## 3 Methods  
### Mud Gas Detection Principle  
Helium has two stable isotopes, \({}^{3}\)He and \({}^{4}\)He. \({}^{4}\)He is radiogenic, and \({}^{3}\)He is cosmogenic. According to its source, helium can be classified into atmospheric helium, crustal helium, and mantle-derived helium, which have different helium isotope ratios, corresponding to 1.4 \(\times\) 10\({}^{-6}\), 2 \(\times\) 10\({}^{-8}\), and 1.1 \(\times\) 10\({}^{-5}\), respectively [7; 9; 28; 29; 30; 31]. Helium in natural gas reservoirs primarily consists of crustal and mantle-derived helium. Since the gas measured during the drilling process is released from the mud, which comes in full contact with air in the mud tank, a mixture of air and gas will form (Figure 2a). Therefore, in order to detect helium anomalies in the mud gas, the minimum detection limit of the instrument needs to be at least greater than the helium concentration in air (5.24 ppm, [7; 32; 33]).  
To meet this requirement, we used a mass spectrometer for helium detection. Compared with other instruments, mass spectrometers have lower detection limits, allow for real-time access to gas information, have a wider application range, and provide automated detection [34; 35]. Different types of mass spectrometers have different detection cycles. Traditional large magnetic mass spectrometers usually perform static measurements, require complex gas purification processes and long detection cycles, with a single test lasting more than two hours, and cannot determine the helium concentration in the mud gas in real time during the drilling process; so, they are not suitable for helium detection during drilling. Compared to large magnetic mass spectrometers, small mass spectrometers are characterized by their ability to perform rapid and dynamic measurements and by their portability. The quadrupole mass spectrometer is one of the most advanced and widely used small mass spectrometers and is very suitable for drilling site detection [19; 20]. The helium detector was designed based on the quadrupole mass spectrometer. During the measurement process, the mud gas followed two paths, one into the chromatograph to measure the concentration of total hydrocarbons, hydrogen, and carbon dioxide, and the other into the helium detector to measure helium concentration (Figure 2b).  
In the determination of the helium isotope ratio (\({}^{3}\)He/\({}^{4}\)He), achieving accurate results is primarily constrained by the \({}^{3}\)He\({}^{+}\) signal value, which is generally several orders of magnitude lower than the \({}^{4}\)He\({}^{+}\) signal value. During the measurement, gas impurities are ionized by the ion source, generating charged ions with the same mass-to-charge ratio as the  
Figure 2: Schematic diagram of the mud gas real-time detection device. (**a**) Degassing process of mud gas in mud tank, (**b**) overall mud gas testing process at drilling site.  
target ion. These ions will interfere with the measurement. The ion interfering with \({}^{3}\)He\({}^{+}\) is HD\({}^{+}\), and that interfering with \({}^{4}\)He\({}^{+}\) is \({}^{12}\)C\({}^{3+}\)[36]. Typically, the resolution of a small mass spectrometer is too low to distinguish \({}^{3}\)He\({}^{+}\) and \({}^{4}\)He\({}^{+}\) from their corresponding interfering ions; only a large magnetic mass spectrometer can do this and thus accurately measure the helium isotope ratio [37]. In the field, the measured \({}^{4}\)He concentration is usually used to represent the total helium concentration due to the low resolution of portable instruments. The determination of helium isotope ratios was beyond the scope of this paper.