Composition, peat-forming vegetation and kerogen paraffinicity of Cenozoic coals: relationship to variations in the petroleum generation potential (hydrogen index)

Research output: Contribution to journalArticle

Abstract

Coals with similar thermal maturity and from the same deposit normally show a considerable range in
petroleum generation potential as measured by the Hydrogen Index (HI). This variation may partly be
related to variations in plant input to the precursor mires and organic matter preservation. It is widely
accepted that some Cenozoic coals and coaly sediments have the potential to generate oil, which is related
to the coal's paraffinicity. Coal paraffinicity is not readily reflected in the bulk HI. In this paper, the
relationships between measured HI and coal composition, coal kerogen paraffinicity and floral input have
been investigated in detail for three sets of coals from Colombia/Venezuela, Indonesia, and Vietnam. The
samples in each coal set are largely of iso-rank. The petroleum generation potential was determined by
Rock–Eval pyrolysis. Reflected light microscopy was used to analyse the organic matter (maceral)
composition and the thermal maturity was determined by vitrinite reflectance (VR) measurements. The
botanical affinity of pollen and spores was analysed by palynology. Coal kerogen paraffinicity was
determined by ruthenium tetroxide-catalysed oxidation (RTCO) followed by chain length analysis and
quantification (mg/g TOC) of the liberated aliphatic chains. The coals are dominated by huminite, in
particular detrohuminite. Only the Vietnamese coals are rich in microscopically visible liptinite. The pollen
and spores suggest that the coals were derived principally fromcomplex angiospermmire vegetations, with
subordinate proportions of ferns that generally grew in a subtropical to tropical climate.Measured HI values
vary considerably, but for the majority of the coals the values lie between approximately 200 mg HC/g TOC
and 300 mg HC/g TOC. Aliphatics yielding monocarboxylic acids dominate in the coal kerogen, whereas
aliphatics yielding dicarboxylic acids are secondary. However, the dicarboxylic acids show that crosslinking
long-chain aliphatics are present in the kerogen structure. All studied coals are paraffinicwith C19–35
aliphatic chains in the kerogen, and the aliphatics in the range C25–35 show that the coals may have the
potential to generate waxy crude oil. The Indonesian coals are richest in long-chain aliphatics and are thus
potentially most oil-prone. Multivariate statistical analysis shows that for the present three sample sets
variations in HI are positively correlated to different combinations of the C10+ aliphatic chains in the
kerogen and the amount of detrohuminite + liptodetrinite and liptinite. Furthermore, part of the HI can be
attributed to hydrogen in compounds with less than 10 carbon atoms, which is the lowest alkyl detected by
RTCO analysis, thus representing a potential for generation of gas and light liquid hydrocarbons. The
measured HI is therefore not always a good indicator for humic coal's potential to source waxy oil. Vegetational influence (palynology) on the variation of HI cannot be shown within the investigated span of variance and for the present coals. However, it is likely that some of the range in measured HI values is caused by floral vairiations not revealed by the palynological analysis and to variations in the preservation of the organic matter.

Details

Authors
  • Henrik Ingermann Petersen
  • Sofie Lindström
  • Hans Peter Nytoft
  • Per Rosenberg
External organisations
  • External Organization - Unknown
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Geology
Original languageEnglish
Pages (from-to)119-134
JournalInternational Journal of Coal Geology
Volume78
Issue number2
StatePublished - 2009
Peer-reviewedYes
Externally publishedYes