Example of a 3D salt interpretation model from offshore Brazil based on the integration of various geoscientific approaches.

The seismic imaging of salt is very complex.

Statoil researchers have developed an integrative, geologically constrained seismic imaging workflow.

This is sorely needed because the quality of PSDM12 (pre-stack depth migration) images around and beneath salt bodies depends entirely on the quality of the salt model, of which there are often several alternatives.

The primary aim of the workflow is thus to combine and integrate seismic interpretation with geological understanding (e.g. basin construction) and complementary geophysical approaches (e.g. gravimetery and magnetometry13 (grav/mag) modelling) to improve the input of seismic velocities into the PSDM software.  

The processing workflow essentially follows a classical Gulf of Mexico-style procedure, involving:

• interpretation of the top of the salt
• artificially inserting the body of the salt beneath it (a procedure known as salt flooding)
• interpreting the base of the salt from the resulting seismic

The workflow has been applied to a Statoil target using a series of interpretative iterations14 between seismic interpretation, grav/mag and basin reconstructions, and seismic processing.
 
Each iteration involves the building of a salt velocity model based on the grav/mag and basin reconstructions and using the velocities as input for depth migration.

The final delivery to the exploration team also includes the results of basin modelling15 and source rock prediction to provide a fuller assessment of prospectivity. 

This workflow is an important step forward, although geoscientists must recognize that each of the afore-mentioned methods and datasets are – to a lesser or greater extent – inherently uncertain.

All possible alternatives, both desirable and undesirable, should thus be considered, and the results must never be forced to meet pre-conceived ideas.

Having an understanding of, and a close relationship with, other geoscience disciplines makes it easier to spot alternative explanations from the start.

Whether exploring in salt-prone or salt-free basins, the key to reducing discovery costs lies in combining geological and geophysical knowledge, methodologies, technologies and results, and ensuring that the products directly impact the decision-making process.