Abstract: Geochemical and Geological Control on Filling History of Eocene Reservoirs, Maracaibo Basin, Venezuela

Margarita Alberdi, Jesus Maguregui, Carmen Toro, Miguel Marquina

Crude oils of Eocene fluvio-deltaic reservoirs in "Bloque V" and "Centro Lago" fields in the center of the Maracaibo Lake show many differences in composition, which are due to stratigraphically and structurally controlled reservoir geometry and a low rate of in-reservoir mixing of at least two successive petroleum charges.

Oils produced from the top of structural highs contain 18(H) oleanane, higher Pr/Ph and C_23-3/C_24-4 ratios, a lower proportion of DBT/P compounds, and clearly different fingerprint patterns in the C₆-C₁₅ range, than those observed in oils produced from the lower parts of the structures. These compositional differences suggest that two source rocks, or two distinctive organic facies within the same Cretaceous La Luna Formation, generated and filled vertically poorly connected Eocene reservoirs.

On the other hand, saturate-biomarkers ratios, triaromatics (C₂₁/C₂₁+C₂₈), n-paraffins (n-C₂₀/n-C₂₉) and n-heptane index suggest that oils in upper reservoirs are slightly less mature than oils in lower reservoirs and, consequently filled the structure first.

Additional evidence from formation water analysis and tectonic basin evolution allow us to interpret at least two petroleum pulses from Cretaceous source rocks during Upper Miocene to present day kitchens located in the Andes foredeep at the southeast of the study area.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela