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Quantitative analysis of variation of organic carbon mass and content in source rock during evolution process
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 36, Issue 4, August 2009 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2009, 36(4): 463–468.
RESEARCH PAPER
Quantitative analysis of variation of organic carbon mass and content in source rock during evolution process Zhou Zongying Exploration & Production Research Institute, Sinopec, Beijing 100083, China
Abstract: According to the mass conservation principle of organic carbon, a mathematical model of organic carbon mass compensation coefficient is established to indicate the variation of organic carbon mass. Considering the influence of organic carbon hydrocarbon generation, hydrocarbon expulsion and rock weight loss, a mathematical model for calculating changes in organic carbon content is also established. Forward method is used to simulate the compensation coefficient of organic carbon mass and the organic carbon content value of source rocks in different evolution stages and hydrocarbon expulsion efficiency. The compensation coefficient variation depends on organic matter type, maturity degree and expulsion efficiency. The maximum compensation coefficients of types ĉ, Ċ1, Ċ2 and ċ organic matter are 2.104, 1.360, 1.169 and 1.099, respectively. There exists a threshold value in hydrocarbon expulsion efficiency in source rocks, type ĉ is 20%, type Ċ1 is 30%, type Ċ2 and type ċ is 60%. When the expulsion efficiency is less than this threshold value, the residual organic carbon content of the source rock is generally higher than the original organic carbon content; when the expulsion efficiency is greater than the value, the residual organic carbon content is generally lower than the original organic carbon content. Under the conditions of full expulsion, the maximum organic carbon reduction of types ĉ, Ċ1, Ċ2 and ċ muddy source rocks is roughly 43%, 20%, 10% and 10%. Key words:
hydrocarbon source rock; organic carbon mass; compensation factor; organic carbon content; mathematical model
Introduction Mass (quantity) and content of organic carbon in rock are two most important organic geochemical indicators for the evaluation of oil/gas resources and source rock in a sedimentary basin[1-4]. When the source rock does not generate and expel hydrocarbon in great quantity before the oil-generating threshold is reached, the organic matter content is referred as original abundance of organic matter. However, the normally measured result is the content of the residual organic matter after hydrocarbon is generated and expelled in the source rock. The residual organic matter in source rock includes dissoluble organic matter (chloroform asphalt “A”) and indissoluble organic matter (kerogen). According to the theory on hydrocarbon generation by kerogen, as the buried depth of the source rock increases continuously, higher and higher geotemperature is experienced. When the threshold value of hydrocarbon generating temperature is reached, the thermal degradation of the kerogen begins and hydrocarbon is generated and expelled in great quantity. As a result of oil/gas expulsion,
the content of organic matter should decreases continuously. As for a highly – over mature source rock, if the residual organic carbon content is used to evaluate the hydrocarbon source rock or calculate the oil/gas resources, a deviation result might occur. For this sake, many researchers have studied on the restoration of the original abundance of organic matter in the process of geological evolution by means of simulated experimental data and a number of methods[5-10]. While the absolute quantity of organic matter decreases as a result of hydrocarbon generation and expulsion, the rock weight is also reduced under the effects of a series of factors such as compaction inducing water drainage and dissolution. It was considered that the reduction of the rock weight could compensate, even exceed the reduction of the absolute quantity of organic matter in the rock under certain conditions, leading to the organic matter abundance basically constant, even rise somewhat. Therefore, debate exists extensively as to whether to restore the residual TOC in all the highly – over mature source rocks or no need at all to restore the original TOC[11-12].
Received date: 27 December 2008; Revised date: 27 April 2009. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the Ministry of Land and Resources Project: Convention petroleum resources appraisal in SINOPEC =3& Copyright © 2009, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
Zhou Zongying. / Petroleum Exploration and Development, 2009, 36(4): 463–468
In the process of thermal evolution of source rock, how does the mass (total quantity) and content of organic carbon change? How is the variation amplitude? Particularly, as to the variation of the organic carbon content, under what condition the residual organic content might be greater than the original organic carbon content? For these questions, in this paper, according to the mass conservation principle of organic carbon, a mathematical model of organic carbon mass compensation coefficient is established to reflect the variation of organic carbon mass. Comprehensively taking into account of the effects of organic carbon hydrocarbon generation, hydrocarbon expulsion and rock weight loss on hydrocarbon content, a mathematical model for calculation of the changes in organic carbon content is also established. And, the forward method is used to simulate and calculate the compensation coefficients of organic carbon mass as well as the organic carbon contents in various types of source rocks at different evolution stages and hydrocarbon expulsion efficiencies. The possible variation scopes and variation amplitudes of the compensation coefficients of organic carbon mass and the organic carbons contents are analyzed quantitatively, providing scientific basis for the objective evaluation of source rocks and the potential of oil/gas resources.
1 Quantitative analysis of variation in organic carbon mass 1.1 Mathematical model for calculation of variation in organic carbon mass When source rock does not generate and expel hydrocarbon in great quantity, the organic carbon mass is assumed as original organic carbon mass. After hydrocarbon is generated and expelled in great quantity, the organic carbon mass is defined as residual organic carbon mass. The result of original organic carbon mass divided by the residual organic carbon mass is defined as the compensation coefficient per unit mass of organic carbon. According to the mass conservation law of substances in chemical reaction, the original organic carbon mass before a source rock generates and expels hydrocarbon in great quantity equals to the sum of the residual organic carbon mass after the thermal evolution is matured plus the organic carbon mass in the expelled oil/gas. Taking 0.83 as the conversion coefficient between hydrocarbon and organic carbon, the following mathematical expressions can be obtained:
mr
m0 0.83Qs Ex m0
(1)
K
m0 mr
(2)
1 1 0.83Qs Ex
where, m0—original organic carbon mass per unit volume of source rock, g; mr—residual organic carbon mass after evolution of original per unit volume of source rock, g; Qs—cumulative hydrocarbon generating volume in source
rock at various stages of evolution (or simulated temperature points), g/g; Ex—hydrocarbon expulsion efficiency of source rock, f; K—compensation coefficient of per unit mass of organic carbon, f. Equation (1) is a model of the relationship between the original organic carbon mass and the residual organic carbon mass as built up based on hydrocarbon generation and expulsion in organic matter; equation (2) reflects the variation rule of organic carbon mass in the geological process of hydrocarbon generation and expulsion in source rock. At a given stage of evolution, the residual organic carbon mass equaled to the result of the original organic carbon mass divided by the compensation coefficient of organic carbon mass, or the original organic carbon mass equals to the product of the residual organic carbon mass at a certain stage of evolution multiplied by the compensation coefficient of organic carbon mass. By means of the compensation coefficient of organic carbon mass, the relationship between the original organic carbon mass and the residual organic carbon mass can be described quantitatively. It is a succinct method, easy for operation. And, the compensation coefficient of organic carbon mass is related only to two parameters, hydrocarbon generating rate of organic carbon and hydrocarbon expulsion efficiency of source rock. The compensation coefficient of organic carbon mass is a concept different from the restitution coefficient of organic carbon as involved often in the evaluation of source rock. The latter is a restitution coefficient related to organic carbon abundance, equivalent to the result of the original organic carbon abundance (content) divided by the residual organic carbon abundance (content). Hydrocarbon yield rate is one of crucial parameters for the evaluation of oil/gas resources by genetic method (especially the basin simulation), and the charts of hydrocarbon yield rate is established on the basis of data obtained in the thermal modeling experiments of source rock samples. To obtain the data of hydrocarbon yield rate in source rocks with different lithology and different type of organic matters, great efforts have been made by relevant research institutions, production companies and scholars in China[12-13]. Most data were acquired from experiments on oil/gas yield rates in various source rocks by means of thermal compression modeling, of which some data of gas yield rates were those of the gas derived by crude cracking completely. In this paper, some typical data of hydrocarbon yield rates in four types of source rocks were collected, including type ĉ, Ċ1, Ċ2 and ċ[13-16]. Hydrocarbon expulsion efficiency refers to the ratio of hydrocarbon expulsion volume to the hydrocarbon generating volume in source rock. The primary affecting factors include the combination mode of source rock and reservoir rock, development of fractures in source rock, type of organic matter, evolution level of organic matter and so on. The hydrocarbon expulsion efficiencies of source rock
Zhou Zongying. / Petroleum Exploration and Development, 2009, 36(4): 463–468
Table 1 Calculated results of compensation coefficients of organic carbon mass at different hydrocarbon expulsion efficiencies in source rocks with different types of organic matters Sample
Immature dark grey mudstone K1 in Well Dishu 1, Hailar Basin (well depth: 1 101.38 m)
Kerogen type
Type I
Mudstone (E2) in Well LF 13-2-1, Zhujiangkou Basin
Type II1
Grey-black mudstone(J) in Well Liushen 1, Kuche Depression
Type II2
Mudstone(E) in Well LingFeng 1, Oujiang Sag, East China Sea
Type III
Ro/%
HC yield rate/ (mg·g-1)
0.29 0.39 0.52 0.60 0.69 0.93 1.24 1.65 2.21 0.64 0.70 0.83 0.94 1.15 1.27 1.92 0.70 0.80 0.90 1.00 1.10 1.30 1.50 1.70 2.00 0.60 0.70 0.80 0.90 1.00 1.10 1.30 1.50 1.70
82.20 101.20 189.40 259.90 466.40 580.60 599.70 621.50 632.30 103.51 97.11 162.77 214.77 251.14 272.39 318.62 16.00 32.00 54.00 76.00 94.00 128.00 147.00 161.00 174.00 40.50 54.50 65.50 74.50 81.60 87.50 96.00 103.00 108.50
vary greatly in different areas. For the convenience of analysis; in this paper, five typical values are used as the hydrocarbon expulsion efficiencies of source rock, i.e. 0, 25%, 50%, 75% and 100%. 1.2 Calculation results of compensation coefficient of organic carbon mass For different hydrocarbon expulsion efficiencies (0, 25%, 50%, 75% and 100%), the compensation coefficients of organic carbon in ĉ, Ċ1, Ċ2 and ċ type of source rocks are calculated by equation (2), using the data of hydrocarbon yield rates in source rocks as acquired in thermal compression modeling (Table 1). From the calculated results, it is illustrated that: (1) When the hydrocarbon expulsion efficiency in a source rock is 0, namely, the source rock is an enclosed system without any substance exchange with outside, oil/gas generated retains completely in the source rock. In this case, the organic carbon mass (quantity) in the source rock keeps constant always in the geological process, i.e. the mass of residual organic carbon in the source rock equals to the original organic carbon mass, the compensation coefficient
Compensation coefficient of organic carbon mass at different HC expulsion efficiencies/f 0 25% 50% 75% 100% 1 1.017 1.035 1.054 1.073 1 1.021 1.044 1.067 1.092 1 1.041 1.085 1.134 1.187 1 1.057 1.121 1.193 1.275 1 1.107 1.240 1.409 1.632 1 1.137 1.317 1.566 1.930 1 1.142 1.331 1.596 1.991 1 1.148 1.348 1.631 2.065 1 1.151 1.356 1.649 2.104 1 1.022 1.045 1.069 1.094 1 1.021 1.042 1.064 1.088 1 1.035 1.072 1.113 1.156 1 1.047 1.098 1.154 1.217 1 1.055 1.116 1.185 1.263 1 1.060 1.127 1.204 1.292 1 1.071 1.152 1.247 1.360 1 1.003 1.007 1.010 1.013 1 1.007 1.013 1.020 1.027 1 1.011 1.023 1.035 1.047 1 1.016 1.033 1.050 1.067 1 1.020 1.041 1.062 1.085 1 1.027 1.056 1.087 1.119 1 1.031 1.065 1.101 1.139 1 1.035 1.072 1.111 1.154 1 1.037 1.078 1.121 1.169 1 1.008 1.017 1.026 1.035 1 1.011 1.023 1.035 1.047 1 1.014 1.028 1.043 1.057 1 1.016 1.032 1.049 1.066 1 1.017 1.035 1.054 1.073 1 1.018 1.038 1.058 1.078 1 1.020 1.041 1.064 1.087 1 1.022 1.045 1.069 1.093 1 1.023 1.047 1.072 1.099
of organic carbon mass equals to 1. (2) When the hydrocarbon expulsion efficiency in a source rock is 100%, namely, the source rock is an open system, oil/gas generated will migrate out completely from the source rock. In this case, the loss of the organic carbon quantity (mass) in the source rock is maximized and the compensation coefficient of organic carbon will be the highest under same or similar conditions in terms of organic matter type and evolution level. Taking the hydrocarbon expulsion coefficients of organic carbon in mudstone with type ĉ organic matter of Well Dishu 1, Hailar Basin as examples, which corresponds to different hydrocarbon expulsion efficiencies at a Ro as 1.24%, for a hydrocarbon expulsion efficiency 25%, the compensation coefficient is 1.142; For a hydrocarbon expulsion efficiency 50%, the compensation coefficient is 1.331; for a hydrocarbon expulsion efficiency 75%, the compensation coefficient is 1.596; when the hydrocarbon expulsion efficiency is 100%, the compensation coefficient reaches the maximum value 1.991. (3) Enclosed and open systems are two extreme cases. In nature, the processes of both hydrocarbon generation and
Zhou Zongying. / Petroleum Exploration and Development, 2009, 36(4): 463–468
expulsion are neither the enclosed systems nor the open systems completely, but some states between the enclosed and open systems. The oil/gas generated in a source rock migrates out partially, resulting in a residual organic carbon mass in the source rock less than the original organic carbon mass and a compensation coefficient of organic carbon mass greater than 1. (4) At a given hydrocarbon expulsion efficiency in a source rock, the better the type of organic matter in the source rock and the higher the thermal evolution level, the higher the compensation coefficient of organic carbon is. (5) When the hydrocarbon yield rate and hydrocarbon expulsion efficiency in a certain type of source rock are maximized, the corresponding compensation coefficient of organic carbon mass is called as the maximum compensation coefficient of organic carbon mass. The maximum compensation coefficients of organic carbon mass vary significantly in source rocks with different types of organic matters. For the source rocks with type ĉ of organic matter, when the maximum hydrocarbon yield rate is 632.30 mg/g and the hydrocarbon expulsion efficiency is 100%, the maximum compensation coefficient of organic carbon mass is as high as 2.104; for the source rocks with type Ċ1 of organic matter, when the maximum hydrocarbon yield rate is 318.62 mg/g and the hydrocarbon expulsion efficiency is 100%, the maximum compensation coefficient of organic carbon mass is 1.360; for the source rocks with type Ċ2 of organic matter, when the maximum hydrocarbon yield rate is 174.00 mg/g and the hydrocarbon expulsion efficiency is 100%, the maximum compensation coefficient of organic carbon mass is 1.169; for the source rocks with type ċ of organic matter, when the maximum hydrocarbon yield rate is 108.50 mg/g and the hydrocarbon expulsion efficiency is 100%, the maximum compensation coefficient of organic carbon mass is 1.099.
the source rock. Thus, the original organic carbon content and the residual organic carbon content can be expressed mathematically as follows:
C0
m0 I0 U w 澠1 I0澡U m0
(3)
Cr
mr IU w (1 I0 ) U mr
(4)
where ij0—original porosity of source rock, f; ij—porosity of source rock, normally calculated from the buried depth-Ro relationship equation and rock compaction equation, f; C0—original organic carbon content, f; Cr—residual organic carbon content, f; ȡw—density of water saturated in pores, 1.0 g/cm3; ȡ—density of rock matrix, 2.8 g/cm3. From equation (3), obtain:
m0
C0 >I0 U w (1 I0 ) U @ 1 C0
From mass equilibrium equation (1) and equation (4), obtain:
Cr
m澠 0 1 0.83Qs Ex ) IU w (1 I0 ) U m澠 0 1 0.83Qs Ex )
(6)
The research results of the previous researchers are used directly for the buried depth-Ro relationship equation and mudstone compaction equation[17]. Namely, the mudstone compaction equation is:
I
54.74 e 0.935 4u10
3
Z
2 Quantitative analysis of variation in organic carbon content
where, Z——Buried depth, m. The buried depth-Ro relationship equation is:
2.1 Mathematic model for calculation of organic carbon content
Z
For the convenience of discussion, at a Ro value as 0.50%, the corresponding porosity of the source rock is regarded as the original porosity; the corresponding organic carbon content and mass are regarded as the original organic carbon content and mass respectively; after an evolution to a certain extent of the source rock (with a Ro greater than 0.50%), the corresponding organic carbon content and mass are regarded as the residual organic carbon content and mass respectively. According to the definition, the organic carbon content equals to the result of organic carbon mass in source rock divided by the sum of the source rock mass (water + rock matrix, in the process of source rock evolution, the mass of rock matrix keeps constant) plus the organic carbon mass in
(5)
(7)
514.801 3711.29 Ro 1 970.27 Ro2 644.054 Ro3 78.194 2 Ro4
(8)
For various original organic carbon contents (0.5%, 1.0%, 2.0%, 5.0%, 10.0% and 15.0%), using data of hydrocarbon yield rates derived from the thermal compression modeling of four types of argillaceous source rocks (typeĉ, Ċ1, Ċ2 and ċ), when the hydrocarbon expulsion efficiency varies in a range from 0 to 100%, equations (5)-(8) can be combined to calculate the variation of residual organic carbon content as maturity changes. Tables 2-5 show the calculated results of residual organic carbon contents in four types of argillaceous source rocks at different hydrocarbon expulsion efficiencies when the original organic carbon content is 1.0%.
Zhou Zongying. / Petroleum Exploration and Development, 2009, 36(4): 463–468
Table 2 Calculated results of residual organic carbon contents in argillaceous source rocks with typeĉorganic matter at different hydrocarbon expulsion efficiencies HC yield Residual organic carbon content at different HC expulsion efficiencies/% rate/(mg·g-1) 0 5% 10% 15% 20% 25% 30% 40% 50% 75% 100% 0.60 58.80 1.017 1.015 1.012 1.010 1.007 1.005 1.002 0.998 0.993 0.980 0.968 0.70 122.00 1.031 1.025 1.020 1.015 1.010 1.005 1.000 0.989 0.979 0.953 0.927 0.80 273.00 1.041 1.029 1.018 1.006 0.994 0.983 0.971 0.948 0.924 0.866 0.807 0.90 497.00 1.049 1.028 1.007 0.985 0.964 0.942 0.921 0.878 0.835 0.727 0.619 1.00 552.00 1.056 1.032 1.008 0.984 0.960 0.936 0.912 0.864 0.816 0.696 0.575 1.10 550.00 1.062 1.038 1.014 0.990 0.966 0.942 0.918 0.870 0.821 0.701 0.580 1.30 480.00 1.071 1.050 1.028 1.007 0.986 0.965 0.944 0.902 0.859 0.753 0.647 1.50 435.00 1.077 1.058 1.039 1.019 1.000 0.981 0.962 0.923 0.884 0.788 0.691 1.70 432.00 1.082 1.063 1.044 1.025 1.005 0.986 0.967 0.928 0.890 0.794 0.697 2.00 418.00 1.088 1.070 1.051 1.032 1.013 0.995 0.976 0.939 0.901 0.807 0.713 2.20 415.00 1.091 1.073 1.054 1.036 1.017 0.998 0.980 0.942 0.905 0.812 0.718 Note: The hydrocarbon yield rates are derived from the thermal compression simulation of the black mudstone (K1qn) in Well Qian 32, Songliao Basin, literature [15]. Ro/%
Table 3 Calculated results of residual organic carbon contents in argillaceous source rocks with type Ċ1 organic matter at different hydrocarbon expulsion efficiencies HC yield rate Residual organic carbon content at different HC expulsion efficiencies /% /(mg·g-1) 0 5% 10% 15% 20% 25% 30% 40% 50% 75% 100% 0.64 103.51 1.023 1.019 1.014 1.010 1.006 1.001 0.997 0.988 0.979 0.958 0.936 0.70 97.11 1.031 1.026 1.022 1.018 1.014 1.010 1.006 0.998 0.989 0.969 0.948 0.83 162.77 1.044 1.037 1.030 1.023 1.016 1.009 1.002 0.988 0.974 0.939 0.904 0.94 214.77 1.052 1.043 1.034 1.024 1.015 1.006 0.997 0.978 0.959 0.913 0.866 1.15 251.14 1.064 1.053 1.042 1.031 1.020 1.009 0.998 0.976 0.954 0.899 0.844 1.27 272.39 1.069 1.058 1.046 1.034 1.022 1.010 0.998 0.974 0.950 0.890 0.830 1.92 318.62 1.087 1.073 1.058 1.044 1.030 1.016 1.001 0.973 0.944 0.873 0.802 Note: The hydrocarbon yield rates are derived from the thermal compression simulation of the mudstone (E2) in Well LF13-2-1, ZhujiangKou Basin, literature[16]. Ro/%
Table 4 Calculated results of residual organic carbon contents in argillaceous source rocks with typeII2 organic matter at different hydrocarbon expulsion efficiencies HC yield rate Residual organic carbon content at different HC expulsion efficiencies /% /(mg·g-1) 0 10% 20% 30% 40% 50% 60% 70% 75% 80% 100% 0.70 16.00 1.031 1.029 1.028 1.026 1.025 1.024 1.022 1.021 1.020 1.020 1.017 0.80 32.00 1.041 1.038 1.036 1.033 1.030 1.027 1.025 1.022 1.021 1.019 1.014 0.90 54.00 1.049 1.045 1.040 1.035 1.031 1.026 1.022 1.017 1.015 1.012 1.003 1.00 76.00 1.056 1.050 1.043 1.036 1.030 1.023 1.017 1.010 1.007 1.003 0.990 1.10 94.00 1.062 1.054 1.045 1.037 1.029 1.021 1.013 1.004 1.000 0.996 0.980 1.30 128.00 1.071 1.059 1.048 1.037 1.026 1.014 1.003 0.992 0.986 0.981 0.958 1.50 147.00 1.077 1.064 1.051 1.038 1.025 1.012 0.999 0.986 0.980 0.973 0.947 1.70 161.00 1.082 1.068 1.054 1.039 1.025 1.011 0.996 0.982 0.975 0.968 0.939 2.00 174.00 1.088 1.073 1.057 1.042 1.026 1.010 0.995 0.979 0.972 0.964 0.933 Note: The hydrocarbon yield rates are derived from the thermal compression simulation of the grey-black mudstone (J) in Well Liushen 1, Kuche Depression, literature [14]. Ro/%
2.2 Analysis of calculated results of variation in organic carbon content From Tables 2-5, it can be seen that: when the hydrocarbon expulsion efficiency is 0, the residual organic carbon contents in all the types of source rocks are greater than the original organic carbon contents. And, the higher the evolution level is (corresponding to deeper buried depth), the higher the residual organic carbon content is. This is because that there is no hydrocarbon expulsion in source rock and the organic carbon mass keeps constant always when the hydrocarbon expulsion efficiency is 0 , but, under compaction, the porosity of source rock decreases and porous water is drained out, leading to rock mass reduced. The calculation results show that: (1) Generally, in argillaceous source rocks with ĉ type organic matter, the
residual organic carbon content is usually greater than the original organic carbon content at a hydrocarbon expulsion efficiency less than 20%, as a result of a reduction of organic carbon mass less than the reduction of rock mass at low hydrocarbon expulsion efficiency. When hydrocarbon expulsion efficiency greater than 20%, the residual organic carbon content is usually less than the original organic carbon content; and the higher the hydrocarbon expulsion efficiency is, the lower the residual organic carbon content is commonly. When the hydrocarbon expulsion efficiency is 100%, the organic carbon content reduces from the original value 1.0% to the minimum 0.575%, with a reduction near 43%. (2) Generally, in argillaceous source rocks with Ċ1 type organic matter, the residual organic carbon content is usually greater than the original organic carbon content at a
Zhou Zongying. / Petroleum Exploration and Development, 2009, 36(4): 463–468
Table 5 Calculated results of residual organic carbon contents in argillaceous source rocks with type III organic matter at different hydrocarbon expulsion efficiencies HC yield rate Residual organic carbon content at different HC expulsion efficiencies /% /(mg·g-1) 0 10% 20% 30% 40% 50% 60% 70% 75% 80% 100% 0.68 30.10 1.028 1.026 1.023 1.021 1.018 1.015 1.013 1.010 1.009 1.008 1.003 0.90 57.30 1.049 1.044 1.040 1.035 1.030 1.025 1.020 1.015 1.012 1.010 1.000 1.05 99.80 1.059 1.051 1.042 1.033 1.024 1.016 1.007 0.998 0.994 0.990 0.972 1.19 126.70 1.066 1.055 1.044 1.033 1.022 1.011 1.000 0.988 0.983 0.977 0.955 1.59 114.40 1.080 1.069 1.059 1.049 1.039 1.029 1.019 1.009 1.003 0.998 0.978 2.11 210.30 1.090 1.071 1.052 1.034 1.015 0.996 0.977 0.958 0.949 0.939 0.901 2.81 207.90 1.098 1.079 1.060 1.042 1.023 1.004 0.985 0.967 0.957 0.948 0.910 Note: The hydrocarbon yield rates are derived from the thermal compression simulation of the grey mudstone (Es3, well depth: 2 315.3 m) in Well Quan 99, Langgu Sag, northern Jizhong Depression, literature[15]. Ro/%
hydrocarbon expulsion efficiency less than 30%. If hydrocarbon expulsion efficiency is greater than 30%, the residual organic carbon content is usually less than the original organic carbon content. When the hydrocarbon expulsion efficiency is 100%, the organic carbon content reduces from the original value 1.0% to the minimum 0.802%, with a reduction near 20%, far less than the reduction amplitude of type I. It is primarily related to a hydrocarbon yield rate of typeĊ1 organic carbon smaller than that of typeĉorganic carbon. (3) Generally, in argillaceous source rocks with Ċ2 and ċ types organic matter, the residual organic carbon content is usually greater than the original organic carbon content at a hydrocarbon expulsion efficiency less than 60%. When hydrocarbon expulsion efficiency is greater than 60%, the residual organic carbon content is usually less than the original organic carbon content. When the hydrocarbon expulsion efficiency is 100%, the typeĊ2 organic carbon content decreases from the original value 1.0% to the minimum 0.933% and the type ċ organic carbon content decreases from the original value 1.0% to the minimum 0.901%, with both reduction of organic carbon contents not over 10%.
3
Conclusions
Organic carbon mass and organic carbon content are two different concepts, the former is an absolute quantity while the latter is a relative quantity as a ratio of organic carbon mass in rock to the rock mass (including organic carbon mass). Even if the residual organic carbon content in source rock is higher than the original organic carbon content, the mass of organic carbon in source rock is still decreasing. In the process of source rock’s evolution, the residual organic carbon mass is inevitably less than the original organic carbon mass as long as there is hydrocarbon expulsion. So, it is necessary to restore the original organic carbon mass by a compensation coefficient of organic carbon mass. In the mudstone with type Ċ1 organic matter from Well LF13-2-1, Zhujiangkou Basin, when Ro is 1.92% and the hydrocarbon expulsion coefficient is 25%, the residual organic carbon content (1.016%) is higher than the original organic carbon content (1.0%) while the original organic carbon mass decreases, with a compensation coefficient of
organic carbon mass as 1.071. The compensation coefficient of organic carbon mass is related to the type of organic matter, thermal evolution level and hydrocarbon expulsion efficiency of source rock. The better the type of organic matter in a source rock is and the higher the thermal evolution level and hydrocarbon expulsion efficiency are, the higher the compensation coefficient of organic carbon is. Under a condition of hydrocarbon fully expulsion (hydrocarbon expulsion efficiency as 100%), the maximum compensation coefficients of organic carbon mass in the source rocks with typesĉ,Ċ1,Ċ2 and ċ organic matters are 2.104, 1.360, 1.169 and 1.099, respectively. There exists a threshold value of hydrocarbon expulsion efficiency in each type of argillaceous source rock: 20% for typeĉ, 30% for type Ċ1, and 60% for typesĊ2 and ċ. When the expulsion efficiency is less than this threshold value, the residual organic carbon content in the source rock is generally higher than the original organic carbon content regardless of the source rock’s evolution level; when the expulsion efficiency is greater than the threshold value, the residual organic carbon content is generally lower than the original organic carbon content. Under the condition of fully expulsion (hydrocarbon expulsion efficiency as 100%), the maximum reduction amplitudes of organic carbon content in argillaceous source rocks with typesĉ, Ċ1 , Ċ2 and ċ organic matters are about 43%, 20%, 10% and 10%, respectively.
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Geology, Sinopec, 1989. [14] Zhao Mengjun, Wang Zhaoming, Pan Wenqing, et al. Lower Palaeozoic source rocks in Manjiaer Sag, Tarim Basin. Petroleum Exploration and Development, 2008, 35(4): 417-423. [15] Qin Jianzhong. Source rocks in China. Beijing: Science Press, 2005. 1-219. [16] Zhu Junzhang, Shi Hesheng, Shu Yu, et al. Heating and pressing
Petrolei Sinica, 1988, 9(1):17-24.
simulation experiments of typical source rocks of Zhu 1
Lu Shuangfang, Fu Xiaotai, Li Qiming. Restoration of the
depression: Discussion of modes of hydrocarbon generation and
organic parameters of the chemical kinetic models for
expellation and TOC recovery coefficients. Natural Gas
generation of hydrocarbon from mature organic matter in the Tarim basin and its significance. Geological Review, 2000, 46(5): 556-560. [10] Qin Jianzhong, Zheng Lunju, Tenger. Study on the restitution coefficient of original total organic carbon for high mature marine hydrocarbon source rocks. Earth Science—Journal of
Geoscience, 2006, 17(4):573-578. [17] Lu Shuangfan, Xue Haitao, Zhong Ningning. Simulating calculation of the variation of organic matter abundance and hydrocarbon-generating potential during geological processes. Geological Review, 2003, 49(3):292-297.
RESEARCH PAPER
Quantitative analysis of variation of organic carbon mass and content in source rock during evolution process Zhou Zongying Exploration & Production Research Institute, Sinopec, Beijing 100083, China
Abstract: According to the mass conservation principle of organic carbon, a mathematical model of organic carbon mass compensation coefficient is established to indicate the variation of organic carbon mass. Considering the influence of organic carbon hydrocarbon generation, hydrocarbon expulsion and rock weight loss, a mathematical model for calculating changes in organic carbon content is also established. Forward method is used to simulate the compensation coefficient of organic carbon mass and the organic carbon content value of source rocks in different evolution stages and hydrocarbon expulsion efficiency. The compensation coefficient variation depends on organic matter type, maturity degree and expulsion efficiency. The maximum compensation coefficients of types ĉ, Ċ1, Ċ2 and ċ organic matter are 2.104, 1.360, 1.169 and 1.099, respectively. There exists a threshold value in hydrocarbon expulsion efficiency in source rocks, type ĉ is 20%, type Ċ1 is 30%, type Ċ2 and type ċ is 60%. When the expulsion efficiency is less than this threshold value, the residual organic carbon content of the source rock is generally higher than the original organic carbon content; when the expulsion efficiency is greater than the value, the residual organic carbon content is generally lower than the original organic carbon content. Under the conditions of full expulsion, the maximum organic carbon reduction of types ĉ, Ċ1, Ċ2 and ċ muddy source rocks is roughly 43%, 20%, 10% and 10%. Key words:
hydrocarbon source rock; organic carbon mass; compensation factor; organic carbon content; mathematical model
Introduction Mass (quantity) and content of organic carbon in rock are two most important organic geochemical indicators for the evaluation of oil/gas resources and source rock in a sedimentary basin[1-4]. When the source rock does not generate and expel hydrocarbon in great quantity before the oil-generating threshold is reached, the organic matter content is referred as original abundance of organic matter. However, the normally measured result is the content of the residual organic matter after hydrocarbon is generated and expelled in the source rock. The residual organic matter in source rock includes dissoluble organic matter (chloroform asphalt “A”) and indissoluble organic matter (kerogen). According to the theory on hydrocarbon generation by kerogen, as the buried depth of the source rock increases continuously, higher and higher geotemperature is experienced. When the threshold value of hydrocarbon generating temperature is reached, the thermal degradation of the kerogen begins and hydrocarbon is generated and expelled in great quantity. As a result of oil/gas expulsion,
the content of organic matter should decreases continuously. As for a highly – over mature source rock, if the residual organic carbon content is used to evaluate the hydrocarbon source rock or calculate the oil/gas resources, a deviation result might occur. For this sake, many researchers have studied on the restoration of the original abundance of organic matter in the process of geological evolution by means of simulated experimental data and a number of methods[5-10]. While the absolute quantity of organic matter decreases as a result of hydrocarbon generation and expulsion, the rock weight is also reduced under the effects of a series of factors such as compaction inducing water drainage and dissolution. It was considered that the reduction of the rock weight could compensate, even exceed the reduction of the absolute quantity of organic matter in the rock under certain conditions, leading to the organic matter abundance basically constant, even rise somewhat. Therefore, debate exists extensively as to whether to restore the residual TOC in all the highly – over mature source rocks or no need at all to restore the original TOC[11-12].
Received date: 27 December 2008; Revised date: 27 April 2009. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the Ministry of Land and Resources Project: Convention petroleum resources appraisal in SINOPEC =3& Copyright © 2009, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
Zhou Zongying. / Petroleum Exploration and Development, 2009, 36(4): 463–468
In the process of thermal evolution of source rock, how does the mass (total quantity) and content of organic carbon change? How is the variation amplitude? Particularly, as to the variation of the organic carbon content, under what condition the residual organic content might be greater than the original organic carbon content? For these questions, in this paper, according to the mass conservation principle of organic carbon, a mathematical model of organic carbon mass compensation coefficient is established to reflect the variation of organic carbon mass. Comprehensively taking into account of the effects of organic carbon hydrocarbon generation, hydrocarbon expulsion and rock weight loss on hydrocarbon content, a mathematical model for calculation of the changes in organic carbon content is also established. And, the forward method is used to simulate and calculate the compensation coefficients of organic carbon mass as well as the organic carbon contents in various types of source rocks at different evolution stages and hydrocarbon expulsion efficiencies. The possible variation scopes and variation amplitudes of the compensation coefficients of organic carbon mass and the organic carbons contents are analyzed quantitatively, providing scientific basis for the objective evaluation of source rocks and the potential of oil/gas resources.
1 Quantitative analysis of variation in organic carbon mass 1.1 Mathematical model for calculation of variation in organic carbon mass When source rock does not generate and expel hydrocarbon in great quantity, the organic carbon mass is assumed as original organic carbon mass. After hydrocarbon is generated and expelled in great quantity, the organic carbon mass is defined as residual organic carbon mass. The result of original organic carbon mass divided by the residual organic carbon mass is defined as the compensation coefficient per unit mass of organic carbon. According to the mass conservation law of substances in chemical reaction, the original organic carbon mass before a source rock generates and expels hydrocarbon in great quantity equals to the sum of the residual organic carbon mass after the thermal evolution is matured plus the organic carbon mass in the expelled oil/gas. Taking 0.83 as the conversion coefficient between hydrocarbon and organic carbon, the following mathematical expressions can be obtained:
mr
m0 0.83Qs Ex m0
(1)
K
m0 mr
(2)
1 1 0.83Qs Ex
where, m0—original organic carbon mass per unit volume of source rock, g; mr—residual organic carbon mass after evolution of original per unit volume of source rock, g; Qs—cumulative hydrocarbon generating volume in source
rock at various stages of evolution (or simulated temperature points), g/g; Ex—hydrocarbon expulsion efficiency of source rock, f; K—compensation coefficient of per unit mass of organic carbon, f. Equation (1) is a model of the relationship between the original organic carbon mass and the residual organic carbon mass as built up based on hydrocarbon generation and expulsion in organic matter; equation (2) reflects the variation rule of organic carbon mass in the geological process of hydrocarbon generation and expulsion in source rock. At a given stage of evolution, the residual organic carbon mass equaled to the result of the original organic carbon mass divided by the compensation coefficient of organic carbon mass, or the original organic carbon mass equals to the product of the residual organic carbon mass at a certain stage of evolution multiplied by the compensation coefficient of organic carbon mass. By means of the compensation coefficient of organic carbon mass, the relationship between the original organic carbon mass and the residual organic carbon mass can be described quantitatively. It is a succinct method, easy for operation. And, the compensation coefficient of organic carbon mass is related only to two parameters, hydrocarbon generating rate of organic carbon and hydrocarbon expulsion efficiency of source rock. The compensation coefficient of organic carbon mass is a concept different from the restitution coefficient of organic carbon as involved often in the evaluation of source rock. The latter is a restitution coefficient related to organic carbon abundance, equivalent to the result of the original organic carbon abundance (content) divided by the residual organic carbon abundance (content). Hydrocarbon yield rate is one of crucial parameters for the evaluation of oil/gas resources by genetic method (especially the basin simulation), and the charts of hydrocarbon yield rate is established on the basis of data obtained in the thermal modeling experiments of source rock samples. To obtain the data of hydrocarbon yield rate in source rocks with different lithology and different type of organic matters, great efforts have been made by relevant research institutions, production companies and scholars in China[12-13]. Most data were acquired from experiments on oil/gas yield rates in various source rocks by means of thermal compression modeling, of which some data of gas yield rates were those of the gas derived by crude cracking completely. In this paper, some typical data of hydrocarbon yield rates in four types of source rocks were collected, including type ĉ, Ċ1, Ċ2 and ċ[13-16]. Hydrocarbon expulsion efficiency refers to the ratio of hydrocarbon expulsion volume to the hydrocarbon generating volume in source rock. The primary affecting factors include the combination mode of source rock and reservoir rock, development of fractures in source rock, type of organic matter, evolution level of organic matter and so on. The hydrocarbon expulsion efficiencies of source rock
Zhou Zongying. / Petroleum Exploration and Development, 2009, 36(4): 463–468
Table 1 Calculated results of compensation coefficients of organic carbon mass at different hydrocarbon expulsion efficiencies in source rocks with different types of organic matters Sample
Immature dark grey mudstone K1 in Well Dishu 1, Hailar Basin (well depth: 1 101.38 m)
Kerogen type
Type I
Mudstone (E2) in Well LF 13-2-1, Zhujiangkou Basin
Type II1
Grey-black mudstone(J) in Well Liushen 1, Kuche Depression
Type II2
Mudstone(E) in Well LingFeng 1, Oujiang Sag, East China Sea
Type III
Ro/%
HC yield rate/ (mg·g-1)
0.29 0.39 0.52 0.60 0.69 0.93 1.24 1.65 2.21 0.64 0.70 0.83 0.94 1.15 1.27 1.92 0.70 0.80 0.90 1.00 1.10 1.30 1.50 1.70 2.00 0.60 0.70 0.80 0.90 1.00 1.10 1.30 1.50 1.70
82.20 101.20 189.40 259.90 466.40 580.60 599.70 621.50 632.30 103.51 97.11 162.77 214.77 251.14 272.39 318.62 16.00 32.00 54.00 76.00 94.00 128.00 147.00 161.00 174.00 40.50 54.50 65.50 74.50 81.60 87.50 96.00 103.00 108.50
vary greatly in different areas. For the convenience of analysis; in this paper, five typical values are used as the hydrocarbon expulsion efficiencies of source rock, i.e. 0, 25%, 50%, 75% and 100%. 1.2 Calculation results of compensation coefficient of organic carbon mass For different hydrocarbon expulsion efficiencies (0, 25%, 50%, 75% and 100%), the compensation coefficients of organic carbon in ĉ, Ċ1, Ċ2 and ċ type of source rocks are calculated by equation (2), using the data of hydrocarbon yield rates in source rocks as acquired in thermal compression modeling (Table 1). From the calculated results, it is illustrated that: (1) When the hydrocarbon expulsion efficiency in a source rock is 0, namely, the source rock is an enclosed system without any substance exchange with outside, oil/gas generated retains completely in the source rock. In this case, the organic carbon mass (quantity) in the source rock keeps constant always in the geological process, i.e. the mass of residual organic carbon in the source rock equals to the original organic carbon mass, the compensation coefficient
Compensation coefficient of organic carbon mass at different HC expulsion efficiencies/f 0 25% 50% 75% 100% 1 1.017 1.035 1.054 1.073 1 1.021 1.044 1.067 1.092 1 1.041 1.085 1.134 1.187 1 1.057 1.121 1.193 1.275 1 1.107 1.240 1.409 1.632 1 1.137 1.317 1.566 1.930 1 1.142 1.331 1.596 1.991 1 1.148 1.348 1.631 2.065 1 1.151 1.356 1.649 2.104 1 1.022 1.045 1.069 1.094 1 1.021 1.042 1.064 1.088 1 1.035 1.072 1.113 1.156 1 1.047 1.098 1.154 1.217 1 1.055 1.116 1.185 1.263 1 1.060 1.127 1.204 1.292 1 1.071 1.152 1.247 1.360 1 1.003 1.007 1.010 1.013 1 1.007 1.013 1.020 1.027 1 1.011 1.023 1.035 1.047 1 1.016 1.033 1.050 1.067 1 1.020 1.041 1.062 1.085 1 1.027 1.056 1.087 1.119 1 1.031 1.065 1.101 1.139 1 1.035 1.072 1.111 1.154 1 1.037 1.078 1.121 1.169 1 1.008 1.017 1.026 1.035 1 1.011 1.023 1.035 1.047 1 1.014 1.028 1.043 1.057 1 1.016 1.032 1.049 1.066 1 1.017 1.035 1.054 1.073 1 1.018 1.038 1.058 1.078 1 1.020 1.041 1.064 1.087 1 1.022 1.045 1.069 1.093 1 1.023 1.047 1.072 1.099
of organic carbon mass equals to 1. (2) When the hydrocarbon expulsion efficiency in a source rock is 100%, namely, the source rock is an open system, oil/gas generated will migrate out completely from the source rock. In this case, the loss of the organic carbon quantity (mass) in the source rock is maximized and the compensation coefficient of organic carbon will be the highest under same or similar conditions in terms of organic matter type and evolution level. Taking the hydrocarbon expulsion coefficients of organic carbon in mudstone with type ĉ organic matter of Well Dishu 1, Hailar Basin as examples, which corresponds to different hydrocarbon expulsion efficiencies at a Ro as 1.24%, for a hydrocarbon expulsion efficiency 25%, the compensation coefficient is 1.142; For a hydrocarbon expulsion efficiency 50%, the compensation coefficient is 1.331; for a hydrocarbon expulsion efficiency 75%, the compensation coefficient is 1.596; when the hydrocarbon expulsion efficiency is 100%, the compensation coefficient reaches the maximum value 1.991. (3) Enclosed and open systems are two extreme cases. In nature, the processes of both hydrocarbon generation and
Zhou Zongying. / Petroleum Exploration and Development, 2009, 36(4): 463–468
expulsion are neither the enclosed systems nor the open systems completely, but some states between the enclosed and open systems. The oil/gas generated in a source rock migrates out partially, resulting in a residual organic carbon mass in the source rock less than the original organic carbon mass and a compensation coefficient of organic carbon mass greater than 1. (4) At a given hydrocarbon expulsion efficiency in a source rock, the better the type of organic matter in the source rock and the higher the thermal evolution level, the higher the compensation coefficient of organic carbon is. (5) When the hydrocarbon yield rate and hydrocarbon expulsion efficiency in a certain type of source rock are maximized, the corresponding compensation coefficient of organic carbon mass is called as the maximum compensation coefficient of organic carbon mass. The maximum compensation coefficients of organic carbon mass vary significantly in source rocks with different types of organic matters. For the source rocks with type ĉ of organic matter, when the maximum hydrocarbon yield rate is 632.30 mg/g and the hydrocarbon expulsion efficiency is 100%, the maximum compensation coefficient of organic carbon mass is as high as 2.104; for the source rocks with type Ċ1 of organic matter, when the maximum hydrocarbon yield rate is 318.62 mg/g and the hydrocarbon expulsion efficiency is 100%, the maximum compensation coefficient of organic carbon mass is 1.360; for the source rocks with type Ċ2 of organic matter, when the maximum hydrocarbon yield rate is 174.00 mg/g and the hydrocarbon expulsion efficiency is 100%, the maximum compensation coefficient of organic carbon mass is 1.169; for the source rocks with type ċ of organic matter, when the maximum hydrocarbon yield rate is 108.50 mg/g and the hydrocarbon expulsion efficiency is 100%, the maximum compensation coefficient of organic carbon mass is 1.099.
the source rock. Thus, the original organic carbon content and the residual organic carbon content can be expressed mathematically as follows:
C0
m0 I0 U w 澠1 I0澡U m0
(3)
Cr
mr IU w (1 I0 ) U mr
(4)
where ij0—original porosity of source rock, f; ij—porosity of source rock, normally calculated from the buried depth-Ro relationship equation and rock compaction equation, f; C0—original organic carbon content, f; Cr—residual organic carbon content, f; ȡw—density of water saturated in pores, 1.0 g/cm3; ȡ—density of rock matrix, 2.8 g/cm3. From equation (3), obtain:
m0
C0 >I0 U w (1 I0 ) U @ 1 C0
From mass equilibrium equation (1) and equation (4), obtain:
Cr
m澠 0 1 0.83Qs Ex ) IU w (1 I0 ) U m澠 0 1 0.83Qs Ex )
(6)
The research results of the previous researchers are used directly for the buried depth-Ro relationship equation and mudstone compaction equation[17]. Namely, the mudstone compaction equation is:
I
54.74 e 0.935 4u10
3
Z
2 Quantitative analysis of variation in organic carbon content
where, Z——Buried depth, m. The buried depth-Ro relationship equation is:
2.1 Mathematic model for calculation of organic carbon content
Z
For the convenience of discussion, at a Ro value as 0.50%, the corresponding porosity of the source rock is regarded as the original porosity; the corresponding organic carbon content and mass are regarded as the original organic carbon content and mass respectively; after an evolution to a certain extent of the source rock (with a Ro greater than 0.50%), the corresponding organic carbon content and mass are regarded as the residual organic carbon content and mass respectively. According to the definition, the organic carbon content equals to the result of organic carbon mass in source rock divided by the sum of the source rock mass (water + rock matrix, in the process of source rock evolution, the mass of rock matrix keeps constant) plus the organic carbon mass in
(5)
(7)
514.801 3711.29 Ro 1 970.27 Ro2 644.054 Ro3 78.194 2 Ro4
(8)
For various original organic carbon contents (0.5%, 1.0%, 2.0%, 5.0%, 10.0% and 15.0%), using data of hydrocarbon yield rates derived from the thermal compression modeling of four types of argillaceous source rocks (typeĉ, Ċ1, Ċ2 and ċ), when the hydrocarbon expulsion efficiency varies in a range from 0 to 100%, equations (5)-(8) can be combined to calculate the variation of residual organic carbon content as maturity changes. Tables 2-5 show the calculated results of residual organic carbon contents in four types of argillaceous source rocks at different hydrocarbon expulsion efficiencies when the original organic carbon content is 1.0%.
Zhou Zongying. / Petroleum Exploration and Development, 2009, 36(4): 463–468
Table 2 Calculated results of residual organic carbon contents in argillaceous source rocks with typeĉorganic matter at different hydrocarbon expulsion efficiencies HC yield Residual organic carbon content at different HC expulsion efficiencies/% rate/(mg·g-1) 0 5% 10% 15% 20% 25% 30% 40% 50% 75% 100% 0.60 58.80 1.017 1.015 1.012 1.010 1.007 1.005 1.002 0.998 0.993 0.980 0.968 0.70 122.00 1.031 1.025 1.020 1.015 1.010 1.005 1.000 0.989 0.979 0.953 0.927 0.80 273.00 1.041 1.029 1.018 1.006 0.994 0.983 0.971 0.948 0.924 0.866 0.807 0.90 497.00 1.049 1.028 1.007 0.985 0.964 0.942 0.921 0.878 0.835 0.727 0.619 1.00 552.00 1.056 1.032 1.008 0.984 0.960 0.936 0.912 0.864 0.816 0.696 0.575 1.10 550.00 1.062 1.038 1.014 0.990 0.966 0.942 0.918 0.870 0.821 0.701 0.580 1.30 480.00 1.071 1.050 1.028 1.007 0.986 0.965 0.944 0.902 0.859 0.753 0.647 1.50 435.00 1.077 1.058 1.039 1.019 1.000 0.981 0.962 0.923 0.884 0.788 0.691 1.70 432.00 1.082 1.063 1.044 1.025 1.005 0.986 0.967 0.928 0.890 0.794 0.697 2.00 418.00 1.088 1.070 1.051 1.032 1.013 0.995 0.976 0.939 0.901 0.807 0.713 2.20 415.00 1.091 1.073 1.054 1.036 1.017 0.998 0.980 0.942 0.905 0.812 0.718 Note: The hydrocarbon yield rates are derived from the thermal compression simulation of the black mudstone (K1qn) in Well Qian 32, Songliao Basin, literature [15]. Ro/%
Table 3 Calculated results of residual organic carbon contents in argillaceous source rocks with type Ċ1 organic matter at different hydrocarbon expulsion efficiencies HC yield rate Residual organic carbon content at different HC expulsion efficiencies /% /(mg·g-1) 0 5% 10% 15% 20% 25% 30% 40% 50% 75% 100% 0.64 103.51 1.023 1.019 1.014 1.010 1.006 1.001 0.997 0.988 0.979 0.958 0.936 0.70 97.11 1.031 1.026 1.022 1.018 1.014 1.010 1.006 0.998 0.989 0.969 0.948 0.83 162.77 1.044 1.037 1.030 1.023 1.016 1.009 1.002 0.988 0.974 0.939 0.904 0.94 214.77 1.052 1.043 1.034 1.024 1.015 1.006 0.997 0.978 0.959 0.913 0.866 1.15 251.14 1.064 1.053 1.042 1.031 1.020 1.009 0.998 0.976 0.954 0.899 0.844 1.27 272.39 1.069 1.058 1.046 1.034 1.022 1.010 0.998 0.974 0.950 0.890 0.830 1.92 318.62 1.087 1.073 1.058 1.044 1.030 1.016 1.001 0.973 0.944 0.873 0.802 Note: The hydrocarbon yield rates are derived from the thermal compression simulation of the mudstone (E2) in Well LF13-2-1, ZhujiangKou Basin, literature[16]. Ro/%
Table 4 Calculated results of residual organic carbon contents in argillaceous source rocks with typeII2 organic matter at different hydrocarbon expulsion efficiencies HC yield rate Residual organic carbon content at different HC expulsion efficiencies /% /(mg·g-1) 0 10% 20% 30% 40% 50% 60% 70% 75% 80% 100% 0.70 16.00 1.031 1.029 1.028 1.026 1.025 1.024 1.022 1.021 1.020 1.020 1.017 0.80 32.00 1.041 1.038 1.036 1.033 1.030 1.027 1.025 1.022 1.021 1.019 1.014 0.90 54.00 1.049 1.045 1.040 1.035 1.031 1.026 1.022 1.017 1.015 1.012 1.003 1.00 76.00 1.056 1.050 1.043 1.036 1.030 1.023 1.017 1.010 1.007 1.003 0.990 1.10 94.00 1.062 1.054 1.045 1.037 1.029 1.021 1.013 1.004 1.000 0.996 0.980 1.30 128.00 1.071 1.059 1.048 1.037 1.026 1.014 1.003 0.992 0.986 0.981 0.958 1.50 147.00 1.077 1.064 1.051 1.038 1.025 1.012 0.999 0.986 0.980 0.973 0.947 1.70 161.00 1.082 1.068 1.054 1.039 1.025 1.011 0.996 0.982 0.975 0.968 0.939 2.00 174.00 1.088 1.073 1.057 1.042 1.026 1.010 0.995 0.979 0.972 0.964 0.933 Note: The hydrocarbon yield rates are derived from the thermal compression simulation of the grey-black mudstone (J) in Well Liushen 1, Kuche Depression, literature [14]. Ro/%
2.2 Analysis of calculated results of variation in organic carbon content From Tables 2-5, it can be seen that: when the hydrocarbon expulsion efficiency is 0, the residual organic carbon contents in all the types of source rocks are greater than the original organic carbon contents. And, the higher the evolution level is (corresponding to deeper buried depth), the higher the residual organic carbon content is. This is because that there is no hydrocarbon expulsion in source rock and the organic carbon mass keeps constant always when the hydrocarbon expulsion efficiency is 0 , but, under compaction, the porosity of source rock decreases and porous water is drained out, leading to rock mass reduced. The calculation results show that: (1) Generally, in argillaceous source rocks with ĉ type organic matter, the
residual organic carbon content is usually greater than the original organic carbon content at a hydrocarbon expulsion efficiency less than 20%, as a result of a reduction of organic carbon mass less than the reduction of rock mass at low hydrocarbon expulsion efficiency. When hydrocarbon expulsion efficiency greater than 20%, the residual organic carbon content is usually less than the original organic carbon content; and the higher the hydrocarbon expulsion efficiency is, the lower the residual organic carbon content is commonly. When the hydrocarbon expulsion efficiency is 100%, the organic carbon content reduces from the original value 1.0% to the minimum 0.575%, with a reduction near 43%. (2) Generally, in argillaceous source rocks with Ċ1 type organic matter, the residual organic carbon content is usually greater than the original organic carbon content at a
Zhou Zongying. / Petroleum Exploration and Development, 2009, 36(4): 463–468
Table 5 Calculated results of residual organic carbon contents in argillaceous source rocks with type III organic matter at different hydrocarbon expulsion efficiencies HC yield rate Residual organic carbon content at different HC expulsion efficiencies /% /(mg·g-1) 0 10% 20% 30% 40% 50% 60% 70% 75% 80% 100% 0.68 30.10 1.028 1.026 1.023 1.021 1.018 1.015 1.013 1.010 1.009 1.008 1.003 0.90 57.30 1.049 1.044 1.040 1.035 1.030 1.025 1.020 1.015 1.012 1.010 1.000 1.05 99.80 1.059 1.051 1.042 1.033 1.024 1.016 1.007 0.998 0.994 0.990 0.972 1.19 126.70 1.066 1.055 1.044 1.033 1.022 1.011 1.000 0.988 0.983 0.977 0.955 1.59 114.40 1.080 1.069 1.059 1.049 1.039 1.029 1.019 1.009 1.003 0.998 0.978 2.11 210.30 1.090 1.071 1.052 1.034 1.015 0.996 0.977 0.958 0.949 0.939 0.901 2.81 207.90 1.098 1.079 1.060 1.042 1.023 1.004 0.985 0.967 0.957 0.948 0.910 Note: The hydrocarbon yield rates are derived from the thermal compression simulation of the grey mudstone (Es3, well depth: 2 315.3 m) in Well Quan 99, Langgu Sag, northern Jizhong Depression, literature[15]. Ro/%
hydrocarbon expulsion efficiency less than 30%. If hydrocarbon expulsion efficiency is greater than 30%, the residual organic carbon content is usually less than the original organic carbon content. When the hydrocarbon expulsion efficiency is 100%, the organic carbon content reduces from the original value 1.0% to the minimum 0.802%, with a reduction near 20%, far less than the reduction amplitude of type I. It is primarily related to a hydrocarbon yield rate of typeĊ1 organic carbon smaller than that of typeĉorganic carbon. (3) Generally, in argillaceous source rocks with Ċ2 and ċ types organic matter, the residual organic carbon content is usually greater than the original organic carbon content at a hydrocarbon expulsion efficiency less than 60%. When hydrocarbon expulsion efficiency is greater than 60%, the residual organic carbon content is usually less than the original organic carbon content. When the hydrocarbon expulsion efficiency is 100%, the typeĊ2 organic carbon content decreases from the original value 1.0% to the minimum 0.933% and the type ċ organic carbon content decreases from the original value 1.0% to the minimum 0.901%, with both reduction of organic carbon contents not over 10%.
3
Conclusions
Organic carbon mass and organic carbon content are two different concepts, the former is an absolute quantity while the latter is a relative quantity as a ratio of organic carbon mass in rock to the rock mass (including organic carbon mass). Even if the residual organic carbon content in source rock is higher than the original organic carbon content, the mass of organic carbon in source rock is still decreasing. In the process of source rock’s evolution, the residual organic carbon mass is inevitably less than the original organic carbon mass as long as there is hydrocarbon expulsion. So, it is necessary to restore the original organic carbon mass by a compensation coefficient of organic carbon mass. In the mudstone with type Ċ1 organic matter from Well LF13-2-1, Zhujiangkou Basin, when Ro is 1.92% and the hydrocarbon expulsion coefficient is 25%, the residual organic carbon content (1.016%) is higher than the original organic carbon content (1.0%) while the original organic carbon mass decreases, with a compensation coefficient of
organic carbon mass as 1.071. The compensation coefficient of organic carbon mass is related to the type of organic matter, thermal evolution level and hydrocarbon expulsion efficiency of source rock. The better the type of organic matter in a source rock is and the higher the thermal evolution level and hydrocarbon expulsion efficiency are, the higher the compensation coefficient of organic carbon is. Under a condition of hydrocarbon fully expulsion (hydrocarbon expulsion efficiency as 100%), the maximum compensation coefficients of organic carbon mass in the source rocks with typesĉ,Ċ1,Ċ2 and ċ organic matters are 2.104, 1.360, 1.169 and 1.099, respectively. There exists a threshold value of hydrocarbon expulsion efficiency in each type of argillaceous source rock: 20% for typeĉ, 30% for type Ċ1, and 60% for typesĊ2 and ċ. When the expulsion efficiency is less than this threshold value, the residual organic carbon content in the source rock is generally higher than the original organic carbon content regardless of the source rock’s evolution level; when the expulsion efficiency is greater than the threshold value, the residual organic carbon content is generally lower than the original organic carbon content. Under the condition of fully expulsion (hydrocarbon expulsion efficiency as 100%), the maximum reduction amplitudes of organic carbon content in argillaceous source rocks with typesĉ, Ċ1 , Ċ2 and ċ organic matters are about 43%, 20%, 10% and 10%, respectively.
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