Controls on the Reservoir Quality of the Mirador Formation, Cusiana Field, Colombia

Edward A. Warren, Andrew J. Pulham

The Mirador Formation is a sandstone-rich depositional unit of Middle to Late Eocene age that is developed over much of the central and western Llanos Basin, Eastern Colombia. In the Cusiana Field (British Petroleum, Ecopetrol, Total and Triton) it comprises the youngest of several major reservoir units. The reservoir quality of Mirador sandstones in the Cusiana Field is characterised by low porosity (typically less than 10 percent) but high permeability (up to 4000 millidarcies) quite unlike anything seen before in British Petroleum producing fields. Furthermore, flow rates in excess of 10,000 barrels of oil per day have been achieved. The unusually low effective porosities of the Mirador sandstones lead to early interpretations of quite complex controls on reservoir qua ity. Two properties that were considered were secondary porosity and conductive fractures.

Detailed petrographic and geochemical analysis reveals that the sandstones are quartzose and quartz cement is the only major porosity reducer. Reservoir quality within the Mirador Formation is controlled by facies (especially detrital clay content) and by grain size.

Comparison with other quartzose sandstones around the world reveals that the porosity and permeability of the Mirador sandstones is similar to other quartz-cemented quartzose sands, such as the Fontainebleau Sandstone (oligocene), Paris Basin, France. Consequently, these Cusiana sandstones appear to be representative of very simple sandstones in which the only major poroperm modifiers are compaction and quartz cement. Other deeply buried quartzose sands may thus retain high permeability at very low porosity.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995