Intramolecular 13C isotope distributions in propane from natural gases and produced in the laboratory
Position-specific isotope analysis of propane provides novel information to understand its sources and sinks.However,little is known about the overall evolution trends because limited data are available.Here,we compared the position-specific isotope values of propane from natural gases with a wide maturity range(reflectance of 1.0％to 3.0％)from China,America,and Brazil,and from simulated gases produced by various types of organic precursors(e.g.,type Ⅰ-Ⅲ kerogen,crude oil,and nC25)in the laboratory.We found that the evolution trends of intramolecular isotope compositions of propane from laboratory-simulated gases differed greatly from those of natural gases.This is likely associated with the fact that intramolecular isotopes of propane in the early maturity phase are observed clearly in laboratory experiments but not in geological basins,especially for propane from oil-type gases.The δ13C values at both central and terminal carbon positions in propane from natural oil-type gases showed little variation(within 2‰-3‰)during maturation.Thus,intramolecular isotopes of propane are a reliable indicator of gas origin.For propane from coal-type gases,there were two stages of evolution.With increasing maturity,the δ13C value first increased rapidly at the central carbon position but remained stable at the terminal carbons.Then the intramolecular isotope compositions approached the theoretical values for propane generated from the cracking of long chain alkanes.This was attributed to a large contribution of isoprenoid organic precursor to propane generation in coal at maturity before the dry gas window stage.The intramolecular isotopes of propane were clearly affected by post-generation processes.With chemical oxidation degradation by high-valence Fe(Mn)oxide and thermal sulfate reduction,the carbon in the central position became heavier twice as fast as that in the terminal positions.Natural gases from the Keshen 8 gas field in the Kuche Depression of the Tarim Basin have been strongly oxidized by high-valence metal oxides,and approximately 50％of the propane has been degraded.Consequently,the central carbon of propane is much heavier than the terminal carbons,with the △13Ccentral at 14.9‰.CO2,which partly originates from alkane oxidation,is 13C depleted(δ13CCO2 of-16.4‰).Natural gases from the Keshen 13 gas field showed originates from alkane oxidation,is higher δ13CCO2(2.5‰)than those from other gas fields in the area.Additionally,the propane samples had normal intramolecular isotope compositions consistent with source rock maturity.That is likely a result of later charging(by about 1-2 Ma)of natural gases in the Keshen 13 gas field than the natural gases in the other gas fields.The dominant origin of natural gases in both the Kekeya and Akemo gas fields of the southwest Tarim Basin is Carboniferous marine shales.Some gases generated from Jurassic/Permian coal at moderate to high maturity have mixed into the Akemo gas field and caused 13C depletion at the central carbon in propane.The propane intramolecular isotope compositions are similar in Ordovician natural gases from the Tazhong 45,Lungudong,Jilake,and Fuman oilfields,and clearly different to those from the Hetianhe gas field.These results demonstrate that the intramolecular carbon isotope ratios of propane will be useful for revealing the origins of natural gases and evaluating the effects of post-generation processes.This knowledge will aid research on the carbon cycle on Earth and other planets.
intramolecular isotopecharging historyTarim Basincoal type gasoil type gaschemical oxidation
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