Geochemical Study of Natural Bitumen, Condensate and Gas Seeps from Sousse Area, Central Tunisia

Natural hydrocarbon seepage has helped petroleum
exploration as a direct indicator of gas and/or oil subsurface
accumulations. Surface macro-seeps are generally an indication of a
fault in an active Petroleum Seepage System belonging to a Total
Petroleum System. This paper describes a case study in which
multiple analytical techniques were used to identify and characterize
trace petroleum-related hydrocarbons and other volatile organic
compounds in groundwater samples collected from Sousse aquifer
(Central Tunisia). The analytical techniques used for analyses of
water samples included gas chromatography-mass spectrometry (GCMS),
capillary GC with flame-ionization detection, Compound
Specific Isotope Analysis, Rock Eval Pyrolysis. The objective of the
study was to confirm the presence of gasoline and other petroleum
products or other volatile organic pollutants in those samples in order
to assess the respective implication of each of the potentially
responsible parties to the contamination of the aquifer. In addition,
the degree of contamination at different depths in the aquifer was also
of interest. The oil and gas seeps have been investigated using
biomarker and stable carbon isotope analyses to perform oil-oil and
oil-source rock correlations. The seepage gases are characterized by
high CH4 content, very low δ13CCH4 values (-71,9 ‰) and high
C1/C1–5 ratios (0.95–1.0), light deuterium–hydrogen isotope ratios (-
198 ‰) and light δ13CC2 and δ13CCO2 values (-23,8‰ and-23,8‰
respectively) indicating a thermogenic origin with the contribution of
the biogenic gas. An organic geochemistry study was carried out on
the more ten oil seep samples. This study includes light hydrocarbon
and biomarkers analyses (hopanes, steranes, n-alkanes, acyclic
isoprenoids, and aromatic steroids) using GC and GC-MS. The
studied samples show at least two distinct families, suggesting two
different types of crude oil origins: the first oil seeps appears to be
highly mature, showing evidence of chemical and/or biological
degradation and was derived from a clay-rich source rock deposited
in suboxic conditions. It has been sourced mainly by the lower
Fahdene (Albian) source rocks. The second oil seeps was derived
from a carbonate-rich source rock deposited in anoxic conditions,
well correlated with the Bahloul (Cenomanian-Turonian) source rock.

• A. Belhaj Mohamed
• M. Saidi
• N. Boucherb
• N. Ourtani
• A. Soltani
• I. Bouazizi
• M. Ben Jrad

• Biomarkers
• oil and gas seeps
• organic geochemistry
• source rock.

[1] Link, W.K., 1952. Significance of oil and gas seeps in world oil
exploration. American Association of Petroleum Geologists Bulletin 36,
1505–1540.
[2] Jones, V.T., Drozd, R.J., 1983. Predictions of oil or gas potential by
near-surface geochemistry. American Association of Petroleum
Geologists Bulletin 67, 932–952.
[3] Abrams, M.A., 2005. Significance of hydrocarbon seepage relative to
petroleum generation and entrapment. Marine and Petroleum Geology
22, 457–477.
[4] Etiope, G., Feyzullayev, A., Baciu, C.L., 2009a. Terrestrial methane
seeps and mud volcanoes:a global perspective of gas origin. Marine and
Petroleum Geology 26, 333–344.
[5] Sami Khomsi, MouradBédir, Mohamed Soussi, Mohamed Ghazi Ben
Jemia, Kmar Ben Ismail-Lattrache., 2006. Mise en évidence en
subsurface d’événements compressifs en Tunisie orientale (Sahel) :
généralité de la phase atlasique en Afrique du Nord. C. R. Geoscience
338 (2006) 41–49.
[6] Layeb M. (1991):Etude géologique, géochimique et minéralogique
régionale des faciès riches en matière organique de la Formation Bahloul
d'âge Cénomano-Turonien dans le domaine de la Tunisie Centrale.
Thèse de Doctorat de spécialité, Faculté des Sciences de Tunis, 209p.
[7] Saidi M. and Inoubli H. (2001): Geochemistry and organic petrography
of proven Tunisian source rocks (poster). 20th International Meeting of
Organic geochemistry (Nancy 2001). [8] Saidi M. (2003): Tunisian source Rocks and oils
Geochemicalcharacteristics and Oil-Source rock correlation. Rapport
interne ETAP.
[9] Saidi M. (1993): Etude géologique et géochimique des roches mères
albo-vraconiennes dans le domaine de la Tunisie Centro-Septentrionale.
Thèse de Doctorat de sépcialité, Université de Tunis II, 135p.
[10] A. Belhaj Mohamed, M. Saidi and B. Ibrahim., 2014. Migration
Pathway Evaluation through Analysis of Hydrocarbon Seeps in the
Pelagian basin, Sahel of Tunisia. 76th EAGE Conference and Exhibition
2014
[11] Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and Occurrence,
second ed. Springer-Verlag, Berlin, 235 pp.
[12] Schoell, M., 1980. The hydrogen and carbon isotopic composition of
methane from natural gases of various origins. Geochimica et
Cosmochimica Acta 44, 649–662.
[13] Schoell, M., 1983. Genetic characterization of natural gases. American
Association of Petroleum Geologists Bulletin 67, 2225–2238.
[14] Whiticar, M.J., Faber, E., Schoell, M., 1986. Biogenic methane
formation in marine and fresh water environments: CO2-reduction vs.
acetate fermentation–Isotope evidence. Geochimica et Cosmochimica
Acta 50, 693–709.
[15] Thompson, K. F. M., 1983, Classification and thermal history of
petroleum based on light hydrocarbons: Geochimica et Cosmochimica
Acta, v. 47, p. 303–316, doi:10.1016/0016-7037(83)90143-6.