Reservoir Performance of Late Eocene Incised Valley Fills, Cusiana Field, Llanos Foothills, Eastern Colombia.

Andy Pulham, Edward Warren, and Jeff App

The Cusiana Field is located in the Llanos Foothills of Eastern Colombia. The principal reservoir is the late Eocene Mirador Formation which comprises >50% of reserves. Currently (September 1995) the Mirador reservoir is providing nearly all of the 150,000bopd of production from the Cusiana Field.

The Mirador reservoir comprises a stack of Incised valley deposits. The fills of the valleys are dominated by quartz arenite sandstones. The average porosity of the valley sandstones is 8% which reflects abundant quartz cement (~14%) and significant compaction during deep burial (~20,000 feet). Single valleys are up to 70 ft thick and exhibit a distinctive bipartite fill that reflects changing energy conditions during filling. Bases of valleys have the coarsest grain size and have sedimentological and trace fossil evidence for deposition in highly stressed, brackish water environments. The upper parts of the valleys are typically finer grained and were deposited in more saline settings.

Despite the low porosity of the Mirador valleys, drill stem tests and production log data show that they have phenomenal performance characteristics. Rates of >=10,000bopd are achieved from single valleys. Bases of the valley fills are the key contributors to flow. Integration of detailed core and pore system analysis with the reservoir performance data shows that the permeability fabric of the Mirador can be explained by original depositional architecture and simple loss of primary porosity. Comparison of Cusiana with other quartz-rich sandstones from around the world suggests that its low porosity/high performance is predictable.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California