25th International Meeting on Organic Geochemistry IMOG 2011

P-308
Evidence of hydrocarbon generation through decarboxylation
reactions: qualitative and quantitative analyses by FTICR-MS
Elodie Salmon 1 , Françoise Behar 2 , Patrick G. Hatcher 1
1 Old Dominion University, Department of Chemistry and Biochemistry, Norfolk, VA, United States of America,
2 IFP Energies Nouvelles, Rueil-Malmaison, France (corresponding author:esalmon@odu.edu)
The aim of the present study is to demonstrate that
decarboxylation reactions are the main processes
occurring during hydrocarbon generation from
kerogen. To ascertain this conclusion, a Type I
kerogen from the Green River formation was
pyrolyzed in a closed system at various T/t conditions.
The NSO compounds generated during the artificial
maturation were collected by successive n-pentane
and dichloromethane (DCM) extractions and analyzed
using electrospray ionization Fourier transform ion
cyclotron resonance mass spectrometry (ESI-FTICR-
MS) to identify dominant series of compounds.
A large variety of 75 homologous series is identified
across the entire m/z range of each mass spectrum
and represents at least 72% of the total peak area of
each spectrum. Several series of polar CHO, CHOS,
and CHON compounds between C12 and C50+ are
observed. During the artificial maturation, the
molecular weights of these C12 - C50+ compounds tend
to decrease with increase severity. The CHO
compounds that are dominant in each spectrum
represent, at low conversion (275°C/9h), 58% and
23% of the total peak area in the n-pentane and DCM
extracts, respectively. Within these CHO compounds,
more than half of are comprised in the 4 series,
CnH2nO2, CnH2n-2O2, CnH2n-2O4, CnH2n-10O2, which all
correspond to carboxylic acid compounds.
Similar to previous studies involving
extraction/derivatization approaches, we show that
the distribution of these carboxylic acid compounds
(Figure 1) detected by ESI-FTICR-MS is directly
correlated to the distribution of hydrocarbons using
the carbon preference index (CPI). This index is
commonly used to characterize oils and source rock
extracts and is defined as the predominance of odd or
even numbered hydrocarbons or carboxylic acids.
The correspondence of CPI between saturated
hydrocarbons (odd-dominated) and saturated
monoacids CnH2nO2 (even-dominated) is suggestive
of a genetic link that involves decarboxylation.
We have quantified the amount of carboxylic acids
that is converted to hydrocarbons using external
(calibration curves) and internal (by standard
additions) calibration of each extract and using
oxygenated model compounds (linear, alicyclic,
saturated, and unsaturated) containing acid and
alcohol groups. We show that almost the entire
amount of hydrocarbons generated during the artificial
maturation can be related to one of the carboxylic
acid compounds extracted from the kerogen. This
strongly suggest that decarboxylation of acid
compounds from kerogen is the main process for the
generation of hydrocarbons.
441