Integrated seismic and well-log analysis for the exploration of stratigraphic traps in the Carbonera Formation, Llanos foreland basin of Colombia

Abstract

The Llanos foreland basin of Colombia is the country's most prolific oil producer, with the most known oil fields found in normal fault traps created during flexure of the foreland basin. The objective of the study is to evaluate the potential of stratigraphic traps in the Late Eocene-Early Miocene Carbonera Formation, a 400-1800 m-thick reservoir unit with numerous traps related to unfaulted, sand-prone fluvial deposits. We integrated 700 km2 of 3D seismic data with eight wells near the ~8600 bcd Cubiro field in the central area of the Llanos foreland. Interpretations of well logs and stratal slices through multi-attribute and iso-frequency amplitude cubes show that the Carbonera Formation contains tectonically-controlled, sinuous channel belts seen on seismic lines as strong, high-amplitude, concave reflections with variable width-to-depth ratio. Well logs through the Carbonera Formation record two regressive-transgressive cycles with greater preservation of non-reservoir, shaly transgressive deposits within the intermediate Carbonera members 5 and 3, and sandy, lowstand deposits in the lower and uppermost Carbonera members 7 and 1, respectively. Seismic and gamma-ray facies analyses allowed the distinction of prospective, sand-rich point bars, scrolls, and basal lag deposits from non-prospective, mud-rich abandoned channels. Stratal slices within the Carbonera Formation show changes from Late Eocene-Early Oligocene, northeast-flowing channel belts of a main axial fluvial system -proposed to be the proto-Meta River-, to a Late Oligocene southeast-flowing tributary channels interpreted as a result of eastward flexural migration, enhanced accommodation from loading of the Eastern Cordillera and a shift from an underfilled to overfilled foreland basin.