## 1 Introduction  
As the intensity and depth of coal mining increase, the resulting gas accidents pose a significant threat to the safety of coal mines [1, 2]. At present, the most effective preventive measure is to carry out CBM extraction in advance for the target coabled [3, 4]. The Chinese government issued \"Interim Provisions on Coal Mine Gas Extraction Standards\", which clearly require that the residual gas content should first be calculated based on the CBM extraction volume, and the actual residual gas content measured after meeting the specified requirements [5, 6]. However, the target coables of most coal mines in China have multiple adjacent coalbeds. To improve the utilization rate of boreholes, the layer-through boreholes are arranged to extract from multi-seams in rocky roadways at the top or bottom plates [7]. However, due to the limitations of the existing technology and equipment development level, technicians can only measure the mixed CBM extraction of multi-seams at the mouth of boreholes and cannot measure the CBM extraction of a single target coabled. In this area, the researchers have proposed various research methods, such as the theoretical analysis method, numerical simulation method and field test method. The extraction weight analogy method is a theoretical analysis method which estimates the extraction ratio based on the thickness and bulk weight, as well as the gas content and radius of each coabled. However, this method does not consider the differences between each coalbed and gas storage, and the calculation result does not conform to the reality of CBM extraction from multi-seams [8]. The normalization method for extracting relevant quantities is also a theoretical analysis method which multiplies and normalizes the relevant quantities of CBM (coalbed thickness, gas content, permeability coefficient), but the method considers the relevant quantities of CBM extraction in a more idealized way, and it does not cover all the influencing factors [9]. The numerical simulation method is used to establish the extraction ratio by modelling the CBM extraction from multi-seams, which is only suitable for mines with simple coalbeds and gas storage, and it is difficult to standardize the parameters for complex mines [10]. The measured flow method is a field test method which is determined by separately measuring the individual extraction volume of the target coalbed and multi-seams [11; 12]. However, this method ignores the occupancy influence of adjacent coalbeds on the extraction space of layer-through boreholes, and a lot of manpower and material resources are needed in the filed study. Therefore, the problem of accurately quantifying the extraction ratio of a target coalbed needs further research. Isotope geochemistry has been employed in sourcing hydrocarbon reservoirs in the oil and gas industry [13], the type of CBM genesis [14] and its geochemical genesis [15; 16]. Some scholars have used a combination of carbon isotopes and chromatographic hydrocarbon fingerprint technology to quantitatively obtain the mixing ratio of various organic genetic types of natural gas [17; 18]. Some other scholars have established a linear relationship between the CBM carbon isotope of mixed gas and the mixing ratio, thus accurately dividing the ratio of CBM and oil-type gas in mixed gas reservoirs [19; 20; 21; 22]. In the Panbei and the Qidong Coal Mines in Anhui Province and the Sijiazhuang mine field in Shanxi Province, geological genesis was investigated by testing the distribution characteristics of stable carbon isotopes in CBM, respectively [23; 24; 25]. The CBM sources in the goat were determined by comparing the isotopic differences between single CBM and mixed gas from multi-seams [26; 27; 28; 29; 30]. The above studies have laid the foundation for the application of isotope geochemistry in determining the extraction ratio of target CBM from multi-seams. In this paper, the use of isotope geochemistry is proposed for the first time in the industry.