Non-Marine, Late Eocene-Oligocene Sequence Stratigraphy and Changing Fluvial Style in the Northern Llanos Foreland Basin of Colombia

The Llanos foreland basin (LFB) of Colombia is the country's most prolific oil producer, with most known oil fields found in anticlines bounded by normal faults. The objective of the study is to assess the reservoir potential of the Late Eocene-Oligocene Carbonera Formation, a 400-1800m-thick reservoir unit of fluvial deposits with many areas of unexplored stratigraphic traps related to unfaulted, sand-filled fluvial channels. We integrated 700 km² of 3D seismic data volumes with 9 wells in the eastern Casanare province near the Jordan oil field, and additional 2D lines tied to 32 wells in a 17 168 km² area near the Rubiales field. Interpretation of reservoir distribution including a gamma-ray facies analysis of wells were combined with interpretation of multiple 3D attributes including coherence, curvature, and spectral decomposition. Well analysis shows that of the eight members of the Carbonera Formation, two members, C7 and C1, are clean, well-sorted fluvial sandstones with porosities up to 20%. Mineralogy, plant debris, and coals indicate floodplain aggradation in members C8, C6, and C4 with brackish water indicating fluvial connectivity to the Caribbean Sea to the northeast. Correlations between wells and attribute maps show that the Carbonera Formation represents tidally influenced fluvial deposits within straight to meandering channel belts seen as strong, high-amplitude, concave reflections on seismic data. Well logs record two regressive- transgressive cycles with higher preservation of lowstand deposits. On seismic time slices, channel belts exhibit: 1) moderate width to depth (W/D) ratio and high sinuosity for the main drainage; 2) low to moderate width to depth (W/D) ratio and low sinuosity for the tributary channels; 3) a decrease of sand content in the middle members of the formation; and 4) wider channels with thicker floodplain deposits and localized sandstone bodies in members C5 and C3. Well subsidence curves from this area of the LFB show an increase of total subsidence due to the rapid uplift of the Eastern Cordillera in Oligocene time and eastward widening of the LFB. The clastic source area for the LFB shifted from the Guyana Shield to the east (represented by the members C7 and C5) to a source area produced by the uplift and erosion of the Eastern Cordillera to the west (represented by the members C3 and C1). Flattened time slices show changes in paleoflow directions of rivers from southwest to northeast, controlled by the eastward migration of the flexural high of the LFB in the Late Eocene–Early Oligocene, to a NW to SE direction of flow in the Middle Oligocene that likely corresponds to the development of tributaries feeding a larger axial fluvial system. Increases in short-lived accommodation of the LFB led to large embayments produced by the ingress of brackish waters up the main fluvial system from the Caribbean Sea.