Three well-defined structural styles have been identified in this 700 km long by 100 km wide region that follows the eastern border of the mountain chains. In the central-northern part of the Cordillera Oriental there are both thin-skin and high-angle, basement involved tectonics. In the southern Andes de Merida and northern Cordillera oriental there are emergent imbricate high-angle faults involving basement. These two structural domains have opposite vergences and they are separated by a left-lateral transfer zone. Finally, the structural style of the northern portion of the Andes de Merida is controlled by the Caribbean allochthonous rocks, which override sedimentary sequences of the continental domain.

The Upper Cretaceous La Luna Formation is the main source rock in the entire region. Hydrocarbon generation in the northern Andes de Merida, occurred initially due to the Middle Eocene Caribbean nappes emplacement overriding sequences with La Luna included and later on, through burial by younger depositional sequences further south until Early Miocene time. In contrast, oil generation in the southern Andes de Merida and northern Cordillera Oriental area was due mainly to sedimentary overburden over the source rock from Late Oligocene to Middle Miocene. To the south, in the central Cordillera Oriental at least two generation stages occurred, the main generation being from the Late Miocene to the present as a combined process of tectonics and sedimentary overburden during and after the Andean uplift.

Using well logs and some conventional cores, a compaction model has been developed that better fits the measured porosity values of the reservoirs. Siliceous cementation of sandy sections results in a rapid decrease in Oligocene to Upper Cretaceous reservoir quality from the basin into the central Cordillera Oriental foothills. This diagenetic phenomenon may be due to tectonic deformation during Middle to Upper Miocene Andean uplift.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas