Subsidence History of the Continental Margin Offshore Northwest Colombia

Germán Y. Ojeda, Alexander Rey, and Andrés Reyes
Instituto Colombiano del Petróleo, Piedecuesta, Colombia

The late Cenozoic tectonostratigraphic development of NW Colombia's offshore basins is the result of the interplay of at least two geologic processes. Continued plate convergence between the Caribbean and South American plates provided the stresses that controlled deformation. In addition, the Magdalena river system, fed by the actively uplifting and eroding Andean orogen, largely controlled offshore depositional systems. To better understand the interaction amongst tectonics and the resulting sedimentary deposits, backstripping analysis of one offshore well was carried out. The resulting subsidence curve, after correction for paleobathymetry and eustasy, reveals three distinct episodes of differential subsidence rate. The earliest event started on, or prior to, 17 Ma and ceased around 13.5 Ma, with an average subsidence rate of ~190 m/My. This period was followed by a relatively quiescent stage between 13.5 Ma and 3.5 Ma, with an average subsidence rate of ~20 m/My. A final episode of increased subsidence started about 3.5 Ma and appears to continue today, with an average subsidence rate of some 120 m/My. The integrated subsidence rate, from seafloor to TD, is about 84 m/My. Rock facies drilled by this well can be interpreted as a function of these three subsidence phases. Sandy layers cluster toward the bottom and top of the well, and correlate closely with the two stages of relatively high subsidence rate. Fine-grained sediments span the period of slow subsidence, only disrupted by sporadic limestone and sandstone. This qualitative correlation represents a potential tool for predicting reservoir age, geometry and lithology offshore Colombia.