Ocena stabilności pokładów soli cechsztyńskiej na wyniesieniu Łeby pod kątem lokalizacji magazynów paliw płynnych

Abstract

Perspectives of Zechstein salt stability on the £eba Elevation and location of liquid fuel storageA b s t r a c t . Three fundamental factors have to be taken into account in considering stability of the salt seams for location of liquid fuel storage: (1) tectonic structure, (2) recent geodynamics and (3) salt rheology under given conditions of pressure and temperature. The first tectonic factor is pretty favorable for salt deposits in the £eba Elevation area. In an interior of the East European Craton, far from the Teisseyre Tornquist fault zone, there is lack of faults that might cross-cut the salt complex. Minor tectonic dislocations as well as salt thickness do not favor halokinetic movements. Also minimal degree of compressive deformation in the Laramide phase of Mid-Polish Trough inversion cause the structure of salt seams to be relatively simple. Present-day geodynamics is also in favor of the salt stability. In accordance with the existing record of recent and historical earthquakes the discussed area is aseismic. Also the recent tectonic stress (differential stress) in the sedimentary cover seems to be small. This is indicated by scarcity of so-called borehole breakouts (compressive failures due to concentration of stress at the borehole wall) as well as the results of numerical simulation. Negligible rate of craton deformation predicted by thermomechanical modeling (< 10-18 s-1) suggests that tectonic stress in rock salt should be less than 1 MPa. Rheological properties of the rock salt depend on pressure and temperature, which, in turn, are a function of depth and lithospheric structure. Salt deposits in the Łeba Elevation are at a depth of approximately 1.000 m, where there are physical conditions in which salt is deforming primarily by dislocation creep mechanism. Only with the participation of free water content additional mechanism of dissolution and precipitation under pressure is triggered. Because of the expected minimal size of tectonic stress in salt complex, differential stress at the wall storage chamber results mainly from the difference between lithostatic load and hydrostatic pressure in the chambers. Laboratory tests of dislocation creep of the rock salt indicate that the strain at the edge of the salt chambers may attain a rate from 10-10 s-1 to 10-9 s-1. These are the rates at which 1% strain is attained in 0.3 to 3 years, which should provide sufficient stability of the salt chambers. Preliminary estimations indicate that the salt deposits located in Łeba Elevation area offer optimal conditions for the construction of liquid fuel storages. Further progress in assessing the geomechanical suitability of the salt complex for the purpose of storage will be possible using numerical simulation models, when the structure is characterized by high quality of seismic images, and rheological salt parameters are determined by mean of laboratory tests of the real core samples from the salt layers