Stratigraphic architecture of incised valleys and unincised channel systems in the Carbonera Formation (C6-C1 members: Upper Oligocene – Lower Miocene), Llanos basin, Colombia

Abstract

This work presents a sequence stratigraphic framework for the Upper Oligocene – Lower Miocene C6-C1 members of the Carbonera Formation in the Llanos basin, Colombia. In the interval corresponding to the C5 member, facies associations that comprise middle shoreface, tidal channels and bars, tidal flat deposits, brackish bay, tidal-fluvial channels and central estuary have been identified through the integration of core descriptions and wireline logs. The vertical assemblage of the recognized facies associations permits the definition of several cycles of transgression and regression, as well as episodes of relative sea-level fall and their consequent fluvial incision. Basinward shifts of facies, lateral changes in well-log motifs and seismic amplitude anomalies allow the interpretation of incised-valley systems overlying deposits associated with marginal and shallow marine environments, which also present channelized sand bodies. Consequently, the sedimentary and stratigraphic architecture is composed of an interspersed set of channels with different genetic origins and contrasting potentials in terms of petroleum production. Maximum flooding surfaces, subaerial unconformities and transgressive ravinement surfaces are the significant stratigraphic surfaces identified, which separate progradational and retrogradational stages and define sequence boundaries.In addition to the C5 member, a sequence stratigraphic model for the interval corresponding to the C6-C1 members of the Carbonera formation is presented based on cuttings, well-log stacking patterns and interpretation of 3D seismic horizon and time slices. In total, nine regressive-transgressive cycles (i.e., genetic stratigraphic sequences) are interpreted for the interval studied. The proposed stratigraphic framework relates oil occurrences and currently producing intervals with stratigraphic surfaces and permits a consistent classification of the petrophysical properties, establishing several families that are genetically related.The sedimentological descriptions of the Carbonera C5 member and the proposed sequence stratigraphic model highlight the marine influence in the deposition of the Carbonera Formation. Furthermore, this work emphasizes the role of relative sea-level fluctuations in deposition and erosion in coastal areas during tectonically active periods in the foreland Llanos basin.