AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

The Origin of Salt-Encased Sediment Packages: Observations From the SE Precaspian Basin (Kazakhstan)


Intrasalt sediment packages containing different amounts of siliciclastic or carbonate sediments or other non-halite evaporites, such as gypsum or anhydrite are common within most salt sequences. Intrasalt sediment packages may have been deposited at different stratigraphic positions with respect to the salt in which they are contained (i.e., before, during, or after deposition of the autochthonous salt) and been incorporated into the salt sequence by different processes. Recognizing intrasalt inclusions and then integrating them into velocity models can improve seismic imaging of adjacent and deeper stratigraphic sections, and, ultimately, help define hydrocarbon plays involving intrasalt or subsalt reservoirs. Furthermore, understanding the origin and evolution of intrasalt sediment packages may yield important insights into the tectonic and geodynamic history of the basin, as well as into our understanding of salt tectonics. Despite the importance of intrasalt sediment packages, there has been no attempt to systematically describe their origin and establish identifying criteria. In this work, we first outline the possible origins of intrasalt sediment packages, addressing questions such as: Were they originated in subsalt, intrasalt, or suprasalt position? How are they incorporated into salt? Subsequently, we examine how sediment packages that originated on top of salt, such as minibasins, can be encased within salt. We propose four key processes by which salt can be emplaced above the minibasins to encase them. These processes include: a) salt expulsion from beneath a minibasin experiencing density-driven subsidence; b) salt expulsion from beneath adjacent subsiding minibasins; c) salt expulsion associated with lateral shortening; d) override of minibasins by a salt sheet sourced from elsewhere. Finally, we present a case study from the SE Precaspian Basin, Kazakhstan, where, using a borehole-constrained 3D seismic reflection dataset, we show how the proposed criteria can be applied in an area with abundant, newly discovered sediment packages encased within salt.