Geochemical and Isotopic Study of Crude Oils from the Orinoco Oil Belt, Venezuela

Verónica Carmona, Wendy Murillo, José Sánchez, José Antonio García, Héctor Henríquez, and Alejandro Martínez
Exploration and Reservoir Characterization Technical Management, PDVSA Intevep

A group of crude oils from the Orinoco Oil Belt, within the Eastern Venezuela basin were analyzed. This representative group of oils is located in the areas of Carabobo (previously Cerro Negro), Ayacucho (previously Hamaca), Junín (previously Zuata) and Boyacá (previously Machete; Fig. 1).

According to their API gravity, the oils analyzed are classified as extra-heavy and heavy. They have similar SARA compositions (Fig 2a), absence of n-paraffins and isoprenoids, and the presence of demethylated hopanes that suggest a moderate to severe level of biodegradation according to Peters´s scale (2005). Exceptionally, some oils from the Boyacá, Junin and Ayacucho area have a unimodal distribution of n-paraffins, and isoprenoids typical of marine organic matter with a pristane/ phytane ratio between 1 to 3.

The maturity of these oils was evaluated with saturate biomarkers and aromatic markers. The percentages of isomerization of C₂₉steranes are between 51 and 68%. The values of equivalent reflectance percentage (%Req) estimated through the methylphenanthrene index (MPI) are between 0.8 and 1.1% (Fig. 2b). The maturity parameters considered indicate that probably these oils were generated by a rock that, during expulsion, was within the oil window, specifically at the peak of hydrocarbon generation.

The distribution of vanadium and nickel in the oils studied indicate the presence of two genetic families (Fig. 2c).The first family corresponds to the Carabobo, Ayacucho and Junín areas, with an average V/Ni ratio of 4.5. The absolute concentrations of vanadium and nickel vary from 98 to 480 mg•Kg^-1 and from 24 to 100 mg•Kg^-1, respectively.

The second family is represented by the Boyacá oils, with an average V/Ni ratio of 7.70. The absolute concentrations of vanadium and nickel vary from 505 to 1137 mgKg^-1 and 75 to 139 mg•Kg^-1, respectively.

The high V/Ni ratios (>4) in the two oil families indicate anoxic depositional conditions for their source rocks (Lewan, 1984). The Boyacá oils were probably generated from marine organic matter deposited under more reducing marine conditions than the oils of the Ayacucho, Junín and Boyacá areas.

The carbon isotopic values of the oils and their saturate, aromatic, resin and asphaltene fractions are between (– 26.5 and –28.0)‰ δ¹³C. The δ¹³C isotopic ratios from saturate and aromatic fractions (Fig. 2d) also indicate that the oils are derived from organic matter that is predominantly marine (Sofer, 1984). The ternary diagram of C₂₇,C₂₈ y C₂₉ steranes (Fig. 2e) shows as well that all these oils are derived from marine organic matter (Peters, 2005).

The isotopic data from the oils and their saturate, aromatic, resin and asphaltene fractions are similar. Only two samples show a significant variation between the whole oil isotopic value and their respective fractions. The oils from the Carabobo and Ayacucho areas show similar results, but more negatives values when compared with oils from Junín and Boyacá. Generally, the oils genetically correlated, and with similar maturity, can have isotopic variations close to 1 ‰ (Peters, 2005). The largest isotopic variation for the δ¹³C in the whole oils of this study was 1.1 ‰. The resins have the largest variation with a value of 1.1 ‰ with respect to the other fractions. The resins fraction (NSO compounds) are highly soluble in water, and the effect of water washing on the NSO fraction is a change towards isotopically more negative values when compared to an unwashed oil (Kuo, 1994).

Owing to the fact that the maturity of the oils of the Orinoco Oil Belt is similar, and that the secondary migration has only a slight influence on the isotopic composition, the δ¹³C isotopic value can be used as correlation tool. Differences in the isotopic curves of Fig. 2f may be due to variations in the organic facies on the source rock. However, the causes of the high dispersion of carbon isotopic values of whole oils and oil fractions of the Boyacá and Junin areas should be studied.

The present study does not attempt to determine possible source rocks. Several formations within the Eastern Venezuela basin should be studied in detail for this purpose.

References

Alberdi, M. and Lafagué, E. (1993). Vertical variations of organic matter content in the Guayuta Group, Upper Cretaceous, Eastern Venezuela Basin. Organic Geochemistry, 20 (4): 425-436.

Kuo, L. C. (1994) An experimental study of crude oil alteration in reservoir rocks by water washing. Organic Geochemistry, 21(5): 465-479.

Lewan, M. (1984). Factors controlling the proportionality of vanadium to nickel in crude oils. Geochimica et Cosmochimica Acta, 48: 2231-2238.

Peters, K. (2005). The biomarker guide (Vol II), 2nd Ed. Cambridge: Press Syndicate of the University of Cambridge. 1155 p.

Sofer, Z. (1984). Stable carbon isotope compositions of crude oils: application to source depositional environments and petroleum alteration. AAPG Bulletin, 68 (1): 31-49.

Tissot B. and Welte, D (1984). Petroleum formation and ocurrence, 2nd Ed. New York: Springer-Verlag. 699 p.

Fig. 1. Localization map of Orinoco Oil Belt.