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The identification of the origin and source of natural gas is always a difficult and hot issue. Hereinto, the maturity identification is one of the most important scientific problems. Many empirical equations have been established to decipher the relationship between the maturity of gas source rocks and the carbon isotopic composition of natural gas. However, these equations proposed often fail to identify the maturity of the source rocks correctly, which in turn prevents the identification of genetic types and source rocks of the natural gas because the petroliferous sedimentary basins in China are complex and diverse, with multiple sets of source rocks and different thermal history. In this paper, the oil-associated gas from the Permian lacustrine source rocks and the coal-derived gas from the Jurassic source rocks in Junggar and Turpan-Hami basins have been investigated to decipher the relationship between the maturity (vitrinite reflectance) of gas source rocks and the carbon isotopic composition of methane. The equations established are δ13C1=25lgRo−42.5 for oil-associated gas, and δ13C1=25lgRo−37.5 for coal-derived gas. These new equations are suitable for the maturity identification of source rocks in most petroliferous basins, and favorable for the identification of the genetic type and source of natural gas, which is very important to improve the geological theory of natural gas. [ABSTRACT FROM AUTHOR]

According to the compositional and isotopic analyses of He, CO 2 and CH 4 of 95 gas samples from the Ordos Basin and 76 gas samples from the Bohaibay Basin in China, this study carried out a detailed investigation on the geochemical characteristics of helium and CO 2 from the cratonic and rift basins, especially the cratonic Ordos Basin and rift Bohaibai Basin in China. Compared to the rift Bohaibay Basin, the cratonic Ordos Basin has lower level of helium and CO 2 , lower R/Ra ratio and lower δ 13 C CO2 maximum value. When the content of CO 2 is < 10%, both cratonic and rift basins have organic CO 2 associated with the hydrocarbon generation process and inorganic CO 2 from the thermal decomposition of carbonates or dissolution of organic acid. Rift basin can also have CO 2 > 20% and δ 13 C CO2 of − 6 ± 2‰ or − 4‰ –− 9‰, which is of volcanic-magmatic or mantle-derived origin, while cratonic basin has no such type of CO 2 . The cratonic basins (Ordos, Sichuan) have CO 2 < 5% and relatively smaller variation range of δ 13 C CO2 , i.e., 25.6‰ (− 0.8‰ –− 26.4‰). The rift basins have higher level of CO 2 , R/Ra ratios and δ 13 C CO2 values. The content of CO 2 can reach 100%, the R/Ra ratio can be up to 6.45 and the variation range of δ 13 C CO2 is 34‰ (7‰ –− 27‰). Bigger variation range of δ 13 C CO2 implies stronger tectonic activities, which is not favorable for the gas accumulation. While smaller variation range means more stable structure, which is favorable for the discovery of large gas fields. Sample in the cratonic basin in China generally has CH 4 / 3 He of 10 9 – 10 12 and R/Ra < 0.1 and in the rift basin it has CH 4 / 3 He of 10 5 – 10 13 and R/Ra of 0.1– 6.45. It is concluded that methane with CH 4 / 3 He ≤ 10 8 is of inorganic origin and methane with CH 4 / 3 He ≥ 10 11 is of organic origin. The regression equation for the CH 4 / 3 He-R/Ra in the cratonic basin is: y = 3E + 11e − 46.28x and it is: y = 4E + 10e − 1.791x in the rift basin. Both Ordos and Bohaibay basins found gas wells with He > 0.05%, which has significant commercial values and provides a clue for the exploration of helium-rich gas fields. Traps with R/Ra > 2 is a favorable clue for the discovery of CO 2 gas fields. Seven CO 2 gas fields have been found in the Bohaibay Basin, but no systematic development has been carried out and only some liquid CO 2 has been injected for tight oil from the Huagou CO 2 gas field. [ABSTRACT FROM AUTHOR]

Abstract: A number of contributions have demonstrated that the stable carbon and hydrogen isotopic compositions together with molecular compositions have a great significance on determination for gas origin and gas generation pathways of biogenic gases. Together with some data from published work, a total of 143 gas samples were compiled from different biogenic gas producing locations in China, to investigate the gas origin and gas generation pathways by the molecular composition, stable carbon and hydrogen isotopes. The results demonstrate that biogenic gases in China are dominated by methane with low level of C2+3 (generally less than 0.5%) and low δ13CCH4 values (generally≤55‰). The relatively high level of C2+3 (up to 11.79%) in the biogenic gases from the Baise and Songliao basins indicates the possible mixture of low-mature thermogenic gases. Biogenic gases from the Jiangzhe coastal area (including Shanghai, Zhejiang, and Jiangsu provinces) and Yingqiong basin have δDCH4 values greater than −190‰, indicating marine salt water depositional environments. However, all other gases have δDCH4 values less than −190‰, possibly indicating a terrestrial freshwater or freshwater to slightly saline water depositional environment. The methanogenic pathway of biogenic gases in China is dominated by biogenic carbonate reduction, though a number of these biogenic gases were generated in a terrestrial freshwater or freshwater to slightly saline water depositional environments. Compared to the bacterial carbonate reduction methanognic pathway in the Luliang basin, gas formation mechanism in the Baoshan basin was more toward bacterial methyl-type fermentation, likely due to the relatively higher geothermal gradient in the Baoshan basin. [Copyright &y& Elsevier]

Abstract: Based on quantum chemistry calculations for normal octane homolytic cracking, a kinetic hydrogen isotope fractionation model for methane, ethane, and propane formation is proposed. The activation energy differences between D-substitute and non-substituted methane, ethane, and propane are 318.6, 281.7, and 280.2cal/mol, respectively. In order to determine the effect of the entropy contribution for hydrogen isotopic substitution, a transition state for ethane bond rupture was determined based on density function theory (DFT) calculations. The kinetic isotope effect (KIE) associated with bond rupture in D and H substituted ethane results in a frequency factor ratio of 1.07. Based on the proposed mathematical model of hydrogen isotope fractionation, one can potentially quantify natural gas thermal maturity from measured hydrogen isotope values. Calculated gas maturity values determined by the proposed mathematical model using δD values in ethane from several basins in the world are in close agreement with similar predictions based on the δ13C composition of ethane. However, gas maturity values calculated from field data of methane and propane using both hydrogen and carbon kinetic isotopic models do not agree as closely. It is possible that δD values in methane may be affected by microbial mixing and that propane values might be more susceptible to hydrogen exchange with water or to analytical errors. Although the model used in this study is quite preliminary, the results demonstrate that kinetic isotope fractionation effects in hydrogen may be useful in quantitative models of natural gas generation, and that δD values in ethane might be more suitable for modeling than comparable values in methane and propane. [Copyright &y& Elsevier]

Abstract: To understand the carbon isotope fractionation during the mineral-catalyzed Fischer-Tropsch synthesis of hydrocarbons under hydrothermal conditions, experiments on formic acid were carried out at 300°C and 35 MPa using gold tubes in the presence of Fe as a catalyst. The experiments were composed of two groups: with and without water. Due to the limited volume of the gold tubes, only methane was available for isotopic analyses among all produced hydrocarbons. The results demonstrate that CO2 is the gas most enriched in 13C whereas methane is the gas most depleted in 13C. Moreover, methane becomes more and more depleted in 13C with an increase in reaction time. The carbon isotopic fractionation between CO2 and CH4 (α(CO2-CH4)) reached 1.052–1.059 at 144 h, which is similar to those of microbial reduction of CO2 to CH4 by methanogenic bacteria (1.048–1.079). This implies that carbon isotopic fractionation during Fischer-Tropsch synthesis is controlled by kinetic isotopic effects. [Copyright &y& Elsevier]