Citation: | YANG Zongcai,XU Xuemin,YANG Jiajia,et al. A Pre-Treatment Method for the Determination of Organic Carbon Isotope Composition in Sedimentary Rocks[J]. Rock and Mineral Analysis,2024,43(6):847−857. DOI: 10.15898/j.ykcs.202403110038 |
The organic carbon in sedimentary rocks is mainly in the form of kerogen, and it is necessary to extract kerogen from samples before obtaining the organic carbon isotope value. The extraction process requires a significant quantity of hazardous chemicals and a long preparation process. Therefore, in daily work, there is an urgent need to develop a more convenient and environmentally friendly pre-treatment method. A simple acid treatment method was established, and 110 sedimentary rock samples with different lithology (limestone, shale, oil shale) and different organic carbon content range (0.83%−35.33%) were selected for comparison experiments of two pretreatment methods. The results show that for 94% of the samples, the difference of carbon isotope values obtained by the acid pretreatment method established in this study and the kerogen extraction method was less than 1.0‰, which met the deviation requirements for repeated measurements, indicating that this pretreatment method can be used to accurately obtain the key geological parameter of an organic carbon isotope value. Furthermore, the organic carbon content and lithology of the samples does not influence the results, demonstrating the applicability of this method to typical geological samples and fulfilling the requirements of geological exploration and investigation. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202403110038.
Significance: The carbon pool in sedimentary rocks comprises both organic and inorganic carbon. Organic carbon predominantly exists in the form of kerogen, which accounts for over 80% of the total organic carbon in these rocks, while inorganic carbon primarily occurs as carbonate[1-2]. There are notable differences in the isotopic values of organic and inorganic carbon, each bearing distinct geological significance[3-6]. Currently, it is widely accepted that the stable carbon isotope value of organic matter is largely determined by its source[7-9], remaining relatively unaffected by thermal evolution. This characteristic renders it a valuable tool for distinguishing types of organic matter in the field of oil and gas geochemistry[10-15].
Since organic carbon in sedimentary rocks primarily exists in the form of kerogen, it is necessary to extract kerogen from samples before obtaining the organic carbon isotope value[30-32].
The preparation process of kerogen is complex and time-consuming, which limits the rapid acquisition of organic carbon isotope data from sedimentary rocks and affects the geological research and exploration evaluations. Furthermore, a significant amount of chemical reagents are utilized in the kerogen preparation process, which can have considerable environmental impacts. Consequently, some researchers have proposed a pretreatment method that only uses hydrochloric acid to replace the pretreatment process of kerogen preparation[33-36]. This method is simple and efficient, and it has garnered significant attention from scholars in recent years. However, the current application objects of this method are mainly modern sediments, while the data for ancient sedimentary rocks are very limited. There is also a lack of systematic comparison of the impact of the two pre-processing methods on the organic carbon isotope value determination of rock samples with different lithology and TOC values. It is not clear whether this simple pretreatment method can completely replace the kerogen extraction method[33-41].
This study established a pre-treatment method for effective separation of organic carbon components from sedimentary rocks based on dilute hydrochloric acid, and experiments were conducted using samples of various lithologies and organic carbon contents, including shale, limestone, and oil shale. The results demonstrate that the organic carbon isotope data obtained through this pre-treatment method are comparable to those acquired via the traditional kerogen extraction method. This research offers a more convenient and environmentally friendly pre-treatment method for isotope research in sedimentary rocks samples.
Methods: 110 typical profile rock samples were collected from Qiangtang Basin, including shale, oil shale and limestone. Each crushed sample was divided into 3 parts for pretreatment and subsequent experimental analysis. The Part 1 sample was used for extraction and preparation of kerogen, with the sample weight of about 100g. Part 2 and Part 3 samples were used to carry out acid treatment with about 5g per sample, and the organic carbon contents and isotope values were measured respectively after treatment.
The total organic carbon (TOC) content of sedimentary rock samples was determined by a Leco CS-744 carbon sulfur analyzer. The EA-IRMS combined system was used to determine the carbon isotope value of the sample. The combustion tube temperature of the element analyzer was set at 950℃, and the reduction tube temperature was set at 600℃. During the test process, the test results of standard substances and repeated samples need to meet the quality requirements of standard methods, and the carbon isotope test error of repeated samples is less than 0.5‰.
Data and Results: The research results indicate that the Δ13C value (δ13Cacid−δ13Cker) of the samples processed by the two methods ranges from −2.8‰ to 1.8‰. A correlation analysis was conducted on the isotope values obtained using these two pre-processing methods, revealing a strong consistency between the two datasets. The correlation followed the linear relationship described by the equation y=0.97x−0.61, with a correlation coefficient of R2=0.97 (Fig.1). Notably, the proportion of samples with Δ13C values less than 1.0‰ constitutes 94% of the total samples. These samples satisfy the repeatability error requirements for carbon isotope determination based on current standards, indicating that the carbon isotope values obtained through different pretreatment methods are highly comparable[2,21,24,30].
The study conducted a comparative analysis of carbon isotope data obtained through two pre-processing methods applied to samples with varying lithology (Fig.2). It was observed that shale samples were significantly more influenced by the different pre-processing methods, and a total of six shale samples exhibit Δ13C values exceeding 1.0%, significantly higher than those of other lithologies. Analysis indicates that this phenomenon may be attributed to either a relatively high clay mineral content in the shale or the presence of non-kerogen organic carbon in these samples[13,32]. However, there is currently little data in this part, and additional experiments are still needed to accurately reveal the reasons for this difference.
In addition, the article compared the conditions of samples with varying total organic carbon (TOC) contents (Fig.3). The results indicate that the TOC content does not consistently influence the differences between the two methods. Specifically, only six samples exhibit differences greater than 1.0‰, with their TOC values primarily ranging from 5.00% to 15.00%. Notably, these samples tend to have a high oil content. During the acid treatment process, an oil film forms around the samples, which hinders the acid from fully contacting and reacting with them, ultimately leading to discrepancies in the measurement results.
The study selected three crucibles with varying water permeability rates to conduct comparative experiments. The results indicate that the differences in water permeability rates of acid treatment containers do not significantly impact the organic carbon isotope values.
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