A method for dividing the sequence of fine-grained sedimentary strata based on the frequency reduction of conventional well logging curves

Abstract

The present invention discloses a method for fine-grained sedimentary strata sequence division based on the frequency reduction of conventional logging curves, comprising the following steps: determining the need for high-precision sequence division; performing frequency reduction processing on each original logging data to obtain the corresponding frequency reduction change broken line; performing comprehensive comparative analysis on the obtained frequency reduction change broken lines to complete the fine-grained sedimentary strata sequence division. The present invention solves the difficult problem of high-precision sequence division of fine-grained sedimentary strata, provides an effective technical method for the study of the distribution, development and evolution of fine-grained sedimentary strata, and the law of oil and gas enrichment, and provides effective technical support for the exploration and deployment of unconventional oil and gas in old oil fields. The method of the present invention is highly operational and easy to master, and is also applicable to various types of sedimentary strata units, and has promotion value.

Description

Technical Field

[0001] The invention relates to the field of unconventional oil and gas exploration, and in particular to a method for dividing fine-grained sedimentary strata sequences based on frequency reduction of conventional logging curves. Background Art

[0002] Fine-grained sedimentary research and oil and gas exploration have been academic hotspots in recent years. Among them, high-precision sequence stratigraphic division of fine-grained sedimentary strata is both a difficult point and a research focus of unconventional oil and gas exploration in the future. Limited by the data and characteristics of fine-grained sedimentary strata, the traditional sequence division method cannot solve this problem: ① Due to the "quasi-homogeneity" characteristics of fine-grained sedimentary strata, the vertical changes of rock-electric combination and cyclic characteristics are not obvious, and it is difficult to divide them based on human experience; ② Due to the limitations of few drilling cores and limited analytical and testing data, it is unrealistic to divide high-precision sequences through systematic mineral composition, elemental geochemistry, paleontology and other data; ③ Due to the limitations of seismic resolution, the sequence stratigraphy theory of three-dimensional seismic is difficult to apply in fine-grained sedimentary strata, especially lacustrine fine-grained sedimentary strata. ④ The use of new logging methods and systematic analytical testing in new drillings is costly on the one hand, and difficult to compare horizontally with old wells on the other hand. ⑤ Using mathematical algorithms to process the energy signals of logging data and divide sequence strata by regular changes in the vertical direction is a harsh constraint on the one hand, and difficult for ordinary geologists to master on the other hand. Summary of the invention

[0003] The purpose of the present invention is to provide a method for dividing the stratigraphic sequence of fine-grained sedimentary strata based on the frequency reduction of conventional logging curves, so as to solve the problem that high-precision stratigraphic sequences of fine-grained sedimentary strata are difficult to effectively divide and identify in old well areas with little coring data and low-quality seismic data.

[0004] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0005] A method for dividing fine-grained sedimentary strata based on conventional well logging frequency reduction includes the following steps:

[0006] Determine the need for high-precision sequence division;

[0007] Perform frequency reduction processing on each original logging data to obtain the corresponding frequency reduction change curve;

[0008] The 3-4 frequency reduction change broken lines obtained will be subjected to comprehensive comparative analysis to complete the division of fine-grained sedimentary stratigraphic sequence.

[0009] The present invention is based on the three-level sequence framework and the electrical response principle of fine-grained sedimentary strata to down-frequency process conventional logging curves, so as to quickly, conveniently, relatively objectively and accurately identify sequence interfaces of different levels (3-6 levels), which is convenient for ordinary geologists to analyze and compare, realize high-precision sequence stratigraphic division and comparison of fine-grained sedimentary strata, and lay the foundation for later geological research and exploration deployment.

[0010] According to the method for dividing a fine-grained sedimentary stratum sequence of the present invention, preferably, the step of determining the need for high-precision sequence division includes:

[0011] Query the existing sequence stratigraphic framework in the study area, clarify the basic characteristics and identification marks of sequence interfaces at all levels, and determine the sequence level of the stratigraphic unit where the fine-grained sedimentary strata are located;

[0012] Determine the need for high-precision sequence stratigraphic division;

[0013] Query the layered data table to determine the top and bottom depths of the stratigraphic units that require high-precision sequence stratigraphic division, and determine the top and bottom boundaries of the logging data body to be processed, so as to process the logging data of the target layer in a targeted manner in the next step.

[0014] The sequence stratigraphic framework of the study area is established through drilling, logging and seismic data, and the sequence level of the stratigraphic unit where the fine-grained sedimentary strata are located is determined. The group and segment stratigraphic units in the stratigraphic system generally represent the third-level sequence. The rock electrical characteristics and seismic reflection characteristics of their interfaces are relatively obvious and easy to identify, compare and track.

[0015] According to the method for dividing the fine-grained sedimentary strata sequence of the present invention, preferably, the determination of the high-precision sequence strata division requirements is carried out according to production requirements and research status. The accuracy reaches the oil layer group or sand layer group, corresponding to the 4th, 5th or higher level sequence.

[0016] According to the method for dividing the sequence of fine-grained sedimentary strata of the present invention, preferably, the step of performing frequency reduction processing on each original logging data to obtain the corresponding frequency reduction change polyline comprises: vertically segmenting the original logging curve defined by the top and bottom boundaries of the target layer according to the extreme value change, that is, dividing it into a plurality of minimum value-maximum value and maximum value-minimum value curve segments from top to bottom;

[0017] The mathematical derivatives of the separated well logging curve segments are taken to obtain the inflection point data of any segment of the curve, which reflects the change of sedimentary environment, i.e. the minimum level sequence interface;

[0018] Record the data of each inflection point to form a new curve, namely the first frequency reduction curve, and complete the first frequency reduction process;

[0019] The above frequency reduction processing is repeated for the first frequency reduction curve until there are only 3 data points left in the final frequency reduction curve, reaching the original stratigraphic level of the target layer. The frequency reduction change line that can achieve the high-precision stratigraphic division goal is the frequency reduction change line of the final logging curve.

[0020] The first frequency reduction processing is performed on the logging data to be processed, that is, the inflection point reflecting the change of the sedimentary environment or the extreme point representing the extreme sedimentary environment in the logging data is obtained to obtain the logging data reflecting the minimum level sequence interface. The logging data of the minimum level sequence interface is frequency-reduced again to obtain the logging data of the higher level sequence interface. And so on, the frequency reduction is performed multiple times until the logging data reflects a level 3 sequence, and the frequency reduction processing is completed. After the original conventional logging data is frequency-reduced, a new set of logging data is obtained, so that the logging curve can be simplified into a frequency reduction change line.

[0021] Generally, a third-level sequence (group or segment stratigraphic unit in the stratigraphic system) needs to be down-sampled 4-5 times according to the stratigraphic thickness. If the fifth down-sampled data reflects the third-level sequence (original sequence level), the fourth down-sampled data reflects the fourth-level sequence (parasequence group), and the third down-sampled data reflects the fifth-level sequence (sand layer group). The second and first down-sampled data are basically consistent with the original curve data, reflecting a higher-precision sequence level. Then the third down-sampled curve represents the final down-sampled change line of the curve, which can meet the current accuracy requirements of geological research.

[0022] According to the method for dividing the sequence of fine-grained sedimentary strata of the present invention, preferably, the original logging curve includes logging data such as natural gamma, natural potential, resistivity, etc.

[0023] All types of logging curves have the possibility of frequency reduction, and the specific choice depends on the number of logging curves actually obtained. Under normal circumstances, the logging data includes natural gamma, natural potential and resistivity logging curves. Frequency reduction uses 3-4 curves, which can be verified with each other.

[0024] According to the method for dividing the fine-grained sedimentary strata sequence of the present invention, preferably, the step of performing comprehensive comparative analysis on the obtained frequency reduction change broken lines to complete the fine-grained sedimentary strata sequence division comprises:

[0025] In the single well stratigraphic column chart, add the obtained frequency reduction change broken lines, and determine the top and bottom interfaces and lake flooding surfaces of each level of sequence according to the broken line changes, analysis and comparison, and complete the high-precision sequence division of the single well;

[0026] The above single-well high-precision sequence division was performed on all wells in the study area;

[0027] On the basis of high-precision sequence division of single wells, high-precision sequence stratigraphic division of the whole area is carried out. Under the control of stratigraphic framework profile, high-precision sequence stratigraphic plane distribution map is further drawn to analyze the distribution law of fine-grained sedimentary strata.

[0028] According to the method for dividing the fine-grained sedimentary strata sequence of the present invention, preferably, after completing the fine-grained sedimentary strata sequence division, the method further includes a step of verifying the obtained division results.

[0029] According to the method for dividing the fine-grained sedimentary stratigraphic sequence of the present invention, preferably, the step of verifying the obtained division result comprises:

[0030] Use traditional methods such as rock electrical changes, sedimentary cycles or comparative marker layers to divide the adjacent wells where fine-grained sediments are relatively underdeveloped or the larger layer section containing fine-grained sediments in the well, and compare and verify the division results with the sequence division results of the fine-grained sedimentary strata to be verified. If the sequence scheme divided by traditional methods such as rock electrical cycles is highly consistent with the sequence scheme obtained in this application, including the large sequence interface and lake flooding surface are basically consistent; the interface characteristics and marker layers are consistent with the interface characteristics and comparative marker layers of the existing sequence framework; then the feasibility of the method of the present invention is confirmed. The verification of this step is equivalent to direct method verification.

[0031] According to the method for dividing the fine-grained sedimentary stratigraphic sequence of the present invention, preferably, the step of verifying the obtained division result further comprises:

[0032] The following other research data are used to support the verification: within the same sequence, the changes in the sedimentary facies of adjacent wells should satisfy Walter's facies law, the distribution of provenance and sand bodies should be continuous, and the seismic facies and seismic reflection characteristics should be consistent. The verification of this step is equivalent to the later theoretical verification.

[0033] According to the method for dividing the fine-grained sedimentary stratigraphic sequence of the present invention, preferably, the step of verifying the obtained division result further comprises:

[0034] Sequence stratigraphic division is verified through reservoir physical properties, oil and gas content, oil, gas and water production capacity, etc. obtained during actual production. For the same sand layer group, reservoir rock and mineral characteristics should be equivalent, oil, gas and water interfaces should be consistent, etc. The verification of this step is equivalent to later practical verification.

[0035] The beneficial effects of the present invention include:

[0036] The method for dividing the sequence of fine-grained sedimentary strata of the present invention can be used to quickly and accurately divide the sequence strata in the case of few new wells in old oil fields, low-quality seismic data, unclear rock-electric changes in fine-grained sedimentary strata, limited analytical data, and difficult prediction of sweet spot reservoirs. The method solves the problem of high-precision sequence division of fine-grained sedimentary strata, provides an effective technical method for the study of the distribution, development and evolution of fine-grained sedimentary strata, and the laws of oil and gas enrichment, and provides effective technical support for the exploration and deployment of unconventional oil and gas in old oil fields. The method of the present invention is highly operational and easy to master, and is also applicable to various types of sedimentary stratum units, and has promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the frequency reduction processing of the SP curve of Shu 118 well in an embodiment of the present invention.

[0038] Figure 2 This is a high-precision sequence division map obtained by using the fine-grained sedimentary stratum sequence division method based on conventional well logging curve frequency reduction in the Shu 112 well in the embodiment of the present invention;

[0039] In the figure, the fourth member of the Shahejie Formation is a third-order sequence, the Dujiatai oil layer and the Gaosheng oil layer are fourth-order sequences, and DuⅠ, DuⅡ, DuⅢ, GaoⅠ, GaoⅡ and GaoⅢ are fifth-order sequences. DETAILED DESCRIPTION

[0040] In order to explain the present invention more clearly, the present invention is further described below in conjunction with preferred embodiments. It should be understood by those skilled in the art that the following specific description is illustrative rather than restrictive, and should not be used to limit the scope of protection of the present invention.

[0041] The embodiment of the present invention provides a method for dividing a fine-grained sedimentary stratum sequence based on a conventional well logging curve frequency reduction, comprising the following steps:

[0042] 1. Determine the need for high-precision sequence division

[0043] 1. Query the existing sequence stratigraphic framework in the study area, clarify the basic characteristics and identification marks of sequence interfaces at all levels, and determine the sequence level of the stratigraphic unit where the fine-grained sedimentary strata are located;

[0044] 2. Determine the need for high-precision sequence division. According to production needs and research status, determine the need for high-precision sequence division, whether the accuracy is oil layer group or sand layer group, corresponding to level 4, level 5 or higher level sequence;

[0045] 3. Determine the top and bottom boundaries of the logging data body to be processed, query the layered data table, and determine the top and bottom depths of the stratigraphic units that require high-precision sequence stratigraphic division, so as to process the logging data of the target layer in a targeted manner in the next step.

[0046] 2. Frequency reduction processing of logging curve

[0047] 1. The original logging curve defined by the top and bottom boundaries of the target layer is segmented vertically according to the extreme value changes, that is, it is divided into several minimum-maximum and maximum-minimum curve segments from top to bottom.

[0048] 2. Perform mathematical derivation on each logging curve segment separated in step 1 to obtain the inflection point data of any segment of the curve, which reflects the change of sedimentary environment, i.e., the minimum level sequence interface.

[0049] 3. Record the data of each inflection point to form a new curve, namely the first frequency reduction curve, and complete the first frequency reduction process.

[0050] 4. For the first frequency reduction curve, repeat steps 1 to 3 to complete the second frequency reduction and form the second frequency reduction curve.

[0051] 5. Perform step 4 again until there are only 3 frequency reduction curve data points left, reaching the original sequence level of the target layer, and then reverse the process step by step. The frequency reduction change line that can achieve the high-precision sequence division goal is the frequency reduction change line of the final well logging curve. Figure 1 As shown, it is the frequency reduction change curve obtained by frequency reduction processing of the SP curve of Well Shu 118.

[0052] Figure 1 In the figure, the spontaneous potential (SP) curve has been downgraded five times, which is consistent with the initial sequence level (the existing sequence framework is reflected in the stratigraphic units on the left side of the figure, among which the fourth member of the Shahejie Formation is a third-order sequence, and the Dujiatai oilfield and Gaosheng oil layer are fourth-order sequences). Then the fifth downgrade change line reflects the third-order sequence, the fourth downgrade change line reflects the fourth-order sequence, the third downgrade change line reflects the fifth-order sequence, and the second and first downgrade change lines reflect smaller-level, higher-precision sequences. Therefore, Figure 1 In the above figure, the sequence cycle reflected by the third frequency drop of the spontaneous potential (SP) curve has realized the high-precision sequence (five-level sequence) division of Shu-118 Well, that is, the two oil layers in the fourth member of the Shahejie Formation can be divided into five sand layers. Then the third frequency drop of the spontaneous potential (SP) curve is the final frequency drop.

[0053] 3. High-precision sequence division

[0054] 1. Perform frequency reduction processing on conventional natural gamma, natural potential, resistivity and other logging data to obtain the frequency reduction change curve.

[0055] 2. In the single well stratigraphic column chart, add the frequency reduction change lines obtained in step 1, determine the top and bottom interfaces and lake flooding surfaces of each level of sequence according to the cycle changes, analysis and comparison, and complete the high-precision sequence division of the single well.

[0056] For example Figure 2 As shown, this is a high-precision sequence division map obtained by using the above method of fine-grained sedimentary strata sequence division based on the frequency reduction of conventional logging curves in Shu 112 Well; Figure 2 The fourth section of Zhongsha is a third-order sequence, the Dujiatai oil layer and Gaosheng oil layer are fourth-order sequences, and DuⅠ, DuⅡ, DuⅢ, GaoⅠ, GaoⅡ and GaoⅢ are fifth-order sequences.

[0057] 3. For all wells in the study area, the above method is used to perform high-precision sequence division of single wells.

[0058] 4. Based on the single well sequence division, high-precision sequence stratigraphic division is carried out for the entire area. Under the control of the stratigraphic framework profile, a high-precision sequence stratigraphic plane distribution map can be further drawn to analyze the distribution law of fine-grained sedimentary strata.

[0059] 4. Verification of the division results

[0060] 1. The feasibility of this method is confirmed by verifying the sequence stratigraphic division of adjacent wells where fine-grained sediments are relatively undeveloped or larger layers containing fine-grained sediments in the well. If the sequence stratigraphic division is highly consistent with the traditional sequence stratigraphic division method based on rock electrical changes, sedimentary cycles or comparative marker layers, the feasibility of this method is confirmed.

[0061] like Figure 2 In the figure, the stratigraphic system on the left is the original sequence stratigraphic framework, in which the Shahejie Formation is a second-order sequence, the fourth segment of Shahejie Formation is a third-order sequence, and the Dujiatai oilfield and Gaosheng oil layer are fourth-order sequences; in the sequence scheme of the SP curve and RT curve frequency reduction change line comprehensive division on the right, the interface between the third-order sequence and the fourth-order sequence is consistent with the interface and lake flooding surface of the original sequence framework (traditional rock electrical cycle division method), which proves that this method is feasible. At the same time, the division of the five sandstone groups of Du I, Du II, Du III, Gao I, Gao II, and Gao III was achieved, that is, the goal of high-precision sequence (five-order sequence) division was achieved.

[0062] 2. Supported by other research data, the changes in sedimentary phases of adjacent wells in the same sequence should satisfy Walter's phase law, the distribution of provenance and sand bodies should be continuous, and the seismic phases and seismic reflection characteristics should be consistent.

[0063] This verification belongs to the subsequent indirect verification. After the sequence division is completed using this method, the sedimentary phase mapping of each sand layer group will be carried out. Therefore, Walter phase law, sand body continuity, and seismic phase consistency are all multi-solution, so high-precision sequence division is a necessary condition.

[0064] 3. The sequence stratigraphic division is verified through the reservoir physical properties, oil and gas content, oil, gas and water production capacity, etc. obtained during the actual production process. For the same sandstone group, the reservoir rock and mineral characteristics should be equivalent, and the oil, gas and water interfaces should be consistent.

[0065] This verification belongs to the follow-up practical verification. There are generally few such data in fine-grained sedimentary layers. For wells with such data, the verification results are consistent with the first verification results.

[0066] Because in the actual production process, high-precision sequence division is only the first step in oil and gas exploration. The sand body distribution and oil and gas content are predicted based on the results of high-precision sequence division, and then the well site is deployed. After drilling, only when the target layer reaches the standard of industrial oil and gas, a series of coring, analysis and testing, oil testing, and mining will be carried out, which can further prove the accuracy of the sequence interface division.

[0067] Therefore, this type of verification is an iterative process of theory-practice-theory.

[0068] The principle of the sequence division method based on the frequency reduction of the well logging curve in the present invention is also applicable to stratigraphic units other than fine-grained sedimentary formations. In the sandstone-mudstone interbedded stratigraphic unit, the high-precision sequence scheme divided by this method is highly consistent with the traditional sequence scheme method divided by methods such as rock electrical cycles, which proves the feasibility of this method. The high-precision sequence division results can be verified based on the reservoir physical properties, oil and gas content, oil, gas and water production capacity, etc. obtained in the actual production process.

[0069] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not limitations on the implementation methods of the present invention. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the protection scope of the present invention.

Claims

1. A method for dividing fine-grained sedimentary strata based on the frequency reduction of conventional well logging curves, characterized in that: The method for dividing the fine-grained sedimentary stratigraphic sequence comprises the following steps: Determine the need for high-precision sequence division; Perform frequency reduction processing on each original logging data to obtain the corresponding frequency reduction change curve; The obtained frequency reduction change broken lines are comprehensively compared and analyzed to complete the division of fine-grained sedimentary stratigraphic sequence; The step of performing frequency reduction processing on each original logging data to obtain the corresponding frequency reduction change curves comprises: The original logging curve defined by the top and bottom boundaries of the target layer is segmented vertically according to the extreme value changes, that is, it is divided into several minimum-maximum and maximum-minimum curve segments from top to bottom; Perform mathematical derivation on each segment of the well logging curve to obtain the inflection point data of any segment of the curve; Record the data of each inflection point to form a new curve, namely the first frequency reduction curve, and complete the first frequency reduction process; The frequency reduction process is repeated for the first frequency reduction curve until there are only 3 data points left in the final frequency reduction curve, reaching the original stratigraphic level of the target layer. The frequency reduction change line that can achieve the high-precision stratigraphic division goal is the frequency reduction change line of the final logging curve.

2. The method for dividing the fine-grained sedimentary stratigraphic sequence according to claim 1, characterized in that: The step of determining the high-precision sequence division requirement comprises: Query the existing sequence stratigraphic framework in the study area, clarify the basic characteristics and identification marks of sequence interfaces at all levels, and determine the sequence level of the stratigraphic unit where the fine-grained sedimentary strata are located; Determine the need for high-precision sequence stratigraphic division; Query the layered data table to determine the top and bottom depths of the stratigraphic units that require high-precision sequence stratigraphic division, and determine the top and bottom boundaries of the well logging data volume to be processed.

3. The method for dividing the fine-grained sedimentary stratigraphic sequence according to claim 2, characterized in that: The determination of the need for high-precision sequence stratigraphic division is based on production needs and research status.

4. The method for dividing the fine-grained sedimentary stratigraphic sequence according to claim 1, characterized in that: The original logging curve includes natural gamma, natural potential and resistivity logging data.

5. The method for dividing the fine-grained sedimentary stratigraphic sequence according to claim 4, characterized in that: The step of performing comprehensive comparative analysis on the obtained frequency reduction change broken lines to complete the division of the fine-grained sedimentary stratigraphic sequence comprises: In the single well stratigraphic column chart, add the obtained frequency reduction change broken lines, and determine the top and bottom interfaces and lake flooding surfaces of each level of sequence according to the broken line changes, analysis and comparison, and complete the high-precision sequence division of the single well; The above single-well high-precision sequence division was performed on all wells in the study area; On the basis of high-precision sequence division of single wells, high-precision sequence stratigraphic division of the whole area is carried out. Under the control of stratigraphic framework profile, high-precision sequence stratigraphic plane distribution map is further drawn to analyze the distribution law of fine-grained sedimentary strata.

6. The method for dividing the fine-grained sedimentary stratigraphic sequence according to claim 5, characterized in that: After completing the fine-grained sedimentary stratum sequence division, the method further includes the step of verifying the obtained division results.

7. The method for dividing the fine-grained sedimentary stratigraphic sequence according to claim 6, characterized in that: The step of verifying the obtained division result comprises: Use traditional methods to perform sequence stratigraphic division on adjacent wells where fine-grained sediments are relatively undeveloped or on larger intervals containing fine-grained sediments in the well, and compare the division results with the sequence stratigraphic division results of the fine-grained sedimentary strata to be verified.

8. The method for dividing the fine-grained sedimentary stratigraphic sequence according to claim 7, characterized in that: The step of verifying the obtained division result also includes: This is supported by the following studies: within the same sequence, the changes in sedimentary facies of adjacent wells satisfy Walter's facies law, the distribution of provenance and sand bodies is continuous, and the seismic facies and seismic reflection characteristics are consistent.

9. The method for dividing the fine-grained sedimentary stratigraphic sequence according to claim 8, characterized in that: The step of verifying the obtained division result also includes: The sequence stratigraphic division is verified through the reservoir physical properties, oil and gas content, and oil, gas and water production capacity obtained during the actual production process.

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