Seismic Forward Modelling of Internally Layered and Structured Salt Diapirs

Abstract

Seismic illumination of the interior of a salt body has not been a priority for most interpreters during hydrocarbon exploration and development. With the increasing necessity of safely storing energy (e.g., H2) or waste (e.g., CO2) in the subsurface, salt bodies, due to their dissolubility, inertness and impermeability, have become an interesting possibility. This study models the seismic response of the internal structure of salt bodies, through Seismic Forward Modeling: the methodology follows the selection of detailed cross-sections of salt diapirs, considering a variety of salt structures with different degrees of deformation and internal layering; then the construction of the elastic properties using a graphical approach; and finally the perform of the Finite Difference Elastic Wave Equation modeling and Migration, to generate a synthetic 2D seismic acquisition and a 2D seismic image. These data are processed using Pre-stack Depth Migration algorithms, such as Kirchhoff and RTM. Some of the geophysical parameters are varied to analyze their respective impact of the results, which is a 2D migrated seismic section, and with them, a reference system to identify the signals and events that appear inside the salt body (if they are real or artifacts), is obtained. The thesis focuses on Upper Permian Zechstein salt structures in the North Sea. Other geological variables, such as degree of salt mobilization, geometry and internal structure of the body, are evaluated to understand their effect in the seismic image around and inside the salt body.

One of the most important findings obtained from this work, is that it is possible to illuminate the most contrasting stringers and layers inside a salt diapir, if they are over the seismic resolution and their geometry is not considerably complex. However, only the first 1000 meters of the diapir can be focused properly, leaving the deeper layers with not enough signal to be interpretable. Also, choosing the halite P-wave velocity works as a good first approximation at the moment of migrating the data, showing the existence of internal layer, but with differences in the real positioning and with some degradation of the signal. Despite the limitations of the method, these results are fundamental for de-risking salt storage projects, where the characterization of the facies and physical properties inside the salt bodies, are mainly achieved from the interpretation of seismic data. Also, a better comprehension of the petroleum system is obtained by modeling the salt structures and getting their image response on the surrounding minibasins, which is essential for de-risking salt-related structural or stratigraphic traps.