Salt, sediment and tar interactions and the origin of tar mounds in the Green Canyon area of the Gulf of Mexico

The Gulf of Mexico (GOM) is a structurally complex basin with a prolific petroleum system. The main topic of concern in this study is the origin of tar and its relation to the formation of tar mounds in the GOM Basin. The Cenozoic structural history of the GOM Basin is directly related to the mobilization of Jurassic salt due to the increase in sediment load on the salt. It is suggested that salt tectonics significantly influences the expulsion of tar via faults to the sea floor.;This thesis study aims to provide evidence or refute previous understanding of the origin and migration asphaltenes to surface tar mounds in the Green Canyon area of the Gulf of Mexico. Tar originates from the first oils produced by source rocks, which are usually rich in asphaltenes. Hence early oils have a higher density and more asphaltenes than later oils. When lighter oils are generated from the source, they migrate up-dip towards the reservoir and displace the early heavy oils. The later lighter gas bearing 'oils allow for the precipitation of asphaltene by "gas stripping" along the migration pathway (Mueller, 2011).;A model for the migration of tar in the study area was developed to explain the occurrence of surface tar mounds, which may further assist in the understanding of subsurface tar mats which are commonly found at the base of allochthonous Sigsbee salt canopy as seen for example in the Mad dog Field (Romo et al., 2007) in GOM. Three-dimensional seismic reflection data from the area has been licensed from WesternGeco for the purpose of this study, which will allow for surface and near-surface seismic interpretations and mapping of tar mounds previously identified at the sea-floor above the Shenzi Field (Williamson et al., 2008). It was found that mounds at the surface where not clearly resolved with conventional 3D seismic data. However, the migration pathways can be resolved.;The result of this research shows two possible theories for the formation of tar mounds on the sea floor based on the evidence provided. The first is supported by the presence of a salt suture at the base of a normal fault that cuts about 8000-9000 feet from the top of salt to the sea floor through the underlying Plio-Pleistocene sections. It is suggested that this salt suture was once an allochthonous salt window that allowed for the migration of asphaltenes to the surface via the overlying normal fault. The second theory is supported by the presence of a sandy carrier bed that runs into the normal fault described above. Tar is believed to have migrated through a salt window outside of the study area and into this carrier bed.;The model developed for these results predicts tar mounds are deposited in the topographic low on the downthrown hanging wall of extensional normal faults. The tar mounds are less than 5 feet high and cover approximately 610 meters (2000 ft.) by 152 meters (500 ft.). This model could also be applied to predicting the location of subsalt tar mats commonly found while drilling at the base of the allochthonous Sigsbee Salt canopy.