25th International Meeting on Organic Geochemistry IMOG 2011

O-37
The seco-oleananes: identification, origin, application and
distributions in late cretaceous/tertiary deltaic petroleum
systems
Olukayode Samuel 1 , Hans Peter Nytoft 2 , Geir Kildahl-Andersen 3 , Jon Eigill Johansen 3
1 Mobil Producing Nigeria (an ExxonMobil subsidiary), Lagos, Nigeria, 2 Geological Survey of Denmark and
Greenland (GEUS), Øster Voldgade 10, DK-1350, Copenhagen K, Denmark, 3 CHIRON AS, Stiklestadvn. 1,
NO-7041, Trondheim, Norway (corresponding author:olukayode.j.samuel@exxonmobil.com)
The recognition of certain molecular compounds that
represent specific processes and products in time and
space over varying biological and chemical conditions
in the geosphere and their successful linkage to their
natural products in the biosphere has permitted the
use of compounds like oleanane, lupane,
gammacerane and bicadinanes in characterising
sedimentary organic matter and petroleum. Four
previously unreported seco-oleananes identified as
A1, A2, B1, B2 and a compound ―C‖ previously
identified as seco-hopane (Schmitter et al., 1982,
Fig.1) all occurring in Late Cretaceous/Tertiary Deltaic
oils of Assam, Beaufort-Mackenzie, Gulf of Mexico,
Niger Delta and Kutei basins are hereby reported.
The ―A‖ and ―B‖ series elute as isomeric doublets (Fig.
1). All the seco-oleananes contain the base peak m/z
123 ion and significant m/z 109 and m/z 137 ions in
their mass spectra. Compound C (seco-hopane) was
found to be present in both non-deltaic (North Sea)
and deltaic oil samples irrespective of the basin,
organic matter source and the source rock age.
Only seco-oleanane B2 was successfully synthesized
under laboratory condition of isomerisation and
hydrogenation of oleanenes mixture from pure lup-
20(29)-ene prepared from birch-bark. The
observations suggest that the natural B2 in the oil was
probably formed under conditions of isomerisation
and hydrogenation of acid catalysed ring-C opened
product of oleanenes derived from angiosperm higher
plant precursors. Such Ring C opening mechanism
can occur at super acidic sites on clay minerals. NMR
data confirm that A2 and B2 are isomeric 8, 14 secooleananes
(i.e. oleanane with an opened C-ring).
NMR also supports structures for B2 and A2 in which
both CH3-26 and CH3-27 have inverted configurations
compared to their suggested oleananoid precursors,
and thus isomerised to thermodynamically more
stable products.
The presence and abundance of A1, A2, B1, B2 show
a correspondence with the oleananes in an oil (Fig. 2),
thus suggesting a similar terrigenous angiosperm
higher plants precursor. K-index (K-index =
(A1+A2+B1+B2)/C) co-varies with oleanane index,
thus permitting K-index‘s usage in discriminating
organofacies among oils in the same basin (Fig. 2).
Fig. 1. M/z 414-123 GC-MS-MS transition showing the
distribution of the seco-oleananes A1, A2, B1, B2 and secohopane
―C‖ in end-member oils of the Niger Delta Basin.
K-Index
14
12
10
8
6
4
2
0
A1
A2
A1 A2
A1
A2
B1
B1
B1
B2
B2
B2
Assam
Beaufort-Mackenzie
Gulf of Mexico
Kutei
Niger Delta
C
C
C
C
NDO02 (Shallow water Terrigenous)
OL index = 0.57
NDO30 (shallow water mixed facie)
OL index = 0.41
NDO23 (Deepwater marine)
OL index = 0.25
North Sea (marine)
OL index = 0.00
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Oleanane Index
Fig. 2. Plot of oleanane and K-indices for some Tertiary
deltaic oils.
REFERENCE:
Schmitter et al., 1982. Occurrence of novel tetracyclic
geochemical markers: 8,14-seco-hopanes in a
Nigerian crude oil. GCA 46, 2345-2350
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