--> ABSTRACT: Definition of a New Petroleum System Based on Oil Chemistry, Llanos Basin, Colombia, by J. C. Ramon, L. Dzou, and A. Holba; #91021 (2010)

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Definition of a New Petroleum System Based on Oil Chemistry, Llanos Basin, Colombia 

RAMON, JUAN CARLOS,  LEON DZOU, and HOLBA, ALBERT

One of the most critical exploration parameters in the Llanos basin is timing of generation relative to trap formation, a parameter strongly dependent on the source rock age. Most previous work relates these oils to the prolific Upper Cretaceous formations outcroppings in the Eastern Cordillera. Recently, it have been shown that producing oils correlate with Tertiary source rocks. In the platform, most charged structures are considered Oligocene or older while in the thrust front are related to the Upper Miocene to recent Andean orogeny.

We propose a significant input to the producing oils from Tertiary source horizons based on the detailed study of the chemical composition and concentration of age-indicator biomarkers in crude oils and oil seeps. Significant hydrocarbon-contribution from a Tertiary source is indicated by the presence of high concentrations of the "flowering plant"-markers oleanane, bicadinanes, and oleanoids, as well as by other age-indicator biomarkers.

Llanos oils also contain marine, algal-derived "C[30] steranes" (i.e., 24-n-propylcholestanes), which are diagnostic for oils generated from marine organic matter, suggesting Cretaceous source rocks. Heavy biodegradation has been particularly common among the first oils to fill reservoirs in central Llanos basin oil fields. Some of the older, altered heavy oils were mixed with a second pulse of oil explaining the wide range of oil gravities and the presence of n-alkanes in heavily biodegraded samples (as indicated by presence of demethylated terpanes) measured in the central Llanos Basin. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.