INFLUENCE OF THE EVAPORITE FORMATION STRUCTURE ON SALT TECTONICS AND HYDROCARBON TRAPS (BY THE RESULTS OF NUMERICAL SIMULATION OF HALOKINESIS IN THE PRE-CASPIAN)

Abstract

Within the Pre-Caspian sedimentary basin, there is certain regularity in the distribution of types of structures of salt tectonics. It is characterized by concentric zoning corresponding to the change in the sedimentation thickness of the evaporite sequence. As it increases from 0 to 6 km towards the center of the basin, stamp (embryonic) salt uplifts are replaced by salt pillows first and then by salt domes and diapirs, finally changing to salt massifs and amoeboid-shaped salt ridges. In addition, diapirs in the sections of the Pre-Caspian basin, drawn from the drilling and seismic data, are shaped like high-amplitude "fingers" with a flat base, which is not a typical picture of the Rayleigh – Taylor instability development. Since halokinesis is the main factor controlling the migration and accumulation of hydrocarbons in the Preaspian region, background and aim of prospecting and exploration require identifying the specifics of the formation of various types of salt structures and the relationship between their location patterns and halokinesis process.Numerical simulation shows that, depending on the instable layer thickness and its relationship with the total thickness of the overlying layers, the instability development occurs at different rates, forming different types of structures. When the thickness of the instable layer is greater than or comparable to the thickness of the denser overburden, there occur the salt masses. A greater thickness of the overlying layer gives rise to the formation of classical mushroom-shaped diapirs. A small-thickness low-density layer first undergoes a full bending as it rises, so that its top and bottom turn out to be morphologically similar to each other, thus giving a misleading impression of ordinary stamp folds. Somewhat greater thickness of the low-density layer leads to the development of "pillows" therein. Detailed modeling made it possible to relate the specific shape of the Pre-Caspian diapirs to the fact that the basal and top horizons of the evaporite formation, being composed mainly of terrigenous, carbonate and sulfate rocks, have a normal, non-inverse density and mask complex diapiric structures of halite-saturated domal cores.