Entrapment of Heavy Oils in the Southern Llanos Basin, Colombia: Controlling Geologic Factors and Potential of a Major Emerging Province

De Freitas, Mario G.¹; Gil, Wilson; Bueno, Lenin; and Fajardo, Andres
¹[email protected]

The Llanos foreland basin of eastern Colombia is the country’s most prolific hydrocarbon province. Although the Southern Llanos Basin (SLB) contains over 10 billion barrels of OOIP (mostly heavy oils), a large portion of it remains underexplored and poorly understood, given its more complex tectonostratigraphic development associated with the Neogene uplift of the adjacent Eastern Cordillera and Macarena Range. The known accumulations occur primarily in two main groups, the Castilla cluster, with dominantly compressional structures for Cretaceous reservoirs in the foothills of the Eastern Cordillera, and the Rubiales cluster, with predominantly subtle extensional fault closures for Upper Oligocene – Lower Miocene reservoirs towards the southeastern flank of the basin.

Recently, important accumulations of heavy and extra-heavy (<10 API) oils are being gradually unraveled in more subtle traps in the SLB, anticipating a major emerging province. A complex interplay of controlling geologic factors account for the poor understanding to date of the existing plays and the assessment of their ultimate potential.

We assessed the distribution and key geologic controls on the oil entrapment in the SLB integrating a large regional dataset including 2D, 3D seismic data, gravity, exploratory and stratigraphic well logs, cores, rock and oil samples and geochemical data.

Cross-plots of structure and isochore maps with temperature data suggest a strong control of the underlying Paleozoic sequence thickness on the temperate gradients, which played an important role in oil migration for the Cretaceous to Neogene reservoirs. Paleotopographic features, especially the preserved abrupt escarpments along the top Paleozoic regional unconformity, played a key control on the distribution of the reservoir units and the creation of stratigraphic traps. The onlapping patterns of the successive sandstone reservoir units towards the basin edge show a series of embayments, which generate large traps where a working combination of reservoir and seals (top and bottom) occur. We envisage a multi-billion barrel OOIP potential in this play type.

A multi-phase oil migration is inferred from the distribution of oil types and mixtures in known accumulations. Viscosity entrapment of extra-heavy oil is inferred to form a belt along the southern edge of SLB, as well as the hydrodynamic trapping associated with recharge areas in the Macarena range, both also containing multi-billion barrel OOIP.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013