Magnesium-salt exploration in the northeastern Netherlands

Abstract

Magnesium-bearing salts occur in the Zechstein Basin in the northeastern Netherlands at a depth of 1500 to 2500 m. Exploration has resulted in the discovery of significant amounts of this salt which will be exploited by solution mining for the production of refractory grade magnesiumoxide. Of the four main evaporite cycles, Zechstein III (the ore-bearing cycle) is fully developed and contains the magnesium salts carnallite (MgCl2.KCI.6H2O), bischofite (MgCl2.6H2O) and kieserite (MgSO4.H2O). The salt sequence has been affected by halokinesis, resulting in salt domes and other structures. The magnesium-bearing layers were thickened towards the top of the structures, thus forming prime targets for exploration. Exploration techniques comprise seismic surveys and rotary drilling. Conventional seismic methods adequately define depth and structural configuration, whilst high-resolution seismic wavelet processing is used for the identification of the magnesium-salt layers on the seismic section and determination of thickness. Seismic sections are converted into acoustic impedance sections which are calibrated from acoustic impedance logs. These in turn are derived from velocity and density measurements in bore holes. The mineral composition, ore grade, and thickness can be accurately determined by petrophysical well-logging: in particular formation density, gamma ray, and sonic logs. Results are calibrated by chemical analysis of drillcores.