SEISMO-STRATIGRAPHY AND 3D MODELLING OF HYDROCARBON LEAKAGE IN THE COLORADO BASIN, OFFSHORE ARGENTINA

The analysis of a dense 2D seismic reflection dataset and data from 12 exploration wells, allowed us to identified the main syn- and post-rift seismo-stratigraphic sequences, as well as a variety of liquid/gas hydrocarbon-leakage indicators, on the continental shelf and slope of the Colorado Basin.The sequence maps (twt) show a shifting configuration from the break-up unconformity (130 Ma) to the present-day seafloor. The break-up unconformity displays a central EW-elongated graben, which prevails on the overlying sequences up to the Miocene. Sedimentation during the basin sag phase (Aptian to Campanian) is mainly located in the central part of the basin. During the Paleocene there is a discrete eastward depocentre migration. An important transgression, accompanied by aggradation and decrease in sedimentation rate, sets on during the Eocene. A dramatic increase in sediment supply takes place from the Oligocene up to the Present, driving a depocentre migration to the outer part of the basin.

Mapping of paleo- and present-day hydrocarbon-leakage features, such as gas chimneys, mud volcanoes and seabed pockmarks, provided insights on the definition of potential migration pathways.We identified two populations of vertical seismic pipes, with diameters from 100 m up to 2000 m. One population is concentrated in the central part of the basin and the pipes end within the Eocene sequences. They seem linked to volcanic intervals drilled by wells. The other population is found on the slope of the basin, the pipes reach up to the seafloor and end in pockmark depressions or seabed mounds. 3D seismic reflection data show that these pockmarks, have diameters from 200 m to 900 m and are 20 m to 100 m depth. They located close to an array of slope submarine channels. A highly vertically-faulted interval has been identified within the Campanian and Paleocene sequences. These faults correspond to a polygonal-fault system that could breach sealing sequences, allowing vertical or sub-vertical fluid flow.

Results from a calibrated 3D migration model indicate that although pre-rift and early Cretaceous source rocks (SR) intervals would be depleted, there exists an active kitchen from the Aptian SR, which might be feeding the observed gas chimneys and seabed pockmarks on the slope of the basin.