Non-marine Sequence Stratigraphy and Changing Fluvial Style in the Northern Llanos Foreland Basin of Colombia
The Llanos foreland basin is Colombia’s major oil producing basin with a production of 480 KBPD. Most oil is found in fluvial channel belts ranging in age from Early Eocene to Oligocene and has been mapped from closely spaced wells or from detailed interpretations of 3D seismic volumes. A major reason for well failure in the Llanos basin has been the inability for interpreters to distinguish non-productive mud-filled channels from productive sand-filled channel belts since both channel types can appear similar on seismic sections. The focus of this study is to document change in fluvial style within a sequence stratigraphic framework to better predict reservoir distribution. In this study, we have integrated 500 km2 of 3D seismic data volumes tied to 15 wells in the eastern Casanare province near the Jordan oil field that image a 400-1800-m-thick section of fluvial sediments of the Late Eocene-Oligocene Carbonera Formation. Source rocks include the underlying Gacheta Formation of late Cretaceous age; reservoirs within the Carbonera include the Carbonera 5 and 7 sandy members that are clean, well sorted sandstone with porosity up to 20%. Horizontal time slices through the Carbonera Formation show fluvial channel belts that flowed obliquely (NW to SE) across the trend of the modern Llanos basin with the source area of sands being continental rocks of the Guyana shield. Deltaic sediments have been described in the distal parts of the fluvial network in the area to the southwest of the study area. Well logs show that sediments were deposited in a coastal plain containing numerous, fluvial channel belts. Coals, in members C1, C3, C5 and C7 of the Carbonera formation indicate humid swampy conditions with rapid rates of subsidence. River patterns seen in plan-view vary from anastomosed to straight. Incised channel belts, greater than 30 meters, are interpreted as incised valleys that may show an estuarine fill, and indicate significant falls of sea-level alternated with times of floodplain aggradation.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California