25th International Meeting on Organic Geochemistry IMOG 2011

P-295
Using diamondoids to unravel alteration processes
Gary Muscio
Chevron, Houston, Texas, United States of America (corresponding author:Gary.Muscio@Chevron.com)
The controls on phase (oil vs. gas) and volumes of
hydrocarbons in petroleum systems is the cumulative
effect of source facies, source rock maturity and
alteration processes. In Tertiary Deltas, source rocks
and reservoirs are typically stratigraphically separated
by a thick sedimentary section. In such basins, postexpulsion
alteration processes can have a significant
effect on the composition of a hydrocarbon phase
migrating from the source kitchen to the trap, resulting
in complex, multi-stage charge histories. Alteration
processes can include phase segregation, mixing of
hydrocarbons derived from source rocks of different
facies and/or maturity, and biodegradation.
Understanding the extent and magnitude of alteration
processes can help predict distribution and quality of
petroleum in an exploration environment.
In addition to classifying source rock facies and
maturity [1, 2], diamondoids have been used to
evaluate alteration processes such as biodegradation
and temperature-induced cracking of liquid
hydrocarbons to gas [3, 4, 5]. Their thermal stability
results in a progressive increase in concentrations in
oils that have been submitted to high levels of thermal
stress during cracking.
A set of oils from a Tertiary deltaic basin was
submitted to a comprehensive geochemical analytical
program, including GC, GCMS and GCIRMS, in order
to characterize the samples for their source facies,
level of thermal maturity and alteration. The present
study focuses on using diamondoids to assess how
the process of oil-to-gas cracking and mixing may
have altered the composition of reservoired oils.
For the diamondoid analyses, samples were selected
that were largely unaffected by biodegradation, in
order to minimize any effects of biodegradation on
concentrations of relevant biomarker compounds. The
oils were derived from reservoirs from different
subregional structural trends in the same basin.
Despite subtle compositional variations, suitable
molecular and isotope data suggest that the oils are
derived from source rocks that were deposited in
similar depositional environments.
Results indicate that while concentrations of
methyldiamantanes are lower compared to published
data from similar depositional settings [6], oils from
different structural trends appear to be affected to
varying degrees by contributions from a highly
cracked, diamondoid-rich source, as reflected by
higher concentrations of diamondoids. Furthermore,
samples from the same well but from different depths
also exhibit varying levels of cracking.
Due to the low temperatures (ranging from 65 to 110
C based on MDT temperature data) and relatively
recent charge, it is unlikely that the oils analysed in
this study have been submitted to in-situ cracking
reactions. Rather, the variations in diamondoid
concentrations are believed to be related to
hydrocarbons migrating from deeper sections where
conditions are more conducive for cracking. Hence,
nature and quality of migration pathways and their
variability between different structural trends play a
significant role.
Additionally, mixing of low mature oil with oil that has
been submitted to cracking has also altered the
composition of selected samples.
References
[1] Schulz L.K., et al. (2001) Org. Geochem. 32, 365-
375.
[2] Chen, J.H, et al. (1996) Org. Geochem. 25, 179-
190.
[3] Grice K., et al. (2000) Org. Geochem. 31, 67-73.
[4] Dahl J.E., et al. (1999) Nature 399, 54-56.
[5] Wei et al. (2006) Org. Geochem. 37, 891-911.
[6] Springer, M.V. et al. (2010) Org. Geochem. 41,
1013-1018.
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