Clayton, C. (1991). "Carbon isotope fractionation during natural gas generation from kerogen." Marine and Petroleum Geology 8(2): 232-240.
In petroleum exploration it is important to be able to determine the origin of any gas which is found. This paper describes a new method of estimating the source-type and maturity of a gas based on a Rayleigh fractionation model. Kerogen is divided conceptually into a labile (dominantly oil-generating) fraction and a refractory, gas-prone, component. [delta]13C of methane from either kerogen type, and ethane, propane and butane for gases from labile kerogen, can be defined as a function of [delta]13C of the gas precursor groups in kerogen, a kinetic isotope fractionation factor, k, and the extent of gas generation. The isotopic ratio of the methane precursors relative to bulk kerogen, determined from laboratory pyrolysis, are -17.5[per mille sign] for labile kerogen and -1.4[per mille sign] for refractory kerogen. Values for ethane, propane and butane from labile kerogen, based on field correlations, are -4.9[per mille sign], -2.2[per mille sign] and -1.6[per mille sign] respectively. The corresponding fractionation factors are 0.9892, 0.9919, 0.9947 and 0.9975 for methane, ethane, propane and butane respectively from labile kerogen, and 0.9984 for methane from refractory kerogen. Using these parameters, summary diagrams are constructed which allow differentiation of these sources from each other and from biogenic gases and cracked oil, and recognition of gases of mixed origin. If an independent estimate of [delta]13C for the source kerogen is possible, then [delta]13C of the gas components can be used to estimate maturity in terms of the Gas Generation Index, the fraction of gas potential which has been realized.
Cramer, B., B. M. Krooss, et al. (1998). "Modelling isotope fractionation during primary cracking of natural gas: a reaction kinetic approach." Chemical Geology 149(3-4): 235-250.
A numerical model has been developed to compute stable carbon isotope variations in natural gas (methane) by calculating and generation as a set of parallel first-order reactions of primary cracking. The goal of this work was to combine the description of isotope fractionation with established kinetic models for gas generation. Stable carbon isotope ratios of methane from sedimentary organic matter are characterized by the initial carbon isotope ratio of methane precursors within the organic matter and by a constant difference in activation energy between and generation from corresponding precursor sites. Methane generation is calculated separately for and . A difference in activation energy automatically implies a temperature dependence of fractionation processes which has not been taken into consideration in previous works. This new model offers a theoretical explanation and mathematical description of the observed variability of [delta]-values of methane during open-system pyrolysis experiments. Carbon isotopes of methane within natural gas of thermogenic origin can be simulated for any geological temperature history. The application of the method to two coaly rock samples of the Pokur formation from northern West Siberia results in simulated carbon isotope values of methane which are very similar to those in the natural gas within the reservoirs of the Pokur formation ([delta]=-42[per mille sign] to -54[per mille sign]). This finding supports a thermogenic origin of the gas at an early stage of maturation.
Lorant, F., A. Prinzhofer, et al. (1998). "Carbon isotopic and molecular constraints on the formation and the expulsion of thermogenic hydrocarbon gases." Chemical Geology 147(3-4): 249-264.
The purpose of this paper is to present a new kinetic model for the generation of hydrocarbon gas, linking isotopic fractionation and molecular compositions. Pyrolysis experiments were performed with a Type II kerogen in a confined system under isothermal and anhydrous conditions. A mathematical formalism is applied to a compositional kinetic scheme using the pyrolysis data. The experimental and numerical simulations show that the δ13C of the hydrocarbon gas species increase and diverge in value at high maturity, and that the C2−C5 become more enriched in 13C than the initial kerogen. Such an experimental isotopic evolution is not observed in most geological cases, where the δ13C of the thermogenic gas hydrocarbons tend to converge when the maturity increases. From a comparison between experimental isotopic data from closed and open systems, we propose that the two different trends--divergence vs. convergence--may be explained by taking into account the residence time of the gas in the source, for a given generation rate. Indeed, the residence time appears to be a strongly controlling factor for the isotopic and molecular genetic signatures (δ13C and dryness) of the thermogenic hydrocarbons. This assumption is tested by comparing modeling results with experiments and natural data using a diagram showing the difference in δ13C of ethane and propane as a function of the ratio. Results show that the evolution trends observed in such a diagram obey a logic depending on both the maturity and the expulsion rate of hydrocarbons.
Tang, Y., J. K. Perry, et al. (2000). "Mathematical modeling of stable carbon isotope ratios in natural gases." Geochimica et Cosmochimica Acta 64(15): 2673-2687.
A new approach is presented for mathematical modeling of stable carbon isotope ratios in hydrocarbon gases based on both theoretical and experimental data. The kinetic model uses a set of parallel first-order gas generation reactions in which the relative cracking rates of isotopically substituted (k*) and unsubstituted (k) bonds are represented by the equation k*/k=(Af*/Af) exp(-[Delta]Ea/RT), where R is the gas constant and T is temperature. Quantum chemistry calculations have been used to estimate the entropic (Af*/Af) and enthalpic ([Delta]Ea) terms for homolytic bond cleavage in a variety of simple molecules. For loss of a methyl group from a short-chain n-alkane (<= C6), for example, we obtain an average [Delta]Ea of 42.0 cal/mol and an average Af*/Af of 1.021. Expressed differently, 13C-methane generation is predicted to be 2.4% (24[per mille sign]) slower than 12C-methane generation (from a short-chain n-alkane) in a sedimentary basin at 200°C but only 0.7% (7[per mille sign]) slower in a laboratory heating experiment at 500°C. Similar calculations carried out for homolytic bond cleavage in other molecules show that with few exceptions, [Delta]Ea varies between 0 and 60 cal/mol and Af*/Af between 1.00 and 1.04. Examination of this larger data set reveals: (1) a weak sigmoid relationship between [Delta]Ea and bond dissociation energy; and (2) a strong positive correlation between [Delta]Ea and Af*/Af. The significance of these findings is illustrated by fitting a kinetic model to chemical and isotopic data for the generation of methane from n-octadecane under isothermal closed-system conditions. For a specific temperature history, the fitted model provides quantitative relationships among methane carbon isotope composition, total methane yield and methane generation rate which may have relevance to the cracking of oil-prone kerogens and crude oil. The observed variability of the kinetic reactivity of various methane source rocks highlights the need to apply and adequately calibrate such models with laboratory data for specific study areas. With this approach isotope data of natural gases can be used not only to estimate the time of gas generation in a sedimentary basin, but also to evaluate the source rock maturities at which specific accumulations were generated, and place constraints on trap charging histories.
Cramer, B., E. Faber, et al. (2002). "Reaction kinetics of stable carbon isotopes in natural gas -- insights from dry, open system pyrolysis experiments." Fuel and Energy Abstracts 43(2): 130-130.
Open system nonisothermal pyrolysis with on-line compound-specific 13C/12C stable-isotope analysis (Py-GC/IRMS) has been performed on three carbonaceous sediments from NW Germany (Carboniferous, Westphalian coal, HI = 286 mgHC/gTOC, Ro = 0.72%), West Siberia (Cretaceous, Cenomanian shale, HI = 192 mgHC/gTOC, Ro = 0.43%), and Malaysia (Tertiary, Miocene coal, HI = 190 mgHC/gTOC, Ro = 0.36%). The study was focused on the generation of methane, ethane, and propane + propene. Measured δ13C-values of pyrolytically generated light hydrocarbons were in the range of δ13C-values commonly observed in thermogenic natural gas (−20 to − 40‰, PDB). While the isotopic composition of the pyrolysis products showed a general enrichment in 13C-species with increasing temperature, the isotopic trends of methane displayed characteristic structures involving reversals in certain temperature intervals. On the basis of the experimental data, reaction kinetic parameters have been derived for each isotopic species of the hydrocarbon gases assuming parallel first-order reactions and an Arrhenius-type temperature dependence. The resulting kinetic parameter sets for the Westphalian coal were then tentatively applied to geologic temperature histories to model the chemical and isotopic composition of natural gas generated and accumulated in reservoirs of the NW German Basin. The isotopic compositions (δ13C-values) of methane computed in this simulation show a good agreement with actual isotopic compositions of the natural gases in NW German gas fields. It is demonstrated that the combination of isotope-specific reaction kinetics with the regional thermal history provides a useful tool to account for variations in the isotopic composition of reservoir gases in the course of the accumulation history. These results indicate that, despite the undisputed differences between laboratory and natural conditions for gas generation, open system nonisothermal pyrolysis provides isotope-specific reaction kinetic parameters that satisfactorily describe the isotope effects associated with thermogenic natural gas generation in geologic systems. Application of these parameters in basin modeling studies permits prediction/reconstruction of isotopic compositions of natural gases with the same level of confidence as commonly applied bulk and compound-specific kinetic parameters.
Galimov, E. M. (2006). "Isotope organic geochemistry." Organic Geochemistry 37(10): 1200-1262.
The present-day state as well as the history of isotope organic geochemistry is reviewed. Theoretical aspects of isotope fractionation in a system of complex organic molecules, fractionation of carbon isotopes in the biosphere, isotopes as applied to study the transformation of organic matter, geochemistry of oil and gas, evolution of the carbonate-organic carbon system and aspects of astrobiology are considered.
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