Analysis of Concentration and Isotope Ratio of Noble Gases in Natural Gas

Natural gas contains a large amount of hydrocarbon gases but only trace amount of noble gases. To measure concentrations of noble gases in natural gas, the active gas components (CH₄, C₂⁺, CO₂, etc.) have to be previously removed. The device of noble gas analytical system was designed for the purification and enrichment of noble gases in natural gas, and their element and isotope ratio were analyzed by quadrupole mass spectrometry and isotopic mass spectrometry, respectively. Dynamic vacuum of this system was better than 1.0×10^-6 Pa. The natural gas was firstly introduced to the analytical system and then exposed to a Zr-Al purifying furnace (held at 350 ℃) for 20 min, during which the hydrocarbons, CO₂ and other active gases could be absorbed, aliquot of purified noble gases were transfered to the gas getters for the second purification (10 min), then ⁴He, ²²Ne, ⁴⁰Ar, ⁸⁴Kr, ¹³²Xe were measured by QMS. At last, separation of He+Ne, Ar and Kr+Xe were basically successful by temperature modulated cold finger, and then noble gases isotopic compositions could be measured. Because the noble gases concentration and isotopic ratio in air were well determined, the air could be employed as a standard sample to determine. The degree of purifying noble gases can reach to 99.9%. The relative standard deviation of He, Ne, Ar, Kr content measured by QMS is less than 10%, isotope ratio has a better accuracy generally less than 5%. Although noble gases are ubiquitous in natural gas, they are more accurate and sensitive than stable carbon isotopes for tracing hydrocarbon formation and evolution because of the chemical inertness and time-accumulating effect. However, isotope compositions of noble gases have only been previously used to study tectonic movements, the age of source rock, and terrestrial heat flow, concentration of noble gases, which is good at determining the geochronology of hydrocarbon accumulation, has not yet been published to our knowledge due to the restriction of analysis method. This analytical technique established for noble gases in natural gas could provide abundant geological and geochemical informations containing 23 data about concentration and isotope composition of He, Ne, Ar, Kr, Xe, which will be a powerful tool for studying the geochemistry of natural gas. Natural gas samples from six wells (T813-1H, T814-1H, T820(K), TP37, TP313H and YK17) of Tahe oil field and YAKELA gas field in Tarim Basin were collected and stored in stainless-steel cylinders. These samples were selected to measure concentration and isotope ratio of noble gases, and the results further clarified the regional tectonic activity differences between Tahe oil field and YAKELA gas field, and the differences on source rock characteristics.