LAUSR

Abstract We use a 3D seismic dataset and wireline logs obtained during hydrocarbon exploration to describe the three-dimensional geometry and characteristics of mafic igneous rocks in the Cerro Dragon oilfield, Golfo San Jorge Basin, southern Argentina. Igneous intrusions occur within Upper Cretaceous and Paleocene sedimentary rocks at depths ranging from 2600 m to a ~200 m below of contemporaneous surface. Three distinctive types of shallow intrusions were recognized (layer-parallel, saucer-shaped, and transgressive sills). The largest sills are layer-parallel, some with estimated volumes up to 9,5 km3, whereas saucer-shaped sills and transgressive sills show widely variable intrusion geometries due to interaction with previous extensional tectonic structures and intrusion-related dikes. Sill intrusions were sourced either through vertical mafic dikes of NNW to NNE orientation, some up to 19 km in length and by sill-sill vertical connections. The presence of three imaged lava flows documents subaerial volcanic events coincident with the deposition of the Sarmiento Formation (late Eocene to early Miocene), whose trajectories were traced for 25–46 km of length. Cenozoic igneous intrusions produce both changes in the temperature history of the basin, and the remobilization of hosted hydrocarbon through the creation of supra-intrusion forced folds and new pathways generated during the igneous emplacement. Besides, sills and dikes act as barriers to both lateral and vertical fluid migration, and thus they need to be considered in petroleum prospect analysis.