on the source area (i.e. the research on the orogenic belt) is weak, and even various provenance analysis methods are directly applied to the basin without such research. The source-sink system research on the combination characteristics of the detrital rock debris in the basin is only limited to the interior of the basin, and the single source-sink research in the basin has no integrity and the error of the obtained results is relatively high. In the large sedimentary basin with complex coupling relationship between basin and mountain, the tectonic background of the source area, the type of parent rock lithology, the distance of material transport and the sediment distribution rule have multiple solutions, and the source-sink process recovery under the constraint of multiple geochemical indexes can overcome the spatial limitation of the traditional paleogeographic reconstruction. SUMMARY

[0006] The present application aims to provide a reconstruction method of paleogeographic pattern of sedimentary basin based on geochemical indexes, so as to solve the technical problem in the prior art that the paleogeographic pattern of the sedimentary basin cannot be comprehensively and quickly and effectively reconstructed due to the independent characterization of the geochemical indexes of the sedimentary basin.

[0007] To solve the above technical problems, the present application specifically provides the following technical scheme:

[0008] A reconstruction method of paleogeographic pattern of sedimentary basin based on geochemical indexes, comprising the following specific steps:

[0009] Step one, the main, trace elements and rare earth element composition analysis and determination of the sampling sample of the sedimentary basin, the tectonic background and the parent rock lithology of the sedimentary basin are distinguished, and the analysis of the secondary components in the rock debris in the selected sampling sample is carried out, and the result of the tectonic background of the sedimentary basin is constrained;

[0010] Step two, the analysis and calculation of the heavy mineral composition proportion of the sampling sample obtains the heavy mineral combination maturity index, and the sediment transport distance of the sedimentary basin is judged;

[0011] Step three, the paleoenvironment reconstruction index of the sedimentary basin is determined by the analysis of the trace elements of the sampling sample, the sediment characteristics of the sedimentary basin are combined, and the sediment dispersion rule indicated by the internal stratum of the sedimentary basin is calculated according to the division of the sedimentary facies;

[0012] Step four, the geochemical indexes obtained in steps one, two and three are combined in a coupling mode, and the paleogeographic pattern of the sedimentary basin is reconstructed.

[0013] As a preferred scheme of the present application, in step one, the stratum sequence framework source at the position of the sampling sample is determined through drilling data and sampling sample characteristic analysis, and the spatial distribution of the unconformity structure under a single tectonic movement is superimposed, so as to obtain the theoretical stratum layered unit under the vertical structure of the sedimentary basin;

[0014] The sediment material abundance of the theoretical stratum layered unit on the vertical structure is determined based on the main, trace elements and rare earth element composition analysis and determination, and the theoretical tectonic background and the parent rock lithology of the sedimentary basin are distinguished.

[0015] As a preferred scheme of the present application, the flow direction of the theoretical stratum layered unit is determined through the inclination and dip angle data of the rock stratum of the sampling sample after identification, determination and correction, and the vertical structure and distribution edge of the theoretical stratum layered unit are constrained by the main paleoactive rock stratum fracture and magmatic activity of the sedimentary basin.

[0016] As a preferred scheme of the present application, the sediment material abundance of the theoretical stratum layered unit on the vertical structure is converged through the analysis of the secondary components in the rock debris in the selected sampling sample, and then the vertical structure constrained theoretical stratum layered unit is superimposed, the abnormal data part of the sediment material abundance is removed, and the result of the tectonic background of the sedimentary basin is constrained.

[0017] As a preferred scheme of the present application, the specific method for converging the sediment material abundance of the theoretical stratum layered unit on the vertical structure through the analysis of the secondary components in the rock debris in the selected sampling sample includes:

[0018] By U-Pb dating of the zircon component of the sampling sample, a difference distribution diagram between the zircon crystallization age CA and the sedimentary age DA of the sediment is drawn, i.e. a cumulative probability distribution function diagram, and the cumulative probability distribution function diagram is used to identify, interpret and converge the sediment material abundance of the theoretical stratigraphic layer unit in the vertical structure.

[0019] As a preferred scheme of the present application, synthetic seismic record production is performed, and the structural deformation of the underlying unconformity, the planar distribution of the unconformity structure type and the sedimentary structure of the overlying unconformity are analyzed on the profile of the theoretical stratigraphic layer unit in the vertical structure of the sedimentary basin.

[0020] The main unconformity structure type and deformation characteristics in the deep buried area are further identified and analyzed qualitatively and semi-quantitatively by using the layer flattening method, and at the same time, the heavy mineral assemblage maturity index is obtained by coupling the analysis and calculation of the proportion of the heavy mineral component of the sampling sample, and the heavy mineral assemblage maturity index is obtained by coupling the analysis and calculation of the proportion of the heavy mineral component of the sampling sample.

[0021] As a preferred scheme of the present application, in step three, on the basis of the paleostructure-paleogeomorphology map of different periods, the seismic attribute and wave impedance profile of the theoretical stratigraphic layer unit are combined according to the sedimentary facies model to form a sedimentary facies belt space controlled by paleostructure.

[0022] The paleostructure-controlled sedimentary facies belt space distribution is further divided by using the analysis of the trace elements of the sampling sample to determine the paleoenvironment reconstruction index of the sedimentary basin, combining the sediment characteristics and sedimentary facies division type of the sedimentary basin, and calculating the sediment dispersion law indicated by the internal strata of the sedimentary basin, and then restoring the paleogeographic pattern reconstruction of the sedimentary basin.

[0023] As a preferred scheme of the present application, the paleoenvironment reconstruction index of the sedimentary basin determined by the analysis of the trace elements of the sampling sample includes but is not limited to paleosalinity, paleowater depth and weathering alteration characteristics.

[0024] The weathering alteration characteristics include chemical alteration index and rock component variation index.

[0025] As a preferred scheme of the present application, the sediment characteristics include but are not limited to sedimentary structure, lithologic combination, detrital component and biological fossil combination.

[0026] As a preferred scheme of the present application, the sedimentary facies model specifically includes dividing the sediment into different sedimentary facies according to the physical properties, particle composition and structural characteristics of the sediment.

[0027] Compared with the prior art, the present application has the following beneficial effects:

[0028] The technical scheme of the present application is designed for the "source, channel and sink" of the sedimentary basin. The "source" mainly includes judging the tectonic background of the provenance area through the content proportion difference of trace elements of sedimentary rock samples and judging the type of source rock of the provenance area through the content proportion difference of major elements. The "channel" is calculated by analyzing and calculating the ZTR index of heavy minerals, so as to judge the size of the sediment transport distance. The "sink" firstly uses trace elements to judge the paleoenvironment reconstruction index, including paleosalinity, paleowater depth and weathering alteration characteristics, and secondly uses strata, structure, sediment and paleontological fossils and other data to restore the sedimentary environment of a certain period in the past, and further provides rich information for paleogeographic reconstruction.

[0029] The paleogeographic reconstruction is expanded from single sedimentary basin research to macro regional area from plate scale. The traditional sedimentary model is fixed by the sedimentary evolution characteristics, and the source-channel-sink multi-type coupling combination model establishment method provided by the present application is more universal. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present application or the technical schemes in the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only exemplary, and other drawings can be obtained by the provided drawings without creative labor for those skilled in the art.

[0031] Figure 1 The flow structure schematic diagram of the embodiment of the present application. DETAILED DESCRIPTION

[0032] The technical schemes in the embodiments of the present application will be clearly and completely described below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the present application.

[0033] As shown in Figure 1 The present application provides a sedimentary basin paleogeographic pattern reconstruction method based on geochemical indicators. Through this method, different combination styles of source-channel-sink spatial coupling modes can be effectively established according to the specific characteristics of key elements such as provenance area tectonic background, provenance type, material transport distance and sediment accumulation mode.

[0034] In order to achieve the above purpose and requirement, the technical scheme of the present application is designed for the "source, channel and sink" of the sedimentary basin.

[0035] Among them, "source" mainly includes judging the tectonic background of the provenance area by the difference of the content proportion of trace elements in the sedimentary rock sample, and judging the type of source rock in the provenance area by the difference of the content proportion of major elements;

[0036] "Channel", ZTR index is calculated by analyzing and calculating the content of heavy minerals, so as to judge the size of sediment transport distance;

[0037] "Confluence", first, the trace elements are used to judge the paleoenvironment reconstruction index, including paleosalinity, paleowater depth, weathering alteration characteristics, secondly, the strata, structure, sediment and paleontological fossils and other data are used to restore the sedimentary environment of a certain period in the past.

[0038] Further, the three parts are coupled to provide rich information for paleogeographic reconstruction.

[0039] Including specific steps:

[0040] Step one, the major, trace element and rare earth element composition of the sampling sample of the sedimentary basin are analyzed and determined, the tectonic background and parent rock lithology of the sedimentary basin are distinguished, and the results of the tectonic background of the sedimentary basin are constrained by analyzing the secondary components in the rock debris of the selected sampling sample;

[0041] Step two, the heavy mineral composition maturity index is obtained by analyzing and calculating the content proportion of the heavy mineral composition of the sampling sample, the size of the sediment transport distance of the sedimentary basin is judged, the heavy mineral is the main object of clastic provenance analysis, different parent rock types will produce specific heavy mineral combination, which is used to trace the provenance. Different heavy minerals have different stability, with the increase of transport distance, the content of stable heavy minerals in the heavy mineral combination gradually increases, and the content of unstable heavy minerals gradually decreases;

[0042] Step three, the trace elements of the sampling sample are analyzed to determine the paleoenvironment reconstruction index of the sedimentary basin, the sediment characteristics of the sedimentary basin are combined, and the sedimentary facies are divided to calculate the sediment dispersion rule indicated by the internal strata of the sedimentary basin;

[0043] Step four, the geochemical indexes obtained in steps one, two and three are combined by coupling method to reconstruct the paleogeographic pattern of the sedimentary basin.

[0044] The sample of the present application is collected from part of field sections and drilling cores in the Ordos Basin. In the sampling process, the sample is selected as much as possible to be weak weathering, alteration and diagenesis and representative. First, the rock sample is pretreated, washed with clean water, dried, then initially ground with a ceramic mortar to remove coarse clastic particles, and finally sieved with a standard sample splitter and sent for determination with an ICP-MS mass spectrometer. Under clean, relative humidity 35-50% and room temperature 18-25℃, the aqueous solution in the sample is introduced into the argon stream, dissociated and ionized in the central region of argon plasma, and in the vacuum system, the linear dynamic change of 9 orders of magnitude from ppt to 1000ppm is obtained by high-speed scanning ions.

[0045] The geochemical composition of the sediment is mainly determined by the mixing ratio of the main source area or the composition of different source areas. The rare earth elements in the sedimentary rock and the inert trace elements such as La, Th, Y, Zr, Co and Ni have non-migration, which is mainly controlled by the rock composition of the source area, and can represent the rare earth characteristics of the source rock of the source area.

[0046] Therefore, the rare earth element characteristics have great influence on the research and discrimination of the source and the tectonic environment, and the analysis results of the rock sample can be judged by using La-Th-Sc, Th-Co-Zr / 10 and Th-Sc-Zr / 10 discrimination diagrams, which can be used to judge the environment of ocean island arc, continental island arc, active continental margin and passive continental margin, and provide information of the composition of the source rock.

[0047] The inert elements La, Th, Hf and the like do not change due to transportation and diagenesis, so the La / Th-Hf discrimination diagram can be used to analyze the material source of the sediment.

[0048] In step one, the stratigraphic sequence framework source of the sampling sample at the location is determined through drilling data and sampling sample characteristic analysis, and the spatial distribution of unconformity structure under a single tectonic movement is superimposed to obtain a theoretical stratigraphic layer unit under vertical structure of the sedimentary basin;

[0049] The abundance of sediment material of the theoretical stratigraphic layer unit on the vertical structure is determined through the determination based on the major, trace element and rare earth element composition analysis, and the theoretical tectonic background and mother rock lithology of the sedimentary basin are discriminated.

[0050] The flow direction of the theoretical stratigraphic layer unit is determined through the inclination and dip angle data of the rock layer of the sampling sample after identification, determination and correction, and the vertical structure and distribution edge of the theoretical stratigraphic layer unit are constrained by the main paleoactive rock fracture and magmatic activity of the sedimentary basin.

[0051] Theoretical stratigraphic layering units are identified in vertical tectonic settings by analyzing the secondary components of the detritus in the sampling samples, and the sediment material abundance of the theoretical stratigraphic layering units in the vertical tectonic settings is converged, and then the theoretical stratigraphic layering units are superimposed based on the vertical tectonic constraints, and the abnormal data of the sediment material abundance is removed, and the results of the tectonic background of the identified sedimentary basin are constrained.

[0052] The specific method for converging the sediment material abundance of the theoretical stratigraphic layering units in the vertical tectonic settings by analyzing the secondary components of the detritus in the sampling samples includes:

[0053] The difference distribution diagram between the zircon crystallization age (CA) and the sedimentary age (DA) of the sediment is plotted by U-Pb dating of the zircon component of the sampling sample, and the cumulative probability distribution function diagram is obtained, and the sediment material abundance of the theoretical stratigraphic layering units in the vertical tectonic settings is identified, explained and converged by using the cumulative probability distribution function diagram.

[0054] Detrital zircon is a secondary component of clastic sedimentary rock, but its physical and chemical elasticity and high concentration of some key trace elements have become an important stage in the analysis of sediment sources and the study of crustal evolution.

[0055] Research shows that the spectrum of detrital zircon reflects the tectonic setting of the sedimentary basin.

[0056] The characteristics of the convergent plate margin are that the zircon age is mostly similar to the sedimentary age of the sediment, while the proportion of the sediment in the collision, extension and intracratonic environment is larger, and the age is larger, reflecting the history of the underlying basement.

[0057] By plotting the difference distribution diagram between the zircon crystallization age (CA) and the sedimentary age (DA) of the sediment, the differences can be solved.

[0058] The specific principle is as follows: when the CA-DA (detrital zircon crystallization age-stratigraphic sedimentary age) value is greater than 150 Ma at 5% cumulative proportion, it represents a stretching extension environment; otherwise, further observation of the CA-DA value at 30% cumulative proportion is needed, when CA-DA is greater than 100 Ma, it represents a collision and compression background, and CA-DA is less than 100 Ma, it represents a convergent orogenic background.

[0059] Synthetic seismic record production is performed, and the structural deformation of the underlying strata, the planar distribution of the unconformity structure type, and the sedimentary structure of the overlying strata of the unconformity are analyzed on the profile of the theoretical stratigraphic layering units in the vertical tectonic settings of the sedimentary basin.

[0060] The main unconformity structure types and deformation characteristics of the deep burial area are further identified and analyzed qualitatively and semi-quantitatively by using the layer flattening method, and at the same time, the heavy mineral assemblage maturity index is obtained by coupling the analysis and calculation of the proportion of heavy mineral composition of the sampling sample, and the points are projected on the plane graph of the theoretical stratigraphic layer unit to obtain the paleostructure-paleogeomorphology map of different periods.

[0061] In step three, on the basis of the paleostructure-paleogeomorphology map of different periods, the seismic attribute of the theoretical stratigraphic layer unit is combined with the wave impedance profile according to the sedimentary facies model to form a sedimentary facies belt spatial distribution controlled by paleostructure;

[0062] The paleoenvironment reconstruction index of the sedimentary basin is determined by analyzing the trace elements of the sampling sample, and the sedimentary facies belt spatial distribution controlled by paleostructure is further divided by combining the sediment characteristics of the sedimentary basin and the sedimentary facies division type, the sediment dispersion law indicated by the stratum inside the sedimentary basin is calculated, and then the paleogeographic pattern reconstruction of the sedimentary basin is restored.

[0063] Determining the paleoenvironment reconstruction index of the sedimentary basin by analyzing the trace elements of the sampling sample includes but is not limited to paleosalinity, paleowater depth, and weathering alteration characteristics.

[0064] Paleosalinity: records the change of the salinity of the water body in the original sediment,

[0065] During the weathering process, Th element is easily adsorbed and retained by clay minerals, and U element is easily lost with the weathering process, and Th / U value can be used as an index to distinguish marine and terrestrial sediments: Th / U content ratio less than 2 is marine environment, 2-7 is transitional semi-saline water environment, and greater than 7 is terrestrial freshwater environment.

[0066] Terrigenous clastic sediments are transported to the ocean by rivers, Sr element is not easy to precipitate in the transportation process, Ba element is easy to combine with sulfate ions in seawater and precipitate, and the Ba content of terrestrial and transitional sedimentary deposits is higher, while the Ba content in the ocean is less.

[0067] Sr / Ba>1 indicates marine salt water, 1-0.6 indicates transitional phase of fresh water-salt water, 0.6-0.3 indicates transitional phase of salt water-fresh water, and Sr / Ba<0.3 indicates terrestrial freshwater.

[0068] Paleo-water depth: Zr element is stable in chemical properties during weathering process, and is easy to deposit in relatively shallow water, and Rb element is active in chemical properties, and is common in clay minerals, and is easy to enrich in relatively deep water and low-energy environment, and Rb / Zr ratio can be used to judge the depth of seawater: the higher the Rb / Zr value, the deeper the water, and the weaker the hydrodynamic condition; the lower the value, the shallower the water, and the stronger the hydrodynamic condition; weathering alteration characteristics: Nesbitt et al. proposed chemical alteration index (CIA = [Al2O3 / (Al2O3+CaO+Na2O+K2O)]*100%) and Cox et al. defined rock composition variation index (ICV = [(Fe2O3+K2O+Na2O+CaO+MgO+MnO+TiO2) / Al2O3), which are the most commonly used indicators to judge the influence of weathering conditions on samples, and can infer the stability of tectonic and climate in the source area.

[0069] The sediment characteristics include but are not limited to sedimentary structure, lithological combination, lithic component and biological fossil combination.

[0070] The sedimentary facies mode includes that the sediments are divided into different sedimentary facies according to the physical properties, particle composition and structural characteristics of the sediments.

[0071] In summary, the present application is based on the basic elements required by source-sink process research, including source (tectonic background, parent rock lithology), channel (transport distance), sink (sedimentary system combination, paleoenvironment index) characteristics, so as to reconstruct the paleogeographic pattern. The method is based on the test data of major and trace rare earth elements, heavy mineral analysis and detrital zircon analysis, combined with solid theoretical basis of sedimentology, and can provide effective sedimentary mode guidance in the reconstruction of paleogeographic pattern of oil and gas bearing sedimentary basin.

[0072] The above examples are only exemplary embodiments of the present application, and are not used to limit the present application, and the protection scope of the present application is defined by the claims. Those skilled in the art can make various modifications or equivalent replacements to the present application within the spirit and protection scope of the present application, and such modifications or equivalent replacements are also regarded as falling within the protection scope of the present application.

Claims

1. A method for reconstructing a paleogeographic pattern of a sedimentary basin based on geochemical indicators, characterized by, Comprise specific steps: Step one, by drilling data and sampling sample characteristics analysis, determine the sampling sample location at the stratigraphic sequence framework source, superimposed on a single tectonic movement unconformable structure of spatial distribution, obtain the theoretical stratigraphic layering unit under the vertical structure of sedimentary basin; The main, trace element and rare earth element composition analysis of the sampling sample of the sedimentary basin is determined, the theoretical stratigraphic layering unit is determined on the vertical structure of the sedimentary material abundance, the theoretical tectonic background and the parent rock lithology of the sedimentary basin are distinguished; The flow direction of the theoretical stratigraphic layering unit is determined by identifying, determining and correcting the rock layer inclination and inclination data of the sampling sample, and the vertical structure and distribution edge of the theoretical stratigraphic layering unit are constrained by the main paleoactive rock fracture and magmatic activity of the sedimentary basin; The secondary components of the rock debris in the sampling sample are analyzed, specifically the U-Pb dating of the zircon composition of the sampling sample, the difference distribution diagram between the zircon crystallization age CA and the sedimentary age DA in the sediment is drawn, that is, the cumulative probability distribution function diagram, the cumulative probability distribution function diagram is used to identify, explain and converge the sedimentary material abundance of the theoretical stratigraphic layering unit on the vertical structure, and then the theoretical stratigraphic layering unit is superimposed based on the vertical structure constraint, the abnormal data part of the sedimentary material abundance is removed, and the determined sedimentary basin tectonic background result is constrained; Step two, the proportion of heavy mineral composition of the sampling sample is analyzed and calculated, and the heavy mineral combination maturity index is obtained; Synthetic seismic record is made, and the tectonic deformation of the underlying formation of unconformity, the plane distribution of unconformable structure type and the sedimentary structure of the overlying formation of unconformity are analyzed on the theoretical stratigraphic layering unit profile under the vertical structure of the sedimentary basin; The layer flattening method is used to qualitatively and semi-quantitatively identify and analyze the main unconformable structure type and deformation characteristics in the deep buried area, and the heavy mineral combination maturity index is coupled to obtain the paleostructure-paleogeomorphology map of different periods on the plane map of the theoretical stratigraphic layering unit, and then the sediment transport distance of the sedimentary basin is judged; Step three, on the basis of the paleostructure-paleogeomorphology map of different periods, according to the sedimentary facies model, the seismic attribute and wave impedance profile of the theoretical stratigraphic layering unit are combined to form the sedimentary facies belt space controlled by paleostructure; The paleoenvironment reconstruction index of the sedimentary basin is determined by analyzing the trace elements of the sampling sample, and the paleoenvironment reconstruction index includes paleosalinity, paleowater depth and weathering alteration characteristics, wherein the weathering alteration characteristics include chemical alteration index and rock composition variation index; Combined with the sediment characteristics and sedimentary facies division of the sedimentary basin, the sediment characteristics include sedimentary structure, lithological combination, debris composition and biological fossil combination, the paleostructure controlled sedimentary facies belt space distribution is further divided, and the sediment dispersion rule indicated by the internal stratum of the sedimentary basin is calculated; Step four, the geochemical indexes obtained in step one, step two and step three are combined by coupling, and the paleogeographic pattern of the sedimentary basin is reconstructed.

2. The method according to claim 1, characterized in that, The sedimentary facies mode includes dividing the sediments into different sedimentary facies according to the physical properties, particle composition and structural features of the sediments.

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