A method for identifying shale paleoenvironment based on elemental geochemical analysis

Abstract

The present invention belongs to the field of oil and gas development technology and discloses a method for identifying shale paleoenvironments based on elemental geochemical analysis: the method comprises the following steps: preparing the necessary elemental geochemical data and calculating the required ratios and indices; restoring the paleohumidity and paleotemperature of the target layer based on the elemental geochemical data; restoring paleoclimate characteristics based on the restored paleohumidity and paleotemperature of a single well, and drawing a vertical paleoclimate evolution characteristic map; analyzing paleosalinity changes through a combination of qualitative and quantitative paleosalinity analysis; performing redox restoration based on redox conditions and hypoxia-oxygen enrichment properties; and drawing a vertical paleowater environment evolution characteristic map based on the restored paleosalinity and redox properties of a single well. The method of the present invention improves the precision and accuracy of paleoenvironmental identification.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas development, and in particular to a method for identifying shale paleoenvironment based on elemental geochemical analysis. Background Art

[0002] As the source rock for shale oil and gas with enormous resource potential, the formation mechanism of organic-rich shales is a key scientific issue in shale oil and gas accumulation. The formation of organic-rich fine-grained sediments typically requires abundant organic matter, favorable organic matter preservation conditions, and subsequent mineral preservation conditions. These conditions are ultimately the result of the combined effects of geological factors such as paleoclimate, paleo-tectonic, and paleo-sedimentary environments.

[0003] The distribution, allocation, and migration of trace elements in strata are closely related to the formation environment of sedimentary rocks. Since the properties of some elements are stable and their occurrence is less affected by subsequent diagenesis, the content of major and trace element components and the ratios of certain elements have been widely used to identify paleoenvironment.

[0004] Commonly used paleoclimate analysis methods, such as the CIA index and the Sr / Cu ratio, can effectively indicate cold, arid, and hot, humid climates. Sr / Ba ratios and other ratios are often used to determine paleosalinity. V / (V+Ni) ratios and other ratios are often used as important indicators of the redox dynamics of sedimentary environments.

[0005] However, the various element identification methods are relatively independent and have not been well combined to restore the ancient environment when shale was formed. Summary of the Invention

[0006] In view of the current status of the paleoenvironment when shale was formed through elemental geochemical analysis, the present invention provides a method for identifying the paleoenvironment of shale based on elemental geochemical analysis. It can comprehensively utilize various element identification methods to identify the paleoenvironment of shale formation, and thus can more accurately restore the paleoenvironment of shale, effectively solving the problems existing in the background technology.

[0007] The present invention provides the following technical solution: a method for identifying shale depositional environments based on elemental geochemical analysis, comprising the following steps:

[0008] Step 1: Prepare the necessary elemental geochemical data and complete the required ratio and index calculations;

[0009] Step 2: Restore the paleohumidity and paleotemperature of the target layer based on elemental geochemical data;

[0010] Step 3: Based on the restoration of paleohumidity and paleotemperature of a single well, restore the paleoclimate characteristics and draw a paleoclimate vertical evolution characteristic map;

[0011] Step 4: Analyze the changes in paleosalinity by combining qualitative and quantitative analysis of paleosalinity;

[0012] Step 5: Redox recovery from two aspects: redox conditions and oxygen-deficient-oxygen-rich properties;

[0013] Step 6: Based on the restoration of paleo-salinity and redox properties of a single well, draw a vertical evolution characteristic map of the paleo-water environment.

[0014] Furthermore, the paleohumidity change curve was estimated based on the change curves of Th / U, Sr / Cu, Fe / Mn, Al / Ti and CIA index.

[0015] Furthermore, the paleotemperature change curve was estimated based on the Mg / Sr change curve.

[0016] Furthermore, by combining paleohumidity and paleotemperature, the paleoclimate characteristics of the target layer are inverted and a paleoclimate vertical evolution characteristic map is drawn.

[0017] Furthermore, the relative level of paleosalinity was determined based on the Sr / Ba and Rb / Sr ratios, and the corresponding change curve was established.

[0018] Furthermore, the paleosalinity was quantitatively calculated based on the boron element method and verified with the Sr / Ba and Rb / Sr change curves.

[0019] Furthermore, based on the quantitative calculation results of paleo-salinity and referring to the modern lake salinity classification standards, paleo-salinity is classified into different levels.

[0020] Furthermore, the redox conditions of the deposition environment were determined based on V / (V+Ni) and Th / U, and the oxygen-deficient and oxygen-rich properties of the deposition environment were analyzed based on V / Cr.

[0021] Furthermore, the redox properties of the sedimentary environment were determined based on the redox conditions and the anoxic-oxic nature of the paleoenvironment.

[0022] Furthermore, based on the restoration of paleo-salinity and redox properties of single wells, a vertical evolution characteristic map of the paleo-water environment was drawn.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] This method for identifying shale paleoenvironments based on elemental geochemical analysis breaks paleoenvironmental identification into two broad areas: paleoclimate and paleowater environment, and four sub-areas: paleohumidity, paleotemperature, paleosalinity, and redox properties. Multiple identification methods are applied to each sub-area, corroborating and constraining each other to improve the precision and accuracy of paleoenvironmental identification. By separating paleoclimate into paleohumidity and paleotemperature and discussing them separately, combining paleohumidity and paleotemperature to identify paleoclimate characteristics achieves higher accuracy than simply using ratios. It can also subdivide warm-humid conditions into warm-humid and warm-semi-humid conditions within the original warm-humid range. In paleosalinity, a combination of qualitative and quantitative analysis significantly improves analytical accuracy. In redox properties, analysis of both redox conditions and anoxic-enriched oxygen properties allows for a more precise subdivision of the original reducing conditions into strongly anoxic-reducing and anoxic-reducing conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Attachment Figure 1 This is an operation flow chart of the present invention;

[0026] Attachment Figure 2 Vertical evolution characteristics of paleoclimate in a single well Figure 1 ;

[0027] Attachment Figure 3 Vertical evolution characteristics of paleoclimate in a single well Figure 2 ;

[0028] Attachment Figure 4 Vertical evolution characteristics of single-well paleowater environment Figure 1 ;

[0029] Attachment Figure 5 Vertical evolution characteristics of single-well paleowater environment Figure 2 . DETAILED DESCRIPTION

[0030] The following will be combined with the appended Figure 1-5 The technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0031] A method for identifying shale paleoenvironment based on elemental geochemical analysis includes preparing necessary elemental geochemical data; calculating various parameters required for analysis based on the corresponding elemental data; estimating paleohumidity curves based on Th / U, Sr / Cu, Fe / Mn, Al / Ti, and CIA index curves; estimating paleotemperature curves based on Mg / Sr curves; analyzing paleoclimate change characteristics by combining paleohumidity and paleotemperature curves; qualitatively analyzing the relative levels of paleosalinity based on Sr / Ba and Rb / Sr ratios, and establishing corresponding curves; quantitatively calculating paleosalinity using the boron element method, verifying it with the Sr / Ba and Rb / Sr curves, and determining the paleosalinity level of the target layer; determining the redox conditions of the paleoenvironment based on V / (V+Ni) and Th / U, and analyzing the anoxic and oxic nature of the paleoenvironment based on V / Cr; determining the redox properties of the paleoenvironment based on the determination of the redox conditions and anoxic-oxic nature of the paleoenvironment; and mapping the vertical evolution characteristics of the paleowater environment based on the restoration of paleosalinity and redox properties in individual wells.

[0032] Among them, comprehensive consideration of multiple element ratio methods to restore paleohumidity and estimate the paleohumidity change curve greatly improved the reliability of paleohumidity analysis and served as an important basis for paleoclimate restoration.

[0033] Among them, the Mg / Sr change curve is used as the main basis for ancient temperature changes.

[0034] Among them, the paleoclimate is divided into two aspects, paleohumidity and paleotemperature, and analyzed separately. Finally, the paleoclimate characteristics are analyzed in combination with each other, which helps to improve the accuracy of paleoclimate restoration and can more intuitively see the changing trend of the paleoclimate.

[0035] Among them, the paleo-salinity changes were qualitatively analyzed based on Sr / Ba and Rb / Sr, and the paleo-salinity was quantitatively analyzed using the boron element method. By combining qualitative and quantitative analysis, the accuracy of paleo-salinity was improved.

[0036] Among them, the redox conditions of the paleoenvironment are determined based on V / (V+Ni) and Th / U, the anoxic and oxygen-rich properties of the paleoenvironment are analyzed based on V / Cr, and then the redox properties of the shale paleoenvironment are comprehensively considered. It is possible to subdivide the original level, thereby improving the accuracy of the redox analysis of the shale paleoenvironment.

[0037] Among them, combining the results of paleo-salinity and redox restoration to draw a vertical evolution map of paleo-water salinity helps to intuitively reflect the relationship between paleo-salinity and redox, and thus reflect the changing trend of shale depositional environment.

[0038] Taking the first member of the Qingshankou Formation in the Changling Sag in the southern Songliao Basin as an example, this paper established vertical evolution maps of the paleoclimate and paleo-water sedimentary environment of two wells, reflecting the changing trends of the paleoclimate and paleo-water sedimentary environment of the first member of the Qingshankou Formation in the Changling Sag in the southern Songliao Basin.

Claims

1. A method for identifying shale depositional environments based on elemental geochemical analysis, characterized in that: The following steps are involved: Step 1: Prepare elemental geochemical data and complete the required ratio and index calculations; Step 2: Restore the paleohumidity and paleotemperature of the target interval based on elemental geochemical data. The paleohumidity change curve is estimated based on the Th / U, Sr / Cu, Fe / Mn, Al / Ti, and CIA index change curves, and the paleotemperature change curve is estimated based on the Mg / Sr change curve. Step 3: Based on the restoration of paleohumidity and paleotemperature of a single well, restore the paleoclimate characteristics and draw a paleoclimate vertical evolution characteristic map; Step 4: Analyze the changes in paleosalinity by combining qualitative and quantitative analysis of paleosalinity. The relative high and low of paleosalinity are determined based on the Sr / Ba and Rb / Sr ratios, and the corresponding change curve is established. Step 5: Redox recovery is performed from two aspects: redox conditions and oxygen-deficient-oxygen-rich properties. The oxygen-deficient-oxygen-rich properties determine the redox conditions of the deposition environment based on V / (V+Ni) and Th / U, and the deposition environment is analyzed based on V / Cr. Step 6: Based on the restoration of paleo-salinity and redox properties of a single well, draw a vertical evolution characteristic map of the paleo-water environment.

2. The method for identifying shale depositional environment based on elemental geochemical analysis according to claim 1, characterized in that: Combine the paleohumidity and paleotemperature described in step 3 to invert the paleoclimate characteristics of the target layer and draw a paleoclimate vertical evolution characteristic map.

3. The method for identifying shale depositional environment based on elemental geochemical analysis according to claim 1, characterized in that: The paleosalinity described in step 4 is quantitatively calculated using the boron element method and verified with the Sr / Ba and Rb / Sr change curves.

4. The method for identifying shale depositional environment based on elemental geochemical analysis according to claim 1, characterized in that: The classification of paleo-salinity described in step 4 is based on the quantitative calculation results of paleo-salinity and refers to the modern lake salinity classification standards.

5. The method for identifying shale depositional environment based on elemental geochemical analysis according to claim 1, characterized in that: Step 5: Based on the redox conditions and anoxic-oxic properties of the paleoenvironment, the redox properties of the sedimentary environment are determined.

6. The method for identifying shale depositional environment based on elemental geochemical analysis according to claim 1, characterized in that: The vertical evolution characteristic map of the ancient water environment described in step 6 is drawn based on the restoration of the ancient salinity and redox properties of a single well.

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