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Petroleum System Variations in the Llanos Basin (Colombia)

Moretti, Isabelle 1; Mora, Cesar 3; Zamora, Wilson 3; Velandia, Mauricio 2; Rodriguez Charry, German 2; Mayorga, Marcela 2
1 Cepsa, ifp, Madrid, Spain.
2 Cepscola, Bogota, Colombia.
3 Gems, Bogota, Colombia.

The so called Llanos, the northern part of the Andes current foreland of Colombia is not a single homogeneous basin. After the compilation of all available data, more that 8000 samples on source rock and fluids, a regional synthesis has been done. It includes the mapping of the richness of the various source rocks, temperature and paelotemperature as well as a model of the petroleum system in 3D. The study highlights that there are various good source rocks in the Llanos. The “official” main one is the Gacheta Fm (Upper cretaceous) however its potential decreases eastward (especially the HI), the Fomeque Fm (below), the Los Cuervos Fm (Paleocene) and all the tertiary shales (Carbonera and Leon Fms) have equivalently high potential in the Oriental Llanos. The Paleozoic shales have also remaining TOC content.

The source rocks, except the Paleozoic ones, are poorly mature eastward and the HC are coming from the deepest part of the basin Westward. In the kitchen usually all levels of maturity are present from the gas window in the Paleozoic to immature in the Leon shales. The expected fluids in the traps are a mixture of the HC expelled from these different source rocks. The timing is an important issue in this basin since biodegradation of the first oil takes place in the still shallow and therefore cold reservoirs

In term of maturity history, the Llanos foreland, could be roughly divided in 3 zones, that we will call south, central and north. The specificities of each one are:

South: early maturation: all over the Llanos foreland the Carbonera Fm (Oligocene) is too thin to allow the Cretaceous to be mature except southward in the Apiay-Chichimene area. There the maturation started during the Miocene and so the age of the structure may influence the charge risk.

Centre: limited current kitchen: due to the shortening during the quaternary, the Yopal thrust now bound the drainage area of the central structures. Maturation of the Cretaceous source rocks started at the early Miocene.

North: Leon Shales partially mature: in the north, the Gayabo Fm is up to 4km thick and therefore thick enough to allow the Leon Fm to be mature, and the deepest source rocks to be in the gas window. The maturation happens during the Gayabo deposition, i.e. during the last 10 Myr.

According to this, timing of hydrocarbon charge and level of hydrocarbon mixtures in the reservoirs changed from south to north affecting the charge and preservation risk for exploratory opportunities.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009