From Facies Analysis, Stratigraphic Surfaces and Depositional Sequences to Stratigraphic Traps in the Eocene – Oligocene Record of the Southern Llanos and Northern Magdalena Basin

By Víctor M. CABALLERO, Guillermo RODRÍGUEZ, Julián F. NARANJO, Andrés MORA and Felipe DE LA PARRA

Pages 203–231

Abstract 

Available outcrop sections, rock cores and well logs as well as previous studies in sedimentary geology and palynology, provide the opportunity to study shallow–marine and continental rock record within the context of unconformity bounded depositional sequences. This approach provides insights to the sedimentary evolution of reservoirs and its properties the Middle Magdalena Basin (MMB) and southern Llanos Basin (SLLB) in Colombia.

This work present the illustration and analysis of facies and facies successions of the Eocene – Oligocene stratigraphic units in the Nuevo Mundo Syncline (NMS) of the northern MMB and the SLLB. The facies analysis support the identification of subaerial unconformities, transgressive ravinement surface, flooding surfaces, depositional environments and depositional sequences. Stratigraphic correlation allows to identify the spatial and temporal distribution of facies, stratigraphic architecture of reservoirs and potential of several types of plays in these basins, besides the common stratigraphic histories.

Thirteen facies that combined into ten facies successions led to identify three depositional sequences in the SLLB and two in the NMS. A strongly developed paleosol and an unconformity at the base of the Eocene units allowed to identify a buried, fossilized landscape in the southern Llanos Basin at the end of the Paleocene, these same relief features have been found in the MMB and Eastern Cordillera (EC). The first depositional sequence comprises the Lower Mirador Upper Mirador and lower part of C8 Member of the Carbonera Formation (MCF) in the SLLB; La Paz and Esmeraldas Formations in the NMS. The second depositional sequence is configured by the Oligocene basal sandstones and the upper part of the C8 MCF in southern Llanos Basin and the Oligocene Mugrosa Formation in the MMB. The third sequence is composed of both the C7 and C6 MCF in the SLLB.

Porosity and permeability values allowed to identify the better reservoir units in these basins and the geometry and lithology below and above the buried landscape allow to explain several types of traps/plays that must be searched for in these basins: paleogeomorphic traps such as buried hills, fluvial and incised valleys and erosional pinchouts; stratigraphic traps already identified such as depositional pitchouts, onlap pinchouts and facies changes.

Keywords: origin and depositional history of sedimentary basins, facies analysis, facies successions, sequence stratigraphy correlation, Llanos Basin, Middle Magdalena Basin, reservoir architecture, stratigraphic trap, reservoir properties.