25th International Meeting on Organic Geochemistry IMOG 2011

P-103
Geochemical characteristics of crude oils from the Murzuq
Basin, Libya
Tarek Hodari, Paul Philp
School of Geology and Geophysics, University of Oklahoma, Norman, United States of America
(corresponding author:pphilp@ou.edu)
The study area is situated within the Murzuq Basin
of Southwest Libya. Murzuq Basin is an intra-cratonic
sag basin situated on the North African Saharan
Platform (Aziz ,2000; Echikh and Sola, 2000; Fello et
al., 2006). The Palaeozoic sediments within the basin,
comprised of Cambrian, Ordovician, Silurian,
Devonian and Carboniferous rocks, are
unconformably overlain by up to 1500m of Mesozoic
continental deposits and Cainozoic sequences. The
maximum depth to Precambrian basement reaches
up to 3500m in the central part of the basin. The lower
Silurian Tanezzuft Formation (hot shale) and Middle-
Late Devonian Awaynat Wanin Formation shale are
thought to be the main source rocks along with other
local source rocks. These have been recognized
within Palaeozoic sequences, and have contributed to
charging of the Palaeozoic and Mesozoic reservoirs in
the Murzuq Basin. Only one effective petroleum
system (Tanezzuft-Mamuniyat) has been found in the
Murzuq Basin to date, although others may remain to
be discovered.
Twenty crude oil samples from the Murzuq Basin
(NC-115 Concession) were used for this study.
Complete removal of asphaltenes from oils was
assured by using n-pentane, and the maltenes were
subsequently fractionated into saturate hydrocarbons,
aromatic hydrocarbons, and polar compounds using
high-performance liquid chromatography (HPLC).
Analytical techniques such as gas chromatography
(GC) and gas chromatography-mass spectrometry
(GCMS) were used to characterize the saturate
fractions. The distributions of n-alkanes, isoprenoids,
terpanes, and steranes were used to determine
organic matter (OM) composition, depositional
environment conditions, and extent of biodegradation.
Results reveal that the elevated values of
nC17/nC25 ratios in the n-alkane profiles suggest a
high input of marine-derived OM if not resulting from
maturity. However, values of Pr/nC17 vs. Ph/nC18
indicate that the crude oils were sourced from a
source rock of mixed marine/terrigenous OM inputs.
The predominance of C29 steranes, with relatively low
concentrations of C27 and C28 steranes, in all of the
oils may suggest higher inputs of terrestrial OM with
some contribution from marine OM and lacustrine
algae. The higher Pr/Ph ratios in all oils suggest that
these sediments were deposited in mixed oxic to suboxic
(high Eh) conditions as indicated by the high
concentrations of C30-diahopane, and high ratios of
diasterane/sterane in most of samples. The presence
of pregnane and homopregnane in all oil samples
suggest that these oils have been sourced from
sediments deposited under hypersaline conditions.
Based on the distributions of n-alkanes in all of
crude oils (ranging from C11 to C32), it appears that
these oils are non-biodegraded. Oil to oil correlations
permit the establishment of one genetic family that
has been divided into three subfamilies based on the
differences in the absolute concentration of
biomarkers, biomarker distribution pattern of steranes
(m/z 217) and triterpanes (m/z 191). These oils are
generally characterized by early to intermediate levels
of thermal maturity as indicated by the ratios of
ββ/(ββ+αα) and 20S/(20S+20R) as well as
22S/(22S+22R), and sourced mainly from source
rocks of mixed marine/terrestrial organic matters with
higher inputs from terrestrial origin (Philp, 1985; Philp
et al., 1992).
Selected References
Aziz, A. (2000) Stratigraphy and hydrocarbon
potential of the lower Palaeozoic succession of
licence NC 115, Murzuq Basin, S.W. Libya.
Symposium on Geological Exploration in Murzuq
Basin (Eds. M.A. Sola and D. Worsley), Elsevier,
Amsterdam, p. 349-368.
Echikh, K., and Sola, M.A. (2000) Geology and
hydrocarbon occurrences in the Murzuq Basin, S.W.
Libya. Symposium on Geological Exploration in
Murzuq Basin (eds. M.A. Sola and D. Worsley),
Elsevier, Amsterdam, p. 175-222.
Fello, N., Lüning, S., Ńtorch, P., Redfern, J. (2006).
Identification of early Llandovery (Silurian) anoxic
palaeo-depressions at the western margin of the
Murzuq Basin (southwest Libya), based on gammaray
spectrometry in surface exposures. GeoArabia 11
(3), p. 101-118.
Philp, R.P. (1985) Fossil Fuel Biomarkers. Elsevier
Science Publishers B.V. New York, p. 294.
Philp, R.P., Chen, J.H., Fu, J.M. and Sheng, G.Y.
(1992) A geochemical investigation of crude oils and
source rocks from Biyang Basin, China. Org.
Geochem. Vol. 18, No. 6, p. 933-945.
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