Fluvial Geomorphology Changes Linked to Tectonic Effects during the Late Eocene — Oligocene in the Northern Llanos Foreland Basin of Colombia
Torrado, Lucia; Mann, Paul; Bhattacharya, Janok
The Llanos foreland basin (LFB) of Colombia is the country's most prolific oil producer. Generally, this oil is found in monocline structures associated to antithetic faults. The objective of the study is to document changes in fluvial style within the Late Eocene-Oligocene Carbonera Formation, a reservoir of 400-1800 m thick of fluvial sediments with good exploration potential of stratigraphic traps. We have integrated 700 km2 of 3-D seismic data volumes tied to 9 wells in the eastern Casanare province near the Jordan oil field, where seismic and gamma ray facies analysis along with multi attribute analysis (coherence, curvature and shale indicator) were conducted for a better understanding of reservoir distribution. The Carbonera Formation is divided into eight members; C7, C5, C3 and C1 are typically clean, and well sorted sandstones with porosity up to 20%. Shales, plant debris and coals indicate floodplain aggradation in the members C8, C6, C4 and C2 with water-mixing influence from a northern connection with the Caribbean. The unit represents river-dominated and tidally-influenced deposits of straight to meandering channel belts seen as strong, high amplitude, concave reflections in the seismic, with a) high to moderate Width/Depth (W/D) ratio and moderate sinuosity for the main drainage, b) low W/D ratio and low sinuosity for the tributary channels, c) sand content decrement in the middle members, and d) wider channels with thicker floodplain deposits in the members C5 and C3 with localized sand bodies. Burial curves calculated from well data, showed an increase of total subsidence due to the initial uplift of the Eastern Cordillera in Oligocene time; widening of the basin occurred, allowing deposition further east. The source area shifted from the Guyana Shield represented by the members C7 and C5, to an increased influence from denudation of the eastern flank of the young Eastern Cordillera, represented by the members C3 and C1. Likewise, horizon time slices show changes in flow direction from southwest to northeast, controlled by the eastward migration of the flexure, to a northwest-southeast flow direction in the Middle Oligocene, interpreted as development of tributaries feeding the major fluvial system. The increase in accommodation and slope lowering resulted in short-lived, extensive embayments of brackish water which periodically flooded the fluvial plain of the precursor of today's Orinoco River.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013