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 ope n-system pyrolysis that allows a simulation of primary cracking (tempe ratures: 20 to 810 degrees C, heating rate: 5 degrees C/min, helium fl ow: 21 ml/min) within the (measured) maturity range 0.3 to 5.4% vitrin ite reflectance. Gases collected during pyrolysis were analyzed for th eir molecular composition and the carbon isotope ratios of methane, et hane 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, ho wever, show significant inversions with increasing maturity, that indi cate an isotopic inhomogeneity of the precursors from which methane is generated. Hydrogen isotope values of methane from Kukersite vary bet ween - 211 and - 84 parts per thousand, whereas, hydrogen isotope rati os of methane from xylite increase from - 314 to - 164 parts per thous and. The data of the pyrolysis experiments have been used to develop k inetic models of hydrocarbon generation that are combined with Rayleig h-distillation models to describe the isotope fractionation between or ganic matter and light hydrocarbons. Carbon isotope fractionation fact ors between kerogens and individual gas components are high for Kukers ite (alpha(CH4-Kuk)=1.017, alpha(C2-Kuk)=1.009, alpha(C3-Kuk)=1.005) a nd low for xylite (alpha(CH4-Xyl)=1.0042, alpha(C2-Xyl)=1.003, alpha(C 3-Xyl)=1.001). Hydrogen isotope fractionation factors are lower for Ku kersite and higher for xylite (alpha(CH4-Kuk)=1.1, alpha(CH4-Xyl)=1.2) Gas generation and isotope models are combined with kinetic models of thermal kerogen alteration. The results of the calculations are compa red to measured data of natural samples from the Delaware and Val Verd e basins (U.S.A.).