Primary cracking of algal and landplant kerogens: Kinetic models of isotope variations in methane, ethane and propane

Samples of an algae-rich kerogen and a xylite were subjected to an open-system pyrolysis that allows a simulation of primary cracking (temperatures: 20 to 810°C, heating rate: 5°C/min, helium flow: 21 ml/min) within the (measured) maturity range 0.3 to 5.4% vitrinite reflectance. Gases collected during pyrolysis were analyzed for their molecular composition and the carbon isotope ratios of methane, ethane and propane. With increasing maturity of the algae-rich kerogen, we observe an increase of the carbon isotope ratios of produced light hydrocarbons. Carbon isotope values of methane derived from xylite, however, show significant inversions with increasing maturity, that indicate an isotopic inhomogeneity of the precursors from which methane is generated. Hydrogen isotope values of methane from Kukersite vary between −211 and −84‰, whereas, hydrogen isotope ratios of methane from xylite increase from −314 to −164‰. The data of the pyrolysis experiments have been used to develop kinetic models of hydrocarbon generation that are combined with Rayleigh-distillation models to describe the isotope fractionation between organic matter and light hydrocarbons. Carbon isotope fractionation factors between kerogens and individual gas components are high for Kukersite (αCH4-Kuk = 1.017, αC2-Kuk = 1.009, αC3-Kuk = 1.005) and low for xylite (αCH4-Xyl = 1.0042, αC2-Xyl = 1.003, αC3-Xyl = 1.001). Hydrogen isotope fractionation factors are lower for Kukersite and higher for xylite (αCH4-Kuk = 1.1, αCH4-Xyl = 1.2). Gas generation and isotope models are combined with kinetic models of thermal kerogen alteration. The results of the calculations are compared to measured data of natural samples from the Delaware and Val Verde basins (U.S.A.).