Abstract: Low Porosity/High Permeability, Deeply-Buried Reservoirs of the Cusiana Field, Llanos Foothills, Eastern Colombia: Controls on Reservoir Quality

Andy Pulham, Edward Warren, Joaquin Naar

The Cusiana Field is a thrust-related anticline located on the eastern edge of the Eastern Cordillera of the Colombian Andes. It comprises three clastic reservoirs: lower Tertiary Mirador and Barco Formations and the Santonian-Campanian Guadalupe Formation.

Sandstone porosity is generally low, averaging approximately 8% in the Mirador and Barco; and approximately 15% in the Guadalupe. Despite such low porosities, permeabilities are excellent and can exceed 1000mD. Detailed petrographic, sedimentological and biostratigraphic analyses have been used to identify the controls and distribution of porosity within the Cusiana Field.

The majority of the reservoir sandstones in Cusiana are near pure quartz arenites cemented by blocky, euhedral quartz overgrowths. Other cement phases, including kaolinite, are minor. The low porosity values throughout the reservoirs can be simply explained by compactional loss and relatively hot cementation by quartz during deep burial (>20,000 ft) The exception to the pure quartz arenites are zones in the Guadalupe reservoir that contain significant quantities (up to 20%) of detrital phosphate. These phosphatic sandstones lack the blocky quartz cements, contain pore lining microcrystalline quartz cement, minor calcite and clays (kaolinite and chlorite). They also have the best porosities (up to 22%). There is a direct relationship between presence and quantity of phosphate in the Guadalupe reservoir and increased reservoir quality.

The Cusiana reservoirs provide a good case study of porosity evolution in deeply buried, quartz-rich sandstones and the contrasts that can occur when a non-quartz component to the sandstones is introduced.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela