The Application of Inverse Modeling to Remove the Gravity Effect of Salt Structures Along the Brazilian Southeastern Marginal Basins
Structural framework delineation is fundamental to understanding the origin and evolution of a sedimentary basin. To achieve this objective, gravity data interpretation and modeling act as very useful tools, giving the opportunity of identifying basin-wide highs and lows and, in some cases, helping other exploratory tools (e.g. seismic processing). On the other hand, it is necessary to keep in mind that gravity is highly influenced by lateral rock density variations, many times masking geological features and hindering the interpretation and delineation of such structures.
Along the southeast Brazilian marginal basins, the geological evolution during the Gondwana break-up process had created the depositional conditions for the deposition of an impressive thick evaporite sequence. During the subsequent basin development, represented by a thick open marine transgressive/regressive megasequence, the original evaporitic layer was deformed, creating a countless number of halokenetic structures, placing the halokinesys as the main geological process responsible for the existence of the most significant petroleum system of Brazil. Despite this important role, the density difference between the most common evaporites that constitute the salt bodies and the siliciclastic/carbonate sediments and the volcanic intrusion creates a disturbance on the gravity field, overprinting the basement structural features and making the interpretation of the deeper basin geometry difficult.
In order to remove such a disagreeable effect, the application of gravity inverse modeling is proposed, by the creation of a layering model based on regional interpretation conducted by the Vale E&P team along the Santos, Campos and Espirito Santo basins. The model was populated by densities from well data and also inferred by the regional geological knowledge, creating a gravity response comparable with the real gravity data. The “back-stripped” gravity maps resulting from this modeling contribute to understanding the structural framework of the basins, as well as to helping the seismic interpretation along areas with poor seismic imaging.