Significance of Isotopically Labile Organic Hydrogen in Thermal Maturation of Organic Matter Page: 3 of 17
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from experiments where 15N-enriched ammonium chloride solutions and source rocks containing kerogen
types I, II, IIS, and III were heated over 5 years at temperatures of 100, 144, and 196 C. The resulting data
corroborate our earlier results from our hydrous pyrolysis experiments (330 C for up to 144 hours) and
prove that ammonium is readily converted abiogenically to organic N in kerogen at temperatures that
approach those in naturally maturing sediments. The propensity of different types of kerogen to react with
NH4' or NH3 increases in the order II < I < IIS < III. Ammonium ions' mobility in formation fluids and
their ability to isotopically interact with kerogen makes them an effective isotopic buffer for sedimentary
organic N. Isotopic exchange tends to (i) homogenize the isotopic compositions of participating pools
of N, (ii) limits the isotopic ranges of organic and inorganic N, and (iii) consequently decouples 615N
from SD. Recognition of N-exchange in thermally matured sediments offers fresh perspectives and
opportunities to constrain the extent and pathways of N-containing geofluid migration, especially when
synoptically characterizing organic and mineral-bound N moieties across gradients of thermal maturity.
Our results were recently published in Organic Geochemistry (Schimmelmann and Lis, 2010).
4.2. D/H isotopic correlation among formation waters, oil, oil fractions, biomarkers, and kerogen
from source rocks
This subproject examined the processes that influence D/H ratios of petroleum and individual
biomarkers in Australian oils. We published our paper in Organic Geochemistry (Schimmelmann et al.,
2004). D/H ratios of bulk oils, saturated, aromatic, and polar fractions, individual n-alkanes, formation
waters and non-exchangeable hydrogen in kerogen from source rocks were measured from six Australian
petroleum basins. Isotopic data for oils, oil sub-fractions and matching kerogens indicate that, in general,
the individual components of an oil all have comparable 6D values, with distinct isotopic clustering of oil
families and source rocks from basins (Fig. 1). The weighted-average 6D value of individual n-alkanes is
usually identical to the saturate fraction and whole oil 6D values. However a consistent trend of increasing
6D with alkane chain length is observed in most oils derived from single sources, so that individual n-
alkanes from one oil can vary in 6D by 30%0 or more. This pattern likely reflects fractionations occurring
during hydrocarbon cracking.
Hydrocarbon 6D values (oils, oil fractions, and biomarkers) are in turn depleted in D by an average of
20%c relative to the source kerogen (Fig. 2). A modest correlation between 6D values for aromatic fractions
of petroleum and formation waters indicates that about 50% of aromatic C-bound H has exchanged with
water. Values of 6D for whole oils and aliphatic and polar subfractions are uncorrelated with formation
water 6D values, indicating that essentially all C-bound H in these fractions has been conserved under
reservoir conditions. On the other hand, 6D values for acyclic isoprenoids and n-alkanes are essentially
identical in all the petroleum systems examined, indicating that the large isotopic differences in these
molecules due to primary biosynthetic effects have been erased. At the same time, variations in 6D of
100%0 or more exist between n-alkanes from closely related oils of the Otway Basin, and apparently
preserve the isotopic signature of a major climatic change. These apparently contradictory lines of evidence
can be explained if (1) hydrogen exchange of C-bound H is extensive during petroleum generation, (2) the
D/H composition of the bulk source rock - including water, organic H, and mineral H - is buffered against
isotopic change, either as a result of its high organic and inorganic H content or by its low permeability, or
both; and (3) the D/H ratios of petroleum hydrocarbons are essentially 'locked in' once an immiscible oil
phase is generated and expelled from source rocks.
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Schimmelmann, Arndt & Mastalerz, Maria. Significance of Isotopically Labile Organic Hydrogen in Thermal Maturation of Organic Matter, report, March 30, 2010; United States. (digital.library.unt.edu/ark:/67531/metadc931558/m1/3/: accessed December 10, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.