Method for determining original organic carbon of outcrop hydrocarbon source rock by using organic element

Abstract

The present application relates to the oil field exploration and development technical field, specifically relates to a kind of method for determining outcrop hydrocarbon source rock original organic carbon using organic element.The present application is based on the systematic sampling of surface outcrop, fresh outcrop, shallow borehole and drilling sample, carries out organic element, TOC and other geochemical analysis test to the hydrocarbon source rock sample of the same interval, uses the change and migration of organic element to characterize sample weathering degree, establishes the quantitative relationship of organic element and TOC correction coefficient index under the action of different weathering degree, realizes the quantitative prediction and scientific evaluation of original organic matter abundance of field outcrop hydrocarbon source rock under different weathering degree condition.The method of the present application takes the migration characteristics of carbon, hydrogen, oxygen and other organic elements as the core for the first time, establishes the quantitative correction model of organic matter abundance under different weathering degree, and provides technical support and theoretical basis for the objective evaluation of organic matter abundance of field outcrop hydrocarbon source rock in low exploration degree area.

Description

Technical Field

[0001] This invention relates to the field of oilfield exploration and development technology, specifically to a method for determining the original organic carbon in outcrop source rocks using organic elements. Background Technology

[0002] The enrichment and preservation of abundant organic matter can effectively control the distribution of oil and gas resources. Oil and gas exploration practice has confirmed that the development conditions of source rocks are key factors in the formation and accumulation of oil and gas, among which the organic matter abundance characteristics of source rocks are crucial for source rock evaluation and the study of oil and gas accumulation conditions. Currently, areas with low to medium exploration levels still exist extensively in my country. Due to limited investment in oil and gas exploration and constraints on the level of geological understanding, drilling data is scarce. Therefore, starting with source rock samples from outcrop development areas at the basin margin has become an important foundation for studying source rocks within the basin.

[0003] Outcrop samples, during geological history, come into contact with surface water, atmosphere, or organisms, undergoing a series of physical, chemical, and biological weathering processes that influence the physical structure, chemical composition, organic chemical characteristics, and hydrocarbon generation potential of source rocks. Differences in samples of the same stratigraphy and lithology between wells and outcrops are closely related to weathering. Weathering of organic-rich mudstones and shale is primarily chemical weathering, causing changes in the rock's mineral and elemental composition. In particular, acidic water promotes element migration and significantly reduces organic matter abundance indicators such as organic carbon content and rock pyrolysis parameters. Therefore, the measured organic carbon content and other organic matter abundance of outcrop samples do not represent the original organic matter abundance of the samples, and correction for the original organic matter abundance must be performed in the evaluation of outcrop source rocks.

[0004] Previous studies have suggested that the impact of weathering on organic matter abundance is related to various factors, including lithology, occurrence, tectonic location, weathering time, and paleoclimate of the outcrop area. Currently, it is difficult to analyze the specific relationship between each influencing factor and organic matter abundance. Previous studies on the impact of weathering on the organic matter abundance of source rocks have focused on the depth of weathering and the degree of influence of weathering on organic carbon (TOC) and other organic matter abundance indicators. These analyses are mostly qualitative or semi-quantitative, and the quantitative relationship between organic matter abundance indicators and the degree of weathering in outcrop samples is not yet fully clear, making it difficult to meet the needs of objective evaluation and research on the organic matter abundance of outcrop source rock samples in the field.

[0005] Chinese invention patent CN103926388B discloses a method for identifying low-abundance effective source rocks. The specific steps are as follows: 1) Obtain pyrolysis parameters and organic matter maturity (Ro) parameters of source rocks of the same lithology from multiple locations and single wells in a certain area; 2) Screen out the pyrolysis parameters and Ro corresponding to low-abundance (TOC≤0.5%) source rocks; 3) Calculate the hydrocarbon generation potential index corresponding to each depth or Ro based on the screened source rock parameters; 4) Draw evolution profiles of the TOC source rock hydrocarbon generation potential index with depth or Ro at the same interval but different ranges; 5) Find the evolution profile where the hydrocarbon expulsion threshold first appears among multiple consecutive TOC source rock hydrocarbon generation potential index evolution profiles with different ranges. The TOC value (or TOC range) corresponding to this evolution profile is the lower limit of TOC for low-abundance effective source rocks. Source rocks with a TOC value above this value are considered low-abundance effective source rocks. This patent requires a large number of vitrinite reflectance measurements. Its goal is to clarify the hydrocarbon generation and expulsion characteristics of samples with different degrees of thermal evolution in order to determine the lower limit of organic matter abundance. However, it cannot reflect the degree of reduction in organic matter abundance caused by weathering.

[0006] Chinese patent application CN109507733A discloses a method and apparatus for predicting the organic matter abundance of source rocks. The method includes: acquiring well logging curves for a target area; calculating the total organic carbon (TOC) value of the target area based on the well logging curves; synthesizing seismic records based on the TOC value of the target area and the well logging curves, and performing well-seismic calibration; extracting seismic attribute characteristics from the wellbore sidetracks based on the well-seismic calibration results, and establishing a relationship between TOC and the seismic attribute characteristics; and establishing the spatial distribution of TOC in the target area based on the relationship. This method requires a large amount of well logging data and seismic data from drilling data. Its goal is to fit the residual organic matter abundance distribution characteristics under present geological conditions through seismic and well logging data. However, it cannot reflect the degree of reduction in organic matter abundance caused by weathering processes and cannot perform weathering correction for organic matter abundance in field outcrops.

[0007] Chinese invention patent CN104697959B discloses a method for calculating the original organic matter abundance recovery coefficient. This method includes: establishing a mathematical relationship between the hydrocarbon generation rate G and vitrinite reflectance Ro for each type of organic matter; determining the relationship between the residual degradation rate Dc and the hydrocarbon generation rate G; assuming the original hydrocarbon generation rate Gy is the same as the hydrocarbon generation rate when the vitrinite reflectance Ro equals 4, deriving the relationship between the original organic matter abundance recovery coefficient Rc of highly evolved source rocks and the vitrinite reflectance Ro based on the relationships between the hydrocarbon generation rate G and the vitrinite reflectance Ro, and the residual degradation rate Dc and the hydrocarbon generation rate G; and using the vitrinite reflectance Ro and the conversion coefficient a of available organic carbon to hydrocarbons, obtaining the original organic matter abundance recovery coefficient Rc of highly evolved source rocks to restore their original organic matter abundance. This patent requires a large amount of vitrinite reflectance measurement results, and the method used is applied to correct for the loss of organic matter abundance caused by thermal evolution. There are two main reasons for the loss of organic matter abundance in mudstone and shale: one is the decrease in organic matter abundance due to the degree of thermal evolution, and the other is the decrease in organic matter abundance during weathering. This method can only reflect the influence of the first factor and cannot reflect the degree of decrease in organic matter abundance caused by weathering, thus it cannot perform weathering correction for organic matter abundance in field outcrops.

[0008] Therefore, it is urgent to establish a method for weathering correction of organic matter abundance in field outcrops to provide support for the objective evaluation of source rocks in outcrop areas and lay the foundation for the scientific prediction of oil and gas potential in the region. Summary of the Invention

[0009] The main objective of this invention is to provide a method for determining the original organic carbon of outcrop source rocks using organic elements. This invention utilizes organic elements to determine the original organic carbon of outcrop source rocks, establishes a method for weathering correction of organic matter abundance in field outcrops, provides support for the objective evaluation of source rocks in outcrop areas, and lays the foundation for the scientific prediction of oil and gas potential in the region.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] This invention provides a method for determining the original organic carbon in outcrop source rocks using organic elements, comprising the following steps:

[0012] Select the outcrop section of the target stratum in the study area or select a section with a relatively vertical orientation, and carry out shallow drilling and coring to conduct systematic sampling;

[0013] The organic elements and total organic carbon of the sampled samples were determined.

[0014] Calculate the H / C atomic ratio and O / C atomic ratio in each sample;

[0015] Determine the unweathered sample and its TOC value;

[0016] Based on the TOC values ​​of each sample and the TOC values ​​of the unweathered sample, the organic matter abundance correction coefficient for each sample was calculated.

[0017] The correlation between the H / C atomic ratio, O / C atomic ratio and organic matter abundance correction coefficient Ki of a series of samples was analyzed, and a regression quantitative model was established.

[0018] Determine the original organic matter abundance of the profile sample.

[0019] Furthermore, when implementing shallow core drilling, the drilling depth is 20-100m.

[0020] Furthermore, dense sampling was conducted near the surface, with sample intervals of 0.1m-0.5m; core sample intervals were 0.2m-1m.

[0021] Furthermore, samples whose O / C atomic ratio and organic carbon determination results tend to be constant as the burial depth changes are identified as unweathered samples.

[0022] Furthermore, the organic matter abundance correction factor Ki for each sample is calculated using the following formula:

[0023] Ki = TOC o / TOC i

[0024] TOC o TOCi is the TOC value of the unweathered sample to be determined.

[0025] Furthermore, the regression quantitative model is a single-factor regression quantitative model of atomic ratio and abundance correction factor.

[0026] Furthermore, if the correlation coefficient of the corresponding single-factor regression quantitative model is low, multi-factor fitting can be performed.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] Based on systematic sampling from surface outcrops, fresh outcrops, shallow boreholes, and well samples, this invention performs geochemical analyses such as organic element and TOC on source rock samples from the same stratum. It utilizes the changes and migrations of organic elements to characterize the weathering degree of the samples, establishes a quantitative relationship between organic elements and TOC correction coefficients under different weathering degrees, and realizes quantitative prediction and scientific evaluation of the original organic matter abundance of outcrop source rocks under different weathering conditions.

[0029] This invention provides the first objective evaluation of the impact of different degrees of weathering on the abundance of organic carbon and other organic matter using organic elements. Based on the migration characteristics of organic elements such as carbon, hydrogen, and oxygen, a quantitative correction model for organic matter abundance under different degrees of weathering was established. This provides technical support and theoretical basis for the objective evaluation of organic matter abundance in outcrops of source rocks in low-exploration areas.

[0030] The method of this invention only requires the determination of three parameters of organic elements: carbon, hydrogen, and oxygen. The steps are simple, easy to operate, and easy to promote and apply. Attached Figure Description

[0031] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0032] Figure 1 This is a flowchart of the method for determining the original organic carbon of outcrop source rocks using organic elements, as described in this invention.

[0033] Figure 2 This is a correlation diagram between sample burial depth and total organic matter (TOC) as described in Example 1 of the present invention;

[0034] Figure 3 This is a graph showing the relationship between the burial depth of the sample and the oxygen-to-carbon ratio (IO / C) as described in Example 1 of this invention.

[0035] Figure 4 This is a graph showing the correlation between the sample burial depth and the hydrogen-to-carbon atomic ratio (IH / C) described in Example 1 of the present invention.

[0036] Figure 5 This is a correlation diagram between the sample burial depth and the TOC weathering correction factor (K) described in Example 1 of the present invention;

[0037] Figure 6 This is a graph showing the correlation between the TOC weathering correction factor (K) and the oxygen-carbon atomic ratio (IO / C) as described in Example 1 of the present invention.

[0038] Figure 7 This is a graph showing the correlation between the TOC weathering correction factor (K) and the oxygen-carbon atomic ratio (IH / C) as described in Example 1 of the present invention. Detailed Implementation

[0039] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0040] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, and / or combinations thereof.

[0041] like Figure 1 As shown, the method for determining the original organic carbon of outcrop source rocks using organic elements includes the following steps:

[0042] Step 1: For the target stratum of the source rock, determine the outcrop area of ​​the source rock stratum through field reconnaissance; prioritize fresh outcrop profiles such as newly built roads and newly opened mines; or select profiles with relatively vertical orientation and carry out shallow drilling for coring, with a recommended drilling depth of 20-100m, the specific depth to be determined by factors such as stratigraphic lithology, age, orientation and weathering time; depending on the actual geological conditions, excavation equipment can also be rented for assistance.

[0043] Step 2: Systematically sample the above-mentioned fresh outcrop samples or the target stratum of shallow boreholes. The sampling interval for sections with better underground preservation conditions can be 0.2m-1m, and the sampling interval for sections closer to the surface can be appropriately increased to 0.1m-0.5m.

[0044] Step 3: For the samples collected by the system, determine the organic elements such as C, H, and O, and at the same time conduct geochemical analysis tests on organic carbon (TOC), pyrolysis, microscopic examination, and the abundance and type of organic matter in whole rock sections.

[0045] Step 4: Calculate the H / C atomic ratio (I0.05) of each sample using the organic element determination results. H / C ), O / C atomic ratio (I O / C ).

[0046] Step 5: Based on the observation that the O / C atomic ratio and organic carbon measurement results tend to remain constant as the burial depth changes, samples are identified as unweathered samples, and their organic carbon value is used as the TOC (Total Organic Carbon). o If the target layer has a stable planar distribution and the adjacent layers have easily identifiable marker layers that can be compared with wells drilled nearby within the basin, the well samples can be used as unweathered samples for systematic sampling.

[0047] Step 6, based on the TOC of each sample (TOC i ) and the TOC value of unweathered samples (TOC o ), calculate the organic matter abundance correction coefficient K for each sample. i The formula is:

[0048] K i=TOC o / TOC i

[0049] Step 7: Analyze the correlation between the H / C atomic ratio, O / C atomic ratio and the organic matter abundance correction coefficient Ki of the series of samples, and establish a regression quantitative model, i.e.

[0050] K i1 =f(I O / C )

[0051] K i2 =f(I H / C )

[0052] If the correlation coefficient of the corresponding single-factor model is low, multi-factor fitting can be performed.

[0053] Right now:

[0054] K = f(I) H / C I O / C )

[0055] Step 8: Determine the organic elements and TOC of other surface outcrop source rock samples in the area, and substitute them into the formula determined in Step 7 to determine the original TOC value of the unweathered rock. o ).

[0056] TOC o =TOC i ×K i

[0057] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0058] Example 1

[0059] The method for determining the original organic carbon of outcrop source rocks using organic elements includes the following steps:

[0060] (1) A nearly vertical section of the Middle Permian strata in the southern part of Jimsar, Junggar Basin, Xinjiang Uygur Autonomous Region was selected. The measured dip angle of the strata was 89°. The studied stratum was the Lucaogou Formation (P2l) of the Middle Permian.

[0061] (2) For the concentrated section of hydrocarbon source rock development in the Lucaogou Formation, shallow boreholes with a depth of 35m were drilled and core samples were taken from the entire well section.

[0062] (3) Take dense samples near the ground surface with a sample interval of 0.1m-0.5m. For core sections with better underground preservation conditions, the sample interval is 0.2m-1m.

[0063] (4) Geochemical analyses related to the abundance of organic matter, such as total organic carbon (TOC), were performed. The organic matter abundance of the sample ranged from 0.78% to 2.35%, and the organic matter type was type I. A correlation diagram between the burial depth and TOC of the P2l sample was compiled. Figure 2 ).

[0064] (5) The contents of organic elements such as C, H, and O in the sample were determined using the method of GB / T 19143-2003. Based on the elemental determination results, the H / C atomic ratio (IH / IC) of the sample was calculated. H / C ) and O / C atomic ratio (I O / C ). Plot the sample burial depth and I H / C I O / C Related images ( Figure 3 , Figure 4 ).

[0065] (6) Based on the principle that samples with a relatively constant O / C atomic ratio and organic carbon determination result as the burial depth changes, are identified as unweathered samples, as shown in the figure above, the weathering influence depth of this profile is approximately 5m-10m. In samples buried at depths of 10m-25m, TOC approaches 2.35%, and the oxygen-carbon atomic ratio is approximately constant at 0.040. Therefore, the TOC value of unweathered samples (TOC...) is... o The value was determined to be 2.35%.

[0066] (7) Based on the TOC of each sample (TOC i ) and the determined TOC value of the unweathered sample (TOC o ), calculate the organic matter abundance correction factor Ki for each sample, using the following formula:

[0067] Ki = TOC o / TOC i

[0068] Based on this, a correlation diagram between sample burial depth and TOC correction coefficient was drawn. Figure 5 ).Depend on Figure 5 It can be seen that the maximum TOC correction coefficient for the sample is 3.01.

[0069] (8) Analyze the H / C atomic ratio (I) of the series of samples. H / C ), O / C atomic ratio (I O / C ) and organic matter abundance correction factor K i correlation ( Figure 6 , Figure 7 A single-factor regression quantitative model of atomic ratio and abundance correction factor was established. Wherein:

[0070] K i1 =1.1639×ln(I O / C )+4.9163(R 2=0.8525)

[0071] K i2 = -2.816 × ln(I H / C -5.1797(R) 2 =0.9064)

[0072] As shown in the figure, I O / C I H / C All of them showed good correlation with the organic carbon correction coefficient within a certain period. Figure 6 In the meantime, as the number of unweathered samples increases, the organic matter abundance correction coefficient approaches 1, and the correlation R... 2 It begins to decrease. Figure 7 middle, I H / C It has a higher correlation with the TOC correction coefficient, but in the TOC stable value distribution area, I H / C The values ​​still exhibit some fluctuations, which may be related to the more significant impact of weathering on hydrogen atoms. Therefore, it can be applied within a certain depth range.

[0073] (9) Based on single factors, binary regression can be performed to establish an organic matter abundance correction and quantification model constrained by C, H and O organic elements.

[0074] K = f(I) H / C I O / C )

[0075] Through multiple regression, the quantitative prediction formula for the weathering correction coefficient of organic matter abundance under multiple constraints is obtained as follows:

[0076] K = 0.619 × ln(I O / C )-1.396×ln(I H / C )-2.139(R 2 =0.922)

[0077] (10) Determine the organic elements and TOC of other surface outcrop source rock samples in the area, and substitute them into the formula determined in step 9 to determine the original unweathered TOC value (TOC). o ).

[0078] TOC o =TOC i ×K i

[0079] Example 2

[0080] The method for determining the original organic carbon of outcrop source rocks using organic elements includes the following steps:

[0081] (1) A nearly vertical section of the Paleogene strata at the Sikeshuhe section in the southwestern part of the Junggar Basin in Xinjiang Uygur Autonomous Region was selected. The measured dip angle of the strata was 80°. The studied stratum is the Paleogene Anjihaihe Formation (E 2-3 a).

[0082] (2) For the concentrated section of source rock development in the Anjihaihe Formation, the fresh outcrop above the surface is about 10m. Shallow boreholes were drilled at the corresponding positions on the ground. Since the strata are not completely vertical, in order to drill core samples from the same section, the dip angle and corresponding depth of the strata were calculated. Two shallow boreholes with depths of 9m and 15m were drilled respectively. Based on the lithological combination of the target strata in the outcrop and the comparison of the core lithology, the samples of the target source rock strata were identified, and the corresponding well sections were cored.

[0083] (3) Dense sampling is carried out in core sections above and near the surface, with a sample interval of 0.1m-0.4m. For core sections with better underground preservation conditions, the sampling interval is 0.4m-1m.

[0084] (4) Geochemical analyses related to organic matter abundance, such as total organic carbon (TOC), were performed. The organic matter abundance of the sample was 0.78%-1.75%, and the organic matter type was type II1. E plotting was then performed. 2-3 a. Correlation between sample burial depth and TOC.

[0085] (5) The contents of organic elements such as C, H, and O in the sample were determined using the method of GB / T 19143-2003. Based on the elemental determination results, the H / C atomic ratio (IH / IC) of the sample was calculated. H / C ) and O / C atomic ratio (I O / C Plot a graph showing the correlation between sample burial depth and oxygen-carbon atomic ratio.

[0086] (6) Based on the principle that samples with a relatively constant O / C atomic ratio and organic carbon determination result as the burial depth changes, are identified as unweathered samples, as shown in the figure above, the weathering influence depth of this profile is approximately 5m-10m. In samples buried at depths of 6m-23m, TOC approaches 1.75%, and the oxygen-carbon atomic ratio is approximately constant at 0.036. Therefore, the TOC value of unweathered samples (TOC...) is... o The value was determined to be 1.75%.

[0087] (7) Based on the TOC of each sample (TOC i ) and the determined TOC value of the unweathered sample (TOC o ), calculate the organic matter abundance correction factor Ki for each sample, using the following formula:

[0088] K i =TOC o / TOC i

[0089] Based on this, a correlation diagram between sample burial depth and TOC correction coefficient was compiled, with the maximum TOC correction coefficient for the sample being 2.61.

[0090] (8) Analyze the H / C atomic ratio, O / C atomic ratio, and organic matter abundance correction factor K of the series of samples. i To determine the correlation between atomic ratio and abundance correction factor, a single-factor regression quantitative model was established. Wherein:

[0091] K i3 =1.705×ln(I O / C )+2.1854(R 2 =0.8991)

[0092] K i4 =9.344×e -20.15 (IH / C)(R 2 =0.7247)

[0093] Because of I H / C The region showed low correlation with abundance factors, so I was adopted. O / C The regression model serves as the formula for calculating the organic matter abundance correction coefficient.

[0094] (9) Determine the organic elements and TOC of other surface outcrop source rock samples in this area, and substitute them into the K value in step 8. i3 The model formula can determine the original TOC value (TOC) of the unweathered area. o ).

[0095] TOC o =TOC i ×K i3

[0096] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for determining the original organic carbon in outcrop source rocks using organic elements, characterized in that, Includes the following steps: Select the outcrop section of the target stratum in the study area or select a section with a relatively vertical orientation, and carry out shallow drilling and coring to conduct systematic sampling; The organic elements and total organic carbon of the sampled samples were determined. Calculate the H / C atomic ratio and O / C atomic ratio in each sample; Determining unweathered samples and their TOC values ​​includes identifying samples whose O / C atomic ratio and organic carbon determination results tend to be constant as the burial depth changes; Based on the TOC values ​​of each sample and the unweathered sample, the organic matter abundance correction factor Ki for each sample is calculated using the following formula: Ki=TOC o / TOC i TOC o To determine the TOC value of the unweathered sample, TOC i This refers to the TOC value of the sample. The correlation between the H / C atomic ratio, O / C atomic ratio and organic matter abundance correction coefficient Ki of a series of samples was analyzed, and a regression quantitative model was established. Determine the original organic matter abundance of the profile sample.

2. The method according to claim 1, characterized in that, When performing shallow core drilling, the drilling depth is 20-100m.

3. The method according to claim 1, characterized in that, Dense sampling was conducted near the surface, with sample intervals of 0.1m-0.5m; core samples were taken at intervals of 0.2m-1m.

4. The method according to claim 1, characterized in that, The regression quantitative model is a single-factor regression quantitative model of atomic ratio and abundance correction factor.

5. The method according to claim 4, characterized in that, If the correlation coefficient of the corresponding single-factor regression quantitative model is low, multi-factor fitting should be performed.

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