F. Galarraga1, F. Urbani2, M. Escobar*3, G. Márquez4,

M. Martínez1 and R. Tocco5

1 Instituto de Ciencias de la Tierra, Universidad Central de Venezuela, Caracas, 3895 1010-A, Venezuela.

2 Escuela de Petróleo, Geología y Minas, Universidad Central de Venezuela, Caracas, 3895 1010-A, Venezuela.

3 Carbozulia S.A., Av. 4 Calle 60, Edif. Marfí-A & Universidad del Zulia, Facultad de Ingeniería, Maracaibo 4002 A, Venezuela.

4 Departamento de Ingeniería Minera, Mecánica y Energética, Universidad de Huelva, Huelva, 21819 Huelva, Spain.

5 Repsol Exploración S.A., Paseo de la Castellana 280, 28046 Madrid, Spain.

The organic geochemistry of eighteen samples of seepage oils from Trujillo State, Western Venezuela, was investigated. These oils are probably derived from the predominantly marine Cretaceous La Luna Formation which is near peak maturity for oil generation. A range of biomarkers (n-alkanes, acyclic isoprenoids, phenanthrene and alkyl-phenanthrenes, as well as dibenzothiophene and alkyl-dibenzothiophenes) were analysed by gas chromatography - mass spectrometry (GC-MS).

The seepage oils have been modified as a result of water washing and biodegradation. A first group of samples were slightly biodegraded with the partial loss of n- and iso-alkanes. Other samples fall into two groups: those that are moderately degraded, with partial depletion of acyclic isoprenoids; and those that are severely altered, as indicated by the partial or total absence of hopanes.

One objective of this work was to evaluate the factors influencing the compositional differences of the three samples sets. In-situ measurements, together with the interpretation of the geochemical data, suggest that diverse factors were responsible for these variations. These included different degrees of alteration due to biodegradation and water washing, as well as differences in flow rate towards the surface.

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