Hydrodynamic stagnation zones: A new play concept for the Llanos Basin, Colombia

Hydraulic heads from a calibrated, three-dimensional, constant-density, ground-water-flow model were used to compute Hubbert oil potentials and infer secondary petroleum migration directions within the Llanos Basin, Colombia. The oil potentials for the C7 reservoir show evidence of the development of two hydrodynamic stagnation zones. Hydrodynamic effects on secondary oil migration are greatest in the eastern Llanos Basin, where structural slopes are lowest and local hydraulic-head gradients drive ground-water flow westward down structural dip. The Rubiales field, a large oil reservoir within the eastern Llanos Basin with no structural closure, is located at the edge of one of these stagnation zones. This oil field hosts heavy oils (12 degrees API) consistent with water washing and biodegradation. The best agreement between model results and field conditions occurred in an oil density of 12 degrees API, suggesting that the Rubiales field position is in dynamic equilibrium with modern hydraulic and oil density conditions. Cross sectional ground-water-flow models indicate that the most likely explanation of observed underpressures are caused by hydrodynamic effects associated with a topography-driven flow system. Late Miocene to present-day ground-water flow likely was an important factor in flushing marine connate porewaters from Tertiary reservoirs. Ground-water recharge along the western margin of the basin could help explain the observed low-temperature gradients (20 degrees C/km). However, upward flow rates were not high enough to account for elevated temperature gradients of 50 degrees C/km to the east.