# Characteristics and Origin of Natural Gas in Yongfeng Sub-Sag of Bogda Mountain Front Belt
## 3 Sample Collection and Analysis  
In this study, a total of 18 rock debris samples of the Lucaogou Formation source rocks were collected from Well Xyd1. The main focus of sample collection was to collect the black mudstone in the lower section of the Lucaogou Formation. The rock debris samples were concentrated at a depth range of 1874 m to 1754 m. The sampling interval was concentrated as much as possible in the high-quality source rock development section and evenly distributed. In total, 83 analysis tests were conducted on the 18 samples mentioned above, including 18 analyses of total organic carbon content (TOC) in the source rocks, 13 analyses of chloroform bitumen A, 13 analyses of rock-eval pyrolysis, and 13 analyses of kerogen vitrinite reflectance (R\({}_{\text{o}}\)). The analysis of organic carbon in the source rocks was carried out using a CS-844 organic carbon analyzer. The sample was burned in a high-temperature oxygen stream to convert organic carbon into carbon dioxide. The total organic carbon (TOC) content was detected and provided using an infrared detector. The analysis of chloroform bitumen A was carried out using a YS fully automatic multifunctional extractor. The chloroform heated in a water bath at 78 \({}^{\circ}\)C was converted into vapor and then cooled through a condenser tube to form a liquid at the top of the sample, which was applied to the sample powder to dissolve the organic matter in the chloroform. By continuously repeating the distillation process, the organic matter in the sample was completely dissolved in the chloroform. Finally, the content of the organic matter that was dissolved in the chloroform from the solid sample was determined using the mass method. The rock-eval pyrolysis analysis of source rocks was carried out using the Rock Eval 6 type source rock evaluation instrument. The testing method involved initially heating the samples to 300 \({}^{\circ}\)C to measure the content of free soluble hydrocarbons (S\({}_{1}\)), and subsequently heating the samples to 600 \({}^{\circ}\)C for the pyrolysis analysis. The content of pyrolysis hydrocarbons (S\({}_{2}\)) was measured, and the temperature at which the yield of pyrolysis hydrocarbons (S\({}_{2}\)) was highest was the peak temperature (T\({}_{\text{max}}\)). The vitrinite reflectance measurement of kerogen was carried out using the MSP400 micro fluorescence spectrometer. The kerogen samples were bonded and polished with a binder to form a thin section, and then measured using a microphotometer under direct light with a wavelength of \(\lambda\) = (546 \(\pm\) 5) nm. The microphotometer converted the reflected light intensity into an electric current through a photomultiplier tube. The output current of the sample was compared with the output current generated by a standard substance with known vitrinite reflectance under the same conditions, in order to obtain the vitrinite reflectance of the tested substance. Fracturing and oil testing were conducted on three selected layers in the Lucaogou Formation of Well Xyd1, and natural gas flow was obtained in all layers. Two natural gas samples were collected at different time periods for each layer of testing and a total of six natural gas samples were collected for the three-layer testing. Natural gas composition was analyzed for the above-mentioned natural gas samples and two carbon isotope analyses were conducted for the upper two layers of the natural gas samples. Natural gas composition was analyzed using an HP 7890A gas chromatograph. The natural gas sample was injected into the gas chromatograph, and the content of each component was separated and determined by the difference in retention time of the different components in the fixed-phase column. The carbon isotope analysis of natural gas was carried out using a Delta V Ad-vantage isotope mass spectrometer. The testing method involved raising the temperature of the gas chromatograph oxidation furnace connected to the isotope mass spectrometer to 850 \({}^{\circ}\)C and passing helium gas through it.