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Pyrolysis-GC characterization of whole rock and kerogen-concentrate samples of immature Jurassic source rocks from NW-Germany

Organic Geochemistry
Publication Date
DOI: 10.1016/0146-6380(87)90006-4
  • Kerogen
  • Source Rock
  • Pyrolysis-Gas Chromatography
  • Hydrogen Index
  • N-Alkanes
  • Earth Science


Abstract Nine rock samples from three Jurassic stratigraphic units of a shallow core from NW Germany were analyzed by pyrolysis-gas chromatography. The units contain a mixed Type-II/III kerogen (Dogger-α), a hydrogen-rich Type-II kerogen (Lias-ϵ), and a hydrogen-poor Type-III kerogen (Lias-δ). All of the kerogen was immature ( R o = 0.5%). Two sets of kerogen concentrates (“AD”: HCl/HF followed by a density separation, and “A”: only acid treatment) prepared from the rock samples were also analyzed to make a detailed comparison of the pyrolysates of rock and corresponding kerogen-concentrates. Hydrogen-index (HI) values of the kerogen concentrates prepared from organic-carbon poor rock were nearly 200% higher than HI values of the rock samples. Changes in HI were minimal for the samples containing Type-II kerogen. The A and AD samples from the C org-poor rock yielded pyrolysates with n-alkane series of very different molecular lengths. Pyrograms of the rock samples had n-alkane series extending to n-C 14; the chromatograms of the A samples reached the n-C 14- nC 20 range. The AD samples from C org-poor rock and all three sample types from the C org-rich rock had n-alkane series up to n-C 29. The benzene/hexane and toluene/heptane ratios for the C org-poor rock and A samples were far higher than for the AD samples, which had ratios similar to those of all three sample types from the C org-rich rocks. These results indicate that choice of kerogen preparation method is critical when C org-poor samples are analyzed.

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