Identification of the Processes Controlling the Distribution and Quality of Oil Accumulations of the Southern Llanos Basin, Colombia

Bernardo, Luis Miguel¹, Jesus Augusto Rodriguez¹ (1) Ecopetrol ICP, Bucaramanga, Colombia

The Llanos basin, a foreland basin with the first heavy and conventional oil reserves in Colombia, has vast heavy-oil potential. In the southern Llanos Basin are two mayor heavy oil fields -- the Castilla and Rubiales, which have been found along with a set of minor oil accumulations (10 and 30 ºAPI). Three main factors control the distribution and quality of oil accumulation in the area. The first is the existence of two distinct crude oils generated by dissimilar source rocks at different times. The first oil was generated by Cretaceous marine source-rocks, which was emplaced during Paleocene to Oligocene time into the original configuration of the basin. Much of this oil trapped in Laramide structures was affected by severe biodegradation, and transformed into heavy-oil. The second distinct oil was generated from Tertiary terrestrial source-rocks and emplaced during the last 10 Ma into the actual configuration of the basin. This second oil emplaced in traps that exceeded 80ºC was isolated from biodegradation keeping its original properties. So, the addition of the second non-biodegraded oil improved the quality of the previously heavily biodegradated oil, forming an oil mixture intermediate between a heavy oil with severe biodegradation, and one with no biodegradation. All the oils accumulated are admixed in varying proportions in the area of this two end members. The third factor was an intense hydrodynamic system that flushed out most of the southern accumulations to the north, resulting in new heavy-oil accumulations within hydrodynamics traps with attractive chemical properties.