Abstract: Cretaceous and Tertiary Petroleum Systems of the Northwest Region of the Eastern Venezuela Basin

Stoufer, S.; Chigne, N.; Santchez, J. H. - PDVSA; Mello, M. R. - Petrobras/ Cenpes

Integration of geological and geochemical data of 110 oil samples recovered from carbonate and sandstone reservoirs, ranging in age from Upper Cretaceous to Tertiary, suggest the presence of Cretaceous and Tertiary petroleum systems in the northwest region of the Maturin Sub-Basin, Eastern Venezuela Basin (Fig. 1).

Sophisticated metastable reaction monitoring MRM-GC-MS for biomarkers and related to organic facies allowed the characterization and age related end-members of each petroleum system identified and provides a semi-quantitative means to determine degrees of oil mixing around the Eastern Venezuelan Basin.

Oils derived from the Cretaceous petroleum system are mainly characterized by: dominance of low molecular weight n-alkanes, medium to high sulfur contents (up to 4.0%), low pristane/phytane ratios (up to 2.5), d¹³C heavier than -28.00%, low oleanane abundance, high abundance of C₂₈ bisnorhopane and C₃₅ extended terpanes, C₃₀ regular steranes and organic sulfur compounds, and low hopane/ sterane ratio (< 6; Fig. 2). They are correlated with Upper Cretaceous organic-rich sediments deposited in a marine-neritic anoxic, semi-restricted environment, with carbonates to siliciclastic facies.

Oils derived from the Tertiary petroleum systems are correlated with Oligo-Miocene organic-rich sediments deposited under marine-deltaic conditions. They are classified in distal and proximal marine-deltaic subtypes according to the facies changes of the source rocks. The most important geochemical characteristics are d¹³ lighter than -28%, very low sulfur contents (<O.1), high pristane/phytane ratios (<2.5), and very high concentrations of higher plant biological markers such oleanane, bicadinanes and Des-A tetracyclic terpanes and high hopane/sterane ratio (>6; Fig. 2).

The hydrocarbons were accumulated in sandstones of the San Juan, Los Jabillos and Oficina Formations during the development of the foreland basin. Shales of the Vidoño, Carapita and Oficina Formations constitute the seals for the accumulations, The overburden rocks are formed by the Carapita Formation (up to 7000 meters) and the sediments overthrusted during the collision of Caribbean and South American plates.

The integration of the oil geochemical data with geological information was a key point in understanding the migration pathways and locating the active source rock pods in the study area.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil