Deformation mechanisms and rheology of Pre-cambrian rocksalt from the South Oman Salt Basin

The Neoproterozoic to Early Cambrian Ara Salt from the South Oman Salt Basin has been investigated for creep behavior and microstructure. The salt behavior is of relevance for oil production out of intra-salt carbonate “stringers”. Test and in-situ conditions are about T = 100°C and p_c = 70 MPa. Strain rate stepping and temperature stepping tests have been performed, with creep rates varying from 5 10⁻⁸ to 10⁻⁵ s⁻¹. All tests were run to a total strain of 25–30%. Both secondary and primary creep parameters have been determined. Secondary creep is basically reached only at rates of 10⁻⁷ s⁻¹ or lower (15 MPa differential stress or lower), still orders of magnitude faster than expected in-situ rates, except perhaps borehole closure related creep during drilling. Microstructural observations revealed that the main deformation mechanism during the tests was dislocation creep associated with grain boundary migration recrystallization. This is evidenced by subgrain size reduction, the development of slip lines and the formation of new strain-free grains growing at the expense of subgrain-rich grains. Comparing literature data, the Ara Salt appears to be quite common (average), both under the microscope as in creep behavior.